Search Results (2,148)

Terahertz (THz) radiation generated through ultrafast transient photocurrent mechanisms has become a cornerstone of modern THz photonics, enabling broadband coherent emission with sub-picosecond temporal resolution. This review provides a comprehensive and mechanism-driven analysis of THz pulse generation via photo-Dember diffusion currents, surface depletion field acceleration, and biased photoconductive antenna architectures. We present a comprehensive comparative analysis of wide- and low-bandgap material platforms, including III–V, II–VI, and group IV semiconductors, as well as two-dimensional materials, topological insulators, and Weyl semimetals, highlighting how their intrinsic properties, such as band structure, carrier mobility, recombination dynamics, doping, and dielectric response, govern their THz emission efficiency, bandwidth, and spectral tunability. Special emphasis is placed on germanium (Ge), which has re-emerged as a highly promising THz source material owing to its high carrier mobility, long diffusion lengths, strain-tunable band structure, and CMOS compatibility. We highlight the roles of doping, strain-induced direct transitions, and several fabrication techniques in controlling the nonlinear photoexcited charge-carrier dynamics in Ge, thereby unlocking enhanced broadband THz performance. Finally, we explore the emerging application prospects of THz radiation, ranging from non-invasive security screening to biochemical sensing and archeological preservation. By bridging fundamental material science with scalable device architectures, this review outlines current challenges, highlights evolving opportunities in novel materials, and charts future directions towards integrated THz technologies. Full article

Li n⁺ and In p⁺ diffusion contacts were fabricated on p-type 12N high-purity germanium (HPGe) single crystals by vacuum evaporation of thin-film sources followed by solid-state thermal diffusion. The effects of diffusion temperature on the near-surface structure, morphology, impurity distribution, and device response were systematically investigated. XRD and Raman analyses show that Li diffusion at 100–300 °C and In diffusion at 600–800 °C preserve the bulk Ge crystal structure, whereas higher diffusion temperatures induce surface roughening, near-surface disordering, and interfacial reactions. SIMS depth profiles combined with diffusion simulations confirm effective inward diffusion of both Li and In, with low-concentration tailing that is consistent with defect-assisted diffusion or interfacial trapping. The sample diffused with Li at 200 °C exhibits the lowest dark current, 8.07 × 10⁻⁸ A at −10 V. The final HPGe device with Li/In diffusion contacts shows a stable synchrotron X-ray photoconductive response, and the net response current increases from 4.48 × 10⁻⁷ to 1.15 × 10⁻⁶ A as the incident photon flux increases. These results demonstrate that low-leakage HPGe diffusion contacts require a balance between diffusion-layer formation and near-surface/interface stability, rather than a simple increase in thermal budget. Full article

This paper presents a two-dimensional magneto-photo-thermoelastic model for a fiber-reinforced anisotropic semiconductor half-space subjected to optical excitation and a variable gravity field. The formulation is developed within the framework of generalized photo-thermoelasticity by considering the coupled interactions among thermal, elastic, carrier-density, electromagnetic, and gravity-induced effects. The constitutive equations of a fiber-reinforced anisotropic medium are employed, while the influences of the magnetic field and gravity are incorporated into the governing equations. A suitable nondimensionalization procedure is introduced, and the resulting coupled system is solved analytically using the normal mode technique and eigenvalue approach. Numerical results are obtained for the temperature, carrier density, displacement components, and stress distributions. The influence of the gravity parameter on the physical fields is investigated in detail. The results indicate that gravity significantly affects the mechanical and stress responses, whereas its effect on the thermal and carrier-density fields is comparatively less pronounced. A comparative study between silicon and germanium semiconductors is also carried out, revealing noticeable differences in the amplitudes and attenuation behavior of the coupled fields due to variations in material properties. The present study provides useful insights into coupled multiphysical interactions in semiconductor structures and may be relevant to applications in optoelectronic devices, photonic systems, smart composite materials, and aerospace technologies. Full article

