Versatile on-chip polarization detection leveraging two-dimensional materials

Polarization is a physical property of light that describes light wave’s oscillation. To describe the polarization of light, conventional polarization-sensitive systems rely on bulky polarizing elements combined with conventional photodetectors, which are costly and can take up significant space. These constraints limit their applicability in scenarios where miniaturization and large-scale integration are desired. Writing in Light: Science & Applications, Liu et al. presents an advance in compact polarization detection: a versatile, on-chip polarization-sensitive detection system that leverages the versatility of two-dimensional (2D) materials¹.

The active polarization detection part in Liu et al.’s work is a WSe₂-based photodetector. Here the presence of the metallic electrode interfaces induces plasmonic effects, exciting hot electrons whose generation strength is determined by the polarization of the incident light. The effect is maximized when the polarization is perpendicular to the metal electrode interface and is minimized when parallel. The polarization-dependent hot carriers generation is not a fact of any intrinsic symmetry breaking of the material, unlike other miniaturized polarization detectors using anisotropic 2D materials. Therefore, this mechanism broadens the selection of materials suitable for polarization-sensitive detection. Although the anisotropic ratio of the WSe₂-based photodetector is only approximately 2, integrating it with a MoS₂-based field-effect transistor (FET) creates an amplification system for polarization detection (ASPD) (Fig. 1). This architecture leverages the amplification in the FET, boosting the anisotropic ratio to over 60 in the infrared (IR) band, achieving practical applicability. The ASPD system demonstrates several standout performance metrics: it has a broadband sensitivity, over 10³ on/off ratio in IR, and fully assembled from 2D materials without the requirements of intrinsic material anisotropy, representing a new prototype of miniaturized, efficient polarization detection system.

Fig. 1  Schematic of the integrated WSe₂ PD and MoS₂ FET. WSe₂ PD responses to different polarizations with different voltages due to plasmonic effect, after which the MoS₂ amplifies the signal and produces prominent polarization-dependent output.

The 2D material ASPD presented in Liu et al’s work is capable of polarization modulated optical communication and image recognition. To demonstrate optical communication, the polarization of incident light is modulated, where where 90° polarization represents “0” and 0° polarization represents “1”. The ASCII codes are successfully encoded, transmitted, and decoded in the work. Besides optical communication, the ASPD provides polarization-resolved imaging capabilities, which are combined with neural networks for an image recognition demonstration.

Liu et al’s work focuses on the detection of linear polarization of light, it remains to be seen whether and how the 2D material based ASPD can be developed for circular polarizations. Miniaturized perovskites photodetectors have been demonstrated for effective direct detection of circularly polarized light, where the chirality of the perovskites leads to a selective absorption depending on the circular polarization^2,3. To fully describe the state of polarization (SoP) of light, one would need the Stokes parameters which can represent any polarization state including linear and circular polarizations. Such representations can be illustrated geometrically on a sphere, known as the Poincare sphere⁴, where each SoP is represented by one point on the sphere. Full-Stokes polarization detection has also been achieved in an on-chip manner. By leveraging the resonant absorption in chiral plasmonic metamaterials and photothermoelectric effect, Dai et al.⁵ design a polarimeter with careful manipulations of the positions of the chiral metamaterials, such that geometrical ellipse parameters can be read out by a three-ports device. The parameters are used to reconstruct the full-Stokes parameters for a complete polarization sensing. The requirement of having different orientations and positions of the chiral metamaterials to extract these parameters poses a challenge to further reduce the footprint of the polarimeter. Another demonstration on full-Stokes detection is based on matrix Fourier optics by Rubin et al⁶. In this work, polarization is treated in a matrix Fourier optics formalism taking the spatially varying polarization into account. Under this formalism, Rubin and coworkers design diffractive gratings which can sort the polarization of the incident light. Imaging optics and sensors are used to capture four images corresponding to the diffraction order, where the pixelized polarization information is encoded in the four images that can be analyzed to fully resolve the polarization. As the demand in ultra-compact full-Stokes polarimeter is increasing, shifting the complexity of hardware to computation has been shown to be promising. Graphene moiré superlattices have been demonstrated to host bulk photovoltaic effect (BPVE) that can produce a photovoltage depending on the polarization of the incident light⁷. The BPVE is discovered to be electrically reconfigurable in the device, thus for a specific polarization, tuning the gate voltages can generate a unique 2D mapping that corresponds to the polarization. The 2D photovoltage mapping is analyzed by convolution neural network (CNN) that is properly trained with many of 2D mappings and their corresponding ground truth polarization, thus the CNN can decode the light polarization from a 2D mapping accurately, although the mapping has not been seen by the CNN in the training process. Because of the reconfigurability of the BPVE in the device and the usage of CNN in the decoding process, the polarimeter itself is only 3 µm by 3 µm, with the capability of full-Stokes polarization detection.

With additional degree of freedoms from spin angular momentum (SAM) and orbital angular momentum (OAM), the complete polarization states are represented by a high order Poincare sphere. Given the multiple dimensions in the complete polarization representation, it is in general challenging to fully resolve all in one measurement. In Yang et al.’s work written in Light: Science & Applications, metasurface is implemented to interact with the incident light with specified polarization, SAM and OAM, translating the unknown light properties into a unique pattern on a transverse plane⁸. By analyzing the intensity and location of the hot spots of the pattern, one can decipher the complete light polarizations. Such a demonstration simplifies the process of polarization detection where multiple unknown light properties are readout by a single measurement. By integrating the metasurfaces with the 2D material devices demonstrated in Liu et al.’s work, new possibilities may show up for compact and efficient polarization detection. With wide spectral range and complete polarization detection, the system could enable enhanced remote sensing and environmental monitoring. Additionally, polarization is critical in health diagnostics and such a miniaturized device can be fit in wearable health sensing devices that could improve personal health care.