A Long-Wave Infrared Circularly Polarized Photodetector Based on an Array of Trapezoidal Silicon Pillars

Abstract

Integrating metasurface-based polarizing filters atop photodetectors enables the expansion of detection capabilities from intensity to polarization, offering significant potential for applications requiring high-precision discrimination in scientific, industrial, and defense sectors. However, such metasurfaces often introduce optical efficiency losses. Here, we present a long-wave infrared (8.6 μm) circularly polarized photodetector capable of direct chiral discrimination, eliminating the need for additional optical components. The polarization selectivity arises from Guided-Mode resonances (GMRs) excited by two horizontally offset right-trapezoidal unit cells within a chiral metasurface. This design exhibits a pronounced transmittance contrast (~100%) between left circularly polarized light (LCP) and right circularly polarized light (RCP) while maintaining fabrication simplicity via a conventional single-step lithographic process. The proposed detector is expected to achieve high-dimensional physical characterization by resolving polarization-encoded vectorial information, demonstrating enhanced performance in complex environments.

1. Introduction

Conventional photodetectors [1] are fundamentally limited to measuring light intensity and can only provide scalar information about optical signals. Although this is sufficient for many applications, in critical fields such as quantum communication [2], remote sensing [3], and biomedical imaging [4], it is necessary to obtain the circular polarization state of light—this chiral vector property carries important information about the interaction of light with matter. Traditional methods for circular polarization detection rely on cascaded optical elements [5] (such as waveplates and linear polarizers), which inevitably lead to system complexity, alignment difficulties, and significant optical losses.

Metasurfaces [6,7,8,9,10] have emerged as a promising platform for manipulating optical signals, enabling flexible control of amplitude, phase, and polarization at subwavelength scales. Chiral metasurfaces [11,12,13,14,15], a subset of metasurfaces, can generate differential electromagnetic responses to circularly polarized light (such as selective absorption, phase modulation, or transmission differences), thereby actively regulating the spin degree of freedom of light. Chiral metasurfaces are generally characterized by two metrics: circular dichroism [16,17,18] and circular polarization extinction ratio [19,20,21], which correspond to the difference and ratio of the signals of left and right circularly polarized light, respectively. Chiral metasurfaces offer an intriguing lightweight alternative to cascaded optical components, as their breaking of mirror symmetry [22,23,24,25] allows for the direct discrimination of circularly polarized light without the need for auxiliary optical devices.

In this paper, we designed a long-wave infrared circularly polarized photodetector (CPPD) with an efficiency close to 100% based on the COMSOL 5.6 simulation platform. Firstly, we introduced the structural composition, material information and optical model configuration of the CPPD, analyzed the influence of the geometric changes in the unit cell structure on circular dichroism, then dissected the characteristics of the electric field distribution map of the chiral metasurface filter and the impact of the thickness of the support layer hs on the coupling between the metasurface and the detector, and finally demonstrated the possible impact of process errors on this device.

2. Materials and Methods

Figure 1a illustrates the three-dimensional structure of the CPPD, which consists of a silicon metasurface on top and a type-II superlattice photodetector [26,27] operating at a wavelength of 8.6 μm at the bottom. As shown in Figure 1b, the silicon metasurface is a periodically arranged array, and its unit cell is composed of two right-angled trapezoidal columns. Figure 1c presents the information on the cross-sectional view of the silicon metasurface in the XY plane, revealing that the lower trapezoid can be obtained by rotating the upper trapezoid 180 degrees counterclockwise. The long-wave infrared type-II superlattice photodetector, from bottom to top, consists of a 350 μm thick GaSb substrate, a 0.67 μm thick GaSb buffer layer, a 0.1 μm thick p-type doped GaSb lower contact layer, approximately 400 periods of InAs/GaSb absorption layers, a 0.16 μm thick AlGaSb barrier layer, a 0.37 μm thick n-type doped InAs upper contact layer, and a 3.1 μm thick MgF₂ antireflection layer. The optical refractive index of the absorption region in the type-II superlattice is estimated by the effective medium theory (EMT) [28,29], and the imaginary part of the refractive index at a wavelength of 8.6 μm is approximately 0.09, as shown in Figure 1d. The absorption rate of the CPPD is obtained by the simulation COMSOL Multiphysics, which is equal to 1 minus the reflectance minus the transmittance. Periodic boundary conditions (PBC) are chosen for the four sides of the optical simulation model, while perfect matching layers are added to the upper and lower end faces to truncate the electromagnetic field. The polarization characteristics of the incident light are controlled by the incident port. The reflectance and transmittance are, respectively, the area integrals of the Poynting vector at the reflection port and the incident port. More details of the simulation model and process can be found in the Supplementary Materials.

