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Abstract
We experimentally demonstrated a corrugated metallic metasurface based tunable perfect absorber for terahertz (THz) frequencies in a total internal reflection geometry. The absorbance is strongly depend on the central layer of this three-layer absorber, which provides a feasible approach to tune the absorption. In particular, there exist an optimal gap that enables a perfect absorption at specific frequency. Due to the simple 1D geometric structure of metasurface, its absorption frequency can be easily tailored over a wide frequency range (0.625–1.499 THz). More importantly, the modulation of the effective refractive index and loss of medium environment can be accepted as an alternative approach for the absorption properties modulation. This prism coupling absorber provides a new route for modulation of the absorption characteristics with potential applications in biological sensing.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, terahertz (THz, the so-called ‘terahertz gap’ between 0.1 THz and 10 THz) radiation as one of the most exciting research hotspot, has attracted much attention due to its wide potential applications in imaging [1], communication [2], astronomy [3], nondestructive inspection [4], particularly in biological and chemical sensing [5,6]. Because of the ability to control electromagnetic waves in a unique manner, metasurfaces (typically referring to spoof plasmon surfaces, SPS) decorated with periodic patterns, such as cylinders [7], V-shaped blocks [8], and split rings [9], have attracted great research interests, especially in metasurfaces perfect absorbers (MPAs). MPAs has become an important role in the realization of terahertz devices, including high-resolution terahertz detection [10], imaging [11], sensing [12], etc.
The first experimental observation of near unity absorption achieved by a metasurface perfect absorber with sandwich structure in the microwave regime were reported by Landy et al. [13] in 2008. Since then, extensive research have focused on the metasurfaces based perfect absorber with multilayer approaches which minimizing the reflectivity via matching the impedance to free-space and simultaneously suppressing the transmission via using a metallic ground plane. By modulating the shapes or sizes of unit cell of metasurfaces, the operating frequency bands of MPAs have been expanded from microwave to terahertz. Meanwhile, on the basis of the tailored response of metasurfaces, various terahertz metasurface absorbers with unique absorption characteristics, such as narrowband [14], broadband [15] or multiband absorption [16], and insensitivity to angle of incidence [17] and polarization [18], have been widely investigated and demonstrated. Nevertheless, most of the current reports mainly focused on the absorption strength modulation rather than the resonance frequency modulation [19]. Although various desirable absorption properties of MPAs in the THz regime have been explored by designing complex geometric patterns, it is very difficult to conduct the resonance frequency modulation and achieve perfect absorption simultaneously, which greatly limits their practical applications. It is also worth noting that an abundant development of absorption behaviors occurs with varying dielectric layer thicknesses [20]. But, MPAs usually fabricated with a stationary dielectric layer thickness, limiting the tunability of the absorption [21].
In this letter, we design a single-frequency terahertz perfect absorber based on metallic metasurfaces decorated with periodic subwavelength trapezoidal grooves. Spoof surface plasmon resonances (SPR) induced on the designed metasurfaces with a polyethylene (PE) prism are considered as an alternative mechanism for the generation of electromagnetic wave absorption. We experimentally demonstrated the absorption response of the MPA to the tunable space layer gap and the depth of grooves. A perfect absorption can be acquired when the space layer reaches the optimal value. Moreover, the resonance frequency can be tune to the lower frequency region by increasing the depth of grooves. Finally, we also demonstrate that the modulation of absorption properties by changing the refractive index and loss inside the grooves.
