1 Introduction

Infrared polarization detection has numerous important applications，including military reconnaissance，quantum communication，cosmology，biomedicine，and remote sensing^［1］. However，traditional polarization detection systems are bulky and complex，hindering the miniaturization and integration of polarization detection. On-chip infrared polarization detectors with the advantages of small size，high responsivity，and high polarization extinction ratio，represent the future of polarization detection^［2］. To realize advanced infrared polarization detectors，we primarily focus on enhancing polarization discrimination and miniaturizing device sizes.

The polarization-sensitive materials offer a straightforward way to realize polarization detection. Polarization detectors based on anisotropic materials have compact structures and require no extra fabrication processes compared to common detectors^［3-11］. Recently，a lot of anisotropic or chiral materials have been proposed for polarization detection. Anisotropic van der Waals materials and heterostructures are studied for linear polarization detection^［12-21］. Chiral hybrid perovskites，topological materials，and other circular-polarization-sensitive materials are demonstrated to be able to distinguish the handedness of circularly polarized light^［22-24］. Although the polarization detectors based on polarization-sensitive materials can realize either linear or circular polarization detection，the choice of these materials is quite limited. In addition，poor chemical stability，low responsivity，and low polarization extinction ratio also hinder the development of detectors based on polarization-sensitive materials.

Thanks to advances in micro- and nano-fabrication techniques，polarization-selective optical coupling structures have been successfully integrated with infrared materials to enhance the performance of polarization detectors^［24-28］. By combining the advantages of polarization-selective optical coupling structures and the anisotropic absorption in materials，the integration of polarization-selective plasmonic microcavities and anisotropic materials exhibits a double enhancement of polarization discrimination^［25］. Recently，configurable photocurrent polarity has also been achieved by manipulating local photocurrent density through plasmonic nanoantennas. The polarity of photocurrent is tunable by light polarization flexibly and an infinite extinction ratio could be realized at the polarity-transition point.

In this review，we will introduce the infrared polarization detectors based on polarization-sensitive materials in Section 1. Then the integration of polarization-selective optical coupling structures will be discussed as follows in Section 2. At last，in Section 3，we talk about the next challenge and opportunity for the detection of full Stokes parameters in the future.

1　Polarization-sensitive materials

Traditional methods of linear or circular polarization detection involve rotating polarizers or waveplates. Most detection materials are polarization-insensitive and can only detect light intensity. The requirement of numerous discrete optical components in traditional polarization detection systems hinders the miniaturization and integration of polarization detection systems. Polarization-sensitive materials have been widely investigated to construct compact and filterless polarization detectors.

1.1　Anisotropic absorption in two-dimensional materials

Two-dimensional materials have been extensively studied in the field of optoelectronics due to their unique optical and electronic properties. Anisotropic absorption in some two-dimensional materials promises sensitivity to linearly polarized light^［2］. These materials offer a straightforward approach to on-chip infrared polarization detectors and demonstrate excellent performance. The polarization detection systems consisting of anisotropic two-dimensional materials have the advantage of miniaturization compared to traditional methods.

In 2020，Lei Tong et al. utilized high-mobility，narrow-bandgap，anisotropic quasi-two-dimensional tellurium（Te）photodetectors to achieve target imaging with a linear polarization extinction ratio greater than 9 at the wavelength of 2.3 μm^［29］. Fig. 1（a）shows the schematic device structure and the polarization-dependent photoresponse. The polarization detection based on individual anisotropic materials is very compact. The characteristics of the polarization detector，including linear polarization extinction ratio and response wavelength，rely on the anisotropic absorption of the material.

图 1. （a）Te的晶体结构和器件结构示意图，在入射功率为6.0 mW，入射波长为2.3 μm时，室温下的净偏振光电流ΔI_ph ^［29］，（b）由b-AsP/WS₂/b-AsP构成的单极势垒范德华异质结光电探测器的示意图，线偏振角依赖的正向偏置和反向偏置光电流^［30］，（c）全斯托克斯偏振测量的示意图，单层-少层MoS₂同质结器件和单层MoS₂器件在0 V和-0.1 V时的螺旋度依赖的光电流的对比^［31］，（d）扭曲双层石墨烯光电探测器的示意图，温度为79 K，入射波长为5 µm时，测量不同顶栅和背栅栅压（V_BG，V_TG）下，四分之一波片角度依赖的光电压，当线偏振角为165°，顶栅栅压为 5.2 V，入射波长5 µm和7.7 µm的线偏光激发时，不同背栅栅压下的光电压^［32］