Silicon carbide (SiC) is the leading wide-bandgap semiconductor material, providing mature doping and device fabrication. Additionally, SiC hosts a multitude of optically active point defects (color centers) and is relevant for many applications in quantum technologies. A crucial step towards harnessing the full potential of the SiC platform includes technologies to create color centers with defined localization and density, e.g., to facilitate their coupling to nano-photonic structures and to observe cooperative effects. Here, silicon vacancy centers and divacancies stand out, as no impurity atom is needed, and high-thermal budget annealing steps can be avoided. We characterize the effect of localized, femtosecond laser irradiation of SiC, investigating surface modifications and photoluminescence, including Raman spectroscopy and optical lifetime measurements. We employ commercial, high-purity, semi-insulating substrates and an industrial-grade laser system to explore broader applicability of the method. As a novel approach, we apply femtosecond laser irradiation to SiC substrates with an epitaxial graphene layer and find that the threshold for photoluminescence due to laser treatment is lowered. Full article

Two-dimensional electron gases (2DEGs) at complex oxide interfaces have emerged as a promising platform for next-generation oxide semiconductor devices, owing to their tunable electronic properties and rich interfacial phenomena. In this work, a LaAlO₃/SrTiO₃/LaTiO₃ (LAO/STO/LTO) trilayer heterostructure is proposed and theoretically investigated using a self-consistent Schrödinger–Poisson framework to examine the effects of multi-interface engineering on quantum confinement and carrier transport. The proposed architecture combines polar-discontinuity-driven electronic reconstruction at the LAO/STO interface with charge-transfer-induced electron accumulation at the STO/LTO interface, forming two coupled 2DEG channels within the SrTiO₃ layer. Compared with conventional single-interface oxide heterostructures, the coupled-interface configuration significantly enhances sheet carrier density and electrical conductivity, with predicted carrier densities approaching 10¹⁴ cm⁻² and gate-tunable conductivities in the range of 10³–10⁴ S cm⁻¹ under idealized operating conditions. The effects of layer thickness, gate bias, temperature, and electrostatic coupling are systematically investigated to establish practical design guidelines for optimizing carrier confinement and transport. A sensitivity analysis incorporating interface trap densities up to 2 × 10¹³ cm⁻² demonstrates that more than 60% of the ideal carrier population is retained under moderate defect concentrations, confirming the robustness of the proposed multi-interface strategy. Although the analytical model represents an upper-bound framework, its predictions are discussed in the context of experimentally relevant limitations, including interface roughness, oxygen vacancies, carrier trapping, and defect-induced scattering. Overall, the proposed LAO/STO/LTO heterostructure provides a predictive framework for engineering high-density, electrically tunable oxide 2DEGs for future nanoelectronic, terahertz, photonic, and energy-related applications. Full article

Metasurfaces, as emerging functional optical coatings, enable precise wavefront manipulation at the subwavelength scale. In this work, we propose a polarization-encoded switchable structured light generator based on a single-layer holographic metasurface composed of silicon nanopillars. By combining Fresnel holography technology and the Pancharatnam–Berry phase modulation principle, two sets of holographic phase distributions for perfect vortex beams corresponding to orthogonal circularly polarized states are superimposed onto a single metasurface. The optimized nanopillar achieves a transmittance of approximately 85% and a polarization conversion efficiency of around 98% at the wavelength of 632.8 nm. By simply adjusting the incident polarization state, the metasurface generates a perfect vortex beam under right-handed circularly polarized light illumination and a cylindrical vector beam under horizontal linearly polarized light illumination, enabling on-demand switching between the two structured light modes. Furthermore, we design and validate a spatially multiplexed perfect vortex beam generator, in which the radius, topological charge, and focal plane position of each channel can be independently controlled. This flexible thin-film metasurface platform offers a promising route toward compact, multifunctional photonic devices for advanced optical integration. Full article

