Background

Optical polarization is one of light’s most fundamental and powerful characteristics, essential for a broad array of applications, including optical communications, remote sensing, cosmology and biomedicine. To accurately capture the Stokes parameters of polarizations, polarimeters base on discrete optical components—such as prisms, lenses, filters, polarizers, and waveplates—have been well-established. However, these bulky and complex optical systems pose challenges for miniaturization, especially as demand grows for compact devices with direct electrical readout on a chip-scale platform. 

Motivation

The Stokes-Mueller formalism is a foundational concept in optics (Fig. 1a), successfully describing the linear relationship between the input and output Stokes vectors of light when it interacts with materials. This formalism typically applies to light fields with well-defined planar wavefronts. However, for detection materials integrated with nanostructures or metasurfaces, incident light transforms into complicated near fields without defined wavefronts, which are then absorbed to generate photocurrent. In such case, the Stokes-Mueller formalism does not apply. Finding a method to retrieve the polarization information directly from the measured photocurrents enable compact devices without the need for bulky optical components. Nevertheless, developing a framework to characterize optoelectronic conversion in polarization detection remains an unsolved problem (Fig. 1b).

Innovation

In this research, our group, led by Prof. X. Chen and Prof. J. Zhou at SITP, CAS, in collaboration with the Prof. C.-W. Qiu group at NUS, has developed a chip-scale, miniaturized full-Stokes polarimeter base on the novel concept of optoelectronic polarization eigenvectors (OPEVs). Inspired by the Stoke-Mueller formalism, we are excited to find the OPEVs, which describes linear relationship between photocurrent and incident Stokes vector, as an optoelectronic analog to Stokes-Muller calculus. Notably, the OPEV concept applies regardless of the type of photonic structures integrated with the detection material or whether the detection material itself is isotropic, anisotropic, or chiral. Here, we demonstrate this with a metasurface-integrated MoS₂ device composed of four subpixels, each corresponding to a distinct OPEV (Fig. 2a and b). The polarization information is encoded by these OPEVs across the four subpixels, which allows direct electrical readout. In the polarization space, the OPEVs of these subpixels form a tetrahedron (Fig. 2c). The magnitude of the polarized photocurrent component for subpixel-I, for example, is represented by color on the surface of the Poincaré sphere (Fig. 2d). 

Validation

Building on the proof-of-concept device described above, the full-Stokes reconstruction is formulated as solving an inverse problem. Here, the photocurrents of four subpixels form a photocurrent vector that represents the projection of the input Stokes vector onto a matrix defined by the OPEVs (Fig. 3a). A mapping relationship is established between the photocurrent vector and the Stokes vector. Consequently, the metasurface-integrated polarimeter was optimized to minimalized the condition number of this matrix according the knowledge of linear algebra. Then, all four Stokes parameters can be reconstructed from the four measured photocurrent values. For greater reconstruction accuracy, a Gaussian process regression model was also employed. As shown in Fig. 3b, randomly selected input and reconstructed Stokes parameters are shown on the Poincaré sphere. Our device validates the broadest coverage of polarization states over the entire Poincaré sphere and achieves the lowest reported full-Stokes reconstruction error (r.m.s.e < 1%) among on-chip in-detector polarimeters (Fig. 3c).

So far, a comprehensive framework for miniaturizing polarimetry has been established and validated. This method shows promise for adaptation to focal plane arrays and for broadening its application to mainstream infrared detection materials—such as MCT, QWIP, InGaAs, and T2SLs—with potential to drive exciting and impactful advancements in the infrared regime.

For further details, please refer to our recent publication in Nature Electronics: “An on-chip full-Stokes polarimeter based on optoelectronic polarization eigenvectors”. https://doi.org/10.1038/s41928-024-01287-w