Zhi-Yuan Zheng

In metal-backed phase-change metasurface absorbers, the opaque reflector suppresses transmission and converts near-unity absorption into a single-port reflection-suppression problem. Near a dominant resonance, this suppression is achieved when radiative leakage is matched to intrinsic loss; phase transition can alter this balance through changes in material dissipation and modal field distribution. Here, we investigate a Si/Ge ₂ Sb ₂ Te ₅ (GST)/Si metasurface on an Au substrate within a mode-resolved critical-coupling framework. At the Γ point, mode-resolved loss analysis identifies three spectrally separated resonant branches whose radiative/non-radiative loss balance evolves differently under GST crystallization. One branch is close to critical coupling in the amorphous state, whereas two initially radiation-dominated branches approach critical coupling at higher crystallization fractions. Together, these spectrally separated branches make high absorption accessible over discontinuous intervals covering approximately 70% of the crystallization-fraction range, while providing linewidth-resolvable, phase-state-selective spectral absorption channels. Near-Γ dispersion analysis further shows that finite- k _x modal-state selection modifies the modal loss balance in a branch- and polarization-dependent manner, giving rise to angularly robust high absorption, polarization-dependent shifts of high-absorption conditions, and extension or reshaping of high-absorption intervals. These results establish a mode-resolved loss-balance picture for designing phase-change metasurface absorbers with broad crystallization-state coverage and for explaining how finite- k _x modal dispersion enables angle-accessible high-absorption states.

Plasmonic devices have been intensively studied in the past few decades. However, the substantial intrinsic ohmic losses imposed by noble metals highly limited further applications of such devices. Herein, we demonstrate the formation of long-lived states via external mode coupling in coupled hybrid surface plasmonic nano-resonators in which improved Q factors have been achieved. By systematically engineering the spatial intervals between three silicon nanorods on a silver substrate, we demonstrate the formation of long-lived states through destructive interference of optical fields. Numerical simulations reveal that, at a critical spacing, the Q factor of a resonant mode is significantly boosted to approximately 450—an eightfold enhancement compared to isolated single-rod resonators. Notably, this drastic improvement in photon lifetime is achieved while maintaining an ultrasmall effective mode volume Veff of the order of 10−4 μm3. These findings provide an alternative strategy to material optimization for mitigating metallic losses, offering a robust framework for developing high-efficiency, low-threshold plasmonic nanolasers, and advanced on-chip photonic components.

High-Q resonant metasurfaces are attractive for narrowband spectral control, and integrating phase-change materials enables their resonant features to be tuned after fabrication. Beyond this material-state modulation, the external excitation condition can provide an additional degree of freedom for controlling the observable resonant response. Here, we propose a GSST-integrated one-dimensional compound grating metasurface for programmable guided-mode-resonance (GMR) tuning in the mid-infrared region. The structure consists of two Si/Ge2Sb2Se4Te1 (GSST)/Si multilayer ridges within one supercell of period A. A lateral displacement between the ridges breaks the half-period translational periodicity of the equally spaced reference grating, increasing the primitive period from A/2 to A. This period-doubling perturbation opens radiative coupling to an otherwise dark folded guided branch and forms an observable displacement-activated folded GMR with a deep transmission notch. Full-wave simulations show that the observable amorphous-to-crystalline tuning span increases from 7.13 nm at normal incidence to 26.20 nm at 10° incidence, while strong transmission suppression is maintained. Eigenmode analysis shows that accessing finite-kx operating points shifts the operating window toward shorter wavelengths along a strongly dispersive folded GMR branch and enlarges the separation between GSST states, rather than increasing the modal field concentration inside the GSST layer. Loss-channel analysis further explains the gradual lifting of the transmission minimum at larger incident angles. These results provide a practical strategy for dual-degree-of-freedom programmable spectral control in phase-change GMR metasurfaces.