Single-crystal scintillators are used to convert ionizing radiation into optical photons in various medical imaging applications. A promising material is cerium (Ce)-doped gadolinium aluminum gallium garnet (GAGG:Ce) inorganic scintillator. Three GAGG:Ce 10 × 10 × 10 mm³ crystals of different light yield (LY) were exposed in X-ray tube voltage range of 50–140 kVp. Their absolute luminescence efficiency (AE) was experimentally calculated. A theoretical model was employed to simulate the propagation of photons traversing through the crystal mass. The model was utilized to estimate the detector quantum gain (DQG) and the percentage of transmission of the optical photons per elementary thickness $\mathrm{k}$. Their suitability with various optical photodetectors was evaluated by means of the spectral matching factor (SMF). GAGG:Ce presented AE values reaching 60.72 E.U. (where 1 E.U. = 1 μWm⁻²/(mRs⁻¹)) at 140 kVp. The parameter $\mathrm{k}$ ranged from 0.99973 to 0.99980. GAGG:Ce emission spectrum is highly compatible with charged-coupled devices (CCD), complementary metal-oxide semiconductors (CMOS) and silicon photomultipliers (SiPM). These findings may further consolidate the use of GAGG:Ce and could contribute to the future optimization of this inorganic scintillator when applied in X-ray imaging modalities, or as a radiation detector. Full article

Integrated sensing and communication (ISAC) technology enables the joint integration of communication and sensing functions through the efficient utilization of spectrum/energy resources. By further incorporating with the reconfigurable intelligent surfaces (RISs), the wireless propagation environment of ISAC systems can be dynamically controlled, thereby enhancing the overall sensing and communication (SAC) performance. In this context, accurate channel estimation is a fundamental prerequisite for the efficient operation of the RIS-assisted ISAC systems. However, the strong coupling between SAC signals from direct and reflected channel links, as well as severe noise interference, limits the SAC channel estimation accuracy. This paper proposes a novel two-stage channel estimation scheme for RIS-assisted ISAC systems, where the direct and reflected SAC channels are respectively estimated in the first and second stages. To mitigate the negative effects of noise interference in each estimation stage, an extreme learning machine-based feature purification module is precisely designed, improving the quality of received SAC signals and generating purified channel features. Then, the dedicated deep neural network adopts the purified channel features to estimate SAC channels. Simulation results demonstrate that, under different signal-to-noise ratio conditions and channel dimensions, the proposed scheme achieves superior estimation accuracy and strong robustness compared to the benchmark methods. Full article

Junction temperature (T_j) is a key parameter that directly governs the optical performance and operational reliability of light-emitting diodes (LEDs), which have become indispensable in modern illumination and display systems. Accurate real-time T_j monitoring is critical for ensuring device longevity and consistent light output. Although the forward voltage method (FVM) remains the industry benchmark, its practical implementation is hindered by the need for costly high-speed switching modules and ultra-low-current calibration sources, restricting its deployment in real-time and cost-sensitive scenarios. To overcome these limitations, we introduce and experimentally validate a non-contact optical method for T_j determination that leverages the normalized integral width (NIW) of the LED emission spectrum as a temperature-sensitive spectral parameter. The underlying principle is that spectral broadening—arising from enhanced carrier thermal excitation and temperature-induced bandgap shrinkage—exhibits a robust and quantifiable linear correlation with T_j. Both theoretical analysis and experimental data confirm that this mechanism underpins the excellent linear correlation between NIW and T_j observed across a wide range of LED types, including monochromatic (red, green, blue) and phosphor-converted white LEDs. A rigorous theoretical analysis establishes the mathematical framework linking NIW to T_j. Experimentally, a measurement system centered on a modified commercial spectrometer was constructed. Extensive testing on a diverse array of power LEDs consistently demonstrates an excellent linear correlation (R² > 0.998) between NIW and T_j under normal drive conditions (e.g., typical operating currents). A comparative analysis against the benchmark FVM, conducted using a Mentor Graphics T3Ster system, demonstrates that the proposed method achieves comparable measurement accuracy, with a maximum deviation of merely 2.1 °C, while substantially reducing system cost and complexity. Validation across diverse LED types confirmed excellent linearity and high repeatability. A comparative analysis with established optical methods (e.g., peak wavelength, blue-white ratio, Raman thermography) further underscores the advantages of the NIW method in terms of cost-effectiveness, measurement speed, and broader applicability. Subsequent evaluation of critical factors, including self-heating, ambient light interference, and spectrometer resolution, demonstrates its robustness. Consequently, the NIW method presents a practical solution for real-time, non-intrusive thermal monitoring, well-suited for industrial LED production and quality control. Full article

