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Proceedings Paper

Sub-wavelength grating as phase retarder: design using modal method and modeling by finite element method
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Paper Abstract

Sub-wavelength dielectric gratings can be used to achieve phase retardation. Due to the vector nature of the devices, scalar theory is not applicable and rigorous calculation methods are required. The modal method proves to be a simple but powerful compromise, between rigorous techniques that are computationally expensive and the scalar theory that is inadequate, for design of such elements. As a proof of concept, a quarter wave plate (QWP) was designed and its behaviour compared against previously published data. Wave plate design requires that the orthogonal polarizations travel in the same direction with appropriate phase delay. It is assumed that light is incident normally on the grating. Floquet-Bloch periodicity ensures that discrete modes get excited within the grating. The number of propagating modes and the propagation constant of the modes can be controlled by the angle of incidence, the ratio of period to the incident wavelength and the fill factor. Modal method characterizes the underlying Eigen modes (/ effective indices) of the orthogonal field components in the sub-wavelength structure. Based on the indices obtained by modal method, height of the grating ridge is deduced. The design gives a high aspect ratio of about 8 for a quarter wave phase retarder. The design is also numerically evaluated by the finite element method. The solver COMSOL was used to visualize how polarization direction evolves with time. The designed QWP could convert linearly polarized light into circularly polarized light and vice versa. This result proves the validity of the design procedure.

Paper Details

Date Published: 2 May 2014
PDF: 7 pages
Proc. SPIE 9130, Micro-Optics 2014, 913010 (2 May 2014); doi: 10.1117/12.2051273
Show Author Affiliations
Gayathri M. Sridharan, Indian Institute of Technology Madras (India)
Shanti Bhattacharya, Indian Institute of Technology Madras (India)

Published in SPIE Proceedings Vol. 9130:
Micro-Optics 2014
Hugo Thienpont; Jürgen Mohr; Hans Zappe; Hirochika Nakajima, Editor(s)

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