Fig. 1. （a）Schematic diagram of tellurium（Te）crystal structure. Schematic diagram of the device structure. At room temperature，the incident power is 6.0 mW，and the net polarized photocurrent ΔI_ph is when the incident wavelength is 2.3 μm^［29］；（b）Schematic diagram of the unipolar barrier van der Waals heterostructure photodetector composed of b-AsP/WS₂/b-AsP. Polar plots of linear polarization angle-dependent forward-bias-driven photocurrent and reverse-bias-driven photocurrent^［30］；（c）Schematic diagram of the full-Stokes polarization measurement setup. Comparison of helicity-dependent photocurrents at 0 V and -0.1 V for homostructure devices and monolayer MoS₂^［31］；（d）Schematic of a twisted double bilayer graphene（TDBG）photodetector. Photovoltage（V_ph）as a function of the quarter-wave plate（QWP）angle（θ）at different gate voltage biases（V_BG，V_TG）measured at T = 79 K and λ = 5 µm. Photovoltage function of V_BG excited by 5 µm and 7.7 µm linearly polarized light（LP）when ψ = 165° and V_TG = 5.2 V^［32］

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In 2022，Wenjie Deng et al. constructed a twisted unipolar barrier van der Waals heterostructure using the anisotropic material b-AsP^［30］. The heterostructure consisted of a b-AsP/WS₂/b-AsP configuration，forming a small barrier for electrons and a high barrier for holes to create a unipolar barrier by band engineering. The bias-switchable photocurrents are dominated by the anisotropic absorption at the top and bottom，respectively. The device realizes full polarization state detection of linearly polarized incident light in the mid-infrared band. The linear polarization extinction ratio is 55 under the bias of 0.1 V. Fig. 1（b）shows the structure diagram of the heterojunction device and the polarization photocurrent under the external bias. Anisotropic two-dimensional materials are commonly used for linear polarization detection，and heterojunctions can be constructed based on these materials to enhance their anisotropic absorption through band engineering. However，achieving circular polarization detection using two-dimensional materials is challenging.

In 2021，Chen Fang et al. demonstrated the use of inherent in-plane and out-of-plane optical anisotropy of MoS₂ to fabricate a full-Stokes polarimeter on a single-layer MoS₂/few-layer MoS₂ homojunction chip. This homojunction on-chip full-Stokes polarimeter is based on valley-dependent optical selection rules in monolayer MoS₂，which induces valley-locked spin-polarized photocurrent known as the circular photogalvanic effect（CPGE）. The response is further enhanced by the monolayer MoS₂/few-layer MoS₂ homojunction，enabling the detection of all four Stokes parameters of incident light at zero bias in the 650 ~ 690 nm wavelength range^［31］. Chen Fang et al. employed mechanical rotation to achieve an oblique incidence of light on the surface of an isotropic single-layer MoS₂/few-layer MoS₂ homojunction. Fig. 1（c）shows the schematic diagram of the homojunction device test and the photocurrent as the function of the angle of the quarter-wave plate.

In 2022，Chao Ma et al. achieved a breakthrough in realizing a tunable mid-infrared bulk photovoltaic effect by utilizing twisted double bilayer graphene（TDBG）at 5 μm and 7.7 μm wavelengths^［32］. The TDBG device benefits from the strong symmetry-breaking properties of the graphene moiré superlattice material，as well as the nonlinear light-matter interaction regulated by quantum geometry. Additionally，the advanced machine learning algorithm，such as a convolutional neural network（CNN），is applied to recognize the double-gate-dependent bulk photovoltaic voltage mapping and achieve the full Stokes polarization reconstruction^［32］.

1.2　linear and circular photogalvanic effect in topology materials

Topological materials exhibit novel optoelectronic phenomena due to their unique electronic band structure，involving the Berry curvature of the electron wavefunction^［32］. Topological materials generate prominent photocurrent under linear and/or circular polarized light with broken inversion symmetry or time-reversal symmetry. These phenomena are known as the linear photogalvanic effect（LPGE）and circular photogalvanic effect（CPGE），respectively. Besides，topological semimetals hold promise for broadband infrared detection due to their gapless band structure at the Weyl point.

