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Abstract
Although advancement in nanofabrication provides the opportunity to realize nanoscale geometries with high resolutions, the scalability and repeatability issues limit their large-scale applications. Lithography-free metamaterial absorbers (LFMAs) are a potential route for the upscaling of these designs. With restricted freedom in their synthesis, the importance of the proper material choice is emphasized. Herein, we provide a comprehensive overview of the recently developed LFMAs, from both design and material perspectives, while considering their most promising applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Routes and materials
Light confinement and harvesting in dimensions much smaller than the wavelength have been the subject of many studies in the field of metamaterial-based perfect absorbers. Metals, at sub-wavelength dimensions, can undergo strong light-matter interaction in a broad spectral range [1]. This happens through the collective oscillation of free conduction electrons, so-called localized surface plasmon resonance (LSPR), or inter-band transitions. However, the realization of plasmonic nano units requires complex and large-scale incompatible routes such as lithography-based techniques. Therefore, recently, the concept of lithography-free metamaterial absorbers (LFMAs) has attracted intensive attention [2,3]. The most common type of these scalable LFMAs is planar multilayer designs, with the dominant absorption mechanism of Fabry-Perot (FP) resonance modes [2]. Metal-insulator-metal (MIM) and MIM-insulator (MIMI) and periodic (MI)N cavity configurations are the most common cavity absorbers. One-dimensional photonic crystal (1D-PC) based configurations are also utilized to achieve light absorption in ultrathin absorbing layers. Nanostructured LFMAs could outweigh these planar designs, especially in applications where both strong absorption and high active surface area are desired. Recently, several innovative fabrication routes have been developed to meet this requirement. Deposition induced structuring [4,5], dewetting induced nanounit formation [6–8], oblique angle deposition of three-dimensional (3D) structures [9–14], direct laser writing [15], template-assisted etching [16,17], and deposition [18,19] are examples of these methods, as shown in Fig. 1. Aside from the importance of the route, the choice of the right material is another prominent factor in LFMA design. Figure 2 outlines the most suitable materials for the building of LFMAs. In the ultraviolet (UV) and visible, a vast variety of semiconductors and metals can be used to achieve this goal. To extend this functionality toward longer ranges such as near-infrared (NIR) and short-wavelength infrared (SWIR), lossy metals such as Ti, Cr, W, and Bi could be utilized in a proper configuration [2]. Moreover, doped metal oxides such as Al-doped ZnO (AZO) [20], Ga-doped ZnO (GZO) [21], In-doped SnO2 (ITO) [22], In-doped CdO (ICO) [23,24], and Ce-doped In2O3 (CIO) [25] that show epsilon-near-zero (ENZ) characteristics in the SWIR range could be used in LFMAs. Metal nitrides such as TiN, ZrN, HfN, and InN could also reveal a plasmonic response in the infrared (IR) range [26–29]. Moreover, these alternative plasmonic materials show tunable plasma frequency in the mid-wavelength infrared (MWIR) range. In longer wavelengths (i.e. MWIR, long-wavelength infrared (LWIR), and far-infrared (FIR)), LFMAs can be realized through the excitation of i) optical phonon in metal oxides [30], ii) plasmonics in highly doped semiconductors [31], iii) plasmonic resonances in two dimensional (2D) materials such as graphene and black phosphorus (BP) [32,33], and iv) phonon polariton modes in polar materials [34].
Fig. 2. Possible absorber materials for the realization of LFMAs in different portions of the optical spectrum.
2. Applications
LFMAs are divided into two material categories: i) metallic and ii) non-metallic. Their applications are sorted based on their operational wavelength range; i) sun blackbody radiation spectrum (sun-BBRS) or so-called solar spectrum covering UV-visible-NIR-SWIR ranges, and ii) earth blackbody radiation spectrum (earth-BBRS) covering SWIR-MWIR-LWIR-FIR.
