Micropolarizing device for long wavelength infrared polarization imaging.

Abstract

The goal of this project is to fabricate a four-state pixelated subwavelength optical device that enables mid-wave infrared (MWIR) or long-wave infrared (LWIR) snapshot polarimetric imaging. The polarization information can help to classify imaged materials and identify objects of interest for numerous remote sensing and military applications.While traditional, sequential polarimetric imaging produces scenes with polarization information through a series of assembled images, snapshot polarimetric imaging collects the spatial distribution of all four Stokes' parameters simultaneously. In this way any noise due to scene movement from one frame to the next is eliminated.We fabricated several arrays of subwavelength components for MWIR polarization imaging applications. Each pixel unit of the array consists of four elements. These elements are micropolarizers with three or four different polarizing axis orientations. The fourth element sometimes has a micro birefringent waveplate on the top of one of the micropolarizers. The 4 linear micropolarizers were fabricated by patterning nano-scale metallic grids on a transparent substrate. A large area birefringent waveplate was fabricated by deeply etching a subwavelength structure into a dielectric substrate. The principle of making linear micropolarizers for long wavelengths is based upon strong anisotropic absorption of light in the nano-metallic grid structures. The nano-metallic grid structures are patterned with different orientations; therefore, the micropolarizers have different polarization axes. The birefringent waveplate is a deeply etched dielectric one-dimensional subwavelength grating; therefore two orthogonally polarized waves have different phase delays. Finally, in this project, we investigated the near field and diffractive effects of the subwavelength element apertures upon detection.The fabricated pixelated polarizers had a measured extinction ratios larger than 100:1 for pixel sizes in the order of 15 μm by 15 μm that exceed by 7 times previously reported devices.The fabricated birefringent diffractive waveplates had a total variation of phase delay rms of 9.41 degrees with an average delay of 80.6 degrees across the MWIR spectral region.We found that diffraction effects change the requirement for separation between focal plane arrays (FPA) micropolarizer arrays and birefringent waveplates arrays, originally in the order of hundreds of microns (which are the typical substrate thickness) to a few microns or less. This new requirement leads us to propose new approaches to fabricate these devices.5

A compact polarization-integrated long wavelength infrared focal plane array based on InAs/GaSb superlattice

“…However, compared with the MIR detector, the detectivity of the LWIR detector is relatively weak due to the high dark current, and the integration of polarization gratings will lose a lot of optical signals, which will further reduce the signal-to-noise ratio of the device [21]. Furthermore, large optical crosstalk leads to a low extinction ratio, and the integration technology of polarization grating and detector are difficult problems in the development of polarization-integrated infrared detector [22,23]. Up until now, those two hard problems have not been solved but the research on LWIR PI-FPA is very rare.…”