The impulse response for a phase-change material Ge₂Sb₂Te₅ (GST)-based photodetector integrated with a silicon-on-insulator (SOI) waveguide is simulated using finite difference time domain method. The current is calculated by solving the drift-diffusion model for short pulse (～10 fs) excitation for both of the stable phases. Full width at half-maximum values of less than 1 ps are found in the investigation. The crystalline GST has higher 3 dB bandwidth than the amorphous GST at a 1550 nm wavelength with responsivities of 21 A/W and 18.5 A/W, respectively, for a 150 nm thick GST layer biased at 2 V. A broad spectrum can be utilized by tuning the device using the phase-change property of material in the near infrared region.

Ultraviolet (UV) detectors with large photosensitive areas are more advantageous in low-level UV detection applications. In this Letter, high-performance 4H-SiC p-i-n avalanche photodiodes (APDs) with large active area (800 μm diameter) are reported. With the optimized epitaxial structure and device fabrication process, a high multiplication gain of 1.4 × 10⁶ is obtained for the devices at room temperature, and the dark current is as low as ～10 pA at low reverse voltages. In addition, record external quantum efficiency of 85.5% at 274 nm is achieved, which is the highest value for the reported SiC APDs. Furthermore, the rejection ratio of UV to visible light reaches about 10⁴. The excellent performance of our devices indicates a tremendous improvement for large-area SiC APD-based UV detectors. Finally, the UV imaging performance of our fabricated 4H-SiC p-i-n APDs is also demonstrated for system-level applications.

An ultra-compact hybrid-integration receiver optical subassembly (ROSA) with four channels is demonstrated in our laboratory with the size of 23.3 mm × 6.0 mm × 6.5 mm. The ROSA is comprised of a planar lightwave circuit (PLC) arrayed waveguide grating (AWG) chip, a top-illuminated positive-intrinsic-negative photodetector array chip, and a three-dimensional microwave circuit that is specially designed for compact packaging. For each transmission lane, the 3 dB bandwidth of the ROSA is up to 20 GHz, and the maximum responsivity is up to 0.53 A/W. The proposed package structure can be used for smaller package sizes and would be an easy assembling solution for 100 GbE optical communication devices.

We report the terahertz (THz) wave generation from a single-color scheme modulated by pre-ionized air plasma via an orthogonal pumping geometry. It is found that the amplitude of the THz signal generated by the pump beam tends to decrease gradually with the increase of the modulation power. We believe that the ponderomotive force plays an important role in the process of the interaction between the pump beam and the pre-ionization beam. The hydrostatic state of the electrostatic separation field caused by the modulation beam will directly affect the generation efficiency of the THz wave. Our results contribute to further understanding of the theoretical mechanism and expanding of the practical applications of THz wave generation and modulation.

Signal distortion due to the non-uniform response of the detector degrades the measurement accuracy of most metrology instruments. In this Letter, we report a newly developed calibration source system for reference-based non-uniformity correction using a laser source, a fiber, and a diffusive module. By applying the Monte Carlo simulation, we show that the transmittance of the system highly depends on the cavity reflection of the diffusive module. We also demonstrate the use of this system to achieve a flat field at a very low non-uniformity (less than 0.2%) with proper illumination intensity, which most costly commercial integrating sphere systems traditionally cannot provide.

Nonlinear dynamics in an optoelectronic delayed feedback semiconductor laser and its application in sensing are studied. We analyze the theories of stability and period of the laser. A route to quasi-periodic bifurcation or a stochastic state from stability is numerically analyzed by shifting the feedback level. The induced dynamics are found to be in one of four distributions (stable, periodic pulsed, period-three pulsed, and undamping oscillating). An external injection into the laser results in the process being more or less the opposite with the conventional optical injection cases. Based on this process or the dynamic regimes, we present a modeling of the incoherent detection sensor using the nonlinear period-one characteristic of the laser. The sensor discriminates the injection light variation as a sensing signal via detecting the behaviors from the period-one laser.

High-speed cameras are widely used in experimental research and industrial measurement. Although multiple cameras are commonly used, whether the cameras are triggered at the same time is typically overlooked. This study measures the startup time difference between two high-speed cameras employing a proposed measuring system. A series of comparative experiments was conducted to consider the complex factors that can lead to a time difference. The system recorded the startup time differences for different combinations of two cameras at different frame rates, and thus acquired the dependence of the time difference on these factors. Suggestions are made on the basis of the experimental results.

Using the first-principles method based on the density functional theory (DFT), the work function of seven different GaN (0001) (1×1) surface models is calculated. The calculation results show that the optimal ratio of Cs to O for activation is between 3∶1 and 4∶1. Then, Cs/O activation and stability testing experiments on reflection-mode negative electron affinity GaN photocathodes are performed. The surface model [GaN (Mg): Cs] Cs-O after being activated with cesium and oxygen is used. The experiment results illustrate that the adsorption of O contained in the residual gas increases the surface potential barrier and the reduction of the effective dipole quantity is the basic cause of the quantum efficiency decay.

We propose and demonstrate a pseudo Fabry–Pérot filter in the terahertz frequency range of 0.1–0.5 THz. It consists of alternative liquid crystal layers and metallic slats. Separate sharp resonant peaks are shown in the simulated transmission spectra, and their positions shift toward higher frequencies when the refractive index of liquid crystal decreases. The measured transmission spectra are consistent with corresponding simulations. Via thermally tuning the refractive index of the filled liquid crystal, the resonant transmission frequencies shift accordingly. The work supplies a novel design for tunable terahertz filters, which would play important roles in terahertz imaging, sensing, high speed communication, and security applications.

Previous research shows that few-cycle laser (FCL) pulses with low energy and without a bias field can be used to coherently detect terahertz (THz) pulses. As we know, it is very difficult to stabilize the carrier envelope phase (CEP) of FCL pulses, i.e., there are some random fluctuations for the CEP. Here we theoretically investigate the influence of such instability on the accuracy of THz detection. Our results show that although there is an optimum CEP for THz detection, the fluctuations of the CEP will lead to terrible thorns on the detected THz waveform. In order to solve this problem, we propose an approach using two few-cycle laser pulses with opposite CEPs, i.e., their CEPs are differed by π.