High- Q metasurfaces enable narrowband spectral control, yet active tuning with phase-change materials is often accompanied by linewidth broadening and reduced spectral contrast. Here we numerically show that a localized Ge 2 Sb 2 Te 5 (GST) patch on an otherwise symmetric silicon metasurface can serve simultaneously as a perturbative radiative coupler and an active tuning element. The patch selectively opens radiative access to an intrinsic dark high- Q mode family of the pristine structure while largely preserving the off-resonant background, yielding five narrow polarization-selective resonances in the 4 μ m to 5 μ m band. As GST evolves from the amorphous to the crystalline state, all resonances redshift, but the accompanying linewidth broadening and spectral-contrast degradation are strongly mode dependent. To quantify this behavior, we combine eigenmode analysis, a GST-region weighted overlap factor η GST , and same-polarization two-port temporal coupled-mode theory. Modes with larger η GST generally exhibit larger spectral shifts but are also more susceptible to absorption-induced contrast degradation. As GST crystallizes, the increased local asymmetric perturbation is expected to promote radiative access to the originally dark high- Q modes, while the concurrent rise in material absorption increases non-radiative loss and reduces the available interference amplitude. Across the resonance family, the maximum shift-to-linewidth ratio R SL reaches 9.08, while a semi-crystalline regime can provide a favorable compromise between tunability and spectral contrast. These results provide a symmetry- and loss-based framework for designing actively tunable high- Q metasurfaces using localized phase-change perturbations to selectively access and engineer intrinsic dark high- Q mode families.

In the past few decades, bound states in the continuum (BICs) have been intensively studied due to their superiority in suppressing radiation losses, highlighting potential applications in lasers, imaging, and sensors. Up to now, BICs have been theoretically and experimentally verified in microdisks, waveguides, and metasurfaces, which offer new perspectives to explore novel physics in such configurations. Among these preliminary research studies, microdisks based on Friedrich–Wintgen BICs (FW-BICs) show unprecedented properties, which are not accessible in the conventional system, allowing the existence of bound states in configurations possessing structural symmetry and mode interference or structures with a lower refractive index than that of the surrounding. However, compared to thoroughly explored cavity physics for conventional microdisks, only the merits of high quality and etchless fabrication procedures in BIC microdisks are confirmed, and further explorations are still absent, significantly limiting the application potential of these structures. In this work, we investigate a new kind of singularity by merging two FW-BICs into one. The resultant state not only exhibits enhanced sensitivity to perturbations, but also shows totally suppressed radiations. We believe that our findings illustrate an effective way to design devices with novel functions and expand the application potential of FW-BIC microdisks.

Al doped α-GaOOH nanorod arrays were grown on FTO via hydrothermal processes by using gallium nitrate and aluminum nitrate mixed aqueous solutions with fixed 1:1 mole ratio as precursors. With increasing the gallium nitrate precursor concentrations, the Ga/Al atom ratios in nanorod arrays increase from 0.36 to 2.08, and the length becomes much longer from 650 nm to 1.04 μm. According to the binding energy difference between Ga 2p_3/2 core level and its background in x-ray photoelectron spectroscopy, the bandgap is estimated to be around 5.3 ± 0.2 eV. Al doped α-GaOOH nanorod array/FTO photoelectrochemical photodetectors show enhanced self-powered solar-blind UV photodetection properties, with the decrease of Ga precursor concentrations. The maximum responsivity at 255 nm is 0.09 mA W⁻¹, and the fastest response time can reach 0.05 s. The improved photoresponse speed is ascribed from much shorter transportation route, accelerated carrier recombination by recombination centers, and smaller charge transfer resistance at the α-GaOOH/electrolyte interface with decreasing the gallium nitrate precursor concentrations. The stability and responsivity should be further improved. Nevertheless, this work firstly demonstrates the realization of self-powered solar-blind UV photodetection for α-GaOOH nanorod arrays on FTO via Al doping.