1. Introduction

Metals play a key role in many photonic and optoelectronic device applications. The large number of free electrons in metals make them highly conductive and suitable for constructing antennas for a wide frequency range, i.e., from the radio frequency (RF) to the optical spectral region. In addition, the collective oscillatory motion of free electrons in a metallic structure driven by an electromagnetic field can lead to strong plasmonic responses, which are associated with high field confinement/enhancement and hence enhanced light-matter interactions. Existing photonic structures and devices mostly employ metals in the solid phase, such as gold (Au), silver (Ag), copper (Cu), and aluminum (Al) [1,2]. These metals have excellent material properties (e.g., high electrical and thermal conductivities) and are widely utilized in devices based on rigid substrates (e.g., semiconductors, glass). However, the solid metals are in general not suitable for applications where large flexibility, stretchability and transformability of the devices are required. In contrast, liquid metals can naturally meet these unconventional requirements of emerging device applications. Here, liquid metals refer to the metals in liquid phase near room temperature. There are only a limited numbers of elemental metals which are in liquid phase near room temperature, including mercury (Hg), gallium (Ga), rubidium (Rb), cesium (Cs) and francium (Fr). A few additional known liquid metals are alloys of low-melting-point elemental metals, as alloys can have lower melting points than the constituent elemental metals. Table 1 summarizes the variety of liquid metals and their melting points. Besides the conductivity and melting point, other properties of the liquid metals should also be taken into account when determining their suitability for specific applications. Hg has high toxicity and high vapor pressure (easy to evaporate at room temperature) and therefore is not suitable for most applications. On the other hand, the alkali metals including Rb, Cs, Fr and the sodium-potassium alloy (NaK) are highly reactive (Cs and Fr are also radioactive), which severely limit the scope of their applications. Fortunately, Ga and Ga-rich alloys do not have these major disadvantages and are currently the only liquid metals suitable for a wide range of applications.

Table 1. Melting points and major drawbacks of near-room-temperature liquid metals

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The melting point of Ga is 29.8°C, which is close to room temperature and hence convenient for many applications. On the other hand, the boiling point of liquid Ga is 2400°C, making liquid Ga also suitable for high-temperature applications [3]. Unlike mercury, the vapor pressure of liquid Ga around room temperature is negligible, which means that liquid Ga does not evaporate. In addition, Ga has low toxicity and some Ga compounds have been approved by the United States Food and Drug Administration as medicine [4]. Ga can form eutectic alloys with other metals which have even lower melting points. The eutectic Ga-In alloy (EGaIn, consisting of 75.5% Ga and 24.5% In by weight) has a melting point about 16°C, whereas the eutectic Ga-In-Sn alloy (Galinstan, 68.5% Ga, 21.5 In, 10% Sn by weight) has a melting point about 13°C. These Ga-rich alloys in their liquid phase share many similar properties with liquid Ga and therefore are suitable for a similar range of applications.

Thanks to their appealing material properties, these Ga-based liquid metals have been extensively used in a wide range of applications, including soft and stretchable electronics, soft robotics, sensors, energy storage, thermal management, drug delivery and tumor therapies. Interested readers are referred to several recent reviews focusing on those different topics [5–11]. In recent years, utilization of Ga-based liquid metals in various photonics applications have also been explored and demonstrated some unique advantages over conventional photonic structures and devices based on solid metals. As shown in Fig. 1, liquid metals can be employed to form micro- and nano-structures suitable for various photonics applications in different spectral regions. In this review, we will focus on discussing the recent developments of liquid-metal-based photonic structures and device technologies as well as the relevant applications. Specifically, we first provide a summary of the material properties of Ga-based liquid metals in Section 2, which is followed by a brief review of the various methods for fabricating macroscopic and microscopic liquid metal structures in Section 3. We highlight the various types of liquid metal photonic structures and devices developed for different spectral regions and applications in Section 4, and finally provide a discussion on the opportunities and challenges of future research in Section 5.

 

Fig. 1. (a) Image of bulk liquid Ga pushed out of a syringe. (b) Image of a liquid EGaIn dipole antenna in a PDMS mold. Adapted with permission from [5]. Copyright the Royal Society of Chemistry 2015. (c) SEM image of liquid Ga NPs formed on a silicon substrate by MBE deposition.

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2. Properties of Ga-based liquid metals

2.1 Phase change and thermal properties

The low melting point of Ga (29.8°C) is mainly due to the significantly anisotropic crystal structure of solid Ga [6]. By alloying Ga with In or In/Sn to form EGaIn or Galinstan, the melting point is further reduced to around 16°C or 11°C, respectively [3]. Furthermore, the melting points of nanostructures of these liquid metals also exhibit a size dependence. Liquid metal nanoparticles (NPs) with diameters significantly smaller than 100 nm exhibit melting points considerably lower than the bulk melting points [3]. When these liquid metals are cooled, they tend to supercool, i.e., remain in the liquid phase even when the temperature drops below their melting points. They usually need to be cooled down to temperatures significantly below their melting points (by tens of °C) to solidify. Such a significant supercooling effect can be a major advantage for liquid-metal-based applications which involve operation temperatures below the melting points. The solidification temperature of a liquid metal depends on several factors, including its purity and the availability of nucleation sites, and thus can vary in a large range. Ga-based liquid metals have relatively high thermal conductivities suitable for a variety of thermal management applications. Liquid Ga has a thermal conductivity of about $30.2\; \textrm{W}/\textrm{m} \cdot \textrm{K}$ near its melting point [12], which is about 50 times the thermal conductivity of water, whereas liquid EGaIn and Galinstan have similar thermal conductivities.

2.2 Mechanical properties

The viscosity of liquid Ga and Ga-rich alloys is approximately 2 mPa·s, which is about twice the viscosity of water. Therefore, these Ga-based liquid metals are relatively fluidic. However, unlike water, liquid metals usually have much larger surface tension thanks to the relatively strong metallic bonding. Liquid Ga has a surface tension of about 700 mN/m, whereas the surface tension of water is only about 73 mN/m at room temperature [13]. The surface tension of EGaIn and Galinstan are somewhat lower than that of liquid Ga, but still in the range around 600 mN/m. As a result of the large surface tension, a small amount of liquid metal on a solid surface tends to bead up, as shown in Fig. 1(a). The fluidity and surface tension of liquid metals are important material properties and have significant influences on a variety of methods for producing micro- and nano-structures of liquid metals, which are discussed in Section 3.

A fresh surface of Ga-based liquid metals exposed to air will quickly form an ultrathin (1 to 3 nm) self-limiting oxide layer [14]. Despite its exceedingly small thickness, the surface oxide layer can have considerable impacts on the surface tension and viscosity of a liquid metal structure [15]. Therefore, the thin surface oxide layer can be exploited to shape micro- and nano-structures of liquid metals and change the wetting and adhesion properties of liquid metals with other solid surfaces. It plays a key role in various techniques for fabricating liquid metal based devices. On the other hand, the thin oxide layer can be readily removed by an acid (e.g., HCl) or base (e.g., NH₄OH).

2.3 Electrical properties

Ga-based liquid metals have relatively high electrical conductivity, making them suitable for a variety of electronics applications in which soft and stretchable components are desired. Near its melting point, liquid Ga has a dc electrical conductivity of about $3.8 \times {10^6}\; \textrm{S}/\textrm{m}$ [16]. The electrical conductivities of EGaIn and Galinstan are similar to that of liquid Ga. Although these values are one order of magnitude lower than those of the best metals such as Au and Cu, they can nevertheless meet the requirements of a wide range of applications. Ga-based liquid metals have been frequently used to realize soft and highly stretchable electronic components and systems with self-healing capability, targeting various applications such as wearable and implantable sensors, electronic skin, and soft robotics. These are active research fields with rapid developments in recent years. Interested readers are referred to several previous reviews which have comprehensively covered the related topics [5,17].

2.4 Optical properties

The optical properties of Ga-based liquid metals are closely linked to their suitability for photonics applications, which are the focus of this review. The optical properties of bulk Ga in both solid and liquid phases have been extensively studied by independent measurements using different techniques (e.g., angle-dependent reflection spectroscopy, spectroscopic ellipsometry, electron energy loss spectroscopy) [18–22]. The plasma frequency of bulk Ga in both solid and liquid phases is approximately 14.1 eV, which is significantly higher than those of Ag and Au and comparable to that of Al. Like most metals, in the frequency range far below the plasma frequency, the permittivity function of Ga is well described by the Drude model. However, in part of the visible (Vis) to near-IR (NIR) spectral region (i.e., wavelength longer than 500 nm), the permittivity functions of solid Ga and liquid Ga are considerably different (see Fig. 2(a)). For solid Ga, additional permittivity dispersion and absorption peaks emerge as a result of the interband transitions between several parallel bands in the band structure [23]. These interband transitions between the parallel bands also occur in other polyvalent metals such as Al and In, which are also group III metals. In addition, recent studies have shown that the polymorphs of solid Ga also exhibit different band structures and hence optical properties [24,25]. On the other hand, these parallel bands and interband transitions do not exist in liquid Ga (note that liquid Ga is not a crystalline material and does not have long range order). Therefore, the permittivity function of liquid Ga can be accurately described by the Drude model throughout the entire optical spectral range (up to vacuum UV) [26].

