The resonances of a normally set-up Fabry-Perot (F-P) optical cavity force the cavity to pass only narrow spectral linewidths separated in frequency by the free spectral range (FSR) of the cavity. Large F-P cavities can be made achromatic by inserting an optical material with strong negative dispersion into the cavity to make the optical length equal for all wavelengths, allowing the F-P benefit of resonant-cavity field buildup to happen over a larger bandwidth. F-P microcavities, however, have not previously been made achromatic.
Now, researchers at CREOL, The College of Optics & Photonics, at the University of Central Florida (Orlando, FL) have devised a way to achromatize a F-P microcavity. Remarkably, they do this without inserting any special optical material into the cavity. Instead, using only linear optical components, they correlate each wavelength with its own angle of incidence (θ as a function of λ) so that a continuous spectrum resonates in the cavity. In one example, a single-order 0.7-nm-wide resonance (spectral line) is, as the researchers say, "de-slanted in spectral-angular space," becoming a 60-nm-wide achromatic resonance spanning several FSRs. The resulting resonant buildup inside the cavity for a large spectral range can be used to enhance linear or nonlinear optical effects, with benefits for photovoltaic cells, white-light microlasers, and other photonic devices. Reference: S. Shabahang et al., arXiv:1612.04328v1 [physics.optics] (Dec. 13, 2016).