In 2018，Su-Yang Xu et al. demonstrated the tunable Berry curvature dipole of single-layer topological insulator WTe₂ to realize observable and electrically switchable CPGE^［33］. The low-energy band structure of monolayer WTe₂ without spin-orbit coupling features tilted two-dimensional Dirac fermions at the Q and Q’ points. The CPGE is induced by interband transitions of the inverted quantum spin Hall（QSH）gap near the Q and Q’ points located close to the bottom of the conduction band. The inverted band structure and tilted crystal lattice of monolayer WTe₂ produce Berry curvature dipoles and strong electric field effects，resulting in an observable and electrically switchable CPGE for circularly polarized light detection.

In 2018，Jiawei Lai et al. developed a self-powered photodetector with broadband capabilities，utilizing a type-II Weyl semimetal T_d-MoTe₂. The anisotropy of this material is wavelength-dependent，with greater anisotropy at excitation wavelengths closer to the Weyl node. T_d-MoTe₂ is a promising material for broadband polarization-sensitive and self-powered photodetection with excellent response. Based on the anisotropy of T_d-MoTe₂，there are anisotropic photocurrent responses at different linear polarization excitation of 10.6 μm，4 μm，and 633 nm，and the linear polarization extinction ratios are 2.72，1.92 and 1.19，respectively^［22］. Afterward in 2022，Junchao Ma et al. conducted a study on the semiconductor material Te. They discovered that under 10.6 μm and 4.0 μm photoexcitation，the material exhibited opposite CPGE directions due to the chirality-dependent optical selection rules for transitions between the valence band and the conduction band of the semiconductor material Te between Weyl cones^［23］.

In 2019，Gavin B. Osterhoudt et al. employed the topological structure of Weyl semimetal TaAs and focused ion beam（FIB）manufacturing technology to achieve the giant bulk photovoltaic effect（BPVE）in the 10.6 µm band at room temperature^［34］. The investigation of the nonlinear optical properties of TaAs revealed that the CPGE is related to electronic chirality in TaAs.

图 2. （a）TaAs器件的伪彩色扫描电子显微镜图像，沿a轴和c轴的随四分之一波片角度变化的光电流^［34］，（b）双栅调控单层WTe₂器件的中红外圆偏光电流测试的示意图，沿a轴的偏振依赖光电流^［33］，（c）Te的晶格结构，在激发波长为10.6 μm（中）和4 μm（下）时，Te最大正、负响应位置的四分之一波片角度依赖的光电流^［23］，（d）MoTe₂器件的光学显微镜照片，T_d相MoTe₂的晶体结构，激发波长为10.6 μm时，各向异性光电流响应的线偏振消光比达到2.72^［22］

Fig. 2. （a）False-color scanning electron microscope image of a TaAs device. Along the a-axis and c-axis，the photocurrent varies with the angle of the quarter-wave plate^［34］；（b）Schematic experimental set-up for detecting the mid-infrared circular photogalvanic effect on a dual-gated monolayer WTe₂ device. Polarization along the a-axis depends on the photocurrent^［33］；（c）The lattice structure of tellurium（Te）. The quarter-wave plate-dependent photocurrent at the position of the maximum positive and negative response of the Te device under 10.6 μm（middle）and 4 μm（bottom）excitation^［23］；（d）Optical microscope image of the MoTe₂ device. Crystal structure of T_d-MoTe₂. Anisotropic photocurrent response with a polarization extinction ratio of 2.72 for linearly polarized excitation at 10.6 μm^［22］

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1.3　Chiral perovskite and organic materials

Chiral materials are defined as objects that cannot be superimposed on their mirror images. Due to their distinct chiral properties，they find diverse applications in fields such as medicine，biology，and quantum technology^［35］. The differential absorption of left-handed circularly polarized light（LCP）and right-handed circularly polarized light（RCP）by chiral perovskite and organic materials offers the opportunity to directly detect circularly polarized light（CPL）.

In 2019，Chao Chen et al. fabricated a CPL detector using chiral organic-inorganic hybrid（α-PEA）PbI₃ perovskite. To synthesize the chiral perovskite，they selected chiral α-phenylethylamine，whose π bond on the benzene ring aids in the positional interaction between the chiral amine and the（PbI₆）^4- matrix，enhancing the CPL-sensitive absorption. The circularly polarized detector exhibited a maximum polarization extinction ratio of 1.1 around the wavelength of 395 nm，a responsivity of 797 mA/W^-1，and a detectivity of 7.1 × 10¹¹ Jones^［36］. The device remained stable for one month. Although direct detection of CPL was achieved，the circular polarization extinction ratio（CPER）is not sufficient.