 

Fig. 2. (a) Relative permittivity functions of Ga in solid and liquid phases. Data obtained from [26]. (b) Imaginary part of relative permittivity from randomly distributed liquid Ga NPs on sapphire substrates. Adapted with permission from [29]. Copyright American Institute of Physics 2007. (c) Hyperspectral cathodoluminescence images of individual Ga NPs at 380, 430, 500, and 800 nm. Adapted with permission from [26]. Copyright American Chemical Society 2015.

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The optical properties of Ga NPs have been studied more recently. The earlier works studied the optical properties of a 2D layer of liquid Ga NPs using spectroscopic ellipsometry [27,28]. The liquid Ga NPs exhibited the shape of a truncated sphere. The effective medium theory and Drude dispersion were applied to analyze the experimental data. Within the measurement spectral range, a peak near 2.5 eV was observed in the parallel component of the imaginary part of the effective permittivity, which was attributed to a surface plasmon resonance (SPR) of the liquid Ga NPs. The SPR shifted to higher energy with decreasing average NP size. However, the theory predicted two SPR modes for a 2D layer of truncated metal spheres, i.e., a longitudinal mode (collective electron oscillations in the plane of the 2D layer) and a transverse mode (collective electron oscillations out of the plane of the 2D layer). The transverse mode should have a much higher energy than the longitudinal mode, but was not observed in this initial study because it was located outside the measurement spectral range. In the later studies, the measurement spectral range was wide enough to observe both SPR modes [29,30]. The transverse mode was located in the energy range of 4 to 5 eV (i.e., in the UV region) for liquid Ga NPs with diameters ranging from 174 nm to 267 nm (Fig. 2(b)). Another more recent study reported the optical responses of individual Ga NPs which were characterized with hyperspectral cathodoluminescence microscopy (Fig. 2(c)) [26]. The findings from analyzing the responses of individual Ga NPs were consistent with those of the previous studies of Ga NP ensembles. In addition, clear evidence of high-order modes and hot spots due to near-field inter-NP coupling were observed. EGaIn and Galinstan in the liquid phase are expected to exhibit similar optical properties as liquid Ga (i.e., described by the Drude model across the entire optical spectral range), although their optical properties have not been studied as extensively and systematically as liquid Ga [31,32].

2.5 Toxicity

The toxicity and environmental impacts of a material plays a crucial role in determining the scope of its applications. Unlike mercury, Ga and Ga-based liquid metals have low toxicity [4,33]. Micro- and nano-particles of Ga-based liquid metals have been investigated for drug delivery and anticancer therapy applications [8,34]. Certain Ga compounds (e.g., gallium nitrate) have been investigated for their therapeutic effects, and some have been approved by FDA as drugs for treating hypercalcemia [4]. More systematic and comprehensive studies of the toxicity and environmental impacts of Ga-based liquid metals should be conducted. Nevertheless, as Ga-based liquid metals do not evaporate in ambient conditions (due to their negligible vapor pressure), the risk of unintentional inhaling of these liquid metals should be extremely low.

3. Methods of fabricating liquid metal structures for device applications

As a result of the fluidic nature of liquid metals, conventional fabrication methods for solid metal structures are either not suitable or need significant adaptations to fabricate liquid metal structures for various applications. On the other hand, the fluidic nature of liquid metals enables new and facile fabrication methods. In this section, we discuss the advantages and limitations of a variety of methods developed for producing liquid metal structures of different scales.

3.1 Molding-based patterning techniques

Liquid metal structures can be formed by introducing liquid metal into a preformed mold [35–38]. If the mold structures are open, liquid metal can be mechanically pressed into the mold (see Fig. 3(a)). The mold containing the desired liquid metal structures can be subsequently sealed [35]. Alternatively, liquid metal can be injected into encapsulated mold structures [36]. The mold can also function as a stencil mask which can be removed after forming the liquid metal structures [38], provided that the liquid metal adequately wets the underlying substrate and maintain its structural integrity without the mold. The molding-based methods have been demonstrated to be capable of patterning liquid metal structures with feature sizes ranging from mm down to µm, and therefore are suitable for realizing reconfigurable RF and terahertz (THz) photonic structures. Furthermore, such reconfigurable liquid metal structures may also be used for tunable plasmonic waveguide applications.

 

Fig. 3. (a) Schematics of the procedure for forming liquid metal structures in a elastomer mold. Adapted with permission from [35]. Copyright Wiley 2014. (b) Schematics of the procedure for forming liquid metal structures based on selective wetting. Adapted with permission from [39]. Copyright Elsevier 2015. (c) Schematic of fabricating liquid metal structures by direct writing. Adapted with permission from [42]. Copyright Wiley 2014. (d) Image of a 3D printed liquid metal structure. The scale bar represents 500 µm. Adapted with permission from [45]. Copyright Wiley 2013.

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3.2 Patterning by selective wetting

Another effective approach to forming well defined liquid metal patterns is selective wetting. Ga-based liquid metals wet numerous metals (such as Au) well, but wet other materials (such as organic materials and PDMS) poorly. Therefore, by defining wettable thin film patterns on a non-wettable substrate, a liquid metal will remain only on the wettable thin film patterns upon making contact with the substrate surface [39–41]. This approach has been used to realize micron-scale liquid metal patterns (see Fig. 3(b)), and in principle should also be suitable for creating nanoscale liquid metal patterns.

3.3 Printing-based patterning techniques

Liquid metal patterns can be directly printed on a substrate using various printing techniques (Fig. 3(c)), such as direct writing [42], inkjet printing [43], microcontact printing [44], and 3D printing techniques [45]. The thin oxide skin of Ga-based liquid metals plays a key role in these printing techniques, as it can provide sufficient adhesion between the liquid metal pattern and the substrate, and stabilize 3D liquid metal structures (Fig. 3(d)). The patterning resolution of the current printing-based techniques are typically limited to tens of μm. However, recent progress has pushed the liquid metal printing resolution into the few µm range [46].

3.4 Fluidic jetting and microfluidic flow focusing

Fluidic jetting refers to injecting liquid metal through a small orifice into another liquid medium, which can lead to formation of liquid metal micro-droplets [47,48]. Fluidic jetting is a facile technique which only requires simple tools such as syringes, but it does not provide precise control of the sizes of the liquid metal droplets. A more sophisticated approach is microfluidic flow focusing which produces micro-droplets of a liquid by forcing simultaneously two or more immiscible liquids through a micro-orifice [49,50]. A microfluidic flow focusing device for producing liquid metal micro-droplets is shown in Fig. 4(a). When the center liquid metal (EGaIn) stream and two outer streams of glycerol/water mixture intersect in the flow focusing orifice, the liquid metal stream breaks into droplets with relatively uniform sizes, which are transported out of the device by the glycerol/water streams. Surfactants can be added in the liquid medium to stabilize the liquid metal micro-droplets and form monodisperse. Furthermore, by introducing ultrasonic acoustic waves into the microfluidic flow focusing device, liquid metal NPs can be produced (Fig. 4(b)) [51].

 

Fig. 4. (a) Optical image and schematic of a microfluidic flow focusing device for producing liquid EGaIn microparticles. Adapted with permission from [49]. Copyright Royal Society of Chemistry 2012. (b) Schematic of a microfluidic flow focusing device combined with ultrasonic wave agitation for producing liquid EGaIn NPs. Adapted with permission from [51]. Copyright Wiley 2018. (c) Schematic illustration of the SLICE process for transforming bulk liquid metal into microparticles and NPs. Adapted with permission from [52]. Copyright Americal Chemical Society 2014. (d) Schematic illustration of the ultrasonication process for transforming bulk liquid metal into NPs. Adapted with permission from [54]. Copyright Wiley 2016. (e) Schematic illustration and SEM images of Ga NPs formed on a silicon substrate by MBE deposition. Adapted with permission from [26]. Copyright American Chemical Society 2015. (f) Schematic illustration of the synthesis of Ga NPs via galvanic replacement reaction of sacrificial Zn NPs. Adapted with permission from [61].Copyright Royal Society of Chemistry 2021.

