Wide-Field-of-View Lyot Filter: FY21 Line-Supported Optical Systems Program

Lyot filters employ differential phase retardation for orthogonal polarizations of light induced by stacks of birefringent waveplates. Lyot filters are widely used for filtering the emission spectra of tunable lasers and for astronomical imaging, particularly for solar observation. Achieving filters with simultaneously high spectral resolution, free spectral range, and field of view, however, remains an outstanding challenge. In this paper, a novel concept for a Lyot filter is described that employs a combination of two materials with opposing signs for both birefringence magnitude and dispersion. An analytical model of this design indicates that a large field of view can be achieved while maintaining high spectral resolution and free spectral range for material combinations that yield a zero-crossing of the birefringence magnitude but a large birefringence dispersion. Candidate material combinations for the novel filter design are surveyed, first with experimental data for common birefringent materials, and subsequently with automated database mining algorithms and first-principles materials theory. The top candidate material pairs are then screened for projected impact on filter performance using the analytical filter design model. Broad screening of candidate materials indicates that the material pair of HgI and GaSe yields a birefringence dispersion of 7 x 10-4 nm-1, sufficient to enable a spectral resolution of 0.05 nm for a practically achievable maximum plate thickness of 1 cm. Approaches for synthesis of single-crystal specimens of top candidate materials are then surveyed, and an apparatus for flux crystallization of candidate materials is described.