2. Design of the corrugated metallic metasurface absorber
Our Otto geometry [22] based metasurfaces absorber is composed of the PE prism (with apex angle β = 120°, refractive index of np = 1.54, and internal incident angle θin = 64°) and the SPS consisting of 1D array of subwavelength trapezoidal grooves with period p, depth h, top width wt, and bottom width wb, as shown in Fig. 1. The processing of the corrugated metallic metasurface is divided into two steps. Firstly, a 500 µm thick silicon wafer (36 mm×20 mm) decorated with trapezoidal trench profile array is obtained through deep reactive ion etching (DRIE). Secondly, a 600 nm thick gold layer is sputtered onto the processed silicon wafer to form the SPS, as represented in Fig. 1. Details of the metasurfaces fabrication are described in Methods. Since the gold layer is much thicker than its skin depth at THz the THz regime (approximately 80 nm at 1 THz), the final metallic surface closely resembles a decorated perfect electrical conductor surface. It is held parallel to the PE prism undersurface and attached to a high precision translation stage which drives the metasurface to control the gap g between PE prism bottom and metasurface, i.e., the free-spaced layer thickness. Because its typical thickness scale is mainly determined by the decaying length of the evanescent field, which is approximately equal to the wavelength of the incident wave (300 µm at 1 THz) [23,24]. Thus, it can be exactly controlled by high precise translation stage. This MPAs with attenuated total reflection (ATR) [25] structure are designed to achieve perfect absorption via minimizing the reflectivity through coupling prism to fulfill the momentum matching between the incident TM-polarized THz waves and the highly-confined spoof surface plasmon polaritons (SSPPs) modes, i.e.,
where ${k_0} = 2\pi f/c,$ is the vacuum wavevector, k∥ is the parallel component of wavevector of the incident wave in the PE prism, and kSSPPs is the parallel components of SSPPs modes. Through the excitation of an evanescent field created by the total internal reflection of THz radiation, part of the THz wave will couple into SSPPs wave traveling along the SPS, thus forming an absorbing dip in the reflected THz spectrum.
Fig. 1. Schematic of THz-MPA with Otto geometry. Through ATR at the base of PE prism, part of the THz wave will couple into SSPPs wave traveling along the SPS, thus forming an absorbing dip in the reflected THz spectrum.
3. Results and discussions
Figure 2(a) shows optical microscope images of four fabricated SPS samples (I, II, III, and IV with period p = 60 µm) with top widths wt of 32, 31, 31, and 45 µm, bottom widths wb of 21, 20, 21, and 27 µm, and depths h of 30, 60, 90, and 90 µm (Note that the groove depths of the fabricated SPSs were measured with a film-thickness meter due to their larger depths). We have studied the SSPPs propagation on four planar gold surfaces decorated with linear array of trapezoidal grooves in the Brillioun zone. Figure 2(b) shows the dispersion relation (scatter plots) for the four samples by FEM calculation with COMSOL Multiphysics software. As a comparison, we also plot the dispersion curves of SPSs consisting of periodic square grooves with weff, i.e., weff = wt = wb, using an effective medium approximation model [26]:
Fig. 2. (a) Optical microscopy images of four corrugated SPS samples with different sizes of trapezoidal groove unit. (b) SSPPs dispersion relation (diamond, roundness, rectangle, and triangle) for sample I, II, III, and IV by FEM calculation with COMSOL software in the Brillioun zone. The curves are the calculation results of SPSs decorated with square grooves using an effective medium approximation model. The pink and black double dots curves indicate the light line and the parallel wavevector k∥, respectively. The insets show the electric field distributions for the unit cell of the four SPS samples near the Brillioun zone.
We utilize a photo-conductive terahertz time-domain spectrometer (THz-TDS) system which filled with nitrogen gas to measure the electromagnetic response of the designed absorber to the free-space layer thickness g. Figure 3(a) shows experimental time-domain signals of the MPA with sample III at varying g (Note that only three representative sets of time-domain signals are plotted and shifted for visual clarity). The reference signal are obtained in the absence of the SPS beneath the prism. As it can be seen, a pronounced ringing has been gradually formed with the decreases of gap, but it will slows down and reduce the spectrum as the gap further decreases. To get a better view of the electromagnetic responses, the measured absorption spectra are plotted in Fig. 3(b). Due to the internal total reflection of the prism, the absorptivity can be calculated by the formula:
Where $R(\omega )={|{{{E(\omega ,\;g)} \mathord{\left/ {\vphantom {{E(\omega ,\;g)} {E(\omega ,\;g \to \infty )}}} \right.} {E(\omega ,\;g \to \infty )}}} |^2},$ is the reflectivity. As is apparent, the absorption spectrum broadens and red-shifts with the decrease of gap, including the resonance frequency fsp changes from 0.655 THz for g = 250 µm to 0.557 THz for g = 130 µm. This is because a portion of the SSPPs field will outcoupling into the prism result in an additional damping of SSPPs at narrower gap [27,28]. Meanwhile, the absorption coefficient appears change that increases first but then drops. In particular, there exists an optimal free-space layer thickness that enables the complete destructive interference of the reflected light and thereby gaining perfect absorption at a specific frequency. A peak absorption of A = 99.97% with fsp = 0.625 THz at g = 190 µm can be observed from the measured absorption spectra. Except for the appearance of characteristic absorption peak, another key feature of SPR is the abrupt phase jump near the resonance frequency. The phase change is defined as the difference in phase of each frequency component of the sample ${\phi _{sam}}$ and reference ${\phi _{ref}}$ spectra, i.e.,Fig. 3. Experimental time-domain signals (a), absorption spectra (b), and phase change spectra (c) of the MPA with sample III at varying free-space coupling gaps g. The measured time-domain signals are shifted for visual clarity.
