Abstract

Embedded sub-wavelength grating structures provide an alternative to present narrowband filter, polarization and laser-mode selector options. By filling in the grooves of a dielectric surface-relief grating with a high index material, the grating region can be made to support leaky modes. Coupling to these leaky modes can occur if an incident field has the proper wavelength, polarization, and angle; this results in resonance effects which are characterized by extreme narrowband, sharp peaks in reflectivity and transmissivity over wavelength and angle. In this paper, general design principles for sub-wavelength grating structures are discussed. We present results based on rigorous diffraction theory that describe the effects of period, duty cycle, thicknesses, refractive indices, and polarization on the location and linewidth of resonant excitation. We have developed designs with band-pass linewidths on the order of Δλ/λ ~ 10⁻⁷, where the peak center wavelength can be shifted by Δλ/λ ≈ 0.2 for angular detunings of Δθ 75°. In addition, with more exotic structures it may be possible to do electrical tuning as well. These results are consistent with principles based on waveguide-coupling concepts. Narrowband filters can be fabricated using current lithographic techniques and together with sputtering and/or chemical deposition.

© 1993 Optical Society of America