Jiadong Yu

To address application demands for long slits, high spectral resolution, and compact system design, traditional Offner-type freeform surface imaging spectrometers face challenges of excessively high grating line density and manufacturing complexity. This paper proposes an innovative design for an off-axis three-mirror imaging spectrometer featuring a grating-prism combination with dispersion-corrected freeform surfaces. Building upon a modified Offner structure, the secondary mirror employs a freeform surface grating while prisms are systematically integrated into the optical path to share dispersion and synergistically correct aberrations. By configuring varying numbers (1–3) of prisms (only one being a freeform surface prism) at three critical locations—the input arm, secondary mirror, and output arm—12 distinct design configurations are achieved. A grating imaging spectrometer was concurrently designed for comparison. The resulting designs were rigorously compared against requirements: a wavelength range spanning from 400 to 1000 nm, a spectral resolution of 3 nm, a slit length of 60 mm, and an F-number of 3. Among these designs, a particular configuration demonstrated modulation transfer (MTF values) surpassing 0.7 at the Nyquist frequency of 34 lp/mm in full wavelengths while maintaining the smile and keystone less than one-tenth that of the pixel size. Tolerance analysis confirms that these freeform surfaces can be manufactured effectively. This design fulfills the criteria for both long slit functionality and compact architecture.

With the advancement of freeform optical design, fabrication, measurement, and assembly technology, the application of freeform optical systems is becoming increasingly widespread. The number of optical components in freeform optical systems has increased from two mirrors to three mirrors and four mirrors, and their application bands have also expanded from long-wave infrared (LWIR) to visible light. This paper designed and developed an off-axis four-mirror freeform telescope for visible light imaging. First, the optical and structural design is carried out according to the specified optical indicators. Then, an opto-mechanical–thermal coupling analysis is performed to verify the athermalization characteristics of the all-aluminum structure. Finally, through optical and mechanical alignment, the RMS of the center field of view of the telescope reaches 0.097λ (λ=632.8nm). Concurrently, thermal imaging experiments were conducted to obtain its resolution plate imaging at 65°C, 40°C, 16°C, and 0°C, with no significant changes observed in the images. Subsequently, the RMS of the telescope center field of view was retested after the thermal imaging experiment, showing a difference of 0.102λ, which demonstrates that the off-axis four-mirror freeform visible light telescope possesses excellent athermalization characteristics and imaging quality.

Freeform optics has emerged as a significant design tool over the past decade. Non-rotationally symmetric optical surfaces enable high-performance imaging systems to achieve a large field of view (FOV), high resolution, low distortion, lightweight construction, and compact structure simultaneously. Advances in single-point diamond-turning fabrication and computer-generated hologram (CGH)-based metrology are gradually making optical imaging with freeform optics more practical. In this paper, we present the development and verification of an unobscured off-axis three-mirror freeform aluminum imaging telescope (FAIT). The design, fabrication, metrology, and assembly processes of the FAIT are discussed in detail. This system achieves an unobscured FOV of 4^∘×4^∘ with a maximum distortion of less than 1%. The optical modulation transfer function (MTF) across the full FOV exceeds 0.7 at the Nyquist spatial frequency of 90 lp/mm within the wavelength range of 400–900 nm. By integrating aluminum freeform reflectors with mechanical and electronic systems, the final FAIT successfully captures high-resolution photographs both on the ground and in near-Earth orbit. The all-freeform mirrors ensure that the FAIT maintains a compact envelope. Furthermore, the all-aluminum optomechanical structure facilitates effective athermalization while preserving cost-effectiveness and lightweight characteristics. Additionally, its weight of approximately 2.9 kg renders the entire FAIT system highly advantageous for space applications.

Gallium nitride (GaN) films with an almost single-crystalline quality, smooth surface, and high optical transmission were prepared at 580 °C using inductively coupled plasma metal-organic chemical vapor deposition (ICP-MOCVD) based on a showerhead structure. The GaN thin films with unintentional doping were used as the active layer in the production of thin-film transistors (TFTs) and metal–semiconductor–metal (MSM) photodetectors. TFTs had on-to-off current ratios of 6.3 × 10³ and their output characteristics demonstrated the proper saturation behavior of transistors. The field-effect mobility was 3.79 cm² (V s)⁻¹. MSMs were visible blind, with a contrast of almost three orders of magnitude between ultraviolet and green light. The maximum responsivity was 34 mA W⁻¹ with a 10 V bias voltage and a 325 nm illumination. When the bias voltage was >2 V, the quantum efficiency and noise equivalent power under 266 nm illumination reached >10% and pW ( H z 1/2 ) − 1 , respectively. These findings suggest that the ICP-MOCVD technique has the potential to be used to fabricate (opto)electronic devices at low temperatures.

The complex device structure and costly preparation process have hindered the development and application of the GaN-based ultraviolet and infrared (UV–IR) dual-color photodetector. In this work, we designed and prepared an Au/GaN-nanopillar-based hot-electron photodetector that can operate in the short-wave infrared range, well below the GaN bandgap energy. A suitable Schottky barrier height was developed for a higher photo-to-dark current ratio by post-etching annealing. The surface plasmons generated by Au/GaN-nanopillar arrays could effectively improve the light absorption efficiency. As a result, compared with the planar device, the responsivity of the Au/GaN-nanopillar device could be enhanced by about two orders of magnitude. With the advantages of a simple structure and easy preparation, the proposed devices are promising candidates for application in UV–IR dual-color photodetection.

Temperature-dependent photoluminescence (TDPL) is mostly employed to evaluate the internal quantum efficiency (IQE) of light-emitting semiconductors. The key assumption of this method is that the IQE is 100% under low temperature (LT) (e.g. 10 K), which is often considered to be reasonable without any verification. This may lead to an obvious contradiction between a considerable IQE value and poor emission intensity, especially when there exists a high threading dislocation density. In this paper, the power-dependent photoluminescence (PDPL), TDPL, and time-resolved photoluminescence are carried out on three InGaN single-quantum-well samples with diverse threading dislocation densities to obtain the IQE and the recombination coefficients. It is found that the IQE at LT must be confirmed by PDPL and used to calibrate the TDPL values. Then, the IQEs obtained from different measurements are in good agreement under the same excitation conditions. A phenomenological model is proposed to explain the evolution of IQE and recombination coefficients with threading dislocation density. This work indicates that the commonly used TDPL is not reliable, as the hypothesis of negligible nonradiative recombination at LT is proven to not be valid for InGaN quantum well samples, especially those with high threading dislocation density (>10¹⁰ cm⁻²).

We propose a hybrid plasmon polariton (HPP) Mach-Zehnder modulator (MZM) structure that combines the tight optical confinement of plasmonic waveguides and narrow slot waveguides with the low loss feature of photonic waveguides. Compared with conventional surface plasmon polariton (SPP) modulators, the HPP modulator exhibits lower propagation loss and better overall performance. Simulations based on the finite difference time-domain (FDTD) and finite element method (FEM) predict a half-wave voltage-length product of 0.078 V·mm for the HPP modulator. Meanwhile, the propagation loss is only 0.2 dB/μm, which is less than half of that for an SPP modulator. The modulation bandwidth of a 10-μm-long HHP modulator exceeds 650 GHz, while the total insertion loss of the device is estimated to be lower than 4 dB.