2.1 Sun-BBRS
Metallic LFMAs. Planar metallic LFMAs can reveal spectrally selective narrowband or broadband light absorption. The narrowband planar LFMAs are commonly utilized in color filtering applications [35]. While simple MIM cavity design can generate additive red-green-blue (RGB) colors in transmissive mode (with amplitudes much below unity), tandem shape cavity architectures can generate high-efficiency RGB colors in reflection mode [36–40]. These planar LFMAs can also show dynamically tunable color generation, through the change in spacer layer index or thickness. For index modulation, phase change materials (PCMs) such as GST [41,42], VO2 [43], Sb2S3 [44], or even Sb [45] can be used as the spacer to generate thermally tunable colors. For nanoscale thickness tuning of the spacer, swelling/deswelling of humidity-sensitive polymers and hydrogels is employed to fabricate dynamic MIM color filters [46–48]. Besides filtering, the narrowband LFMAs have potential application in colorimetric sensing platforms such as bio-, and gas-sensing, where the external stimuli shifts the resonance peak of the LFMA [12,49,50]. However, in sensing, a high surface area is desired to maximize the interaction of light with the surrounding environment. Thus, nanostructures such as dewetted or oblique angle deposited plasmonic units offer higher sensitivities [51,52].
On the other side, the broadband LFMAs can harvest a large portion of the solar spectrum, which is desired for photoconversion applications. In metals, photoconversion functionality can be acquired using the excitation and extraction of energetic hot electrons. However, due to the femtosecond relaxation of hot electrons with a dominant mechanism, called Landau damping, only energetic carriers can traverse the Schottky barrier [53]. This photoemission efficiency is even less in planar and large particle sizes [54] where absorption is due to non-resonant inter-band transitions rather than the LSPRs. Despite intensive efforts to find alternative high performance and low-cost plasmonic elements such as Al [55–57], conductive oxides [27,58,59], transition metal nitrides/carbides (such as TiN) [60,61], and doped semiconductors [62–64], the efficiency of hot electron designs is still low. The same obstacles are present in the hot electron based photodetection and photoelectrochemical water splitting (PEC-WS) applications [65]. The alternative route is to use these metallic LFMAs in the application platforms where metals are not the photoactive layer such as thermal photovoltaic (TPV) and steam generation. In TPV, a broadband LFMA absorbs photons, transforms them into heat, and reradiates it using a selective emitter with a spectral emission peak overlapping with the PV bandgap. Based on theoretical calculations, a solar TPV, with an ideal design of absorber and emitter, can achieve efficiencies exceeding the Shockley-Queisser limit [66]. The narrowband emitter can also be realized with a multilayer dielectric-based 1D-PC design, making the overall system a lithography-free architecture [67]. Solar-driven steam generation is another promising area for broadband LFMAs. Advancements in this area have proven efficiencies as high as 90% at 4-sun intensity (4 kW m−2) [68]. Different from TPV, in this application, a high area is a must to maximize the heat transfer between the LFMA and water.
Non-metallic LFMAs. Similar to metallic ones, the dominant application of non-metallic LFMAs in sun-BBRS is photoconversion and semiconductors are its main building blocks [69]. In most semiconductors, the carrier diffusion length is much shorter than the light penetration depth. Therefore, the bulk recombination of carriers limits their efficiency. However, sub-wavelength semiconductor-based LFMAs can offer optically thick, electrically thin platforms suitable for photoconversion systems, such as PV, photodetection, and PEC-WS [67]. Planar metal-semiconductor (MS) and metal-dielectric-semiconductor (MDS) cavity designs are proven to achieve near-unity absorption in deep sub-wavelength semiconductor thicknesses [70–80]. In 2D monolayer semiconductors, dielectric-based 1D-PC and Bragg reflector designs are the right strategy to confine the entire power in the position of the 2D layer and efficiently harvest it [81–83]. Besides the optical response, the electrical characteristics of these semiconductors are also a prominent factor that affects the performance outcome. Ideally, a crystalline semiconductor film should be grown in ultrathin dimensions, but it is a challenging task. The transfer of external crystalline thin films [79,84] and the CVD growth of 2D semiconductors can provide high-quality thin films. In PEC-WS, two more factors should be adopted in the design of these LFMAs; i) surface area and ii) long-term structural integrity (or stability) of the photoelectrode [69]. While planar LFMAs have already been used as photoanode and photocathode components in PEC-WS [84,85], nanostructured designs with a higher water-semiconductor interface can improve the activity. To have a high surface area with good crystallinity, template-assisted etching of crystalline semiconductor host seems a promising approach [16]. A thin protection layer with proper band alignment could be utilized to solve the stability issue.
