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.

Unidirectional guided resonances (UGRs) in periodic metasurfaces have recently attracted research interest because of their ability to achieve unidirectional radiation in all-dielectric structures without metal reflectors, which offers new possibilities for efficient grating couplers and unidirectional lasers. Here, we propose a hybrid metasurface consisting of silicon and ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST) phase change material for controlled UGR generation in the mid-infrared region. Leveraging GST’s phase-change properties to modulate the optical response of the metasurface, we achieve tunable generation of the UGR, which is demonstrated to carry a topological charge of +1. Moreover, by adjusting the degree of GST phase transition, continuous tuning of the radiation asymmetry ratio from ${10}^4$ to 1 is achieved for a specific in-plane momentum and operating wavelength. These findings offer a promising avenue for dynamically controllable UGRs, with potential applications in tunable on-chip optical couplers and light sources.

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.

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.

Unidirectional guided resonances (UGRs) provide a compact route to highly directional radiation without metallic reflectors, but their realization usually relies on precise structural tuning and is often restricted to isolated points in momentum or parameter space. Here, we propose a Ge₂Sb₂Se₄Te₁ (GSST)-based phase-change metasurface that enables nonvolatile spectral tuning of UGR-like resonances by controlling the momentum-space evolution of paired circular-polarization singularities. Instead of relying on an exact C-point merging condition at a single phase state, the proposed structure controls the approach and departure of a symmetry-related C-point pair relative to the $k_y=0$ line, with the closest sampled approach occurring near an intermediate crystallization state. This near-merging evolution selectively suppresses air-side radiation, giving rise to continuously tunable UGR-like resonances with persistent high directionality. At the high-directionality operating point selected for each of the crystallization states considered, the eigenwavelength shifts by $202.2\mathrm{nm}$, exceeding three times the largest eigenmode linewidth. Meanwhile, the downward-to-upward radiation asymmetry ratio $\eta$ consistently exceeds ${10}^2$ over the investigated phase states and reaches approximately $2.4\times {10}^4$ near the optimal crystallization state. A self-consistent fixed-angle analysis further shows that a fixed substrate-side direction of ${16.52}^{\circ }$, equivalent to ${22.93}^{\circ }$ in air, retains $119.6\le \eta \le 191.3$ throughout all 11 states considered while preserving a $186.6\mathrm{nm}$ tuning range. Despite increased crystalline-state absorption, the intrinsic substrate-side modal branching fraction remains at least $75.7\%$. These results establish a nonvolatile strategy for combining spectrally resolvable tuning, persistent directional radiation, and substantial dominant-side modal out-coupling in a single phase-change metasurface.