Optical characteristics of deeply modulated gratings

Studies of grating properties have a long and distinguished history. Much of this work has been confined to relatively shallow gratings with sinusoidal profiles. We report an experimental study of both zero-order and first-order reflectivities of Agcoated photoresist gratings at 633 nm, with particular attention to surface plasma wave (SPW) mode couplings and grating polarization effects. Gratings studied had periods from 380 to 840 nm and depths to 1000 nm. For gratings with only one allowed SPW coupling, as the grating depth is increased beyond the optimal depth for coupling into the SPW, and SPW coupling gradually decreases and the resonance angle shifts and broadens. Simultaneously, the grating diffraction efficiency increases. For optimal gratings, <1% of the incident TM-polarized light is reflected by the grating. In contrast, the grating reflectivity is over 80% for TE-polarized light. Such high polarization ratios offer the potential of inexpensive compact nondispersive polarizing optics. For gratings with 2 orders coupling to the SPW mode, a complex behavior is observed. As the depth is increased, the resonance angles appear to initially attract and then repel each other. The first-order diffraction also shows a strong coupling to the SPW modes.