טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentDahan Nir
SubjectSpace-variant Polarization Manipulation of a Thermal
Emission by a Subwavelength Grating Supporting
Surface Phonon-Polaritons
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Erez Hasman


Abstract

Thermal emission from absorbing material is considered to be incoherent and unpolarized, and accordingly is regarded as spontaneous emission. The surface properties of the absorbing material have a profound impact on the emission's optical properties, and can be manipulated to produce a partially coherent and partially polarized radiating emission. A connection between the emission and the surface properties is established by studying the excitation of surface phonon-polaritons (SPPs). The underlying microscopic origin of the SPPs is a mechanical vibration of atoms. A surface polariton has a longer wavevector than the light waves propagating along the surface at the same frequency. For this reason, they are called “nonradiative” surface polaritons. By coupling the surface polaritons with the propagating wave, by means of an additional prism or grating, one can produce either increased resonant absorption or emission. Because SPPs are able to be excited only by TM-polarized propagating waves, the emission's properties have to be polarization dependent.

Space-variant polarization manipulation of enhanced nondirectional thermal emission in a narrow spectral peak is presented. The emission is attributed to surface phonon-polaritons excitation from space-variant subwavelength SiO2 gratings. Polarization manipulation is obtained by discretely controlling the local orientation of the grating. We experimentally demonstrate thermal emission in an axially symmetric polarization distribution. Theoretical calculations based on rigorous coupled-wave analysis are presented along with experimental results. We utilize this phenomenon to encrypt an image in the polarization state of an infrared thermal radiation. The grating acts as a space-variant coupler between the propagating waves and the SPPs. Decryption of the image is obtained by measuring the polarization state of the thermal radiation and applying the correct key. Unlike other polarization-based encryption schemes, this is a passive one in that it does not require a light source. Finally, theoretical results of a further research are presented. We show that the behavior of the angular emission can be determined according to the grating’s parameters that are introduced on the surface. An enhanced emission in a narrow spectral peak and a well defined direction is theoretically presented.

This phenomenon can be exploited in a variety of applications such as thermal polarization imaging, optical encryption, spatially modulated heat transfer, and the formation of high-efficiency thermal sources.