Pixelless infrared imaging devices based on optoelectronic up-conversion offer a compact and scalable alternative to conventional focal plane arrays; however, their performance is fundamentally limited by lateral carrier diffusion, image smearing, and the trade-off between spatial resolution and conversion efficiency. Existing systems employing quantum well infrared phototransistors (QWIPTs) integrated with light-emitting diodes (LEDs) suffer from degraded modulation transfer function (MTF) at high spatial frequencies and restricted design flexibility. In this article, a quantum cascade detector (QCD)–LED pixelless imaging architecture is proposed and comprehensively modeled as a next-generation alternative. A unified analytical framework is developed to describe carrier concentration, modulation transfer function, image resolution, and image conversion efficiency (ICE) in QCD-LED systems under spatially modulated far-infrared illumination. The models explicitly account for cascade transport, diffusion–drift dynamics, photon recycling, and radiative recombination, enabling direct comparison with conventional QWIPT-LED imagers. Numerical investigations reveal that multi-stage cascade transport significantly suppresses lateral carrier spreading, resulting in a pronounced enhancement in spatial-frequency response. The proposed QCD-LED architecture demonstrates a >32.5% improvement in maximum MTF, a 32.5% increase in conversion efficiency, and a 25% enhancement in response speed, while maintaining comparable or improved image resolution. An optimal performance is achieved for a 10-stage quantum cascade detector with a 2.5 μm period length and a radiative-to-nonradiative lifetime ratio of 0.999, yielding a figure of merit (R × ICE) of 30.99, outperforming previously reported QWIPT-LED systems. Experimental validation confirms excellent agreement with theoretical predictions (R² = 0.989), particularly at high spatial frequencies where QCD-LED devices exhibit more than 140% improvement in contrast transfer. These results establish quantum cascade detector-based pixelless imagers as a robust platform for high-resolution, high-speed infrared imaging, offering superior spatial fidelity and design flexibility for next-generation optoelectronic imaging systems. Full article

Semiconductor lasers are key devices in optical communication, optical storage and other fields. Achieving single-mode, high-power and high-beam-quality output is an important research goal in laser technology. Conventional slab-coupled optical waveguide lasers can effectively suppress higher-order modes, but suffer from a low optical confinement factor and limited ridge width. This paper proposes a 1064 nm deep-etched wide-ridge waveguide slab-coupled photonic crystal semiconductor laser. A photonic crystal structure is introduced into the conventional slab-coupled optical waveguide. The optical field and mode characteristics are analyzed by numerical simulation, verifying the feasibility of the structure in improving the optical confinement factor and realizing single-mode operation with a deep-etched wide-ridge waveguide. Different from traditional photonic crystal lasers, this design provides a new route for high-performance photonic crystal semiconductor laser design. Full article

We propose a three-dimensional (3D) integrated optical switch that leverages liquid crystal (LC) birefringence to achieve reconfigurable light routing for particular suitability for quantum optical communication. In our design, the large refractive index contrast between an LC’s ordinary (n_o) and extraordinary (n_e) indices is exploited by using n_o as an effective cladding and n_e as the core of voltage-controlled waveguides. This allows dynamic waveguide formation not only in-plane (horizontal routing on chip) but also vertically through stacked polymer layers, realizing a 3D switching architecture beyond traditional planar photonic circuits. A prototype multi-layer structure on a silicon substrate is described, incorporating alternating polymer cladding and core films with embedded LC cells that act as switchable waveguide segments. Simulations confirm that the LC switch can confine and direct light between different layers with low loss, enabling compact 3 × 3 and potentially up to 10 × 10 port-count switching matrices. The device is electrically driven (no moving parts) and can be operated at low voltages, ensuring compatibility with photonic integrated circuit fabrication. The simulated LC response time on ON/OFF is on the order of 1.1 ms/45 ms, which is slower than MEMS or electro-optic switches, but, however, sufficient for quantum key distribution and other quantum network applications where ultrafast switching is not required. Overall, this LC cell-based 3D optical switch offers a promising route toward scalable, low-loss photonic switching nodes for next-generation quantum communication networks. Full article

