Polarization detection is an essential topic due to enriching applications including safe optical communication, remote sensing, polarization imaging and biomedical applications1. Polarization, unlike intensity of the light, cannot be directly detected by conventional photodetectors. Currently, the widely used polarization detection methods require bulky optical components such as polarizers and waveplates, which make it challenging for device integration and minimization. Flat optics based on plasmonic structure open a new path for polarization detection with ultra-compact size 2-5. Polarization detection in MIR range is especially attractive due to wide applications in biomedical fields like cancer detection and molecule chirality detection. Yet, MIR polarization detection is even more challenging than that in visible and NIR due to the material absorption limitations. Here we present the theoretical modeling and experimental demonstration of MIR polarization detection based on integrated plasmoinc flat optics composed of optical antenna and nanogratings. Our technique provides complete measurement of full stokes parameters and thus enables the detection of light with any polarization state, including partially polarized light. Moreover, it has the advantages of being ultracompact, capable to work in MIR range with high extinction ratio and easy to integrate with photodetectors. The MIR polarization detector consists of 6 detection units, including 4 nanograting units and 2 circularly polarized light detection units. According to our theoretical modeling, the nanograting units and the CP detection units show high extinction ratio for linearly and circular polarized input light in MIR, respectively. We have also demonstrated experimentally circularly polarized light detection with extinction ratio of 6.2 and linearly polarized light detection with extinction ratio of 45.5. With all 6 elements, we have performed full-stokes polarization measurement of arbitrary polarization states. The measured Stokes Parameters are reasonably well consistent to the input polarization of the light. The average error of S1, S2, S3 is 0.035, 0.025, and 0.104, respectively. And the average error of DOLP and DOCP is 0.036 and 0.103, respectively. The device performance can be further improved by increasing the extinction ratio of the linearly and circular polarization detection units through optimization of design parameters as well as fabrication processes. The detector we proposed can be easily redesigned to any wavelength from NIR to MIR by changing the design parameters of the optical antennas, which is promising for multi-wavelength or broadband polarization detection.
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