Durable silicon–silicon nitride films have been created that exhibit low angle shift and reduced s and p polarization
separation. The more common silicon dioxide – metal oxide films often have performance problems at large angles.
They are inherently sensitive to angle of incidence and thus are prone to alignment issues and cone angle effects of the
incident light. In contrast, silicon–silicon nitride films have much higher average optical indices and thus are ideal for
applications were spectral form and placement are critical at large angles of incidence and when cone angle
considerations are important. In addition, the difference in the spectral performance between the s and p polarizations is
greatly reduced with silicon–silicon nitride films, offering more alignment flexibility when polarized sources are
required. The silicon–silicon nitride films were found to be environmentally durable, and can be applied to a variety of
substrates and substrate geometries, including plano, spherical domes, and complex parabolic surfaces.
Infrared optics are often expected to perform over a wide range of angles of incidence, over a selected bandpass. For the best performance, it is often desired that the shift of spectral features, and the splitting between s and p polarizations, be minimized. These issues can be mitigated to a large extent by design, particularly over a narrow range of angles. However, the availability of high index materials throughout a stack design can greatly improve the performance at a given coating thickness, or greatly reduce the overall thickness required to achieve a design. Here we discuss several designs that have been achieved via hydrogenated silicon in multilayers, which demonstrate improved performance at oblique angles of incidence.
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