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Abstract
Novel manipulation techniques for the propagation of electromagnetic waves based on metamaterials can only be performed in narrow operating bands, and this drawback is a major challenge for developing metamaterial-based practical applications. We demonstrate that the scattering of metamaterials can be switched and that their operating band can be tuned by introducing liquid metal in the design of functional metamaterials. The proposed liquid metal-based metamaterial is composed of a copper wire pair and a tiny pipe filled with a liquid metal, namely eutectic gallium-indium. The interference of the sharp magnetic resonance of the copper wire pair and the broad dipolar mode of the liquid metal rod lead to an electromagnetically induced transparency (EIT)-like spectrum. We experimentally demonstrate that this EIT-like behavior can be switched on or off by exploiting the fluidity of the liquid metal, which is useful for multi-frequency modulators. These findings will hopefully promote the development of fluid matter-based metamaterials for extending the operating band of novel electromagnetic functions.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterials (MMs) have attracted a great deal of attention in recent years in the fields of physics and material engineering because their properties can be arbitrarily tailored by rationally designing their microstructure (meta-atoms or meta-molecules) [1,2]. Owing to their unique properties, such as negative permittivity, negative permeability, and negative refraction [3], MMs have been investigated for applications in diffraction-unlimited imaging [4–6], invisible cloaking [7], sub-wavelength cavities [8–11], polarization manipulation [12], and electromagnetic absorbers ranging from the microwave band to visible frequencies [13–20]. Furthermore, quantum metamaterials are emerging as new developments in this field [21–23], which may play an important role in improving light-matter interactions for fundamental physics and applications [24–26]. Some novel optical phenomena [27], such as electromagnetically induced transparency (EIT), can be classically mimicked in metamaterials with specifically designed meta-molecules for manipulating classical waves.
EIT originates from the quantum interference between two transition pathways within atoms or molecules in laser fields. It is a promising technique for eliminating the effects of a medium on radiation [28]. The steep dispersion of EIT can result in slow light and enhanced light-matter interactions. However, the extreme environment required for quantum EIT prevents it from being used in practical applications. The spectral characteristics of EIT have been reproduced in several classical structures [29–31]. Particularly, in recent works, artificially structured metamaterials were proposed for the demonstration of the classical analogue to EIT in plasmonic metamaterials [28,32–39]. Metamaterials provide a novel platform for realizing the classical analogue of quantum behaviors easily. One of the drawbacks of metamaterial-based EIT is its narrow operating band, which may limit the practical applications of metamaterial analogues to EIT. It is high desirable to extend or modulate the operating band of metamaterials by incorporating adjustable mechanisms when building metamaterial-based devices. Various meta-devices made of certain materials that respond to external stimuli (including semiconductors, superconductors, electronic components, and flexible substrate) have been proposed in the past few years to realize versatile switchable and frequency-agile functionalities [39–46]. Gu et al. showed that active optical control of metamaterial-induced transparency can be realized by integrating photoconductive silicon into the metamaterial unit cell [39]. In a previous work, we incorporated a PIN diode component in an EIT metamaterial to realize an electromagnetic modulator that can operate at multiple frequencies [44]. It has been demonstrated that the group delay of terahertz light can be dynamically controlled in active EIT metamaterials under a small gate voltage by combining diatomic metamaterials with gated single-layer graphene [46,47].
Recently, liquid metamaterials have been demonstrated using dielectrics and metals in their liquid states [48–52], in which tenability can be achieved by exploiting the fluidity of the liquid materials for continuously controlling the geometry of the metamaterials [53]. In this paper, we demonstrate switchable EIT-like behavior by integrating liquid metal in a meta-molecule. The liquid metal alloy eutectic gallium-indium (EGaIn) was employed in our study because it is suitable for the formation of stable metamaterial structures at room temperature [54]. We experimentally found that the observed EIT-like behavior can be switched on or off by controlling the formation of a liquid metal cut-wire structure in a plastic pipe. By exploiting this, a multiple-frequency electromagnetic modulator capable of transmitting significantly modulated microwaves was implemented.
