# Metasurface-Based Polarimeters
## 3 Metasurface-Based Polarimeters
### Photodetector Integrated Polarimeters  
In the previous sections, the SOPs are mainly determined by the measured optical signals. An as-yet unexplored milestone in the field of metasurface-based polarimeters is to achieve an integrated electronic device, which can electronically probe the polarization states. Owing to the fascinating properties of metasurfaces, several photodetector integrated polarimeters have been proposed and demonstrated by integrating meta-atoms with semiconductor elements [95; 96]. In this section, we will review some of the recent progress in photodetector integrated polarimeters. As a good example, Figure 9a schematically shows a silicon photodetector integrated with a set of plasmonic structures that can be used as either a broadband linear-Stokes polarimeter or a narrowband full-Stokes polarimeter, capable of determining the complete state of polarization of a light beam [95]. Specifically, in this silicon-based Schottky detector, the Schottky barrier is created by using a few nanometer thick chromium layer between the n-type silicon substrate and the gold layer. The gold contact is patterned with four linear subwavelength slits at different orientations and two subwavelength coaxial apertures surrounded by spiral grooves with opposite twists, which have  
Figure 8: Silicon-based orbital angular momentum (OAM) Spectropolarimeter. (**a**) Schematic set-up of an OAM spectropolarimeter metasurface (OSPM) of 50 \(\upmu\)m diameter illuminated by an arbitrarily polarized polychromatic light source. The image captured by the CCD was obtained for an elliptically polarized beam at \(\lambda_{1,23}=550\), 590 and 633 nm with topological charge, \(l=0\); (**b–g**) Measured far-field intensities for different wavelengths, polarizations, and OAMs. Note, \(\left|\lambda,P,l\right\rangle\) stands for the incident beam state, representing the wavelength, the polarization, and the OAM, respectively. Reproduced with permission from [80], Copyright Macmillan Publishers Ltd: Nature Light: Science and Applications, 2017.  
different optical responses to the incident polarizations. In this way, these differently shaped plasmonic structures patterned in the Au film can work as the polarization filters for the six extreme polarizations (\(|x\rangle\), \(|y\rangle\), \(|a\rangle\), \(|b\rangle\), \(|r\rangle\) and \(|l\rangle\)). Given the linear relationship between the generated photocurrent and the light intensity behind the filters in the silicon substrate, the required intensity measurements can effectively be translated into a set of photocurrent measurements. Figure 9b shows the fabricated linear Stokes detector in which four linear slits are milled into the gold film with focused ion beam (FIB) milling. By raster scanning the laser beam over the device, the photocurrent images of the linear Stokes detector at \(\lambda=830\,\mathrm{nm}\) are obtained, confirming that the light absorption in the silicon is dominated by the portion of the light polarized normal to each slit (Figure 9c). The photocurrent response of a single slit at a given polarization is defined as the maximum measured photocurrent found in scanning the beam over the slit. If the direction of the incident electric field is continuously varied with a half wave plate, the photocurrent response of a single slit changes accordingly, revealing the linear polarization-dependent response of the slit, as shown in Figure 9d. Therefore the arrangement of four slits can be used for linear Stokes polarimetry with which one can distinguish between linear, circular, and elliptical states of polarization and determine the degree of linear polarization for partially polarized light over a broadband wavelength range.