The application of parity-time (PT) symmetry in optics, especially
PT-symmetry breaking, has attracted considerable attention as a novel approach
to controlling light propagation. Here, we report optical limiting by two
coupled optical cavities with a PT-symmetric spectrum of reflectionless modes.
The optical limiting is related to broken PT symmetry due to light-induced
changes in one of the cavities. Our experimental implementation is a
three-mirror resonator of alternating layers of ZnS and cryolite with a
PT-symmetric spectral degeneracy of two reflectionless modes. The optical
limiting is demonstrated by measurements of single 532-nm 6-ns laser pulses. At
fluences below 10 mJ/cm2, the multilayer exhibits a flat-top passband at 532
nm. At higher fluences, laser heating combined with the thermo-optic effect in
ZnS leads to cavity detuning and PT-symmetry breaking of the reflectionless
modes. As a result, the entire multilayer structure quickly becomes highly
reflective, protecting itself from laser-induced damage. The cavity detuning
mechanism can differ at much higher limiting thresholds and include
nonlinearity.Comment: 17 pages, 5 figure