3. Results

Figure 2a shows the transmission spectrum of the chiral metasurface in the wavelength range from 8.0 μm to 9.2 μm. In case of the LCP incidence, the transmission first increases, then decreases, and then increases again, with the maximum transmission of 0.97 corresponding to the operating wavelength of 8.6 μm. For RCP, there is a significant attenuation at the wavelength of 8.6 μm, and the transmission is already close to 0. Clearly, near the wavelength of 8.6 μm, the chiral metasurface exhibits distinct transmission characteristics for different chiral circularly polarized light. As shown in Figure 2b, the circular polarization extinction ratio (CPER) spectrum of the silicon metasurface filter shows the characteristics of a Dirac function. At the wavelength near 8.6 μm, CPER is a sharp peak, and on both sides far from the 8.6 μm wavelength, the value of CPER rapidly approaches 0. Here, $\mathrm{C}\mathrm{P}\mathrm{E}\mathrm{R}=10\times \mathrm{l}\mathrm{o}\mathrm{g}(T_{\mathrm{L}\mathrm{C}\mathrm{P}}/T_{\mathrm{R}\mathrm{C}\mathrm{P}})$, where $T_{\mathrm{L}\mathrm{C}\mathrm{P}}$ and $T_{\mathrm{R}\mathrm{C}\mathrm{P}}$ are the transmission of the chiral metasurface for LCP and RCP, respectively.

3.1. The Influence of Geometric Structures in Metasurfaces

Generally, the geometric structure of the unit cell of the metasurface can directly determine the peak position, line shape and intensity of the circular dichroism (CD) spectrum, and its asymmetric parameters (such as displacement Δx, tilt angle θ) have a close mapping relationship with the CD value. Figure 3a–d, respectively, show the influence of the geometric changes in the unit cell of the chiral metasurface on the CD spectrum, where $\mathrm{C}\mathrm{D}=T_{\mathrm{L}\mathrm{C}\mathrm{P}}-T_{\mathrm{R}\mathrm{C}\mathrm{P}}$. Figure 3a shows the case where the unit cell of the metasurface is two aligned rectangles, which is a typical C₂ symmetric structure. Figure 3e shows the CD spectrum of the metasurface corresponding to the structure in Figure 3a, which is a constant zero, reflecting the fact that the unit cell of the metasurface is a non-chiral symmetric structure. Compared with Figure 3a,b introduces a horizontal displacement dx, which makes the unit cell of the metasurface transition from a non-chiral symmetric structure to a chiral symmetric structure. Figure 3f shows the CD spectrum of the geometric structure in Figure 3b, which presents a typical double-hump function spectrum shape, with the maximum CD peak value being approximately 0.2. In Figure 3c, triangles are added to both sides of the rectangle, introducing the concept of a tilted structure. As shown in Figure 3g, a huge CD peak (>0.8) appears at a wavelength of 8.6 μm, indicating that the metasurface has acquired significant chiral symmetry after undergoing dual changes in displacement and tilt angle. The structures in Figure 3c,d satisfy the mirror symmetry relationship about the YZ plane, which means that the metasurface’s control over the transmission of chiral beams has been reversed; that is, the CD values at each wavelength in Figure 3g,h are opposite to each other.

3.2. Analysis of the Electric Field Distribution of Metasurfaces

Observing the near-field intensity distribution of metasurfaces is also an effective means to analyze the optical resonance modes. Figure 4a shows the electric field intensity distribution of the metasurface on different XY cross-sections, all sharing the same color scale. All six sub-figures indicate a strong electric field localization phenomenon in the air gap between adjacent unit cells along the Y-axis, which is represented by the red areas in the images. Additionally, as shown in Figure 4b,c, there is a clear linear relationship between the length of the air gap, ∆l, and the incident wavelength, which exhibits the typical characteristics of the GMRs. The red dashed circles and black boxes, respectively, indicate the cases corresponding to the metasurface structure in Figure 1, which simultaneously correspond to the extremely high LCP transmission and extremely low RCP transmission. Figure 4d shows the electric field localization contrast (ELC) of the chiral metasurface for different chiral CPs, which quantitatively measures the concentration degree of the electromagnetic field energy in Figure 4a. The maximum ELC corresponds to a wavelength of 8.6 μm, indicating a direct correlation between the resonance wavelength and the distribution of electric field hotspots. Here, $\mathrm{E}\mathrm{L}\mathrm{C}={Q_{\mathrm{L}\mathrm{C}\mathrm{P}}-Q}_{\mathrm{R}\mathrm{C}\mathrm{P}}$, $Q_{\mathrm{L}\mathrm{C}\mathrm{P}}$ = $\int {\displaystyle \frac{\left|E_{\mathrm{L}}\right|}{E_0}}dV/I_{max}$, $Q_{\mathrm{R}\mathrm{C}\mathrm{P}}$ = $\int {\displaystyle \frac{\left|E_{\mathrm{R}}\right|}{E_0}}dV/I_{max}$, where $dV$ corresponds to the volume occupied by the metasurface. $I_{max}$ is the normalization term in the denominator and is the maximum value of $\int {\displaystyle \frac{\left|E_{\mathrm{L}}\right|}{E_0}}dV$. E₀ corresponds to the electric field intensity of the incident light. E_L and E_R, respectively, correspond to the electric field intensities when the incident light is LCP and RCP.

