Dielectric resonant grating structures for narrow-band filtering applications

Dielectric resonant grating structures are investigated for use in narrow-band filtering applications. An interference-waveguide approach is developed to investigate the response of infinite and finite length structures operating at oblique and normal incidence under plane wave and finite beam illumination. Rigorous analysis and experimental results are used to verify the approach. The interference-waveguide approach clearly identifies the mechanisms responsible for the resonances as well as the relationships and tradeoffs between the physical parameters of the structure and its performance as a filter. The performance of a resonant grating reflection filter operating at oblique incidence is shown to be limited by its spectral linewidth. The narrower the spectral linewidth the longer the filter must be to achieve near optimal reflection efficiency. Furthermore, at oblique incidence the angular selectivity is directly related to the spectral linewidth resulting in inefficient filtering of finite beams especially in the IR. At normal incidence energy can be laterally confined within the structure about the point of incidence. Because of this, structure length is less of a concern. In addition, associated with the confinement is a broadening of the angular selectivity, which permits efficient filtering of finite beams even in the IR. The insight gained from this analysis provides clues on how the parameters of the structure can be manipulated to obtain desirable characteristics; we demonstrate a narrow-band spectral filter with an angular selectivity hundreds of times broader than what could be achieved at oblique incidence, a narrow-band flat-top spectral filter with a broadened angular selectivity exhibiting a pass band ratio that is greater than five times that of a single resonant grating reflection filter, and a dual grating geometry capable of varying the spectral linewidth from nanometers to theoretically zero.