We have developed a new method for optical limiting using a system of coupled optical cavities with a PTsymmetric 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.
Optical limiters transmit low intensity input light while blocking input light with the intensity exceeding certain limiting threshold. Conventional passive limiters utilize nonlinear optical materials, which are transparent at low light intensity and turn absorptive at high intensity. Strong nonlinear absorption, though, can result in over- heating and destruction of the limiter. Another problem is that the limiting threshold provided by the available optical material with nonlinear absorption is too high for many applications. To address the above problems, the nonlinear material can be incorporated in a photonic structure with engineered dispersion. At low intensity, the photonic structure can display resonant transmission via localized mode(s), while at high intensity the resonant transmission can disappear, and the entire stack can become highly re ective (not absorptive) within a broad frequency range. In the proposed design, the transition from the resonant transmission at low intensity to nearly total re ectivity at high intensity does not rely on nonlinear absorption; instead, it requires only a modest change in the refractive index of the nonlinear material. The latter implies a dramatic increase in the dynamic range of the limiter. The main idea is to eliminate the high-intensity resonant transmission by decoupling the localized (resonant) modes from the input light, rather than suppressing those modes using nonlinear absorption. Similar approach can be used for light modulation and switching.
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