2. Results and discussion
A schematic of the proposed EIT-like metamaterial is shown in Fig. 1. It consisted of a cut wire made of a liquid metal alloy and a wire pair made of copper film (with a thickness of 18 μm). These metallic structures were placed on a PCB substrate made of 1.0-mm-thick Teflon with a permittivity of 2.65. The lateral sizes of the structure were 47.54 mm × 22.14 mm. Because of the fluidity of the liquid metal used at room temperature, we used a plastic pipe to restrict the flow of liquid metal and thus form a liquid metal-based cut wire. The inside diameter of the plastic pipe was 1 mm. The pipe was connected to a “Y”-shaped injection. Liquid metal and water were injected through pipe 1 and pipe 2, respectively. These injections were controlled using peristaltic pumps connected to the two arms of the “Y”-shaped port. Additionally, the length l of the liquid metal-based cut wire was be regulated by altering the pressure of the pump. Symmetric copper wires were printed on the substrate. The length and width of the structure were w1 = 18 mm and w2 = 5.5 mm, respectively, and the line width was w = 1 mm. The gap between the two metal wires was g = 1 mm.
Fig. 1 Schematic of the liquid metal-based EIT metamaterial. The proposed metamaterial consists of a cut wire made of a liquid metal alloy and wire pairs made of copper. The substrate slab is made of Teflon, and the geometric sizes of the structure are marked with red arrows.
We first considered the spectral response of the coupled metamaterial design. EIT-like behavior can be numerically characterized by solving Maxwell’s equations using finite element methods within a PEC-surrounded air box (to simulate our experimental setup, the sample was characterized in a standard WR-187 waveguide). The transmission spectra of the liquid metal-based cut-wire, the wire pair, and the coupled metamaterial structure are presented in Fig. 2. For the metamaterial with only the left liquid metal cut wire at a specific length of l = 21.6 mm, it can be seen that there is a resonant dip at approximately 4.79 GHz on the transmission spectra (red curve). This resonance is a broad electric-dipolar mode, as deduced from our investigation on the local-fields. This dipolar mode is similar to the bright mode of wires in plasmonically induced transparency systems, which provides an opaque spectral range for the formation of the EIT analogue. Then, we considered the properties of the paired wires. Their transmission spectra are represented by the blue curve, in which a sharp resonance dip at approximately 4.84 GHz can be observed. This resonance overlaps with the broad electric-dipolar mode, and it was verified to be a magnetic-dipolar mode via local-field investigations. After we identified these two overlapped resonances, we further studied the case of a metamaterial comprised of both the left liquid-metal cut wire and the right wire pair; the resulting transmission spectra is shown by the green curve. It can be seen that the original transmission dip of the metallic wire pair changed to a transmission peak at the same frequency with the appearance of the liquid-metal cut wire, and this transparency was accompanied by two transmission dips on both sides of the peak. These peak/dips are an exact physical representation of the constructive and destructive interferences in EIT.
Fig. 2 Simulated transmission spectra of the EIT structure: liquid-metal column by itself (red curve), wire pair by itself (blue curve), and the proposed coupled structure (green curve).
In order to observe the interactions of the two parts of the proposed metamaterial structure and analyze the operating principle of the EIT-like phenomenon clearly, we obtained a schematic diagram of the surface current distribution via calculations made in CST Microwave Studio, as shown in Fig. 3. It can be seen that, at the frequency of the EIT peak, the electric-dipolar mode excited from the left liquid-metal cut wire is extremely depressed by the magnetic-dipolar mode exited from the right wire pair. This destructive interference between the two resonant modes leads to the occurrence of the observed EIT-like phenomenon.