In 2020，A. Ishii et al. fabricated a CPL detector using the helical one-dimensional（1D）structure of lead halide perovskite，which is composed of naphthyl ethylamine-based chiral organic cations^［37］. The 1D structure consists of face-sharing（PbI₆）^4- octahedral chains，and R-（+）- and S-（-）-1-（1-naphthyl）ethylamine were chosen as the chiral cations in the 1D perovskites. The helicity of the 1D structure is primarily influenced by the chiral cations，resulting in an extremely high CPER. This work reported the highest CPER（25.4 at the wavelength of 395 nm）of perovskite based circular polarization detectors.

In 2021，Zhen Liu et al. incorporated chiral organic ligands into the inorganic octahedral framework（PbX₆）^4- of perovskite to create an optically active chiral hybrid perovskite（CHP）with efficient charge transport^［38］. The chiral ligand has a large π bond on the benzene ring，facilitating the Coulomb interaction between the chiral amine and the（PbI₆）^4- matrix，which enhances CPL-sensitive absorption. They fabricated CPL detectors with high photocurrent and polarization selectivity using CHP single-crystal nanowire arrays. The CPL detectors exhibited a CPER of 1.27，an on-off ratio of 1.8×10⁴，and a responsivity of 1.4 A/W^-1 at 510 nm with a bias voltage of 5 V.

In 2022，Yang Cao et al. created a new van der Waals heterojunction by combining a two-dimensional chiral hybrid perovskite material（MBA）₂PbI₄ with black phosphorus（BP）^［39］. The interfacing perovskite provides an inflow of numerous photogenerated carriers to BP，to reinforce the charge carrier transfer，separation，and transport processes. The responsivity and photogain of BP in heterostructures are boosted by almost one order of magnitude with respect to BP alone，which is more obvious under excitation above the bandgap of perovskite. The linear polarized light extinction ratio of the van der Waals heterojunction reaches 9 at a wavelength of 1 550 nm.

图 3. （a）用于圆偏振光直接探测的手性杂化钙钛矿单晶阵列设计^［38］，（b）PbI₃光电探测器的示意图，（R- and S-α- PEA）PbI₃的晶体结构，（R-，S-，and rac-α- PEA）PbI₃薄膜的圆二色性和吸收光谱^［36］，（c）螺旋一维钙钛矿基光电探测器示意图，（R-NEA）PbI₃器件在入射波长为395 nm，光强为1.0 mW cm^-2时，左旋圆偏振和右旋圆偏振的电流-电压曲线^［37］，（d）基于BP和手性钙钛矿MPI的范德华异质结光电探测器的示意图及其晶体结构，MPI/BP异质结在1550 nm光照下，电子和空穴转移过程，MPI/BP异质结器件和单独BP器件的输出曲线，及其在入射功率为100 μW，入射波长为1550 nm时，归一化的偏振分辨光电流^［39］

Fig. 3. （a）Chiral hybrid perovskite（CHP）single-crystal array design for high-performance CPL direct photodetection^［38］；（b）Schematic of the photodetector. Crystal structure of（R- and S-α-PEA）PbI₃. Circular dichroism（CD）and absorbance spectra of（R-，S-，and rac-α-PEA）PbI₃ thin films^［36］；（c）Schematic diagram of a helical one-dimensional perovskite-based photodetector. J-V curves of（R-NEA）PbI₃ device under LCP and RCP with a wavelength of 395 nm and an intensity of 1.0 mW cm^-2［37］；（d）Schematic diagram and crystal structure of vdW heterostructure photodetector device based on BP and chiral perovskite MPI. Electron and hole transfer process of MPI/BP heterostructure under 1550 nm illumination. Heterostructure output curves and individual BPs. Polar plot of the normalized polarization photocurrent measured at an illumination power of 100 μW and a wavelength of 1550 nm^［39］

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2　Integration of polarization-selective optical coupling structures

In the previous section，the detection of linearly and/or circularly polarized light is based on polarization-sensitive materials，such as anisotropic two-dimensional materials，topological materials，and chiral perovskites or organic materials. However，the choice of these materials is quite limited. Poor chemical stability，low responsivity，and low polarization extinction ratio are the main problems for polarization detectors based on polarization-sensitive materials. On the other hand，artificial micro-nano optical structures show great potential in controlling the interaction between polarized light and matter. The polarization detectors with polarization-selective optical coupling structures，as well as the integration with anisotropic materials，show better performance in responsivity and polarization extinction ratio.