2.2 Earth-BBRS
Metallic LFMAs. In most metals, the LSPR excitation and interband transition happen in the UV-visible range. However, lossy metals in (MI)N and metal-1D-PC configurations, can reveal resonant light absorption in the SWIR-LWIR range [37,86–89]. These planar absorbers can act as selective/broadband thermal emitters with possible applications in radiative cooling and thermal camouflage [90–93]. Integration of these LFMAs with PCMs offers dynamically tunable IR emitters/absorbers [94–96]. Such a design could even enable passive radiative thermostat functionality by adjusting the visible and IR absorption/emission [97].
Non-metallic LFMAs. Highly doped semiconductors, doped metal oxides, metal nitrides/carbides, and 2D materials such as graphene, and BP are alternative plasmonic materials in MWIR, LWIR, and FIR range [34]. Heavily doped semiconductors such as Ge, Si, III-V compounds are appealing alternatives to metals for IR plasmonics [31]. Their plasma frequency can be tuned chemically, optically, or electrically across a broad range. Semiconductor-based hyperbolic metamaterials, made of doped-undoped pairs, can be developed to realize narrowband directional IR emission [98,99] and ultrafast and low power all-optical switching [100]. Doped metal oxides have the advantage of visible transparency, making them compatible materials for spacecraft cooling and thermal camouflage [11,101]. 2D materials with exceptional electrical and optical properties are another attractive category in IR plasmonics. Graphene, as the most famous member of this group, can support guided plasmonic modes in the IR range, where the spectral position of these modes can be effectively tuned by its chemical potential (µ) [33,102–104]. Graphene monolayer-based LFMAs have been realized using the excitation of Tamm plasmons in 1D-PC structures [105–107]. Moreover, the use of distributed Bragg reflector (DBR) in a cavity design, to couple light into the graphene monolayer, has been demonstrated [104]. A similar design strategy can be utilized to achieve LFMA using a monolayer of BP [108]. Due to their high carrier mobility, LFMAs made of these 2D materials coupled with narrow bandgap semiconductors have promising applications in IR photodetector designs [109,110]. Different from graphene, the BP monolayer shows strong in-plane anisotropy, used to design polarization selective absorbers [111]. Moreover, different from common narrow bandgap semiconductors, BP has a tunable bandgap from 2 eV (in monolayer) to 0.3 eV (in bulk). Based on recent work [112], a vertical electric field can tune the bandgap of BP and provide a wide tunability (from 3.7 to 7.7 µm) in its photoluminescence (and consequently absorption peak) spectral response. The last category of non-metallic materials is polar dielectrics. These materials are used to make LFMAs in a wide range spanning from MWIR to FIR [2]. The IR optical response of these materials (such as hBN, SiC, AlN, GaN, GaP, and α-MoO3) are dominated by highly reflective reststrahlen (RS) bands that are located between the longitudinal and transverse optical phonons. Within these RS bands, light can couple with optical phonons to support surface phonon-polariton (SPhPs) modes. LFMAs made of these polar materials are commonly made in 1D-PC and DBR cavity configurations and have been mainly used to realize coherent monochromatic, and directional thermal emitters [113–115]. The hybrid use of these polar materials with PCMs and graphene can provide tunable IR absorbers to be used in radiative cooling, adaptive thermal camouflage, and modulators [116,117]. Moreover, some of these polar materials including BP, hBN, and α-MoO3 have strong in-plane and out-of-plane anisotropy. This could be utilized to realize lithography-free polarization converters in the IR region [118].
3. Outlook
In summary, besides the development of low-cost, facile, and extendable routes, material exploration is an essential key to realizing cm-scale high-performance LFMAs. The integration of the right elements with large-scale compatible nanofabrication routes will put this technology one step closer to industrialization. In photoconversion, a deep understanding of hot electron dynamics, exploring new materials with a long hot electron lifetime, synthesis of ultrathin high mobility crystalline semiconductors, and the development of designer heterojunctions are some of the key goals to be followed. Spectrally-selective thermal emitters are another area of high interest, due to the lack of cost-effective, narrow-band light sources in the MWIR and LWIR. TPV technology and steam generation could also widen the opportunities for the use of these LFMAs in the green energy industry. The integration of these large-scale compatible designs with microfluidic channels can offer a real-time and label-free detection platform for bio-agents with high sensitivity and reliability.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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