As shown in Fig. 3(c), the phase change spectrum has a region with a sharp jump near the absorption peak. Interestingly, the direction of phase jump transits from positive to negative when the gap reduce to 180 µm. And it does that because of the modulation of the optimal resonance condition (i.e., R passes through the point R = 0) caused by the tiny change of coupling gap [29]. Thus, the direction change of phase jump further proves that perfect absorption can be achieved via properly adjusting the thickness of the free-space layer.
Figure 4 shows experimental absorption spectra 4(a) and phase change spectra 4(b) of the MPAs with SPS samples of different depths and widths (I, II, III, and IV, as shown in Fig. 2(a)) at their respective relatively optimal coupling gaps (g = 75, 130, 190, and 215 µm). As it can be seen, all absorption spectra exhibit near perfect absorption along with a sharp phase jump. The absorption values we acquired for the four SPS samples are 99.99%, 99.52%, 99.97%, and 99.50%, respectively. These experimental results are a good validation of the previous inference that perfect absorption can be acquired by properly modulating the thickness of the coupling layer. Meanwhile, their resonance frequencies agree well with the theoretical predictions by the SSPPs dispersion relation (as shown in the Fig. 2(b)). Specifically, we can see a dramatic red-shift in the absorption peaks as the groove depth increases, which reduces from 1.499 THz for h = 30 µm to 0.625 THz for h = 90 µm. The emergence of the absorption frequency shifts is attributed to the lower asymptotic frequency of SSPPs dispersion relation. Previous theoretical studies have showed that the high-order SSPPs mode with the order of m can be supported, when h is large enough, i.e., h > mp (integer m is the order of the mode) [24]. But higher order SSPPs usually excited with a narrower coupling gap due to their higher asymptotic frequency, increasing the difficulties of the excitation of SSPPs and the controlling of the absorption by the central layer of the absorber. Also, the experimental results of sample III and IV (which shifts from 0.625 THz for wt = 30 µm to 0.550 THz for wt = 45 µm) verify our theoretical analysis that a small red-shift of resonance frequency occurs with the groove width increases. But the modulation length of groove width is very limited by the groove period. For comparison, their corresponding absorption and phase change were calculated via 2D numerical simulation by COMSOL Multiphysics software. The inset of Fig. 4(c) shows the unit cell of the 2D simulation model. A periodic boundary condition was applied along the direction of periodicity to simulate a linear array of trapezoidal grooves. Besides, the gold film was simulated using a transition boundary condition at the grooves surface; the permittivity of gold at THz frequencies is described by the Drude model with plasma frequency ωp = 1.367×1016 rad/s and scattering frequency γp = 4.072×1013 rad/s (Ref. [30]). As seen from Fig. 4(c) and 4(d), the variation trend of the simulated absorption spectra and phase change spectra with different depths and widths are in agreement with the measurements. The main difference between the calculations and experiments lies in the full width at half maximum (FWHM) of the absorption spectra. The primary reasons are the increase in absorption losses caused by the formation of hot spots at the structural imperfections in the SPS samples and diffraction effects resulting from the scattering of the SSPPs modes at the edges of the SPS. Meanwhile, the structural imperfections of SPS samples also lead to the small shift of the optimal resonance frequency. The frequency modulation response of the simple 1D resonator structure demonstrates that such MPA has a significant design flexibility that the resonance frequency can be precisely adjusted by the groove depth.
Fig. 4. Experimental absorption spectra (a), and phase change spectra (b) of the MPA with SPS samples of different depths and widths (I, II, III, and IV) at their respective relatively optimal coupling gaps g. The corresponding simulated absorption spectra (c), and phase change spectra (d). The inset shows the unit cell of the 2D simulation model.