3.3. The Influence of the Thickness of the Support Layer (hs)

In the previous sections, we focused on analyzing the transmission characteristics of chiral metasurfaces for different circularly polarized lights. In this subsection, we mainly discuss the influence of the thickness hs of the support layer (MgF2) between the chiral metasurface and the type-II superlattice photodetector on chirality. One fact must be noted, which is that we have pre-assumed the metasurface and the photodetector to be two independent sub-modules when designing the CPPD, so hs must be carefully scanned to ensure that the coupling effect between them is not a negative feedback. Figure 5a depicts the impact of hs on the CPPD at a wavelength of 8.6 μm. The absorption spectrum of LCP has a typical fluctuating characteristic, while the absorption spectrum of RCP first decreases and then remains basically unchanged. CPER is also a fluctuating curve, with the maximum CPER value corresponding to hs of 3.1 μm. Here, $\mathrm{CPER}=10\times \mathrm{l}\mathrm{o}\mathrm{g}(A_{\mathrm{LCP}}/A_{\mathrm{RCP}})$, where $A_{\mathrm{LCP}}$ and $A_{\mathrm{RCP}}$ are the absorption rate of the chiral metasurface filter for LCP and RCP, respectively. Figure 5b shows the absorption spectrum and CPER spectrum of the CPPD when hs = 3.1 μm. The maximum LCP absorption rate (≈0.33) and CPER value (≈28 dB) both occur at the operating wavelength of 8.6 μm. Figure 5c–f show the electric field distribution in the detector region under different CP incident conditions. It can be found that a large amount of electric field energy from the upper region penetrates into the absorption layer corresponding to LCP. However, in the case of RCP incidence, the upper metasurface acts like a perfect reflector, and almost no free photons can enter the absorption region, which is represented by the blue color indicating low electromagnetic energy in the picture.

3.4. Analysis of Potential Process Errors

Since the chirality of the polarization detector is determined by the nanostructure pattern of the metasurface, the influence of line errors cannot be ignored. In semiconductor processing, it is impossible to achieve an ideal rectangle, and instead, a circular arc replaces the right angle of the rectangle. This is a result of the combined effects of photolithography diffraction, microloading during etching, and the natural tendency of surface energy minimization. Figure 6a shows the influence of the radius of the inscribed circle in the silicon metasurface on the polarization detector. When the radius is less than 100 nm, the absorption rates of LCP and RCP remain almost unchanged. When the radius is greater than 100 nm, the two absorption rates approach each other as the radius increases. Additionally, over-etching and under-etching phenomena are often encountered during semiconductor etching, and the resulting etching depth errors also require additional attention. The inset in Figure 6b shows the over-etching phenomenon, where not only is the silicon completely etched away, but also the underlying substrate material is patterned. As shown in Figure 6b, the change caused by the etching error ∆h is weak. However, a completely different phenomenon occurs in Figure 6c. For LCP, when the current etching error ∆h is within the range of 0 nm to 60 nm, the absorption rate decreases significantly as ∆h increases. When ∆h is greater than 60 nm, the absorption rate remains almost unchanged. Moreover, for RCP, the absorption rate increases as ∆h increases. When ∆h equals 150 nm, the absorption rates of RCP and LCP are the same, indicating that the detector has lost its chiral detection capability. Therefore, it is very necessary to intentionally over-etch for a few more seconds during etching to guarantee that the etching process unequivocally reaches the over-etching state.

Table 1. A comparison of the performance of chiral metasurfaces.

Structure	Publication	Wavelength (μm)	CD	CPER (dB)
π-shaped [32]	2024	1.55, 2.55	0.7, 0.75	~10, ~10
Monoclinic Lattices [33]	2024	1.6	0.3	/
Broken circle [34]	2025	1.35	0.17	/
Graphene and silicon [35]	2024	250	0.9	9
Double Si columns [36]	2024	1.01	~0.9	6
Swastika-shaped [37]	2024	0.53	0.26	3
Double-layered gold [38]	2025	1.25	0.55	~8
This work		8.6	~1	27

presents the performance of some chiral metasurfaces published in the recent two years. It can be found that the CD and CPER of the metasurfaces in the manuscripts are both excellent. Particularly for the CPER index, our work has basically led by one order of magnitude.

4. Discussion

We have developed a novel CPPD capable of direct chiral discrimination in free-space optical systems, comprising a silicon-based chiral metasurface integrated with a type-II superlattice photodetector. By precisely engineering the air gap dimension (Δl) between adjacent meta-atoms, our device achieves near 100% CD at the operational wavelength of 8.6 μm through excitation of optimized GMRs modes. This CPPD scheme provides access to the chirality of photons—a critical light parameter that remains inaccessible to conventional intensity-based photodetectors, thereby opening new possibilities in quantum optics and interdisciplinary applications. Future research directions will focus on two key aspects, namely (1) bandwidth enhancement to enable spectrally broadband operation and (2) integration of chiral metasurfaces with dual-color or multicolor detection systems, to simultaneously achieve multi photon acquisition and polarization-resolved measurements.