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3.5 Shearing and ultrasonication

Bulk liquid metals can be broken down into microparticles and NPs using two facile techniques: shearing [52] and ultrasonication [53,54]. The technique named SLICE (shearing liquids into complex particles) employs a rotary tool rotating at high speed in a liquid environment to create large shearing forces which breaks down a bulk liquid metal into smaller and smaller particles (illustrated in Fig. 4(c)), until a limit is reached. On the other hand, ultrasonication can generate vapor-filled cavities (cavitation) in a liquid medium. The collapse of these cavities produces shock waves which break down nearby bulk liquid metal into NPs (see Fig. 4(d)). This method can produce NPs with diameters in the range of tens of nm with high yield. Surfactants can be added in the liquid medium to stabilize the liquid metal NPs and prevent coalescence [55]. Although the shearing and ultrasonication techniques are straightforward and effective methods for producing liquid metal microparticles and NPs, currently they cannot provide precise control on the particle sizes.

3.6 Physical vapor deposition

Physical vapor deposition (PVD) is another effective and reliable approach to producing liquid metal NPs. PVD of Ga or Ga-rich alloys on a cold substrate does not lead to the formation of uniform thin films. Instead, the metal atoms tend to move around on the substrate and form nucleation sites, which gradually grow into NPs as the deposition time increases (see Fig. 4(e)). Neighboring NPs can also coalesce when they become large enough to directly contact each other. PVD of Ga and EGaIn can be carried out using a molecular beam epitaxy (MBE) system [26,29]. In addition, conventional physical vapor deposition techniques such as thermal evaporation and electron-beam evaporation can achieve similar results [56]. These PVD techniques cannot achieve precise NP size control either. Nevertheless, a key advantage of these techniques is the formation of liquid metal NPs directly on a substrate, which are more convenient for many device applications compared to the techniques for producing NPs in a liquid medium.

3.7 Laser ablation

Liquid metal NPs can be produced using femtosecond (fs) laser ablation. For instance, intense fs laser pulses focused on the surface of a Ga film immersed in a liquid medium can eject Ga NPs from the Ga film surface, which are consequently dispersed in the liquid medium [57]. In addition, fs laser ablation can also eject Ga NPs from the surface of a Ga film in air, and these NPs can be collected directly by a substrate [58]. In sharp contrast to the PVD processes which typically deposit hemispherical Ga NPs on a substrate, the Ga NPs produced by laser ablation are nearly spherical, which lead to significant changes in the plasmonic responses. Besides producing liquid metal NPs, laser ablation has also been employed to directly pattern macroscopic liquid metal structures [59].

3.8 Chemical synthesis

Ga NPs can be chemically synthesized via different types of reactions. For example, thermal decomposition of Ga-based organometallic compounds in hot solvents is a cost-effective method for producing Ga NPs with relatively well controlled sizes [60]. Using thermal decomposition of Ga-alkylamides, Yarema et al. achieved colloidal Ga NPs synthesis with narrow size distributions as low as 7-8%. Another facile method for chemical synthesis of Ga NPs is galvanic replacement reaction. Gao et al. developed a cost-effective process to form relatively uniform Ga NPs through the galvanic replacement of sacrificial Zn NP seeds in a GaCl₃ solution (see Fig. 4(f)) [61]. The synthesis process involves no highly active reagents or special equipment.

4. Applications of liquid metals in photonics

Since the optical properties of Ga-based liquid metals share significant similarity with those of noble metals widely used in various photonic structures and devices, these liquid metals are suitable for many photonics applications. In addition, the highly deformable/transformable nature of liquid metal structures and the facile fabrication techniques discussed in the previous section can bring unique benefits to different applications and enable new possibilities beyond what can be achieved with solid metals. A variety of liquid-metal-based photonic devices and applications have been demonstrated in recent years. Nevertheless, compared to the research on conventional solid-metal-based photonics, liquid-metal photonics is still a nascent research field with vast unexplored potential. In this section, we focus on the recent advances of liquid-metal-based photonics for different spectral regions and applications.

4.1 Liquid-metal-based plasmonics for IR-Vis-UV spectral region

4.1.1 Plasmonic responses of nanostructured liquid metals

The appealing material properties of Ga-based liquid metals make them suitable for a wide range of plasmonics applications (see Fig. 5). The high plasma frequency of Ga and Ga-rich alloys is a clear advantage for realizing UV plasmonic structures. The disappearance of interband absorption in the liquid phase of these metals significantly reduces the optical loss in the visible spectral region [26]. In addition, the lack of domain boundaries and surface roughness in liquid metal nanostructures leads to longer carrier relaxation time and hence lower loss for surface plasmons. The self-limiting gallium oxide layer on the surface of Ga-based liquid metal NPs prevents the oxidation of the NPs, so that such NPs can be stable in ambient conditions for months or even years, which is a clear advantage over NPs made of Ag and Al. Furthermore, the plasmonic responses of Ga-based liquid metal NPs remain stable across a wide temperature range, from significantly below room temperature to hundreds of °C [29].

 

Fig. 5. (a) Schematics and simulated field distributions of the in-plane (longitudinal) and out-of-plane (transverse) SPR modes of liquid Ga NPs on a substrate. Adapted with permission from [26]. Copyright American Chemical Society 2015. (b) Schematics of the fabrication process of Ga gratings and the reflection spectrum change of the Ga gratings induced by the Ga solid-liquid phase transition. Adapted with permission from [66]. Copyright Americal Chemical Society 2012. (c) SEM images from a 30°-tilted view of Ga NPs after anodization for different durations and after thermal oxidation at 300 °C for 5 min. Adapted with permission from [65]. Copyright Chen et al. 2022. (d) SEM images of the Al nanostructured templates of different pit diameters (first column), Ga NPs on Al templates (second column) and on flat Si (third). Adapted with permission from [71]. Copyright Catalán-Gómez et al. 2020. (e) Left: schematic representing the flux and temperature conditions to obtain core–shell, alloy, or phase-segregated GaMg NPs. Middle: Real-time evolution of the imaginary part of the pseudodielectric dielectric function during GaMg NP deposition. Right: Dependence of SPR energy of GaMg alloy NPs of various compositions with a diameter of approximately 55 nm. Adapted with permission from [74]. Copyright Wiley 2011.

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A large percentage of previous studies of the localized surface plasmon resonance (LSPR) of liquid metal NPs were conducted on NPs formed on a flat substrate using the PVD techniques discussed in Section 3, whereas several other effective approaches have also been exploited to realize liquid metal nanostructures with strong plasmonic responses [58,62]. The PVD techniques usually produce liquid metal NPs with the shape of truncated sphere or hemisphere on a substrate, whereas femtosecond laser ablation can deposit spherical liquid metal NPs on a substrate [58]. The difference between the in-plane and out-of-plane dimensions of hemispherical NP results in two LSPR modes, i.e., the longitudinal mode (in-plane oscillation) which has a lower frequency, and the transverse mode (out-of-plane oscillation) which has a higher frequency (see Fig. 5(a)) [26,29]. Light at normal incidence can only excite the longitudinal mode, whereas the transverse mode can be excited by the p-polarization component of light at oblique incidence and its strength is sensitive to the angle of incidence [63]. Both LSPR modes can be tuned across a wide spectral range by varying the size of the NPs. Therefore, taking advantage of the large frequency splitting between the longitudinal and transverse LSRP modes and their size-dependent frequency tunability, Ga-based liquid metal NPs can support strong LSPR covering the entire NIR to UV spectral range, which is significantly wider than the spectral range that can be covered by NPs based on noble metals such as Au and Ag. Although a noble metal NP with a similar shape would also support both the longitudinal and transverse modes at two different frequencies, one of the two modes may be severely damped by the interband transitions in the noble metal.

Previous studies have shown that the thin (<3 nm) self-limiting native gallium oxide layer on the surface of the Ga-based liquid metal NPs only introduces a small redshift of the LSPRs [29]. Nevertheless, intentional thermal oxidation to increase the surface oxide layer thickness has been shown to be an effective method to achieve larger frequency tuning range of the LSPRs without significantly affecting the strength of the resonances [64]. Electrochemical surface anodization was explored as another effective approach to achieve much larger tunability of the LSPRs of liquid metal NPs [65]. A brief surface anodization process (a few seconds) applied to hemispherical Ga NPs on Si substrate has been shown to cause nanoscale dimple-like textures on the NPs’ surface (Fig. 5(c)), which in turn lead to a large change of the NPs’ LSPRs, with up to 77 nm redshift achieved (about 15% relative change in wavelength). The dimple-textured NP surface also features a large number of hotspots with high field enhancement.

