From weak to strong coupling: quasi-BIC metasurfaces for mid-infrared light-matter interactions.

Thanks to their giant, yet tunable, Q-factor resonances, all-dielectric metasurfaces supporting the quasi-bound states in the continuum (q-BIC) resonances are well-suited to provide a promising platform for quantum-coherent light-matter interactions. Yet, the strong coupling regime, characterized by the hybrid light-matter states - polaritons, has not yet been fully explored in the mid-infrared regime. This paper investigates the parameter space of vibrational strong coupling (VSC) between material and metasurface cavities supporting q-BIC resonances in the mid-infrared spectral range. We outline the effects of transition dipole strength, damping rate, and the number of molecules coupled to a single cavity, as well as the cavity damping rates, to understand their respective impacts on VSC. By tuning the Q-factor of the metasurface and material parameters, a new transition light-matter coupling zone is introduced, bridging the gap between weak and strong coupling, where polaritons form but their linewidths prohibit their spectral identification. The study further identifies the effects of cavity linewidth on polariton peak separability in strongly coupled systems, highlighting that the cavities with smaller nonradiative losses and narrower linewidths facilitate better polariton separability. Moreover, we found that matching cavity and material loss, satisfying the critical strong coupling condition, enhances the coupling strength between cavity and material. Overall, these findings can guide the design of photonic cavities suited for VSC experiments, contributing to the burgeoning fields of polaritonic chemistry, light-mediated modulation of chemical reactivity, and highly sensitive molecular spectroscopy.

Highly nonlinear bored core hexagonal photonic crystal fiber (BC-HPCF) with ultra-high negative dispersion for fiber optic transmission system.

In this paper, we propose a bored core hexagonal photonic crystal fiber (BC-HPCF) which obtains ultra-high negative dispersion and large nonlinearity simultaneously. The aim of the proposed design is to achieve the desired optical properties by using circular air holes only to make the fiber simple and manufacturable. To investigate the light guiding properties of the proposed BC-HPCF, finite element method (FEM) with circular perfectly matched boundary layer (PML) is used. According to numerical simulation, it is possible to obtain a large value of negative dispersion of -2102 ps·nm-1·km-1 and large value of nonlinearity of 111.6 W-1 · km-1 at optimum wavelength of 1550 nm. In addition, ±2% deviation in optical characteristics is evaluated and reported in order to study the practical feasibility of the proposed BC-HPCF. The large negative dispersion and high nonlinearity of our proposed design make it a strong candidate for optical broadband communication, super continuum generation, and sensing.

Impact of Kerr nonlinearity on the whispering gallery modes of a microsphere.

To describe the temporal evolution of the mode amplitude of a spherical microcavity, a nonlinear equation is developed by considering loss and Kerr nonlinear effect as perturbations. In order to study the impact of Kerr nonlinearity, the tensor components of χ((3)) in spherical coordinates are calculated. To describe the impact of Kerr nonlinearity, effective mode volume and effective nonlinear coefficient are defined. We found that the resonant modes undergo a negative frequency shift proportional to the injected energy, consistant with the reported experimental observations.