Space-variant polarization manipulation of a thermal emission by a polar material subwavelength grating supporting surface phonon- polaritons Erez Hasman ∗ , Nir Dahan, Avi Niv, Gabriel Biener, Vladimir Kleiner Optical Engineering Laboratory, Faculty of Mechanical Engineering Technion-Israel Institute of Technology, Haifa 32000, Israel ABSTRACT Space-variant polarization manipulation of enhanced omnidirectional thermal emission in a narrow spectral peak is presented. The emission is attributed to surface phonon-polariton excitation from space-variant subwavelength SiO 2 gratings. Polarization manipulation was obtained by discretely controlling the local orientation of the grating. We experimentally demonstrated thermal emission in an axially symmetric polarization distribution. We show that by coupling surface phonon-polaritons to a propagating field, large anisotropy of the emissivity is obtained within a narrow spectral range. We experimentally demonstrate this effect by fabricating a space-variant subwavelength grating on a SiO 2 substrate to encrypt an image in the polarization state of a thermal radiation field. Theoretical calculations based on rigorous coupled-wave analysis are presented along with experimental results. Keywords: Surface waves, phonon, polarization, space-variant, grating, encryption 1. INTRODUCTION Surface phonon-polaritons (SPPs) are attributed to the coupling of an electromagnetic field with phonons at the interface of two homogeneous media. This resonant phenomenon typically occurs in the infrared region of the spectrum and involves only TM radiation. The wavenumber of SPPs is larger than that of a free-space propagating wave at the same frequency, and thus SPPs are nonradiative. Investigation of SPPs in the far field is performed by coupling the SPPs' evanescent field to a propagating field, using a prism or a grating coupler. 1,2 Recently, it was shown that by etching a uniform grating on a SiC substrate, a highly directional peak of thermal emission was achieved. Furthermore, spectral resonance and nondirectional emission were observed at certain frequencies. 2,3 We introduce a theoretical and experimental investigation of space-variant polarization- dependent thermal emission by exploiting the polarization dependence of the SPPs. Computer-generated subwavelength gratings etched on fused silica (SiO 2 ) substrates are used to generate space-variant polarization radiation. 4 As a first step, we designed a grating to enhance the nondirectional thermal emission to form a narrow spectral peak for TM polarization. We were then able to experimentally demonstrate space-variant polarization manipulation of thermal emission by discretely controlling the local orientation of the grating. We utilize it to encrypt an image in the polarization state of an infrared thermal radiation. Polarization state manipulation of the thermal emission was achieved by controlling the local orientation of the space- variant subwavelength grating. 5 The grating act as a space-variant coupler between the propagating waves and the SPPs. Decryption of the image was obtained by measuring the polarization state of the thermal radiation and applying the correct key. Polarization encryption provides additional flexibility to conventional phase- and amplitude-based encryption methods. This feature is advantageous as it makes the optical encryption more secure. 6-8 Furthermore, unlike other polarization-based encryption schemes, ours is a passive one in that it does not require a light source. 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. 9 ∗ Erez Hasman e-mail address: mehasman@tx.technion.ac.il; phone: 972-4-8292916; fax: 972-4-8295711 home page: www.technion.ac.il/optics Nanomanipulation with Light II, edited by David L. Andrews, Proc. of SPIE Vol. 6131, 61310L, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.642884 Proc. of SPIE Vol. 6131 61310L-1