2.1　Polarization-selective optical coupling structures

Plasmonic structures play an important role in the interaction between light and matter. They enhance the polarization-dependent optoelectronic coupling through resonant excitation of localized surface plasmons. Therefore，plasmonic structures are important tools for achieving polarization-selective coupling. Different resonances with the enhanced localized optical field can be realized under specific polarizations of the incident light and then the polarization light is discriminated. Integration of the polarization-selective optical coupling structures and infrared detection materials can greatly improve polarization detection performance.

In 2014，Qian Li et al. introduced a new approach to creating a grating plasmonic microcavity quantum well infrared detector by combining a single quantum well with a grating plasmonic microcavity^［40］. The quantum well is employed as the active structure within the near-field region of the plasmonic effect enhanced cavity，with the localized surface plasmon（LSP）mode and the surface plasmon polariton（SPP）mode being controlled. The artificial plasma modulation significantly influences the propagation and distribution of light within the plasma microcavity，resulting in a high infrared polarization resolution capability for the grating plasmonic microcavity quantum well infrared detection device. Moreover，the device achieves an extinction ratio of 65 at 14.7 µm.

In 2015，Wei Li et al. utilized a periodic array of chiral metamolecules comprised of a ‘Z’-shaped silver antenna on a poly（methyl methacrylate）spacer and an optically thick silver backplane to create a chiral plasmonic nanostructure with hot electron injection^［41］. Plasmonic nanostructures with engineered chirality can differentiate between left-handed circularly polarized（LCP）and right-handed circularly polarized（RCP）light，and photodetection is based on hot electron injection into silicon. The chiral plasmonic nanostructure circular polarization detector achieves a polarization extinction ratio of 3.4 at the wavelength of 1 340 nm.

In 2019，Mengjia Wang et al. employed gold-coated helical carbon nanowire end-fired and dipolar aperture nanoantennas to fabricate circularly polarized photodetectors by rotating surface plasmons on the subwavelength scale and utilizing optical spin-orbit interactions^［42］. The device allows for adjustable polarization control with a CPER of 64 at 1 550 nm，indicating that circularly polarized light can be locally achieved by applying the concept of a helical traveling wave antenna to a single plasmonic nanoantenna.

In 2020，Qiao Jiang et al. utilized an asymmetric n-shaped gold nanoantenna chiral plasmonic metasurface integrated with a single layer of MoSe₂ to create an ultra-thin circular polarimeter^［43］. The chiral plasmonic metasurface is utilized to selectively detect circularly polarized light，while the two-dimensional material determines the working wavelength range（visible to near-infrared）. The circular polarization-dependent photocurrent response based on the left-handed and right-handed metasurfaces is verified. The ultra-thin circular polarimeter achieves an extinction ratio of 1.74 for the left-handed metasurface and 1.9 for the right-handed metasurface at the wavelength of 790 nm.

图 4. （a）光栅等离激元微腔集成的量子阱红外探测器，微腔结构截面以及量子阱红外探测器的扫描电子显微镜图像，入射波长为14.2 ~ 14.9 µm时，随入射光偏振角变化的光电流平均强度^［40］，（b）手性超材料及圆偏光探测器的示意图，实测的超材料圆二色光谱，左手性超材料（蓝色）和右手性超材料（红色）^［41］，（c）螺旋行波纳米天线的示意图及其工作原理，螺旋行波纳米天线输出光束的椭圆率系数和圆偏振消光比的实验光谱，入射波长为 1.55 μm和1.64 μm时的偏振态分析^［42］，（d）单层MoSe₂和手性等离激元超表面集成的复合结构示意图，左手性和右手性等离激元超表面的吸收光谱^［43］

Fig. 4. （a）SEM image of the cleaved facet of the cavity structure. SEM image of PCQWID，a grating plasmonic microcavity quantum well infrared detector. The relationship between the average intensity of the photocurrent measured at the wavelength of 14.2 ~ 14.9 µm and the polarization angle of the incident light^［40］；（b）Schematic of the chiral metamaterial and CPL detector. Experimentally measured circular dichroism spectra of the left-handed（LH，blue）and right-handed（RH，red）metamaterials^［41］；（c）HTN schematic and how it works. The ellipticity factor of the output beam of the helical traveling wave nanoantenna HTN and the experimental spectrum of the DOCP. Polarization state analysis at wavelengths of 1.55 μm and 1.64 μm^［42］；（d）Schematic illustration of the hybrid structure consisting of a chiral plasmonic metasurface and monolayer MoSe₂. Optical absorption spectra of left-handed and right-handed plasmonic metasurfaces illuminated by light.^［43］

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2.2　Integration of anisotropic material and polarization-selective optical coupling structures

Combining the advantages of polarization-selective optical coupling structures and the anisotropic absorption in materials，the integration of polarization-selective plasmonic cavities and anisotropic materials exhibits a double enhancement of polarization discrimination.