Vivekchand et al. demonstrated one-dimensional (1D) Ga plasmonic gratings (400 nm periodicity) which can resonantly couple free-space incident light in the visible spectral region to propagating SPPs in the gratings, and achieved dynamic modulation of SPP excitation via thermally induced reversible solid-liquid Ga phase transitions [66]. The 1D Ga gratings were fabricated by molding liquid Ga with a cured polyurethane template. The Ga gratings in the liquid phase exhibited significantly higher SPP coupling efficiencies (62% vs 24%) and narrower resonance widths (22 nm vs 75 nm) compared to that in the solid phase (see Fig. 5(b)). Since solid Ga has multiple phases with sufficiently different optical properties, the phase polymorphism of Ga can be utilized to enhance the tunability of the phase-dependent responses of Ga-based plasmonic structures [24].

4.1.2 Substrate engineering for liquid metal plasmonic nanostructures

The substrate supporting the liquid metal NPs can impose a strong influence on the plasmonic characteristics of the NPs. The influence of the substrate is not only owing to its refractive index, but can also stem from other types of interactions between the substrate and the NPs. For instance, the formation of liquid metal NPs on a substrate during a PVD process depends on the wetting between the liquid metal and the substrate surface, therefore different substrate materials can lead to different NP shapes and size distributions and consequently different LSPR characteristics. Losurdo et al. observed that the interaction between Ga NPs and the substrate material can affect not only the shape but also the phase of the Ga NPs [67]. Specifically, Ga NPs supported by crystalline sapphire substrate are close to hemispheres and possess two coexisting phases across a wide temperature range (180 K to 800 K): a solid core in $\gamma $-phase and a liquid shell. On the other hand, Ga NPs on amorphous glass substrate are more spherical (the contact angle is close to 150°) and only in the liquid phase above the NP melting point. When the Ga NPs are synthesized on a chemically reactive substrate such as Si, a gallium silicide interfacial layer is formed that lowers the interfacial energy and promotes wetting, leading to Ga NPs with a relatively small contact angle of about 40°. These substrate-induced morphological features and phase configurations have strong influences on the plasmonic responses of the Ga NPs. In another study, Wu et al. investigated the formation of liquid Ga NPs on the surfaces of polar semiconductors with a hexagonal wurtzite crystal structure, including SiC, GaN and ZnO [68]. These polar semiconductors have spontaneous polarization fields along the c-axis of their crystal structures, and hence a polarization-induced surface charge of opposing signs are present at the two different polar surfaces of each semiconductor (e.g., the Si-polar and C-polar surfaces for SiC). These surface charges affect the wettability and mobility of liquid Ga on these surfaces, which in turn govern the formation process of liquid Ga NPs. Different NP shapes, sizes and densities can be realized depending on the choice of polar surface, which lead to different NP LSPR responses. A later study investigated the influence of a graphene/SiC substrate on the properties of liquid Ga NPs [69]. It was revealed that the graphene layers covering the SiC surface had negligible influence on the geometrical properties (shape, size, density) of the formed Ga NPs, but caused significant damping and a small blue shift of the LSPR of the Ga NPs when compared to the Ga NPs formed on a SiC substrate under the same conditions. Ga NPs significantly enhanced the graphene Raman peaks, which was found to be a result of both the plasmonic field enhancement and the charge transfer between graphene and Ga NPs. Another study investigated the formation of liquid Ga NPs on graphene coated SiO₂/Si and quartz substrate and observed that the monolayer graphene significantly improved the size uniformity of the Ga NPs and increased the interparticle distance [70]. The size uniformity improvement is attributed to the higher mobility of Ga on the graphene surface than on the SiO₂ surface, which facilitates the coalescence of smaller NPs to form larger NPs. To significantly improve the size uniformity of liquid metal NPs, Catalan-Gomez et al. employed nanostructured Al substrates which had hexagonal arrays of shallow pits and were fabricated by removing the surface oxide layer of porous anodic alumina substrates. Highly ordered periodic arrays of Ga NPs with relatively uniform sizes were realized on such substrates using the thermal evaporation technique [62]. The sizes and ordered arrangements of Ga NPs can be well controlled by tuning both the Al substrate fabrication process and the quantity of Ga deposition (Fig. 5(d)). The LSPRs of these ordered Ga NPs can be tuned across the entire UV to IR spectral range and have much stronger intensities and narrower linewidths than those of the Ga NPs formed on a flat Si substrate with random distributions [71]. Plasmonic coupling between neighboring NPs can also be more precisely tailored.

4.1.3 Composition engineering for liquid metal plasmonic nanostructures

Another straightforward approach to tune the plasmonic characteristics of liquid metal NPs is to introduce additional elemental metals to form alloy NPs or core-shell structured NPs [72,73]. For example, sequential evaporation of Ga and Mg can be used to realize Ga-core-Mg-shell NPs. Mg has significantly lower plasma frequency compared to that of Ga. By adjusting the volume ratio between the Ga core and the Mg shell while maintaining the overall NP size, the LSPR frequency of the core-shell NP can be tuned continuously across the UV to NIR spectral range. On the other hand, simultaneously depositing Ga and Mg after an initial Ga seeding step can lead to formation of Ga-Mg alloy NPs [74]. The SPR frequencies of the alloy NPs redshift as the composition of Mg increases (Fig. 5(e)). The Ga/Mg alloy NPs are thermally stable, with no tendency of segregation below 300°C. As liquid EGaIn has optical properties similar to those of liquid Ga, EGaIn is also suitable for a range of plasmonics applications. Blaber et al. theoretically investigated the optical properties and plasmonic performance of Ga and Ga-In alloys in both solid and liquid phases using the density functional theory-molecular dynamics (DFT-MD) method, and compared their calculations with experimental data which showed good agreement [75]. The two key plasmonic performance metrics, i.e., the field localization of LSPR and the propagation length of SPP, were found to moderately decrease as the atomic percentage of In in the Ga-In alloy increased. However, the performance difference was not significant between the pure liquid Ga and the commonly used liquid EGaIn with 14.8% In (atomic percentage). Hybrid NP structures consisting of In and Ga have also been explored [76]. As In and Ga were sequentially deposited on the substrate, the indium oxide surface layer prevented alloying of Ga and In. Instead, liquid Ga NPs were formed on the surface of the existing In NPs. The LSPRs of such hybrid NP system exhibited resonance broadening and redshift, which were attributed to the plasmonic coupling between NPs. Varying the mass ratio between In and Ga resulted in different morphological configurations of the hybrid NP system, which in turn led to large tuning of the LSRP characteristics.

4.1.4 Applications based on liquid metal plasmonics

Surface-enhanced Raman scattering (SERS) sensing

Wu et al. achieved the first demonstration of SERS based on liquid Ga NPs [77]. In their work, Ga NPs were formed on a sapphire substrate using MBE. By adjusting the deposition time, several samples with different NP average sizes and densities were prepared, and their SERS performance were compared. The Raman dye cresyl fast violet was used as the model analyte to characterize the SERS performance. A 633 nm HeNe laser was used as the excitation source. The sample with the highest NP density but smallest average NP size produced the largest SERS signal. The overall Raman signal enhancement factor reached 80, which was the average value across the area of the beam spot, whereas the enhancement due to individual NPs were expected to be orders of magnitude higher. The limit of detection (LOD) for the analyte concentration was shown to be better than 10 ppm. In a subsequent study, Yang et al. took advantage of the UV plasmonic responses of liquid Ga NPs and demonstrated high-performance SERS under an UV excitation (325 nm) [78]. Using UV excitation for Raman spectroscopy is intrinsically advantageous because the Raman scattering cross section scales as the fourth power of the excitation frequency. Crystal violet was used as the model analyte in this study. Systematic measurement and data analysis revealed that the Raman enhancement factor at individual hot spots reached as high as 10⁷. This large enhancement factor was facilitated by the bimodal distribution of Ga NPs: larger NPs were surrounded by smaller NPs, and therefore hot spots were formed in the gaps between the larger NPs and smaller NPs (Fig. 6(a)). However, the high UV field enhancement likely led to damage of the analyte molecules, which was manifested as a fast temporal decay of the SERS signal in the time scale of seconds. In addition to SERS and the surface-enhanced photodegradation, significant surface-enhanced fluorescence was also observed. Gao et al. demonstrated synthesis of spherical liquid Ga NPs with controllable size distributions based on galvanic replacement reaction from sacrificial Zn NPs, and employed these Ga NPs to achieve SERS detection of Rhodamine 6 G with a LOD of 10⁻⁶ M and an average enhancement factor exceeding 10⁵ [61]. The large enhancement factor was partially owing to the aggregation of Ga NPs in the analyte solution during solvent evaporation, which led to the formation of abundant hotspots with sub-10 nm gap sizes. Chen et al. demonstrated that a facile process combining sonication and mechanical stirring without addition of surfactants could produce liquid Galinstan NPs with their surfaces coated by nanospikes, which provided more localized hot spots suitable for SERS applications [79]. They distributed such nanospike-decorated Galinstan NPs on a Si substrate and further deposited a thin gold film to realize cost-effective SERS substrates. Rhodamine 6 G was used as the model analyte the characterize the performance of these SERS substrates and a LOD of 10⁻⁷ M was achieved.

