We propose a novel infrared (IR) metamaterial absorber by using chalcogenide glass (ChG) material. The merits of ChG are zero extinction coefficient, excellent IR transparency, high third-order nonlinearity, adjustable refraction index, dispersion, and energy bandgap in the IR wavelength range. Therefore, it is very suitable for metamaterial designs used in widespread applications, such as solar cells, environmental sensors, wearable electronic devices, optoelectronics, and so on. This ChG-based metamaterial absorber is configured with a cyclic ring-disk (CRD) structure. By tailoring the geometrical structures of CRD, the corresponding resonant wavelength and intensity of the reflection spectra could be attenuated and modified. The maximum quality factor (Q-factor) is 625. These unique characteristics of this IR metamaterial absorber with a CRD structure can be used as a tunable IR filter, variable optical attenuator (VOA), and multi-resonance switch. For the applicability of the proposed IR metamaterial absorber, the proposed device is surrounded with different environmental media. The electromagnetic responses indicate that the proposed IR metamaterial absorber can be a high-efficiency environmental sensor with a correlation coefficient of 0.99999.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metamaterials are artificial materials composed of periodic or aperiodic structures to have extraordinary electromagnetic characterizations. By properly tailoring the feature size of metamaterial, it can be realized electro-optic devices spanning the wavelength range from microwave, infrared (IR), visible to ultraviolet (UV) caused from the transformation optics [1,2]. In view of metamaterials having unique electromagnetic characterizations that cannot found in nature materials, metamaterials have great potentialities in the development of novel optical devices, such as optical waveguide, filter, switch, nanophotonic, sensor, high-resolution imaging, and so on. Moreover, the development of metamaterials can greatly reduce the volume and weight of conventional optical devices . Recently, there have been reported many literatures for metamaterial-based optoelectronic devices. However, they are almost passive devices and less researches demonstrated active devices by using metamaterials. Although there are abundant results of passive and active devices for the potential applications, such as sensors [3–7], absorbers [8–14], solar cells [15,16], switches  and filters , their optical performances are not perfect for widespread applications with high portability, applicability, and cost-effectiveness characterizations. Such characteristics of active metamaterial-based devices might minimize limitations of time and space in the uses of energy harvesting, emitter, detector, photovoltaic cell, spectroscopy, sensor and other fields.
In the reported literatures, the uses of materials for metamaterials are including silver (Ag), gold (Au), aluminum (Al), and copper (Cu). Among of these materials, Ag and Au are the two most often used for metamaterial applications than aluminum (Al) and copper (Cu) due to their relatively small ohmic losses or high conductivity in IR wavelength range. However, Ag material will suffer the degradation in the fabrication process. The critical value of uniform Ag film-thickness is less than 25 nm  and the losses of Ag are strongly dependent on the surface roughness . Furthermore, the IR spectra are easily absorbed in Ag and Au materials resulting in low efficiency of electromagnetic response. In this study, we propose a novel IR metamaterial absorber by using chalcogenide glass (ChG) materials. ChG materials are amorphous compound containing the chalcogen elements (S, Se, Te) and exhibit wide IR transparency windows. They are easily synthesized to be a thin-film layer and their compositional flexibility allows the tuning of optical properties making them ideal for optoelectronics applications. The characteristics of ChG materials are zero extinction coefficient, excellent IR transparency, high third-order nonlinearity, adjustable refraction index and energy bandgap in IR wavelength range . Moreover, the feature sizes are less than 2 μm for the ChG with a high nonlinear coefficient, the dispersion of ChG is basically close to zero in near-IR and mid-IR wavelength ranges . These extraordinary characteristics can be utilized micro-electro-mechanical systems (MEMS) technique to modify the electromagnetic response of ChG-based metamaterial absorber with reconfigurable geometrical dimensions. In addition, ChG-based metamaterial absorber can prevent the high absorption comes from traditional metallic materials in IR wavelength range.
Herein, ChG-based metamaterial absorber is composed of an inner disk and an outer ring configuration, which is called the circular ring-disk (CRD) structure on Au/Si substrate. By tailoring the geometrical parameters including refraction index of ChG, CRD radius, gap between inner disk and outer ring, and horizontal relative position of inner disk and outer ring, the resonant frequency and intensity of ChG-based metamaterial absorber can be modified to perform a multi-resonance switch with an ultra-high Q-factor. To further investigate the proposed device can be used in practical applications, ChG-based metamaterial absorber is surrounded with different environmental media. It can be a high-precision refraction index sensor.