We have also investigated the absorption response of the MPA in different dielectric environment. Figure 5 shows the experimental absorption spectra 5(a) and phase change spectra 5(b) of the MPA with sample I in which its grooves filled with water. As it can be seen, there appear a similar perfect absorption peak with a bandwidth of about 0.35 THz and an absorptivity of more than 90%, instead of the complete attenuation of terahertz signal. Compared with that of air, the absorption peak broadens and red shifts markedly as water filling the grooves. This is due to the increase of the effective refractive index and loss of dielectric environment with the addition of water. At the same time, we also can observe a similar phase jump in a wider frequency range, which proves again the excitation of SSPPs mode, as shown in Fig. 5(b). Thus, the effective refractive index and loss modulation can be accepted as an alternative approach for the absorption properties modulation. This design method is also useful for the design of metasurface modulators in other frequency band such as microwave region [31]. In addition, it is important to note that the conventional MPAs with direct irradiation modes, reaches its limit when applied to discern analytes dissolved in polar liquids due to their extremely high absorption in THz regime [32]. But this new MPAs combined with ATR technique provides a potential route to overcome the limitation due to the shorter interaction length of evanescent wave [33], which is particularly important for biological sensing [34,35].
Fig. 5. Experimental absorption spectra (a), and phase change spectra (b) of the MPAs with sample I in which its grooves filled with water and air.
4. Conclusions
In this letter, we have experimentally realized a perfect metasurface absorber with ATR structure. It has been demonstrated that a continuous evolution of absorption resonances occurs with the decreasing free-space layer thickness, which provides a feasible approach to tune the absorption. Specifically, the direction of phase jump will change under a certain coupling gap, which indicates that the perfect absorption (i.e., A = 100%) can be achieved by properly modulating the coupling gap. Experiments indicate that all our four samples resulted in almost perfect absorption effect, with absorption efficiency at 99.99%, 99.52%, 99.97% and 99.50% respectively. Moreover, we also demonstrated that the near perfect absorption can be acquired over a wide frequency range (0.625–1.499 THz) just by modulating the depth of the groove. These experimental results confirmed that the asymptotic frequency of SPSs dispersion relation is quite sensitive to the groove depth. The emerging of 1D corrugated metasurfaces solves the problems of frequency modulation of most MPAs with complicated geometries. More importantly, similar results can be achieved by changing the refractive index and loss inside the grooves, benefit from the high sensitivity of SSPPs wave to the dielectric environment. Besides, our experimental results also inform that the strong absorption of electromagnetic field energy by polar liquid can be alleviate effectively by prism coupling. This means that MPA can be utilized as a good platform for THz-SPR sensing, especially in biological sensing.
5. Methods
SPSs consisting of a periodic trapezoidal groove array were fabricated via conventional deep reactive ion etching (DRIE) on 500 µm thick silicon wafers. Before the DRIE process, a thick layer of AZ 5214E positive photoresist was coated onto the Si substrate via spin coating process. The AZ 5214E was spun at 2000 rpm for 30 s followed by a soft bake at 100 ℃ for 60 s, giving a final photoresist thickness of approximately 2 µm. Then, the photoresist is exposed to UV light via a pre-fabricated photomask in a mask aligner (Karl Suss, MA6/BA6) with a dosage of 1000 mJ/cm2 and developed with AZ 400K developer to reveal the etched mask of groove array. After that, the DRIE using Bosch process was performed in deep silicon etching equipment (Alcatel) with an radio frequency of 13.56 MHz, a chamber pressure of 10 mTorr, and a gas flow rate of 450 sccm to give a silicon groove array with varying depths and an overall area of 20 mm × 35 mm. Finally, 600 nm of gold is sputtered onto the silicon groove array (Explorer–14) to give the final SPS.
Funding
National Natural Science Foundation of China (51675103); Fujian Provincial Science and Technology Project (2019I0004); State Key Laboratory of Mechanical System and Vibration (MSV-2018-07); Shanghai Natural Science Fund (18ZR1414200).
Acknowledgments
Thanks for the discussions with Dr. Gaofeng Zheng of Xiamen University, China.
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