Metalenses based on optical metasurfaces enable wavefront manipulation using subwavelength nanostructures and provide a promising route toward compact and integrated optical systems. However, strong chromatic aberration caused by wavelength-dependent phase responses remains a major obstacle for practical metalens applications in the visible regime. In this work, we present the design and analysis of an achromatic metalens operating in the visible spectrum using silicon nitride (Si₃N₄) dielectric metasurfaces. The metalens employs a phase-engineering strategy based on propagation-phase modulation of polarization-independent nanostructures. By constructing a unit-cell phase library through systematic parameter scanning, the phase responses at different wavelengths are accurately mapped. An interleaved arrangement strategy is introduced, where meta-atoms designed for different target wavelengths are alternately distributed within a single metalens aperture, enabling multi-wavelength phase compensation without increasing the structural complexity. Numerical simulations demonstrate that the proposed metalens achieves near-coincident focal positions across a broad visible-wavelength range. The focal length variation is significantly suppressed compared with conventional single-wavelength metalenses. The metalens exhibits stable focusing behavior with symmetric focal spots, consistent focal sizes, and improved chromatic tolerance. The results confirm that the interleaved design effectively mitigates chromatic focal shift while maintaining high transmission efficiency. This study provides a practical and scalable approach to achieving achromatic focusing in visible-wavelength metalenses. The proposed Si₃N₄-based interleaved design offers strong potential for compact imaging systems, integrated photonics, and visible-light optical devices. Full article

Topological photonic crystals provide an important platform for robust light transport and light-field manipulation. To meet the demands for developing multifunctional and densely integrated photonic circuits, it is necessary to flexibly control light flow with multi-degrees of freedom while maintaining strong topological protection. In this work, we investigate multifunctional topological photonic crystals based on hybrid topological domain walls, which support valley-polarized chiral edge states (VCES). Based on hybrid domain walls, we design two types of compact topological photonic devices. By exploiting direction-selective coupling between valley edge states (VES) and VCES, we construct nonreciprocal coupled waveguides with a nonreciprocal transmission ratio of 10 dB and output-port isolation ratio of more than 30 dB. Moreover, through different configurations of the direction of external magnetic field, we construct a multi-channel selective routing device that enables the configurable transport of valley-polarized electromagnetic waves among multiple channels. Hybrid topological waveguides provide a foundation for designing novel photonic devices, offering the potential for realizing multifunctional integrated topological photonic networks in both classical and quantum regimes. Full article

The ability to detect individual photons with exquisite temporal precision has transformed single-photon avalanche diodes (SPADs) into indispensable tools across photonics, with quantum communication emerging as one of their most demanding frontiers. In recent years, device engineering breakthroughs, including refined junction geometries, advanced avalanche quenching schemes and scalable array integration, have redefined the limits of SPAD performance. Parallel advances in fabrication precision and CMOS-compatible architectures have not only expanded spectral sensitivity from the visible to the near-infrared but also enabled systematic suppression of dark count rates, afterpulsing, and timing jitter. These developments have directly impacted the feasibility and robustness of quantum key distribution and time-correlated single-photon counting, where detection efficiency and noise suppression determine system fidelity. This review unifies recent progress in SPAD technology, weaving together innovations in device design, fabrication strategies, and parameter optimization to reveal their collective influence on next-generation quantum-enabled photonic systems. Looking forward, the convergence of hybrid material platforms, on-chip photonic–electronic co-integration, and intelligent quenching control is poised to elevate SPAD performance to meet, and potentially exceed, the stringent requirements of future quantum communication infrastructures. Full article