2. Designs and methods
Figure 1 shows the schematic drawing of proposed ChG-based metamaterial absorber with CRD structure on Au/Si substrate. The thickness of Au layer is 400 nm. The denotations are diameter of outer ring (R), diameter of inner disk (r), gap between inner disk and outer ring (g), horizontal displacement of outer ring (d), refraction index of air (n0), and refraction index of ChG (nm), respectively. The line width of ring (w) and period of ChG (p) are kept at 0.3 μm and 3 μm, respectively. The inner disk is fixed on Au/Si substrate and the outer ring is designed to be suspended and adjustable. The gap between inner disk and outer ring and horizontal displacement of outer ring can be modified by using MEMS-based electrostatic or electrothermal actuation mechanism to realize tunable metamaterial. According to the propagation and attenuation of surface plasmon polaritons (SPPs) on metamaterial absorber, it can generate the collective oscillation of free electrons within metamaterial absorber when the wavevector of SPPs is equal to that of incident electromagnetic wave  and then the incident electromagnetic energy will be absorbed by these metamaterial structures. We propose an active tuning approach to control ChG-based metamaterial absorber by using Lumerical Solution’s finite difference time domain (FDTD) based simulations to study the optical properties of device. The propagation direction of incident light is set to be perpendicular to the x-y plane in the numerical simulations. Periodic boundary conditions are also adopted in the x and y directions and perfectly matched layer (PML) boundaries conditions are assumed in the z direction. The mesh precision is 0.5 nm and the minimum clearance size is 0.1 μm. The reflection (R) and transmission (T) of lights are calculated by two monitors set on both sides of device. The corresponding absorption (A) is defined as A = 1-R-T. Since transmission is inhibited by the opaque bottom Au layer, and then the absorption can be directly obtained by A = 1-R. It means the high absorption obtained by the low reflection.
3. Results and discussions
Figure 2 shows the reflection spectra of ChG-based metamaterial absorber with CRD structure by changing the refraction index of ChG (nm) from 2.0 to 3.4. The geometrical dimensions are kept at R = 1.2 μm, r = 0.8 μm, g = 0, and d = 0.4 μm, respectively. The tuning range of resonance is 0.42 μm from 3.11 μm to 3.53 μm. The trend and bandwidth of resonances are linear and broader by changing the refraction index of ChG. The resonant intensities are increased gradually at the condition of nm changing from 2 to 2.4 and then decreased gradually at the condition of nm changing from 2.4 to 3.4. The maximum resonance is under the condition of nm = 2.4. The corresponding Q-factor is calculated as 567. The electric (E) and magnetic (H) field distributions for the condition of nm = 2.4 are shown in Fig. 2(b) and 2(c), respectively. It can be seen the most of electromagnetic energy is focused within the ring and disk of CRD structure.
The resonance shifts of ChG-based metamaterial absorber with CRD structure by changing r and R are shown in Fig. 3(a) and 3(b), respectively. In Fig. 3(a), the reflection spectra are red-shifted, while the resonances become strongest by increasing r from 0.6 μm to 1.1 μm. The geometrical parameters are kept at R = 1.2 μm, g = 0, and d = 0.4 μm, respectively. The corresponding Q-factors are calculated in the range of 440 to 567. These values are higher than that reported in literatures [24–26]. In Fig. 3(b), the reflection spectra are almost constant, which variation of resonances is less than 200 nm by increasing R from 0.8 μm to 1.2 μm. The geometrical parameters are kept at r = 0.8 μm, g = 0, and d = 0.4 μm, respectively. The resonant intensity is enhanced by increasing R. The strongest resonance is under the condition of R = 1.2 μm, which is ideal perfect reflection. According to above-mentioned results, it can be concluded that the optimized geometrical parameters of ChG-based metamaterial absorber with CRD structure are nm = 2.4, r = 0.8 μm, and R = 1.2 μm to exhibit a strongest resonance and an ultra-high Q-factor.
Figure 4 shows the reflection spectra of ChG-based metamaterial absorber with CRD structure by changing the gap between inner disk and outer ring (g) from 0 μm to 3 μm. The other geometrical parameters are kept at R = 1.2 μm, r = 0.8 μm, nm = 2.4, and d = 0.4 μm, respectively. The resonance is blue-shifted by increasing g parameter. The effective tuning range of resonance is between g = 0 μm to 2 μm. According to the theory of gap-mode resonance , the Q-factor of device can be enhanced by changing g parameter. The maximum Q-factor is 625 at the condition of g = 1 μm. The corresponding E and H-fields distributions are shown in Fig. 4(b) and 4(c), respectively. The electromagnetic energy is focused on the gap of inner disk and outer ring of CRD structure. The optimized geometrical parameters to earn an ultra-high Q-factor are R = 1.2 μm, r = 0.8 μm, nm = 2.4, d = 0.4 μm, and g = 1 μm, respectively.