In 2018，Yuwei Zhou et al. integrated an array of anisotropic plasmonic microcavity（PMC）with a quantum well infrared detector. PMC structures manipulate photonic modes at a sub-wavelength scale to enhance the photoelectric coupling and increase the absorption of quantum wells^［44］. The double polarization selection mechanism of the PMC and the quantum wells enhance the linear polarization extinction ratio up to 136 in the long wavelength infrared regime. Fig. 5（a）shows the schematic diagram of the three-dimensional simulation of the plasma microcavity quantum well infrared detector and the reconstruction of the Stokes parameters.

图 5. （a）等离激元微腔集成的量子阱红外探测器的三维仿真示意图，（b）Stokes参数的解析^［44］，（c）非对称复合结构示意图，（d）左旋圆偏振和右旋圆偏振入射下，各向异性介质复合结构的吸收和反射光谱^［45］

Fig. 5. （a）Schematic diagram of 3D simulation of plasmonic microcavity quantum well infrared detector.（b）Resolution of Stokes parameters^［44］；（c）Schematic diagram of the asymmetric composite structure.（d）Absorption and reflection spectra of anisotropic dielectric composite structures under LCP and RCP illumination^［45］

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In 2020，Zeshi Chu et al. integrated asymmetric metamaterials on quantum wells for a long-wave infrared circular polarization detector. Based on the double polarization selection mechanism，a CPER of 14 is obtained^［45］. This value is 10 times higher than that of the asymmetric metamaterial integrated HgCdTe detector. HgCdTe is the most widely used infrared detection material^［46-47］. However，the CPER of the asymmetric metamaterial integrated HgCdTe detector is much lower since there is no double polarization selection. The double polarization selection mechanism is generally applicable to a variety of devices. When the asymmetric metamaterial is integrated with another anisotropic infrared detection material（InAsSb nanowire array），the CPER of this device is 12.6 in the mid-infrared region. In comparison，when the same asymmetric metamaterial is integrated into an InAsSb bulk material，the CPER is only 1.7. In addition，the integration of anisotropic optoelectronic materials and asymmetric metamaterials yields an improved CPER without compromising the absorptivity of active materials. High-intensity photonic modes excited by the asymmetric metamaterials can increase the absorption in the active materials by several times. Fig. 5（b）shows the schematic composite structure of the asymmetric metamaterial and the detection material. and the absorption and reflection spectra of the anisotropic composite structure under left-handed circularly polarized light and right-handed circularly polarized light.

The bowtie antenna and aligned single-walled carbon nanotube（SWCNT）films integrated infrared detector，proposed by our research group，can be utilized for highly polarization-sensitive far-infrared detection，with a polarization extinction ratio exceeding 13 600 at a resonance frequency of 0.5 THz^［48］. Our proposed plasmonic microcavity-integrated graphene photodetector for polarization detection achieves a polarization extinction ratio of approximately 30 at a wavelength of 1.55 µm^［49-50］. In addition，the research group has also designed quantum well infrared photodetectors（QWIPs）integrated with all-semiconductor strip plasmonic cavities with high polarization sensitivity，and the polarization extinction ratio exceeds 900 in the terahertz band^［51］. In the long-wave range，our proposed asymmetric metamaterial integrated quantum well（anisotropic material）has a CPER of approximately 14 ^［45］. This CPER is twice as high as that observed for graphene integrated with chiral plasmonic nanoantenna electrodes in the mid-infrared band^［52］.

2.3　Configurable photocurrent polarity by the optical structure

By integrating polarization-sensitive materials with micro-nano optical structures，high responsivity，and polarization extinction ratio has been achieved. Recently，configurable photocurrent polarity has also been realized by integrating plasmonic nanoantennas. The polarity of photocurrent can be tuned by light polarization flexibly and an infinite extinction ratio is realized at the polarity-transition point. In such cases，the traditional definition of polarization extinction ratio is no longer applicable，and the corresponding extinction ratio at the photocurrent polarity-transition approaches infinity.

