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.
We investigate the effect of fabrication tolerances on photonic multimode waveguides operating in the vicinity of a third-order exceptional point degeneracy (EPD), known as a stationary inflection point (SIP). An EPD is a point in the parameter space where two or more Bloch eigenmodes coalesce in an infinite periodic waveguide, and at an SIP three modes coalesce to form the frozen mode. Waveguides operating near an SIP exhibit slow-light behavior in finite-length waveguides with anomalous cubic scaling of the group delay with waveguide length. The frozen mode facilitates stronger light-matter interactions in active media, resulting in a significant increase in the effective gain within the cavity. However, systems operating near an EPD are also exceptionally sensitive to fabrication deviations. In this work, we explore wave propagation and the impact of various fabrication imperfections in analytic models and in fabricated photonic chips for three mirrorless devices operating near an SIP. To advance the concept of the SIP laser, we also analyze how the addition of gain and loss affects the SIP performance. Our results show that while minor deviations from the ideal parameters can prevent perfect mode coalescence at the EPD, the frozen mode remains resilient to small perturbations and a significant degree of mode degeneracy prevails. These findings provide critical insights into the design and fabrication of passive and active photonic devices operating near high-order EPDs, paving the way for their practical implementation in a wide range of applications.
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.
We demonstrate that a family of metamaterials, with a designed complex permittivity and permeability such that their index of refraction is real, have anomalous scattering features as opposed to their lossless passive counterparts with the same index of refraction.
We introduce a class of unidirectional lasing modes associated with the frozen mode regime of non-reciprocal slow-wave
structures.1 Such asymmetric modes can only exist in cavities with broken time-reversal and space inversion
symmetries. The lasing frequency coincides with a spectral stationary inflection point of the underlying passive
structure and it is virtually independent of the size of the cavity. These unidirectional lasers can be indispensable
components of photonic integrated circuitry.
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