To experimentally validate the characteristics of the simulated EIT-like spectrum of the hybrid solid/liquid metal metamaterial, a sample with the same geometric parameters as those shown in Fig. 1 was fabricated, as shown in Fig. 4(a). The flow and injection of liquid metal in the tiny pipe was controlled with a peristaltic pump connected to the “Y”-shaped connector. We set the length of the liquid metal-based cut wire as l = 21.6 mm. The sample was fixed in a standard waveguide and the scattering coefficients of the sample were measured with a vector network analyzer through a two-port set connected to the waveguide (as shown in Fig. 4(b)). The measured transmission spectra for the proposed metamaterials, namely the copper wire pair by itself, the liquid-metal cut wire by itself, and the hybrid liquid/solid metal structure, are presented in Fig. 4(c). It is obvious that we have demonstrated the EIT-like spectrum of the hybrid metamaterial made of a liquid-metal cut wire and a solid metal wire pair. The measured results agree well with our numerical predictions shown in Fig. 2. The inset of Fig. 4(c) plots the measured group delay, from which we can see a maximum group delay of 3.81ns at the transparent window of EIT (4.85GHz) and strong dispersion around the transmission peak. The enhanced group delay at the EIT transmission peak indicates potential application in slow wave.
Fig. 4 (a) Photograph of the fabricated sample. (b) Setup for controlling the flow of liquid metal and the measurement setup. (c) Measured transmission spectra for the metamaterials: the copper wire pair by itself, the liquid-metal cut wire by itself, and the hybrid liquid/solid metal structure.
Liquid metals were introduced as inclusions in metamaterials because the fluidity of liquid materials can be employed for restructuring the geometry of metamaterials. In our study, we exploited the resulting switchable wave propagation by controlling the inclusions in the tiny pipe in the metamaterial sample. We injected water through the second pipe of the “Y”-shaped injection port using a pump to eliminate the liquid-metal cut wire. As expected, the EIT-like peak vanished and there was a new narrow-band resonant dip at the same frequency shown in the transmission spectrum (Fig. 5(a)). As shown in Fig. 5(a), the intensity of transmission was modulated from −1.84 dB to −17.24 dB at 4.91 GHz, from −38.74 dB to −1.36 dB at 4.57 GHz, and from −32.4 dB to −0.99 dB at 5.12 GHz, respectively. Moreover, this switchable behavior can be modeled in numerical simulations, as shown in Fig. 5(b). Both the theoretical results and the experimental proof suggest that the proposed hybrid metamaterial made of liquid and solid metals can be used as a multiple-band electromagnetic modulator. We mainly concerned on the stationary modulation of electromagnetic waves in the liquid metal based reconfigurable metasurface in this paper. It is worth noting that the liquid metal with desired size can be injected in the pipe or washed away for switching purpose within 1 s. The response time of the liquid metal structure can be further improved in the configuration of microfluid chips [53].
Fig. 5 Measured (a) and simulated (b) transmission spectra of switchable EIT-like behavior in a liquid metal-based and water-based metamaterial.
3. Conclusion
In summary, we proposed and experimentally demonstrated that a hybrid metamaterial made of liquid and solid metallic structures can be employed to obtain a switchable EIT-like response. The observed EIT-like transmission arises from the coherent interference of the resonant modes of the wire pair and the broad electric-dipolar mode of the liquid-metal cut wire. We demonstrated that the EIT-like spectrum can be tuned or switched by exploiting the fluidity of the liquid metal. Transmission through the metamaterial can be significantly modulated at the characteristic peak/dips of EIT-like spectrum, and the demonstrated multi-frequency modulation based on a controllable EIT analogue presents benefits for extending the frequency range of metamaterial-based modulators. Liquid metal-based metamaterials provide a new route for reconfigurable metamaterials.
Funding
National Natural Science Foundation of China (NSFC) (61771402, 61505164, 11674266, 11372248), the Shenzhen Science and Technology Innovation (JCYJ20170817162221169), the Natural Science Foundation of Shaanxi Province (2018JM6024, 2017JM6094, 2017JQ5116), the Hong Kong Scholars Program (XJ2017006), and the Fundamental Research Funds for the Central Universities (Grant Nos. 3102018jgc008, 3102017zy033, 3102018zy045).
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