图 6. （a）热电堆单元结构的扫描电子显微镜图像，在入射波长为7.9 µm，光强为270 W cm^-2时，实测随入射光椭圆率角χ变化的热电堆电动势电压值（黑点），及其与理论的归一化Stokes参数S₃分量的比较^［53］，（b）自旋调控的单向等离激元波导片上电学探测器示意图，采用Soleil-Babinet可变相位延迟器将线偏光转换为椭圆偏振态、正交的线偏振态以及左旋和右旋圆偏振态^［54］，（c）超表面调控的石墨烯光电探测器示意图，暗态和光照下的电流-电压曲线^［55］，（d）纳米天线调控的半金属光电探测器示意图，不同栅电压下随偏振角变化的光电压，实测（圆圈符号）和拟合（虚线）^［56］，（e）石墨烯薄片上非手性等离激元纳米结构光响应的对称性分析，不同源漏偏置下的电流-电压曲线，圆偏振光探测器在庞加莱球上的表示，其光电压只正比于入射光Stokes参数的S₃分量^［57］

Fig. 6. （a）Scanning electron microscope image of a thermopile element. Measured thermoelectric reactor emf voltage（black dots）as a function of incident light ellipticity angle χ compared to normalized S₃ Stokes parameters at a wavelength of 7.9 µm and a light intensity of 270 W cm^-2［53］；（b）Device schematic for spin-controlled unidirectional plasmonic waveguide on-chip electrical detection. A Soleil-Babinet variable phase retarder was employed to convert linearly polarized laser radiation into left and right circular polarization states of intermediate elliptical and orthogonal linear polarization states^［54］；（c）Schematic of a metasurface-mediated graphene photodetector. I-V curves of the device under dark and light conditions^［55］；（d）Schematic of a nanoantenna-mediated semimetal photodetector. Measured（symbols）and fitted（dashed lines）photovoltage versus polarization angle for different gate voltages^［56］；（e）Symmetry analysis of the photoresponse of an achiral plasmonic nanostructure located on a graphene sheet. Measured I-V curve with drain-source bias. Illustration of a CPL-specific photodetector in a Poincaré sphere，where the photovoltage V_ph depends only on the fourth Stokes parameter S₃ of the incident light^［57］

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In 2016，Feng Lu et al. placed the thermal junction of a thermocouple at the center of an optical antenna to create an antenna-coupled thermopile photodetector^［53］. The device is only sensitive to light ellipticity，and an antenna-coupled thermopile photodetector is used to convert the degree of circular polarization of light into a DC voltage proportional to the S₃ Stokes parameter of the incident radiation. The detector produces a bipolar voltage output that is proportional to the S₃ Stokes parameter of the incident light in the 7 ~ 9 µm wavelength range. The detector design is completely achiral，indicating that the incident light’s chirality is converted into either the current direction or the DC voltage sign in the detector.

In 2019，Martin Thomaschewski et al. combined strong light-matter interactions in plasmons with semiconductor technology based on spin-orbit interactions in achiral plasmonic nanocircuits^［54］. They achieved this by integrating two gold-germanium-gold chiral plasmonic waveguide photodetectors，resulting in a compact polarimeter capable of detecting the circular polarization light.

In 2021，Jingxuan Wei et al. developed nanoantenna-mediated few-layer graphene photodetectors. The device allows for configurable switching between unipolar and bipolar polarization dependence of linear polarization response in the mid-infrared region through vectorial and nonlocal photoresponses^［55-56］. The orientation of the nanoantenna in the device can be adjusted to vary the polarization extinction ratio from positive to negative，covering all possible values，with the polarization extinction ratio approaching infinity at the polarity-transition point. This enables linearly polarized light detection with a range from finite to infinite extinction ratio.

Recently，in 2022，Jingxuan Wei et al. developed a mid-infrared circular polarization detection device by integrating plasmonic nanostructure arrays and graphene ribbons^［57］. The geometric arrangement of plasmonic nanostructures enhances circularly polarized light detection，and the photocurrent generated by the achiral structure achieves a CPER of 84 under zero bias in the mid-infrared band at room temperature. The detection of CPL should be robust，with immunity against the ubiquitous unpolarized and linearly polarized light.

