Achieving simultaneous narrowband chiral absorption, near-unity circular dichroism (CD), and high-quality factor ( Q ) in the visible regime remains a major challenge for polarization-selective photonic devices, narrowband filtering, and chiral sensing. Here, we propose a visible-light metasurface absorber based on symmetry-protected quasi-bound states in the continuum (quasi-BICs) for ultranarrowband chiral absorption. The proposed metasurface enables near-perfect absorption for one circular polarization while strongly suppressing the opposite handedness. To elucidate the physical origin of the chiroptical response, we investigate two complementary symmetry-breaking mechanisms. In the intrinsically chiral configuration, in-plane displacement of dielectric nanodimers induces strong chiral absorption under normal incidence, yielding CD≈0.99 and Q factors as high as 4.2×10 4 . In the extrinsically chiral configuration, a geometrically symmetric dimer array acquires chirality under oblique illumination, where symmetry breaking arises from the incident wavevector and excites a chiral resonance. Furthermore, we show that reducing structural asymmetry simultaneously enhances the Q factor and improves the angular robustness of the chiral response. The resonance behavior is accurately captured by temporal coupled-mode theory (TCMT) and corroborated by strongly confined electromagnetic fields within the dielectric resonators at resonance. These results establish a practical platform for ultra-high- Q chiral photonic devices operating in the visible regime.
In this work, we investigate the impact of polarization space exploration strategies on the detection of mode-locking regimes in a rectangular pulsed passive mode-locked (PML) fiber laser. A comparative study between pseudo-random and Sobol quasi-random sampling is performed for EPC voltage selection. The Sobol sequence demonstrates superior space-filling characteristics, resulting in faster detection of mode-locking and a higher diversity of accessible regimes, including fundamental, harmonic, dual-pulse, and pulse-bunching states. Following the initial detection of mode-locking, a hill-climbing optimization method with a customized objective function is implemented to drive the system toward stable fundamental mode-locking (FML). The objective function is designed to suppress harmonic or multi-pulse regimes while simultaneously promoting pulse compression and peak amplitude enhancement. Experimental results show improvement of pulse characteristics across iterations, including reduction of pulse width and increase in peak amplitude with the increase in the objective function. The proposed method is further shown to maintain stable FML operation over a period of 65 min by adaptively adjusting the polarization for small environmental perturbations. The combined quasi-random exploration and hill climbing provides a simple, computationally efficient, and robust approach for automatic and stable FML operation in PML fiber lasers.
Through the comparison of a suite of energy transfer rate parameters obtained from a series of solution-doped Tm 3+ -aluminosilicate glass fibers with a Tm 3+ -aluminosilicate fiber made using nanoparticle doping, we show quantitative evidence of doping non-uniformity. In relation to Tm 3+ , we find that energy transfer upconversion is more sensitive to spatial non-uniformities in the rare earth ion doping distribution compared to cross relaxation, which may be explained by high localization of excited Tm 3+ ions, leading to enhancement of the relative strength of the relatively weaker upconversion processes compared to the strong primary cross-relaxation process. Measurements of the fluorescence decay of the 3 F 4 , 3 H 4 , and 1 G 4 levels were completed, in addition to measurement of the 470 nm fluorescence rise time under 790 nm pumping. An analysis of these results using our method of extracting the energy transfer rate parameters enabled quantification of the rates of the energy transfer and energy migration processes relevant to the operation of 790 nm pumped Tm 3+ -doped fiber lasers emitting in the two-micron band.
We report nonlinear dynamics jointly controlled by cavity detuning and intracavity Kerr nonlinearity in a synchronously pumped femtosecond MgO:PPLN optical parametric oscillator pumped at 1030 nm. Scanning the cavity length reveals nine well-separated oscillation peaks, all preserving the fundamental pump repetition rate inherited from the pump laser, measured to be 79.86 MHz at the representative peaks examined (peaks 0, ±2, and ±4), with continuous idler tuning available across 2.15–2.45 µm. The off-center states exhibit comb-like spectra and periodic pulse-train autocorrelations whose modulation periods follow an approximate Fourier-conjugate scaling, consistent with coherent multi-pulse formation. Displacing an intracavity ZnSe crystal away from the focus strongly reduces the effective Kerr phase while preserving the net group-delay dispersion. Under this reduced-Kerr condition and near the central synchronization peak, the OPO enters a dissipative-soliton-like chirped-pulse regime, producing a steep-edged 85-nm-wide spectrum and a strongly chirped 780 fs pulse compressible to 154 fs, corresponding to a reduction of the time–bandwidth product from ≈4.1 to ≈0.81. With the cavity baseplate actively temperature stabilized, the representative operating states examined (peaks 0, +2, and +4) are maintained for at least 8 h, with a root-mean-square output-power fluctuation of 0.75% at the central peak. Cavity detuning and Kerr phase thus play complementary roles in selecting the operating regime of a synchronously pumped OPO.
A model for the intensity-dependent optical properties of dye-doped transparent material is presented. This semi-classical model is built on the well-known rate equations for electronic population densities in a four-energy-level electronic system. This model is suitable for addressing the broadband nature of the dye molecules' transitions and the source. Also, it includes the saturation effect and the enhancement of absorption and emission decay rates, due to the presence of photonic states. The proposed material model is applied to investigate lasing phenomena in small core-shell particles and their hexagonal lattices, for which the optical response can be determined analytically. A frequency-domain numerical method is employed to solve the coupled Maxwell-Liouville equations, yielding self-consistent electronic populations, material properties, and both near- and far-field distributions. Numerical predictions for the lasing threshold are obtained and interpreted within a robust theoretical framework. For the lattices examined, the theory demonstrates that the complex polarizability of individual nanoparticles interacts with the effective polarizability of the lattice, thereby controlling or even eliminating the lasing threshold. Consequently, threshold-less amplification is achievable in non-propagating lattice modes and can be extended to light amplification in passive systems without intrinsic gain.
In this paper, a compact graphene-based electro-optic modulator is designed and simulated that uses photonic crystal cavities for achieving high-performance operation in photonic circuits. The proposed structure consists of a silicon bus waveguide coupled to two photonic crystal cavities, each incorporating a stack of graphene and Al 2 O 3 layers as a central defect. By tuning the graphene chemical potential through an applied voltage, the permittivity of the defect region is modulated, enabling dynamic control over the resonant wavelength. Finite-difference time-domain simulations demonstrate that the interaction between the bus waveguide and the dual cavities significantly enhances destructive interference at resonance, leading to a modulation depth of 19.6 dB with an insertion loss of only 0.8 dB. The active footprint of the device is 12.2µm 2 , ensuring suitability for large-scale photonic integration. Additionally, a configuration employing asymmetric photonic crystal–waveguide separations is proposed, enabling dual-wavelength modulation, which offers the potential for multiplexed signal processing. Compared to previously reported modulators, the proposed design achieves superior performance in terms of modulation depth, low insertion loss, and device compactness. These results highlight the potential of graphene–photonic crystal hybrid structures as a promising platform for next-generation on-chip optical communication and computing systems.