 

Fig. 6. (a) Top: size distribution and SEM image of a SERS substrate with liquid Ga NPs. Bottom: Raman spectra obtained from three SERS substrates with different liquid Ga NP size distributions. The inset shows the LSPR characteristics of the three SERS substrates. Adapted with permission from [78]. Copyright Americal Chemical Society 2013. (b) Schematic illustrations of the liquid Ga NP-based DNA biosensor. Adapted with permission from [81]. Copyright Royal Society of Chemistry 2016. (c) Left: SEM image of liquid Ga NPs on MoS₂ flakes on a sapphire substrate. Right: Photoluminescence spectra from MoS₂ flakes with and without liquid Ga NPs. Adapted with permission from [82]. Copyright Royal Society of Chemistry 2019. (d) Upper left: schematic of a hemispherical liquid Ga NP functioning as a plasmonic antenna and a photocatalytic nanoreactor for hydrogen dissociation, storage, and hydrogen spillover as well as oxygen-reverse spillover. Upper right: Near-field enhancement profile of the hemispherical liquid Ga NP. Lower: SEM and TEM images of the Ga/Al₂O₃ interface showing the formation of Ga₂O₃ localized at the interface upon hydrogen interaction. Adapted with permission from [83]. Copyright Wiley 2021.

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Refractive index sensing

Liquid metal NPs have also been utilized for refractive index sensing applications. For example, Marin et al. demonstrated immunosensors for glutathione (GSH) by employing hemispherical Ga NPs on a silicon substrate [80]. The surface of the Ga NPs was chemically functionalized with a thiol monolayer which had an activated ester termination for forming amide bonds with the GSH-specific antibodies. After incubating such Ga NP immunosensors in GSH solutions of different concentrations, the quantitative sensing was conducted using ellipsometry to extract the analyte-associated changes in the pseudodielectric function at the condition of reversal of polarization handedness (RPH). This specific ellipsometry-based technique was used because the slope of the pseudodielectric function of the Ga-NP-on-silicon composite layer at the RPH condition was extremely sensitive to the medium surrounding the NPs. The experimental results showed that the slope of the pseudodielectric function at the RPH condition was linearly proportional to the GSH concentration in the range between 50 nM to 800 nM. This GSH immunosensor achieved a LOD of 10 nM and a limit of quantification of 50 nM, which were superior to the performance of standard ELISA kits. A subsequent work from the same group further developed this Ga NP based refractometric sensor platform for label-free DNA and single nucleotide polymorphism sensing applications [81]. DNA sensing was achieved by functionalizing the Ga NP surface with a thiolated capture probe sequence (see Fig. 6(b)). The target DNA sequences could be quantified over the range of 10 pM to 3.0 nM with a LOD of 6.0 pM. The sensitivity of this DNA sensor was sufficiently high which allowed the detection of a single nucleotide polymorphism (a point mutation in a DNA sequence), because a single mismatched nucleobase pair led to a slightly distorted double helix structure, which in turn resulted in a detectable optical response (i.e., the pseudodielectric function change). The clinical potential of this sensor platform was further highlighted by the demonstration of the detection of a specific gene mutation associated with cystic fibrosis in human genomic DNA extracted from blood cells. Further advancement of this sensor platform can be achieved by improving the size uniformity of the Ga NPs via optimization of the substrate [70].

Fluorescence and photoluminescence enhancement

In addition to SERS, the LSPRs of liquid metal NPs have also been exploited to enhance other light matter interactions, such as fluorescence and photoluminescence (PL). Tang et al. synthesized EGaIn NPs in water using a microfluidics platform in an ultrasonic bath environment [51]. They further employed the galvanic replacement method to realize even smaller Ag NPs on the surface of EGaIn NPs. The EGaIn NPs were stabilized by surface functionalization with bPEG polymer with trithiocarbonate groups, which also allowed for further biofunctionalization with desired biomolecules (e.g., proteins and antibodies) since bPEG molecules can form amide bonds with various biomolecules. As a proof-of-concept demonstration, fluorophore labelled anti-rabbit immunoglobulin G was successfully grafted on the EGaIn NPs, which was evidenced by the strong fluorescence signal observed from the functionalized EGaIn NP samples. Catalan-Gomez et al. formed hemispherical Ga NPs directly on MoS₂ flakes grown on sapphire substrate and observed that the Ga NPs led to enhancement of the PL from MoS₂ by approximately an order of magnitude [82]. The systematic experimental investigation showed that the PL enhancement was mainly due to the excitation of longitudinal LSPR mode of the Ga NPs, as the enhancement reached its peak value when the excitation laser frequency matched that of the longitudinal LSPR mode (Fig. 6(c)). In addition, the PL mapping analysis indicated significant charge transfer between the Ga NPs and MoS₂ flakes.

Photocatalysis

Liquid Ga NPs have recently been exploited as plasmon-enhanced photocatalyst for hydrogen sensing and storage applications. Theoretical calculations showed that Ga should be an appealing material for hydrogen technologies because Ga can dissociate H₂ by charge transfer processes at relatively low temperatures. Losurdo et al. demonstrated that Ga NPs on a sapphire substrate functioned as nano-antennas whose LSPR hot spots could facilitate electronic and chemical interactions between Ga and hydrogen, which were also sensitive to temperature [83]. In the temperature range of 25°C to 200°C, the dissociative adsorption of hydrogen at liquid Ga NP surface and the interstitial diffusion of hydrogen inside liquid Ga NP were the dominant processes, which led to a reversible blueshift of the LSPR position. These hydrogenation processes were mainly driven by the transverse LSPR hot spots, and were associated with a low activation energy of 1.5 meV which was two orders of magnitude lower than the activation energy for thermal diffusion of hydrogen in Ga. In the temperature range of 200°C to 600°C, the formation of Ga-H hydrides occurred which increased the electron density in the Ga NPs (hydrogen was electron donor) and led to a non-reversible blueshift of both the longitudinal and transverse LSPRs. When the temperature increased to above 600°C, the longitudinal LSPR induced plasmonic photocatalysis of Ga₂O₃ formation at the interface between the Ga NPs and sapphire substrate (see Fig. 6(d)), which resulted in an irreversible redshift and damping of both the longitudinal and transverse LSPRs. The reaction yielding Ga₂O₃ was a result of simultaneous hydrogen spillover and reverse oxygen spillover. It was further demonstrated that utilizing the reversible spectral changes of the liquid Ga NPs for H₂ sensing in the temperature range below 200°C could achieve a fast response (about 5 seconds) and a low LOD of 5 ppm, which was among the best reported H₂ sensing LOD.

4.2. Liquid-metal-based nonlinear optics for IR-Vis-UV spectral region

The low melting points of Ga and Ga-rich alloys together with the drastic permittivity change associated with the solid-liquid phase transition of these metals make them suitable for nonlinear optics applications in the IR-Vis spectral region, such as all-optical switching. An early study by Bennett et al. [84] showed that the reflectivity at 1550 nm of a Ga/silica interface (located at a Ga-coated optical fiber end) changed drastically when Ga underwent the solid-liquid phase transition, i.e., from about 50% when Ga was in the solid phase to nearly 90% when Ga was in the liquid phase. Furthermore, a large reversible reflectivity change could also be induced by a relatively weak pump laser pulse (average power of a few mW) coupled into the optical fiber when the Ga temperature was kept a few degrees below its melting point, hence achieving optically induced intensity modulation of a probe beam by up to 30%. The underlying mechanism of this optically induced reversible reflectivity change was likely due to the formation of a thin interfacial layer of liquid Ga upon optical excitation, which recrystallized into solid Ga after the optical excitation stopped [84–86]. More systematic investigations of the dynamics of the light-induced reflectivity change at the Ga-silica interface were conducted using nanosecond, picosecond and femtosecond laser pulses (see Fig. 7(a)). These experiments revealed that the large reflectivity change was broadband, with a significant spectral response spanning at least from 480 nm to 1800nm. The optical excitation induced reflectivity increase exhibited a two-step process: an initial fast rise in reflectivity within 2‒4 ps which was fluence dependent, followed by a slower increase in a time scale of 300‒500 ps after the laser pulse. On the other hand, the relaxation of the reflectivity change occurred on a much longer time scale, ranging from ns to µs, and was strongly temperature dependent: the recovery time was proportional to ${({{T_m} - T} )^{ - 1}}$, where T is the sample temperature and ${T_m}$ is the Ga melting point$\; $. Therefore, longer recovery time was needed when the sample temperature was closer to the Ga melting point [86]. This effect was demonstrated to be fully reproducible. Such nonlinear liquefying Ga mirrors were later employed by the same group to realize passively Q-switched fiber lasers which generated ns pulses at 1550 nm (Er-doped) and 1030 nm (Yb-doped) (Fig. 7(b)) [87]. A composite material of Ga/Al on a silica substrate was demonstrated to exhibit similar nonlinear responses associated with the solid-liquid phase transition of Ga [88]. This Ga/Al composite material was realized by letting liquid Ga infiltrate the grain boundaries of a thin (250 nm) Al film deposited on a silica substrate, so that a thin nanolayer of Ga formed between the silica substrate and the Al film. Such a composite material exhibited a uniformly high-quality interface with the silica substrate. Optical excitation induced reflectivity change of the mirror-like Ga/Al-silica interface reached about 20%.

