A metasurface-based miniaturized spectropolarimeter design

Abstract

This research is focused on the feasibility assessment of a novel technology of metasurfaces in order to design a spectropolarimeter with the thickness of a few micrometers integrated on the detector array. The motivation behind the research is miniaturization of state-of-the-art spectropolarimeters. The requirements for the instrument are derived from SPEXone instrument, as a reference instrument. Beside the miniaturization, two additional requirements are set as a goal: possibility to measure full-Stokes vector of polarization (including circular polarization), and to improve the spectropolarimetric resolution. The reduction in the volume of the instrument is not the main advantage, but also reduction in the number of optical components with respect to the state-of-the-art. A spectropolarimeter with fewer components allows better stability of performances over different external conditions like temperature change, and easier alignment of the whole system.
In recent years the dielectric metasurfaces attract a lot of attention. Firstly, due to its compatibility to CMOS detector technology, which allows integration of thin layers of metasurfaces directly on the detector array, and secondly due to high transmission of the dielectric materials. The metasurface is an array of scatterers with period and size of scatterers smaller than the wavelength. With proper design, the complete control of electromagnetic waves is possible, including amplitude, phase, and polarization at subwavelength resolution. This is convenient because diffraction is not present at that scale. That enables to design optical devices like lenses, waveplates, polarizing beam splitters with similar performances as current technology, but with extreme reduction in thickness. In this thesis, a systematic electromagnetic performance analysis of metasurfaces is presented. Two types of metasurfaces are designed, first acting as a waveplate, and second as a linear polarizer at different spectral wavelengths. Also, a trade-off of the most suitable spectropolarimetric techniques is done, which led to the Division of Focal Plane (DoFP) type of polarimeter. The combination of these two layers of metasurfaces can modulate the polarization state of light in form of the intensity of light, which is measured on detector pixels. It is shown that with this technique, four unique pixels, each with different polarization modulation, are required to reconstruct four unknowns, the Stokes vector, which fully describes the state of polarization of incident light. Furthermore, the spectral information is measured by designing different sets of four pixels to filter out different parts of the spectrum. The result is two-dimensional detector array which measures spectrum in the vertical axis, and spatial information in the horizontal axis. The spatial information in the vertical axis is achieved by implementing the push broom concept of a satellite. Moreover, the important spectropolarimetric parameters like spectropolarimetric accuracy, spectropolarimetric resolution, and operating spectrum range are estimated. Also, a complete overview of the instrument, which consists of a telescope unit, a bandpass spectral filter and the metasurface integrated on the detector array, is shown. In the end, the performance of the complete system is compared with the reference instrument SPEXone, in order to assess the potential of this concept.