To investigate our proposed device to have high flexibility, the influence of horizontal displacement of outer ring (d) is compared and discussed to perform multi-resonances switch function. Figure 5 shows the reflection spectra of ChG-based metamaterial absorber with CRD structure by changing d parameter from 0.4 μm to 0 μm. The geometrical parameters are kept at R = 1.2 μm, r = 0.8 μm, g = 0 μm, and nm = 2.4, respectively. There are three resonances at the wavelengths of 3.075 μm, 3.125 μm, and 3.175 μm, respectively. For the condition of d = 0.4 μm, there is a resonance at 3.125 μm wavelength caused from CRD is asymmetrical structure, whose corresponding Q-factor is 479. By shrinking d parameter, the resonance will be vanished gradually at 3.125 μm wavelength, while the adjacent both resonances (at 3.075 μm and 3.175 μm wavelengths) will be generated and sharped gradually. In Fig. 5(a)–5(c), there is almost only one resonance at 3.125 μm wavelength that resonant intensity is decreased gradually. The maximum Q-factor is calculated as 567. By continuously changing d parameter equals the values of 0.25 μm, 0.20 μm, and 0.15 μm, there has three resonances at 3.075 μm, 3.125 μm, and 3.175 μm wavelengths as shown in Fig. 5(d)–5(f). While the outer ring of CRD is close to the central disk with d = 0.10 μm, d = 0.05 μm, and then touches each other (d = 0 μm), the central resonance (3.125 μm wavelength) is gradually vanished and the adjacent both resonances become stronger as shown in Fig. 5(g)–5(i). The maximum Q-factors are 439 and 489 for resonances at 3.075 μm and 3.175 μm wavelengths, respectively. These resonances exhibit high-performance attenuation and switch functions that can be used for variable optical attenuator (VOA) and multi-resonance switch applications. The corresponding E-field and H-field distributions of ChG-based metamaterial absorber with CRD structure under the conditions of d = 0.40 μm, d = 0.20 μm, and d = 0 μm are shown in Fig. 6(a)–6(c), respectively. That can be explained by the distributions of electromagnetic energy within the gap between inner disk and outer ring of CRD, which is decreased and possesses single, triple and dual resonances for the conditions of d = 0.40 μm, d = 0.20 μm, and d = 0 μm, respectively. Therefore, the proposed ChG-based metamaterial absorber with CRD structure exhibits multi-resonance switch function by changing the horizontal displacement of outer ring.
To further prove the applicability of proposed ChG-based metamaterial absorber with CRD structure, the proposed device is designed to be surrounded with different environmental media to demonstrate for the high-efficiency environmental sensor application. Figure 7 shows the reflection spectra of ChG-based metamaterial absorber with CRD structure by changing ambient reflection index (n0) from 1.0 to 1.9. The geometrical parameters are kept at R = 1.2 μm, r = 0.8 μm, nm = 2.4, g = 0 μm, d = 0.4 μm, respectively. The resonant intensity is decreased gradually by increasing n0. The Q-factors are calculated in the range of 265 to 370. The corresponding relationship of resonance and n0 is summarized in Fig. 7(b). The trend is linear, whose correlation coefficient is 0.99999. This value is better than those reported in literatures [23–26]. Such design of ChG-based metamaterial absorber with CRD structure can be used as a high-efficiency environmental sensor.
In conclusion, we present a novel IR metamaterial absorber composed of ChG material with CRD structure. The device possesses tunabilities of resonance and reflection intensity. By tailoring the geometrical dimensions, the reflection spectra of ChG-based metamaterial absorber with CRD structure can be attenuated and modified for the uses of tunable filter, VOA, and multi-resonance switch. The optimized geometrical parameters are R = 1.2 μm, r = 0.8 μm, nm = 2.4. The reflection spectra can be tuned and switched by single, dual, and triple resonances by changing the distance of inner disk and outer ring of CRD structure. The maximum Q-factors are 625 and 567 for the applications of IR filter and multi-resonance switch, respectively. To further demonstrate the proposed device can be used as high-efficiency environmental sensor, device is designed to be surrounded with different environmental media with different ambient refraction indexes from 1.0 to 1.9. The Q-factors are in the range of 265 to 370 with a linear relationship, whose corresponding correlation coefficient is 0.99999. Such proposed ChG-based metamaterial absorber with CRD structure exhibits unique characteristics for high-performance optoelectronic devices, which will open an avenue to widespread applications, such as filter, VOA, multi-resonance switch, thermal emitter, photodetector, environmental sensor, and so on.
Research grants of 100 Talents Program of Sun Yat-Sen University (76120-18831103); Science and Technology Planning Project of Guangdong Province (2017B010123005).
The authors acknowledge the State Key Laboratory of Optoelectronic Materials and Technologies of Sun Yat-Sen University for the use of simulation codes.
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