# Metasurface-Based Polarimeters
## 3 Metasurface-Based Polarimeters
### Photodetector Integrated Polarimeters
To determine the handedness of a circularly or elliptically polarized light and also degree of circular polarization for partially polarized light, two subwavelength coaxial apertures surrounded by spiral grooves with opposite twists have been designed, which respond differently to CP light of opposite handedness due to geometric phase effects (Figure 9e,f) [97; 98]. Very recently, a CP light detector was demonstrated by combining a chiral metasurface with hot electron injection, enabling a potential application for polarimetry [96]. As shown in Figure 9g, the proposed chiral metasurface--consisting of the chiral plasmonic meta-atom array, dielectric spacer, and metal backplane--can perfectly absorb CP light with one particular handedness while largely reflecting the opposite component. Within the metasurface, the continuous 'Z'-shaped silver antenna allows for an electrical connection with a silver bus serving as the electrode (Figure 9h). The whole device is realized by placing an n-type silicon wafer in contact with the antenna layer, forming a Schottky barrier, as shown in Figure 9i. As such, the device can selectively generate hot electrons and produce a photocurrent signal depending on the handedness of light. By illuminating the metasurfaces with a CP laser and measuring photocurrent as a function of the laser handedness and wavelength, the photoresponse spectra of the devices were obtained (Figure 9j,k). In general, the photoresponsivity spectrum matches well with the measured absorption spectrum. When the metasurface is left-handed, it absorbs LCP light and reflects RCP light, resulting in dominating photoresponsivity for LCP light. Once the handedness is switched to right, it generates a stronger current signal for RCP light. Furthermore, the large distinction in the photocurrent for LCP and RCP light corresponds to the large CD (Figure 9j). Therefore, by including both right-handed and left-handed surface patterns, the sensor can differentiate between LCP and RCP light. Figure 9: Photodetector integrated polarimeters. **(a)** Schematic of the proposed plasmonic polarimeter that consists of six differently shaped plasmonic slit structures patterned into a gold film on top of a silicon-based Schottky detector; **(b)** SEM image of a part of the Stokes detector; **(c)** Measured photocurrent map of the linear Stokes detector for a linearly polarized incident beam. The angle between the incident electric field and the horizontal slit is 7\({}^{\circ}\) and the illumination wavelength is 830 nm. The photocurrent is normalized to the maximum measured photocurrent; **(d)** Normalized measured photocurrent of the horizontal slit as a function of the polarization angle of the incident linearly polarized light; **(e)** SEM image of a coaxial aperture surrounded by an right-handed spiral groove fabricated with focused ion beam (FIB) milling; **(f)** Normalized measured photocurrent of the coaxial aperture shown in **(e)** as a function of the phase difference between \(E_{x}\) and \(E_{y}\); **(a–f)** Reproduced with permission from [95], Copyright Science Wise Publishing § De Gruyter Berlin, 2012; **(g)** Schematic of the chiral metasurface consisting of a chiral plasmonic meta-molecule array, dielectric spacer, and metal backplane; **(h)** Schematic of the circularly polarized (CP) light detector consisting of a chiral metasurface integrated with a semiconductor that serves as a hot electron acceptor; **(i)** Energy band diagram of the CP light detector; **(j,k)** Experimentally measured (dots) and theoretically calculated (solid curve) photoresponsivity spectra under LCP (blue) and RCP (red) illumination for left-handed (j) and right-handed (k) metasurfaces; **(l)** Photocurrent polarization discrimination ratio spectra of left-handed and right-handed metasurfaces; **(g–l)** Reproduced with permission from [96], Copyright Macmillan Publishers Ltd.: Nature Communications, 2015.