Double-Sided Metasurfaces for Dual Band Mid-Wave and Long-Wave Infrared Reflectors

1. Introduction

Mid-wave infrared (MWIR, λ ≈ 3 to 5 µm) and long-wave infrared (LWIR, λ ≈ 8 to 14 µm) technologies play important common roles in thermal imaging, spectroscopy and remote sensing [1,2]. These wavelengths correspond to transparent atmospheric windows allowing high transmission of thermal radiation. MWIR imaging systems are capable of excellent resolution with enhanced sensitivity and contrast to temperature difference [3] while LWIR systems offer high quality imaging of near-ambient temperature objects and complex environmental conditions [4]. As a synergistic approach to merging these technologies, recent efforts have focused on combining MWIR and LWIR capabilities to yield enhanced performance and broader spectral coverage [5,6].

Meanwhile, optical metasurface technologies have been rapidly advanced through the implementation of resonant photonic lattices (PLs) [7,8]. Contrasting traditional thin-film technology, the PLs enable a single layer operation for various optical components including band-pass filters [9], wideband reflectors [10] and polarizers [11]. Particularly, for the MWIR and LWIR bands, guided-mode resonance (GMR) fashioned PLs are alternatives to traditional multi-layer designs based on quarter-wavelength thickness [12]. However, designing optical components that simultaneously operate effectively in both the MWIR and LWIR bands remains a considerable challenging in metasurface technology. In this work, we introduce an approach for dual band operation via double-sided metasurface structures. As an essential optical component, we focus on design and analysis of MWIR and LWIR reflectors, potentially useful for dual-band operation in various application fields.

2. Double-Side Metasurface Reflector

As shown in Figure 1, double-sided metasurfaces enable high reflection in MWIR and LWIR bands. The dual-band reflection is attributed to cooperative interaction between two distinct resonant reflectors, optimally designed for each MWIR (3-5 μm) and LWIR (8-12 μm) region. When light illuminates the device, the first metasurface reflector generates a high reflection band in the MWIR spectral region while partially reflecting light in LWIR region, as depicted in (i). In an ideal lossless system, the remaining light entirely passes through the substrate and interacts with the second reflector, as shown in (ii). Thereafter, the LWIR light is mostly reflected into air. As a result, seen in (iii), light undergoes high reflection in both MWIR and LWIR regions. Regarding infrared transparent materials such as germanium (Ge), calcium difluoride (CaF₂), potassium bromide (KBr), here, we successfully develop and analyze double-sided metasurfaces for dual band MWIR and LWIR reflectors.

3. Numerical Modelling

Dual band reflectors as proposed here can be constructed using two individually optimized resonant wideband reflectors. In this work, we employ particle swarm optimization (PSO) algorithms [13] to achieve a high reflection in each 3-5 μm MWIR and 8-12 μm LWIR band. For calculation of the reflection, a rigorous coupled-wave analysis (RCWA) method [14] is used with a commercial software package [15]. As a simple construction, we treat a one-dimensional (1D) grating array based on high refractive index dielectric material.

Figure 2a presents the schematic of the 1D MWIR grating reflector. In this design, we incorporate a homogeneous sublayer beneath the one-dimensional (1D) grating using the same high refractive index material. This forms a zero-contrast grating (ZCG), which facilitates superior wideband reflection to achieve [16]. The grating parameter set {${\mathsf{\Lambda }}_1,F_1,d_{g1},d_{h1}$} determines the geometry of the 1D ZCG, where the ${\mathsf{\Lambda }}_1$, $F_1$, $d_{g1}$ and $d_{h1}$ denote a period, fill factor, grating depth, and sublayer thickness of the first part of the metasurface. By setting the refractive indices (n_h=4, n_s=1.4) and using TM polarization (i.e., electric field is perpendicular to the grooves), we determine the optimal parameter set as {${\mathsf{\Lambda }}_1$= 1.73 μm, $F_1$=0.73, $d_{g1}$=0.91 μm, $d_{h1}$=0.5 μm} for the MWIR reflector. The simulated zeroth-order reflectance (R₀) spectrum demonstrates a high reflection band spanning from 3.06 to 4.85 μm, with R₀ values exceeding 0.97 remarkably achieved by the single grating layer. The wideband reflection is attributed to the merging of multiple guided-mode resonance (GMR) modes in the subwavelength regime [17]. At longer wavelengths, high reflection does not appear because GMRs are not excited.