 

Fig. 7. (a) Transient light-induced reflectivity increase in Ga films on Si measured with 150-fs 800-nm pump and probe pulses at various pump energy densities. Adapted with permission from [85]. Copyright Optical Society of America 2001. (b) Top: schematic of the passively Q-switched fiber laser cavities, consisting of a liquefying Ga mirror as the nonlinear element. Bottom: Output power characteristic of the erbium fiber laser with a liquefying Ga mirror, showing the region of stable Q-switching, and a typical output pulse obtained at a pump power of 1.09 W (inset). Adapted with permission from [87]. Copyright American Institute of Physics 1999. (c) Unit cell design of a Ga-backplane/Si₃N₄/gold-disc metasurface absorber with a resonant wavelength of 1310 nm. (d) Top: absolute 1550 nm reflectivity of the metasurface in (c) as a function of time during and after excitation with a 500 µs, 9.5 µW/µm² pump pulse at 1310 nm, for various sample temperatures. Bottom: maximum induced 1550 nm reflectivity change for various 1310 nm pump intensities as a function of sample temperature. The inset shows reflectivity relaxation time as a function of temperature and pump intensity. (c)&(d) adapted with permission from [91]. Copyright American Institute of Physics 2015. (e) Simplified generic diagram illustrating quaternary memory functionality in a Ga NP, employing four different phases, each labeled as a unique logical state. (f) Quaternary memory functionality of the Ga NP, with the particle state monitored using the pump-probe technique. (e)&(f) adapted with permission from [94]. Copyright American Physical Society 2007.

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The optical excitation induced Ga phase transition was further exploited by Krasavin et al. to achieve strong modulation of the efficiency for exciting SPPs on a Ga/MgF₂ interface [89]. In this experiment, a thin Ga film was formed on the surface of a MgF₂ coated BK7 glass prism by pressing a bead of liquid Ga on the prism surface and solidifying it. Therefore, the system was in the Otto configuration and a light wave in the p-polarization undergoing total internal reflection at the glass/MgF₂ interface could efficiently couple to a SPP wave at the Ga/MgF₂ interface, leading to a reflectivity dip in the reflection spectrum. When a pump laser pulse incident on the Ga/MgF₂ interface induced a temporary phase transition of the Ga surface layer from the solid phase to the liquid phase, the SPP coupling efficiency was significantly changed, leading to a dramatic increase of the reflected beam intensity. The magnitude of the effect was sensitive to the sample temperature and the pump pulse fluence. The reflectivity change increased with the pump fluence up to a point and then saturated. The contrast of the reflectivity ${R_{\textrm{on}}}/{R_{\textrm{off}}}$ reached up to 9.4 (${R_{\textrm{on}}}$ and ${R_{\textrm{on}}}$ are the reflectivity when the pump laser beam is on and off, respectively). Consistent with the previous studies, the increase of the reflectivity occurred in the ps time scale after the arrival of the pump pulse, whereas the relaxation of this reflectivity increase occurred on the time scale of tens to hundreds of ns. Exploiting this effect, a plasmonic waveguide based switch/modulator was proposed and theoretically analyzed [90]. The system consisted of two Au-on-silica plasmonic waveguides connected by a Ga-on-silica switch/modulator section. The transmission of an SPP wave between the two Au-on-silica waveguides can be tuned by inducing the Ga phase change in the Ga-on-silica section via either changing the temperature or using an optical excitation. More recently, the optical excitation induced Ga phase transition was exploited to realize a tunable metasurface structure [91]. This “perfect absorber” type metasurface consisted of an array of patch antenna unit cells. Each patch antenna unit cell included a gold disk patch separated by a 50 nm thick silicon nitride dielectric layer from a continuous Ga ground plane (Fig. 7(c)). The solid-liquid phase transition of the Ga ground plane induced a significant change in the spectral response of the metasurface, especially in the near-IR region. The resonant characteristics of the metasurface further enhanced the optical nonlinearity due to the Ga phase change by an order of magnitude. Reversible tuning of the metasurface reflection/absorption could be achieved by light-induced transient interfacial melting of the Ga film under relatively low-intensity excitations. For example, a > 50% relative change in reflectivity at 1550 nm was achieved under an illumination intensity of <20 ${\mathrm{\mu} \mathrm{W}}/\mathrm{\mu }{\textrm{m}^2}$ at 1310 nm (see Fig. 7(d)).

More complex light-induced phase transition behaviors and the associated nonlinearities were found in Ga NPs owing to the polymorphism of Ga. For an ensemble of Ga NPs (with an average diameter of about 50 nm) formed on the surface of a single-mode optical fiber end, pump-probe measurements revealed light-induced reflectivity change of a few percentage, which peaked when the sample temperature was about 170 K and fell to zero at about 200 K [92]. The peak of the light-induced reflectivity change of Ga NPs occurring at such a low temperature was in agreement with the low solid-liquid phase transition temperature of Ga NPs. The light-induced reflectivity change of Ga NPs also exhibited a large and complex temperature hysteresis in a heating-cooling cycle, which was attributed to three main factors: the size distribution of the NPs, the polymorphism of solid Ga, and the fact that phase transitions of NPs usually take the form of a dynamic phase coexistence extending across a temperature interval. As the true nonlinearity characteristics associated with the phase transition of a Ga NP could be masked by the inhomogeneous characteristics of an ensemble of nonidentical Ga NPs, further studies by Soares et al. focused on single Ga NP [93,94]. A single Ga NP was formed on the tapered aperture (100 nm) of a gold-coated silica optical fiber. As the sample temperature increased from 180 K to 260 K, several reversible light-induced reflectivity change peaks were observed, which corresponded to a sequence of transitions between different phases of the Ga NP, i.e., $\gamma $-phase to $\varepsilon $-phase, $\varepsilon $-phase to $\delta $-phase, $\delta $-phase to $\beta $-phase, and $\beta $-phase to liquid phase [93]. Note that the $\alpha $-phase which is the stable phase for bulk solid Ga below its melting point is not present in small Ga NPs. Both positive and negative reflectivity changes were observed. Specifically, the $\gamma $-phase to $\varepsilon $-phase transition and the $\beta $-phase to liquid phase transition induced reflectivity increase, whereas the $\varepsilon $-phase to $\delta $-phase transition and the $\delta $-phase to $\beta $-phase transition were associated with reflectivity reduction. It was further demonstrated that such a single Ga NP could be employed as an all-optical quaternary phase-change memory (Fig. 7(e)) [94]. A relatively weak single laser pulse of a few pJ energy was shown to be sufficient to induce the phase transition from the ground memory state ($\gamma $-phase) to any other memory state ($\varepsilon $-, $\beta $- and liquid phases), hence performing the writing operation (Fig. 7(f)). The pump-probe technique was used to read out the reflectivity change associated with the NP’s structural phase. Erasing the memory (recovering the $\gamma $-phase) was achieved by lowering the temperature to 100 K and turning off all the laser excitation.

Liquid Ga NPs have also been investigated for nonlinear light generation. Xiang et al. fabricated spherical Ga NPs using femtosecond laser ablation and subsequently studied the nonlinear responses of these Ga NPs under femtosecond laser excitation [57]. When the spherical Ga NPs were transferred onto a glass substrate, they only produced second harmonic generation signal. However, when the spherical Ga NPs were located on a silver film, white light emission from the Ga NPs was observed. Analysis of the reflection spectra of the Ga-NP-on-silver samples showed that a Fano resonance originating from the interference between the broad-band mirror-image-induced magnetic dipole mode and the narrow-band gap plasmon mode was formed. The gap plasmon mode was localized in the nanometric gap between the liquid Ga NP and the silver film. When either the magnetic dipole resonance or the Fano resonance was excited by the femtosecond laser pulses, efficient white light emission was generated even at relatively low excitation irradiances. Systematic spectral analysis of the white light emission identified its origin as hot-electron intraband luminescence, and the lifetime derived from the luminescence decay was found to be below the resolution limit of the instrument (10 ps), indicating fast relaxation of hot electrons through photon emission.

