Ghost surface polaritons (GSPs) in anisotropic materials exhibit an unusual bi-state nature with a complex-valued out-of-plane wavevector, providing unique opportunities for nanoscale light manipulation. However, the exploration and application of GSPs have been limited by the scarcity of natural crystals. Here, we revisit GSPs from the perspective of transformation optics. By introducing a shift parameter, we establish a mapping between material tensors and ghost modes, demonstrating the existence of GSPs at various transformed uniaxial interfaces. It is further shown that the wavefront can be modulated via shift parameters, enabling precise control of GSP propagation. Our study provides new insight into GSPs and establishes a versatile framework for their flexible manipulation.
Electromagnetic wave propagation and scattering in materials with time-variant parameters are subjects of an active field of research. The theory is usually restricted to transverse-electric or transverse-magnetic modes in one- or two-dimensional settings. Here we discuss the theory of three-dimensional, multi-component electromagnetic waves in a homogeneous, time-variant material. We present 3 & times; 3 causal and acausal Green's matrices, and a 6 & times; 6 propagator matrix. We derive several fundamental properties such as the conservation of momentum density for a piecewise continuous, time-variant material, symmetry properties of the propagator matrix, and a relation between the causal and acausal Green's matrices. These properties are used in derivation of a general wave-field representation (the counterpart of the Kirchhoff integral for a time-invariant material) and an expression for Green's function retrieval with space correlations (the counterpart of Green's function retrieval with time correlations in a time-invariant material).
Silicon nitride (Si3N4) has emerged as a compelling platform for visible-light, quantum-photonic, and nonlinear-optical applications because of its broad transparency window, low propagation loss, and negligible two-photon absorption. However, the moderate refractive index of Si3N4 poses a challenge for efficient fiber-to-chip coupling using grating couplers. Although advantageous for wafer-scale testing and flexible layouts, Si3N4 grating couplers suffer from low grating strength, which limits their coupling efficiency. Here, we experimentally demonstrate high-efficiency grating couplers on a hybrid amorphous-silicon/silicon-nitride (a-Si/Si3N4) platform. Uniform devices are presented for TE and TM polarizations in both the O-and C-bands, while apodized-focalizing designs are demonstrated for both polarizations in the O-band and for TE polarization in the C-band. The fabricated devices achieved record coupling efficiencies of-1.6 dB and-1.2 dB in the O-band for TE and TM polarizations, respectively, and-1.3 dB and-2.9 dB in the C-band for TE and TM polarizations, respectively. The measured peak wavelengths deviated from their nominal values, primarily because of variations in the refractive index of the fabricated Si3N4 and the grating duty cycles relative to their design values. These shifts are well reproduced by simulations incorporating fabrication biases and can be compensated for by adjusting grating parameters or the fiber coupling angle. To the best of our knowledge, these are the highest coupling efficiencies demonstrated to date for silicon nitride grating couplers in the O-band for both polarizations and in the C-band for TM polarization. For TE polarization in the C-band, fabrication deviations shifted the peak coupling wavelength to 1512 nm, resulting in the highest efficiency reported to date for a silicon nitride grating coupler operating in the S-band. These results demonstrate a practical approach for achieving low-loss fiber-to-chip coupling in Si3N4 photonic integrated circuits, addressing a longstanding challenge in the platform and supporting the development of scalable systems for datacom, telecom, nonlinear photonics, and quantum technologies.
Time-modulated metasurfaces (TMMs) enable low-observable and reconfigurable radar deception, but existing approaches remain limited in synthesizing complex range-velocity (R-V) false-target clusters because of forward-designed modulation schemes and single-platform operation. This paper presents a task-oriented framework for multidimensional false-target synthesis against frequency-modulated continuous-wave (FMCW) radars. An analytical model explicitly connects TMM temporal modulation with the FMCW R-V estimation chain. Intra-frame modulation reshapes the echo spectrum to generate programmable range offsets, while inter-frame control modifies the slow-time response to generate programmable velocity offsets. The modulation-sequence design is then formulated as a combinatorial inverse-design problem and solved using Physics-Informed Initialization-Assisted Particle Swarm Optimization (PIIA-PSO), which exploits analytical R-V relations to construct a physically informative initial population, thereby reducing ineffective early-stage exploration and improving convergence efficiency toward high-quality realizable solutions. The framework is further extended to distributed multi-platform cooperation to increase spatial degrees of freedom and enable coherent energy aggregation. Results show range and velocity errors below 10% for single-platform synthesis and improve the overall SSIM by 7-10 percentage points under distributed configurations, demonstrating enhanced synthesis fidelity, flexibility, and scalability.
Halide perovskites have demonstrated both short-term and long-term synaptic plasticity behaviors through various physical mechanisms, making them suitable for neuromorphic electronics. However, most studies rely on distinct materials or device architectures optimized for a single mechanism, limiting the realization of both forms of synaptic plasticity within a unified material system. Here, we report ionic-ferroelectric halide perovskite (IFHP) artificial synapses that integrate two fundamentally different resistance-switching processes within a single material. This functionality is enabled by the ferroelectric Dion-Jacobson perovskite (4-AMP)PbI4 (4-AMP = 4-(aminomethyl)piperidinium), which exhibits ion-migration-dominated behavior below the coercive voltage (VC similar to 3.5 V) and ferroelectric polarization switching above. Accordingly, the device exhibits volatile paired-pulse facilitation without stable non-volatile memory retention under sub-coercive stimulation, while exhibiting stable non-volatile memory above VC. The ferroelectric-polarization-driven long-term plasticity is further supported by extended retention over 2 & times; 104 s, cycling operation, and device-to-device reproducibility. In addition, the IFHP artificial synapse emulates bio-inspired associative learning and nociceptive sensory functions, highlighting its potential as neuromorphic hardware. These results establish a single-material pathway for voltage-programmable mixed plasticity, addressing a key challenge in halide perovskite neuromorphic hardware.
