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In this paper, a dual-resonances mid-infrared all-dielectric metasurface sensor based on asymmetric cross dimer, which is driven by quasi-bound states in the continuum (QBIC), is proposed and investigated. The metasurface sensor maintains the total permittivity constant when the asymmetric parameter is adjusted, thereby ensuring the stability of the QBIC resonance wavelengths, which exhibit Q-factors of 6351 and 13561, respectively. The multiple decompositions and electromagnetic field distributions reveal that the toroidal dipole is the dominant component of the dual-resonance modes. The sensitivities to the refractive index are 3559 nm/RIU and 1146 nm/RIU, with corresponding figures of merit of 4449 RIU -1 and 2453 RIU -1 , respectively. Further numerical simulations have demonstrated a strong coupling phenomenon between the QBIC and the molecular vibrations of polymethyl methacrylate (PMMA), resulting in a significant enhancement of the infrared absorption signal. With the 50-nm-thick PMMA layer, the enhancement of molecular signal is 90.614%.

Exceptional points (EPs) display peculiar degeneracies, where complex eigenvalues and associated eigenvectors coalesce simultaneously, resulting in a defective Hamiltonian. Meanwhile, the negative imaginary part of the energy eigenvalues related to a finite spectral linewidth at the resonant energy, which could provide a solution to tackle the isolation bandwidth limitation of MRR-based optical isolators without sacrificing the insertion loss. Here, a second-order EP 2 system constructed by SiN-based cascaded racetrack resonators is proposed, while the metal strip operating as an integrated electromagnet provides magnetic fields required for non-reciprocal phase shifting (NRPS). Owing to the existence of the NRPS perturbation, the system is pushed away from EP and consequently triggers complex frequency splitting, resulting in the isolation bandwidth proportional to the square-root perturbation instead. The results show that the isolation bandwidth of the EP isolator is increased by 163% and 22% compared to single-racetrack and cascaded-racetrack isolators with 2.85 dB insertion loss and 34.3 dB isolation ratio, respectively. The presented EP-based optical isolator shows tremendous potential for high-density monolithic integration and packaging.

A polarization beam-splitting multimode filter using pixelated waveguides has been presented and experimentally demonstrated in this paper. Finite difference time domain method and direct binary search optimization algorithm are employed to optimize pixelated waveguides to realize compact size, broad bandwidth, large extinction ratio, low insertion loss, and good polarization extinction ratio. Measurement results show that, in a wavelength range from 1520 to 1560 nm, for the fabricated device working at transverse-electric polarization, the measured insertion loss is less than 1.23 dB and extinction ratio is larger than 15.14 dB, while for transverse-magnetic polarization, the corresponding insertion loss lower than 0.74 dB and extinction ratio greater than 15.50 dB are realized. The measured polarization extinction ratio larger than 15.02 dB is achieved. The device's length is only 15.4 µm.