Another promising approach to nonlinear light generation using liquid metal structures is to combine their unique mechanical properties with their advantageous optical/plasmonic properties. For example, Maksymov and Greentree theoretically studied the possibility of realizing dynamically reconfigurable plasmonic nanoantennas by inducing capillary oscillations of liquid metal NPs [95]. The capillary oscillations of the liquid metal NPs can be driven mechanically or electrically with a tunable frequency ranging from several MHz to several GHz. The time-varying plasmonic responses of such oscillating liquid metal NPs can lead to giant acousto-optic nonlinearities, which can be further exploited to generate light at new frequencies or even realize frequency combs [96]. Another work by Boyd et al. investigated the feasibility of producing beamed UV sonoluminescence by inducing air bubble collapse near the deformable surface of liquid metal microparticles [97].

4.3 Liquid-metal-based photonics for mid-infrared (MIR), THz and beyond

Ga-based liquid metals have also been employed to realize photonic structures and devices operating in wavelength ranges beyond NIR, such as the MIR, THz as well as microwave spectral regions. As the critical dimensions of photonic structures for these spectral regions are typically in the µm scale or larger, such photonic structures made of liquid metals feature large fluidity, flexibility and transformability, which are crucial advantages over solid metals. These unique properties of liquid metal structures have been exploited to achieve unconventional device fabrication and highly reconfigurable and self-healing photonic devices.

4.3.1 Liquid-metal-based MIR photonics

The fluidic nature of liquid metals allows the liquid metals to conformally cover the surfaces of other structures, which provides a simple and effective way to form metal coatings in ambient condition and without the need of conventional metal deposition tools such as thermal/electron-beam evaporators or sputterers. Miao et al. exploited this unique advantage of liquid metal to realize mid-infrared nanophotonic resonators which function as high-performance molecular sensors based on the surface-enhanced infrared absorption (SEIRA) sensing mechanism [98]. These resonant nanophotonic structures are essentially arrays of nanopatch antennas with a common liquid Ga ground plane (Fig. 8(a)). The sensor chips consisting of arrays of gold nanopatches on a glass substrate were first fabricated using standard a nanofabrication process. To perform SEIRA sensing, a thin layer of analyte was introduced to the surface of the gold nanopatches on the sensor chip, and subsequently the sensor chip was placed on the surface of liquid Ga so that the thin analyte film was sandwiched between the gold nanopatches and the continuous liquid Ga film, hence forming the complete nanopatch antenna structures. When the analyte film thickness is in the few nm range, the hot spots of such a nanopatch antenna are distributed in the nanometric gap between the gold nanopatch and the liquid Ga (Fig. 8(b)), which feature not only exceedingly high field confinement and enhancement but also excellent overlap with the analyte film. Therefore, these nanopatch antennas can lead to drastically enhanced light-analyte interaction and produce strong SEIRA signals. In addition, as the analyte was introduced to the desired locations (i.e., gold nanopatch surface) before the nanometric hot spots were formed (by covering the surface with liquid Ga), a high analyte delivery efficiency was achieved. A self-assembled monolayer of 1-octadecanethiol (ODT) molecules was used as the model analyte to demonstrate the performance of these liquid Ga nanophotonic SEIRA sensors, which achieved superior sensitivity compared to previously reported SEIRA sensors for sensing monolayer ODT (see Fig. 8(c)) [98]. These experimental results show that employing liquid metal to form hybrid nanophotonic structures is a simple and effective strategy to resolve the long-standing dilemma of trading off analyte delivery efficiency against field confinement/enhancement, which often limits the performance of many conventional nanophotonic sensors based on solid metals.

 

Fig. 8. (a) Schematic illustration of the liquid-Ga-based nanopatch antenna SEIRA sensor. (b) Simulated field enhancement profile of the liquid Ga-based nanopatch antenna SEIRA sensor. (c) Left: measured reflection spectra of liquid-Ga-based SEIRA sensors with monolayer ODT showing the ODT vibrational modes. Right: Extracted net SEIRA signals associated with the monolayer ODT. (a)-(c) adapted with permission from [98]. Copyright Wiley 2022. (d) Top: SEM images of 400 nm Ga evaporated onto treated PDMS under high vacuum and low vacuum, respectively. Bottom: SEM images showing a liquid Ga metasurface structure at 0% and 50% strain of the PDMS substrate, respectively. (e) Experimental (left) and simulated (right) reflection spectra for 2 µm period square metasurfaces with 1.5 µm liquid Ga disks at various strains using light polarized perpendicular to the tensile axis. (d)&(e) adapted with permission from [99]. Copyright Wiley 2020.

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Currently, a major hurdle for realizing micrometer or submicrometer scale patterned structures of liquid metals for applications such as MIR photonics is that the conventional metal thin film deposition and lift-off processes do not work well for liquid metals. This is mainly because deposition of liquid metal on a substrate usually leads to formation of non-percolated liquid metal particles rather than a continuous thin film, which is a result of the high surface tension of liquid metals, as discussed in the previous section. To overcome this challenge, Martin-Monier et al. recently demonstrated an effective technique to achieve deposition of percolated liquid metal thin film (with film thickness on the order of 100 nm) by carefully engineering the substrate surface states and controlling the oxidation dynamics during the liquid metal deposition [99]. Combining this liquid metal thin film deposition technique with standard photolithography and lift-off processes, MIR metasurfaces with micrometer scale unit cells (i.e., liquid metal disks) were realized on stretchable PDMS substrate (see Fig. 8(d)-(e)), which allowed for significant strain-dependent spectral tunability. A large strain (e.g., 50%) of the PDMS substrate resulted in significant changes in the shape of the liquid metal disks (Fig. 8(d)), which in turn led to substantial shift in the resonance frequency of the metasurface (Fig. 8(e)).

4.3.2 Liquid-metal-based THz and RF photonics

As discussed in the Section 3, the various techniques for patterning liquid metal structures can achieve critical feature sizes down to the few-µm scale, which provide sufficient spatial resolution for creating photonic structures (such as antennas) with characteristic responses in the THz region (0.1 to 10 THz) and beyond (e.g., millimeter wave, microwave). As liquid metals are flexible and deformable, liquid-metal-based THz to microwave photonic structures can be highly reconfigurable and suitable for realizing adaptive and multifunctional devices and systems for next-generation wireless communications and sensing technologies. In recent years, several groups demonstrated flexible and reconfigurable liquid-metal-based THz resonators, antennas and metasurfaces realized by molding-based patterning techniques [100–106].

The first demonstration of liquid-metal-based THz photonic structures was reported by Wang et al. who fabricated a period aperture array in a liquid EGaIn thin film (∼75 µm thickness) by injecting EGaIn into an elastomeric PDMS mold [100]. Resonantly enhanced THz transmission (in the range of 0.2 THz to 0.4 THz) through the aperture array was observed, which was attributed to the extraordinary optical transmission effect facilitated by the propagating SPPs in the perforated EGaIn film. Furthermore, stretching of the PMDS mold by up to 13% led to corresponding changes in the aperture geometry and array periodicity, which in turn resulted in significant tuning of the THz transmission spectrum.

