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

Gayathri M. Sridharan; Shanti Bhattacharya

doi:10.1117/12.2051273

2 May 2014 Sub-wavelength grating as phase retarder: design using modal method and modeling by finite element method

Gayathri M. Sridharan, Shanti Bhattacharya

Proceedings Volume 9130, Micro-Optics 2014; 913010 (2014) https://doi.org/10.1117/12.2051273
Event: SPIE Photonics Europe, 2014, Brussels, Belgium

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.

Citation Download Citation

Gayathri M. Sridharan and Shanti Bhattacharya "Sub-wavelength grating as phase retarder: design using modal method and modeling by finite element method", Proc. SPIE 9130, Micro-Optics 2014, 913010 (2 May 2014); https://doi.org/10.1117/12.2051273

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