Researchers at the Air Force Research Laboratory (AFRL; Wright-Patterson Air Force Base, OH), Wyle (Dayton, OH), SelectTech Services Corporation (Centerville, OH), and Wesleyan University (Middletown, CT) have created the first optical limiter that operates primarily via reflectivity rather than absorption.1
Optical limiters are useful in some laser systems because they pass low-intensity light but then switch off and block all light if the intensity exceeds a certain level. Such a property can be useful either to prevent damage to components further along in the laser/optical system, or to achieve some other desired experimental effect.
However, blocking high optical intensities by absorption can destroy the optical limiter itself. Thus, the AFRL reflective device is a substantial advance. (Another advantage: the reflected high-intensity light could conceivably be used for other experimental purposes.)
The limiter contains alternating amorphous silicon dioxide (SiO2) and silicon nitride (Si3N4) layers, along with a single gallium arsenide (GaAs) layer in the middle (the "defect" layer). At low intensities, the device has high transmittance due to a resonance caused by the defect mode, but when the intensity passes a certain level, two-photon absorption in the defect layer wrecks the resonance, causing the whole device to become highly reflective.
Note that, although the defect layer becomes absorbing at higher intensities, it does not have to absorb a lot because it is part of a now-reflective thin-film Bragg structure.
While the experimental device operated over a band centered at about 1600 nm due to the optical properties of GaAs, the researchers say that choosing other materials can allow other wavelength ranges.
1. Jarrett H. Vella et al., "Experimental Realization of a Reflective Optical Limiter," arXiv:1510.08028v1 [physics.optics], submitted on 27 Oct. 2015; http://arxiv.org/pdf/1510.08028v1.pdf