In several following works from the same group, different techniques for manipulating liquid metals in PDMS-based microfluidic channels were explored and various reconfigurable THz antennas and metasurface structures were realized [101–103]. In one study, concentric rings of EGaIn were formed around a single aperture in a gold-coated stainless steel film by injecting EGaIn into the individual PDMS microfluidic channels using a syringe, leading to a bullseye structure [101]. The EGaIn rings can also be individually removed from the microfluidic channels using the syringe. The transmission of incident THz wave through the single aperture was dynamically modified by the different configurations of concentric EGaIn rings. In another study, a reconfigurable liquid-metal-based metamaterial consisting of arrays of liquid metal rings in PDMS microfluidic channels was demonstrated [102]. As the PDMS mold was gas permeable, individual liquid metal rings could be removed by applying a small amount of HCl solution on the corresponding location of the PDMS mold, so that HCl vapor could enter the microfluidic channel and remove the surface oxide layer of the EGaIn structure, causing the EGaIn to retract. The unfilled microfluidic channels could be refilled by injecting EGaIn again. Therefore, this EGaIn-based metasurface was reconfigurable on the single unit cell level. A more sophisticated technique for reconfiguring liquid metal structures in microfluidic channels which enables more precise structural changes is to introduce variation of the microfluidic channel width and use pressure to control which parts of the microfluidic channel are filled with liquid metal [103]. To inject liquid metal into a narrow part of the microfluidic channel, a higher pressure is required. Wang et al. designed the microfluidic channel structure shown in Fig. 9(a). The C-shaped split-ring was filled by EGaIn at a relatively low pressure (255 kPa) for injecting the liquid metal into the microfluidic channel. Upon increasing the pressure to 296 kPa, EGaIn entered the narrower part of the microfluidic channel, hence transforming the spit-ring to a closed ring structure. By further increasing the pressure to 386 kPa, the liquid metal was injected into the narrowest part of the microfluidic channel as well as the wider channels on the other side, which corresponded to another substantial structural transformation. These three different structure configurations corresponded to drastically different THz spectral responses. In addition, as the surface oxide layer stabilized the shape of EGaIn in the microfluidic channels, all three structure configurations remained unperturbed after the inject pressure was released, and therefore the device also functioned as a pressure memory. The EGaIn structure could be reset to its original configuration by applying HCl solution on the surface of the PDMS mold, which caused the surface oxide layer to be quickly dissolved and the liquid metal to retract into either the inlet or the outlet. Song et al. demonstrated frequency-tunable THz metasurface perfect absorbers by controlling the height of liquid metal micro-pillars which are the unit cells of the metasurface (see Fig. 9(b)) [104]. The absorption peak frequency could be continuously tuned from 0.246 THz to 0.415 THz. Zhang et al. developed a liquid metal metasurface structure which exhibited chiral THz response when its PDMS mold was deformed by a pressure chamber underneath (see Fig. 9(c)) [105]. The polarization rotation angle could be tuned from 0° to 14° at 0.19 THz when the metasurface was reconfigured from the flat state to the bent state. It has also been demonstrated that liquid-metal-based structures can be used to realize free-standing solid-metal THz photonic structures by solidifying the liquid metal structures in their PDMS molds and subsequently removing the molds [106].

 

Fig. 9. (a) Top: image of a portion of the split ring resonator array and its transmission spectrum. Bottom: image of a portion of the closed ring resonator array and its transmission spectrum. The closed ring resonators are transformed from the split ring resonators by applying higher pressure to inject liquid metal into the narrower channel section. Adapted with permission from [103]. Copyright Optical Society of America 2014. (b) Left: Schematic of the liquid-metal-based metasurface consisting of the liquid-metal-pillar array embedded in silicon cavities. Upper right: Absorption spectra with TE mode (red line) and TM mode (blue dot) when the height of liquid-metal-pillars is 70 µm. Lower right: Absorption color map for the TM mode when the height of the liquid-metal-pillars is tuned from 30 µm to 90 µm. Adapted with permission from [104]. Copyright Song et al. 2017. (c) Schematic illustrations of a deformable liquid-metal-based THz metasurface and the mechanism for inducing deformation and hence spectral response change. Reprinted with permission from [105]. Copyright Optical Society of America 2021. (d) Images of a liquid-metal antenna being stretched, rolled and cut. The antenna self-heals in response to sharp cuts. Adapted with permission from [107]. Copyright Wiley 2009. (e) Left: Schematic illustration of a randomly addressable metasurface as a flat lens with tunable focal distance when resonant properties of the split rings in the array are altered by changing the metal filling fraction. Right: Simulation and experimental results showing the tuning of the metasurface lens’ focus when the spatial phase distribution (the fourth column) is changed. Adapted with permission from [115]. Copyright Wiley 2015.

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In addition to applications in THz photonics, Ga-based liquid metals have also been frequently used to realize a variety of tunable millimeter wave and microwave photonic devices (e.g., antenna [107–110], metasurface [111,112], metamaterial [113,114], phased array [111], flat lens (Fig. 9(e)) [115], absorber [113] and sensor [114,116]), which have much larger characteristic dimensions (in the mm scale) than THz photonic structures. The fabrication of these liquid-metal-based RF photonic devices were mainly based on the molding method. Several structural reconfiguration mechanisms (including mechanical deformation, pressure controlled liquid injection/withdrawal, electrochemically controlled capillarity) have been exploited for tuning the optical responses (e.g., resonant frequency, amplitude, phase) of these photonic structures. Another unique advantage of employing liquid metals is that their fluidity facilitates self-healing of the photonic structures in response to structural damages (see Fig. 9(d)) [107]. Furthermore, liquid metal structures can be integrated with other solid metal structures to achieve tunable hybrid devices. For example, by stacking a flexible printed circuit board with Jerusalem cross resonators on a flexible PDMS substrate with microfluidic channels filled by liquid EGaIn, Kim et al. realized a wideband-switchable absorber by using the liquid metal lines as adjustable inductance sources [113]. The frequency range with over 90% absorption was 7.43 GHz to 14.34 GHz when the channels were empty, and became 5.62 GHz to 7.3 GHz when the channels were filled liquid metal. By integrating liquid metal channels above copper split-ring resonators (SRRs), Rafi et al. demonstrated a high-performance frequency-tunable resonator [109]. When varying the amount of liquid metal introduced into the system, the resonant frequency was continuously adjusted from 3.4 to 2.3 GHz and the resonance amplitude and quality factor experienced less than 10% variation across the entire frequency tuning range. Wiltshire et al. developed a microwave resonator array integrated with liquid metal lines for dielectric sensing applications [116]. Galinstan moved through the microfluidic channels above the copper SRR array and functioned as a capacitive load to the nearby SRRs, lowering their resonant frequency. The resonant frequency shift revealed the dielectric constant of the target material between the liquid metal and the selected SRR.

5. Outlook

The unique advantages of near-room-temperature Ga-based liquid metals over conventional solid metals have been exploited for a wide range of applications in photonics as well as many other areas. Nevertheless, the research and development of liquid-metal-based technologies and applications are still in the early stages. An abundance of new opportunities and possibilities associated with liquid metals await further explorations, which also entail new discoveries to be made to overcome various challenges. More systematic and in-depth studies of the synthesis and material properties of liquid metals are imperative. For example, although elemental Ga, EGaIn and Galinstan are the most frequently used liquid metals for various photonics applications, the optical properties of EGaIn and Galinstan in the NIR-Vis-UV spectral region are not as extensively studied as those of elemental Ga. In addition, mixing Ga with other metals may lead to more types of liquid metal alloys with widely tailorable material (optical) properties, which in turn enable new applications in photonics. On the other hand, development of technologies for more controlled patterning of liquid metal microstructures and nanostructures will enable a large number of new possibilities. As discussed in the previous sections, although relatively large liquid metal structures can be patterned using a variety of techniques, microscopic liquid metal structures are currently realized with poor control of their sizes, shapes and locations. To considerably expand the photonics applications of liquid metals in the entire IR to UV spectral region, reliable techniques for realizing liquid metal structures with critical dimensions on the order of 1 µm or less are required. Although currently the molding based techniques have been used to pattern liquid metal structures with critical dimensions larger than a few µm, it has been demonstrated that liquid metal can be encapsulated in nanometric channels such as a carbon nanotube [117,118]. Another highly desirable but elusive aspect regarding liquid metal microscopic structures is the capability to dynamically reconfigure their structures (e.g., shape and orientation), which may enable highly tunable photonic devices operating in the IR to UV spectral region. The fluidity of liquid metals allows for high reconfigurability for relatively large liquid metal structures. However, the small size and large surface tension of a liquid metal microscopic structure makes changing its shape in real time a much more challenging task. Inducing capillary oscillation is a promising approach to realizing time-varying shape change of liquid metal microscopic structures [95,96]. On the other hand, on-demand switching between different static configurations of liquid metal microscopic structures are also highly desirable. Therefore, more fundamental studies and technology development targeting dynamical reconfiguration of liquid metal microscopic structures should be conducted.

6. Summary

The appealing properties of Ga-based liquid metals, including their near-room-temperature melting points, high plasma frequencies, low toxicity and negligible vapor pressure, make them suitable for a broad range of photonics applications. In particular, the high plasma frequency of liquid Ga makes these liquid metals highly suitable for UV plasmonics. In addition, the fluidic nature and good transformability of liquid metals clearly distinguish them from solid metals and enable unconventional photonic structures and tuning mechanisms. Both macroscopic and microscopic liquid metal structures can be realized using a variety of techniques beyond those for fabricating solid metal structures. The facile approaches to forming liquid metal nanoparticles with strong optical and plasmonic responses facilitate the development of high-performance and cost-effective nanophotonic systems for a variety of applications such as sensing and nonlinear optics. Macroscopic liquid-metal-based photonic structures are highly flexible and reconfigurable, leading to dynamically tunable THz and RF photonic devices with self-healing capability. Nevertheless, the research field of liquid-metal-based photonics is in its infancy and many potential opportunities await further exploration. With the development of new techniques to achieve more precise control of the morphology, positioning and reconfigurability of liquid metal micro- and nano-structures, liquid metals will likely find a much broader range of applications in photonics across a wide spectral range.