For supporting high reflection in the LWIR region, as shown in Figure 2b, the second metasurface is designed by increasing the grating period where the optimal parameter set is {${\mathsf{\Lambda }}_2$= 6 μm, $F_2$=0.38, $d_{g2}$=1.7 μm, $d_{h2}$=0.33 μm} under TE polarization (i.e., electric field is parallel to the grooves). In the simulated R₀ spectrum, wideband reflection is observed from 8.54 to 12.22 μm, with high reflectance (R₀>0.99). As the second metasurface, it can efficiently reflect LWIR light transmitted from the first metasurface.

Figure 2c shows a double-sided metasurface composed the of two individual reflectors from Figures 2a and 2b. The substrate thickness (d) is set to 1 mm, which is a commonly used value. To ensure that each reflector operates with the proper polarization, two 1D grating are aligned in orthogonal directions under linearly polarized light illumination. When the incident electric field is perpendicular to the first grating grooves, the first and second metasurface interact with TM and TE polarized electromagnetic waves, respectively. The simulation result of the R₀ spectrum clearly shows dual MWIR and LWIR reflection with high fluctuations due to Fabry-Pérot (FP) resonances.

4. Analysis

To explain a mechanism of the dual band reflector, we provide an analytical formula based on a simplified optical configuration as illustrated in Figure 3a. The total output R₀ is derived from the sum of the nth reflectance ($R_{\left(n\right)}$) between the two metasurface which can be expressed by (1) to (3).

$$R_0={\displaystyle \sum }_{n=0}^{\infty }{R}_{\left(n\right)}$$

(1)

$$=R_1+R_2{\left(1-R_1\right)}^2{e}^{-2\alpha d}{\displaystyle \sum }_{n=0}^{\infty }{\left(R_1{R}_2{e}^{-2\alpha d}\right)}^n$$

(2)

$$=R_1+{\displaystyle \frac{R_2{\left(1-R_1\right)}^2{e}^{-2\alpha d}}{1-R_1{R}_2{e}^{-2\alpha d}}}$$

(3)

The R₁ and R₂ represent the reflectance of the first and the second metasurface, and α denotes the absorption coefficient of the interlayer (i.e., $\mathsf{\alpha }=4\mathsf{\pi }\mathrm{k}⁄\mathsf{\lambda }$, k being the extinction coefficient of the complex refractive index). In the equation, a phase interference is ignored to avoid FP resonance effects.

Figure 3b shows calculation results obtained by solving (3) using the R₁ and R₂ reflectance from each R₀ spectrum of Figures 2a and 2b. Comparing Figure 2c, which is in case of a lossless interlayer (extinction coefficient k=0, depicted by the blue bold line), the analytical results match to the simulation (RCWA) results, except for the dense oscillation. In realistic fabrication, where mathematically perfect parallelism between two interfaces is unlikely to be formed, FP interference effects will be diminished. Thus, the double-sided metasurface can cooperate the two individual reflectors without the rapid fluctuation seen in the RCWA result, producing a response similar to that predicted by the analytical model. However, when the interlayer is lossy material, the dual-band reflection capability is significantly degraded. As shown in red bold line, which indicates analytical result for k = 0.001, R₀ dramatically decreases in the LWIR region while matching the RCWA results (red thin line). This substantial reduction is caused by the high absorption of the thick interlayer (d=1 mm) as evidenced by the absorption (A) spectrum (i.e., RCWA results, depicted by the green thin line). The LWIR light is substantially absorbed during the long propagation, multi-wavelength path in the interlayer, whereas the MWIR light is predominantly reflected not penetrating the interlayer. Unfortunately, for this reason, it is hard to operate the second reflector when utilizing lossy materials for the interlayer.

5. Conclusions

In summary, we have introduced a dual-band reflector that incorporates double-sided metasurfaces. This innovative design pertains to the cooperation of two GMR reflectors, where two metasurface are combined with an interlayer positioned between them. The top and bottom metasurfaces can be independently designed to achieve the desired spectral response and both functionalities are integrated into a single device with a transparent interlayer. For the MWIR and LWIR reflection bands, two 1D ZCG structures are optimized by utilizing Ge grating layers on a KBr substrate. The Ge/KBr/Ge design demonstrates wideband reflection from 3.3 to 4.77 μm (MWIR region) and 8.85 to 11 μm (LWIR region). In addition, we provide an analytical formula, which will be useful to estimate output spectral response of double-sided metasurfaces in this device class.

Physical construction of the proposed device is challenging due to the necessity of protecting one surface during the pattering of the other. Nevertheless, fabrication is feasible with careful processing and procedures. Alternate implementation is serial arrangement of individual resonators on separate substrates; corresponding devices have not yet been designed. The basic concept of dual reflection band operation is extendable to other spectral regions spanning 400-6000 nm wherein the material selection limitation is not as extreme as in the LWIR band. For polarization independence, the GMR resonators may be patterned as 2D metastructures.