Optical limiters based on PT-symmetry breaking of reflectionless modes

We have developed a new method for optical limiting using a system of coupled optical cavities with a PT-symmetric spectrum of reflectionless modes. The optical limiting occurs when the PT symmetry is broken due to the thermo-optic effect in one of the cavities. In our experiment, we used a two-cavity resonator with PT-symmetric spectral degeneracy of reflectionless modes created from alternating layers of cryolite and ZnS. We demonstrated optical limiting by measuring a single 532-nm 6-ns laser pulse. Our experimental results are supported by thermo-optical simulations, which provide deeper insight into the dynamics of the limiting process. Compared to existing limiter designs, our optical limiter offers a customizable limiting threshold, high damage threshold, nanosecond activation time, and broadband laser protection. Additionally, we have shown a method to achieve an even broader transmission spectral bandwidth by implementing this concept in a four-cavity resonator with greater coupling strength using similar materials.