RESUMEN
Optical cavities transmit light only at discrete resonant frequencies, which are well-separated in micro-structures. Despite attempts at the construction of planar 'white-light cavities', the benefits accrued upon optically interacting with a cavity - such as resonant field buildup - have remained confined to narrow linewidths. Here, we demonstrate achromatic optical transmission through a planar Fabry-Pérot micro-cavity via angularly multiplexed phase-matching that exploits a bio-inspired grating configuration. By correlating each wavelength with an appropriate angle of incidence, a continuous spectrum resonates and the micro-cavity is rendered transparent. The locus of a single-order 0.7-nm-wide resonance is de-slanted in spectral-angular space to become a 60-nm-wide achromatic resonance spanning multiple cavity free-spectral-ranges. The result is an 'omni-resonant' planar micro-cavity in which light resonates continuously over a broad spectral span. This approach severs the link between the resonance bandwidth and the cavity-photon lifetime, thereby promising resonant enhancement of linear and nonlinear optical effects over broad bandwidths in ultrathin devices.
RESUMEN
Although optical absorption is an intrinsic materials property, it can be manipulated through structural modification. Coherent perfect absorption increases absorption to 100% interferometrically but is typically realized only over narrow bandwidths using two laser beams with fixed phase relationship. We show that engineering a thin film's photonic environment severs the link between the effective absorption of the film and its intrinsic absorption while eliminating, in principle, bandwidth restrictions. Employing thin aperiodic dielectric mirrors, we demonstrate coherent perfect absorption in a 2 µm thick film of polycrystalline silicon using a single incoherent beam of light at all the resonances across a spectrally flat, octave-spanning near-infrared spectrum, ≈800-1600 nm. Critically, these mirrors have wavelength-dependent reflectivity devised to counterbalance the decline in silicon's intrinsic absorption at long wavelengths.
RESUMEN
Coherent perfect absorption (CPA) is the phenomenon where a linear system with low intrinsic loss strongly absorbs two incident beams but only weakly absorbs either beam when incident separately. We present an analytical model that captures the relevant physics of CPA in one-dimensional photonic structures. This model elucidates an absorption-mediated interference effect that underlies CPA-an effect that is normally forbidden in Hermitian systems but is allowed when conservation of energy is violated due to the inclusion of loss. By studying a planar cavity model, we identify the optimal mirror reflectivity to guarantee CPA in the cavity at resonances extending in principle over any desired bandwidth. As a concrete example, we design a resonator that produces CPA in a 1-µm-thick layer of silicon over a 200-nm bandwidth in the near-infrared.