ULTRASENSITIVE SWITCHING BETWEEN RESONANT REFLECTION AND ABSORPTION IN PERIODIC GRATINGS

Guided-Mode Resonance (GMR) efiects in transparent periodic gratings possess a number of remarkable phenomena. GMRs exhibit strong features in the optical spectrum, i.e., dips, peaks, cusps, and may attain extremely high Q-factors. In some cases, resonant re∞ection with the e-ciency equal to unity can be observed. We demonstrate that the introduction of small losses in the structure can drastically modify its optical response by causing strong absorption resonances. Unity re∞ection in loss-free structures can be almost completely converted into unity absorption peaks as soon as very small losses are introduced. Even thin absorbing fllms in the structure (or in its vicinity) can lead to such strong resonant absorption efiects. The resonances may exhibit a negligible spectral shift, but a signiflcant variation in the magnitude when losses are slightly altered, which is highly attractive for sensor and switch applications. Absorption peaks experience a resonant behavior with respect to both frequency and material losses. We show that the width of the absorption peaks decreases and approaches the width of the re∞ection peaks, as losses decrease. Thus, high-Q resonances can be observed. The absorption resonances also possess strong angular dependence; they may split and signiflcantly increase in magnitude for a slightly inclined incidence. We elucidate the resonant re∞ection/absorption efiects theoretically and provide numerical examples.