图 7. （a）偏振计的扫描电子显微镜图像，随四分之一波片角度变化的光响应^［58］，（b）器件的扫描电子显微镜图像，仿真和实测的0°和135°线偏振分量随二分之一波片角度变化的强度，以及左旋和右旋圆偏振分量随四分之一波片角度变化的强度^［59］，（c）光纤端面堆叠结构的伪彩色扫描电子显微镜图像，扭曲黑磷单元结构随四分之一波片角度变化的光电流^［60］，（d）高速相干光通信中，片上相位解调的示意图，对于单个纳米盘和两个双纳米盘，实测和仿真的不同波导端口输出的归一化强度^［61］，（e）共振热电光响应的器件结构设计示意图，5个典型器件不同的随偏振角变化的光响应，仿真（实线）和实测（圆形符号），4种典型器件不同的随四分之一波片角度变化的光响应，仿真（实线）和实测（圆形符号）^［62］

Fig. 7. （a）SEM image of a polarimeter. Polar plot of photoresponse as a function of quarter-wave plate（QWP）rotation angle^［58］；（b）SEM image of the device. Simulations and experiments examine the intensity of the 0° and 135° linearly polarized light（LP）components as a function of the half-wave plate（HWP）. Simulations and experiments examine the intensity of different QWP angles for left-handed circularly polarized light（LCP）and right-handed circularly polarized light（RCP）components^［59］；（c）False-color SEM image of a fiber end-face stack. One cycle photocurrent of the twisted BP cell as a function of QWP angle^［60］；（d）Schematic diagram of on-chip phase demodulation for high-speed coherent optical communication. The normalized output intensities of different waveguide ports for a single nanodisk element and two double nanodisk elements were experimentally measured and simulated^［61］；（e）Structural design of resonant thermoelectric photoresponse. Polarization-angle-dependent photoresponse simulations（lines）and measurements（symbols）for five typical devices. Simulated（line）and measured（symbol）photoresponses of four typical devices as a function of QWP angle^［62］

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3　Challenge and opportunity: on-chip full-stokes detection

On-chip infrared polarization detection has been extensively studied nowadays. The perception of a single polarization state has been thoroughly studied. Full Stokes detection that includes all polarization information becomes a challenge and opportunity.

In 2020，Lingfei Li et al. developed four metasurface-integrated graphene-silicon photodetectors based on the geometric chirality and anisotropy of the metasurface for circular and linear polarization-resolved light responses^［58］. The photodetector enables full Stokes parameter detection for arbitrary polarized incident infrared light at 1 550 nm.

In 2021，Changyu Zhou et al. coupled four single-mode silicon waveguides to a circle-like polarization distinguishing device on an insulating silicon substrate to create an on-chip optical polarimeter capable of measuring arbitrary polarization states^［59］.

In 2022，Yifeng Xiong et al. developed a fiber-integrated polarimeter by vertically stacking three photodetection units based on two-dimensional vdW materials on the fiber end face^［60］. The polarimeter consists of six layers of van der Waals materials stacked vertically to form three photodetection units. Two anisotropic BP units are twisted stacked for polarized light sensing，and a bismuth selenide（Bi₂Se₃）layer provides power calibration. The polarimeter features self-power-calibration，self-driven，and ambiguity-free detection of linearly and circularly polarized light by breaking symmetry-induced LPGEs and CPGEs in the two BP units.

In 2022，Ting Lei et al. coupled a network of single-mode Si waveguides to an insulating silicon substrate to achieve on-chip high-speed coherent optical signal detection based on photon spin-orbit interactions and enable full Stokes parameter measurement of incident light^［61］.

In 2022，Mingjin Dai et al. integrated plasmonic chiral materials and two-dimensional thermoelectric materials to prepare an on-chip mid-infrared photodetector^［62］. The photodetector is based on the polarization-dependent photothermal effect in chiral plasmonic metamaterial-mediated and the Seebeck effect of two-dimensional thermoelectric materials. It enables the detection of linearly polarized and circularly polarized light and realizes on-chip full Stokes detection in the mid-infrared band at room temperature. However，the average measurement errors are large. The error of S₁，S₂，and S₃ are 14.2%，15.2%，and 5.4%，respectively. More accurate on-chip full-Stokes detection is desired.

4　Conclusion

Many different approaches and significant efforts have been dedicated to the advances of on-chip infrared polarization detectors. These polarization detectors can be realized by polarization-sensitive materials including anisotropic two-dimensional materials，topological materials，chiral materials，and integrated polarization-selective optical coupling structures. When polarization-selective optical coupling structures and polarization selective detection materials are combined in a proper way，high polarization discrimination can be achieved. With the development of artificial metamaterials-mediated detectors，the polarity of photocurrent can be flexibly tuned by light polarization，and an infinite extinction ratio could be realized at the polarity-transition point. In conclusion，on-chip polarization detection by integrating anisotropic materials and optical structure has received widespread attention. In the future，on-chip full Stokes parameters detection with high accuracy of all polarization states covering the full Poincare sphere presents challenges and opportunities.