Advanced Photonics Research最新文献

Gallium nitride (GaN)-based semiconductor laser diodes (LDs) have garnered significant attention due to their promising applications. However, high-power LDs face serious degradation issues that limit their practical use. This study investigates the degradation factors of 437 nm and 6.3 W LDs by comparing light–current–voltage (L–I–V) characteristics, transmission electron microscopy (TEM), cathodoluminescence (CL), and secondary ion mass spectroscopy (SIMS) before and after 1000-h aging. The diffusion of mirror coating from the resonant cavity surface is identified as a key factor contributing to high-power LD degradation, which has not been reported in milliwatt-level LDs. Meanwhile, the mechanisms behind the LD degradation are profiled and summarized together with the diffusion and other factors. On basis of the mechanism exploration, an anti-aging technology for high-power GaN-based LDs is developed by using aluminum nitride for passivation layer and sapphire materials for mirror film. This anti-aging technology has been verified, and a nearly ten-time degradation suppression is achieved from 1000 h. This study elucidates the degradation mechanisms of high-power GaN LDs and provides an effective technology to extend their lifespan, thereby prompting the practical applications of high-power LDs.

Optically pumped spintronic terahertz emitters (STEs) have, in less than a decade, strongly impacted terahertz (THz) source technology, by the combination of their Fourier-limited ultrafast response, their phononless emission spectrum and their wavelength-independent operation. However, the intrinsic strength of the inverse spin Hall effect governing these devices introduces a challenge: the optical-to-terahertz conversion efficiency is considerably lower than traditional sources. It is therefore primordial to maximize at least their electromagnetic efficiency independently of the spin dynamics at play. Using a rigorous time-domain treatment of the electromagnetic generation and extraction processes, an optimized design is presented and experimentally confirmed. With respect to the strongest reported spintronic THz emitters it achieves a 250% enhancement of the emitted THz field and therefore an 8 dB increase of emitted power. This experimental achievement brings STE close to the symbolic barrier of mW levels. The design strategy is generically applicable to any kind of ultrafast spin-to-charge conversion (S2C) system. On a broader level, our work highlights how a rigorous handling of the purely electromagnetic aspects of THz spintronic devices can uncover overlooked aspects of their operation and lead to substantial improvements.

Organic photodiodes (OPDs) have made remarkable strides and now poised to surpass traditional silicon photodiodes (PDs) in various aspects including linear dynamic range (LDR), detectivity, wavelength selectivity, and versatility.^[1] Tunable mechanical and optoelectronic properties of organic semiconductors, coupled with lower process costs, have propelled OPDs into the spotlight across fields such as wearable light fidelity systems, flexible image sensors, and biomedical imaging.^[2–5] While most advanced organic imaging systems to date rely on polymer-based solution processes, challenges such as the use of toxic organic solvents and reproducibility issues hinder their commercialization.^[6,7] Vacuum-processed OPDs offer a promising alternative, boasting eco-friendliness and compatibility with large-scale fabrication facilities.^[8,9] In this review, recent advancements and challenges in vacuum-processed OPDs, an area that has received less attention compared to solution-processed counterparts, are explored. Herein, four primary pathways for development of vacuum-processed OPDs are outlined: 1) ultraviolet-selective OPDs, 2) visible-light-selective OPDs, 3) near-infrared or short-wave-infrared-sensitive OPDs, and 4) addressing challenges such as higher noise currents compared to inorganic PDs. In this review, it is aimed to furnish readers with a comprehensive understanding of vacuum-processed OPDs, spanning from materials design to device engineering.

Carbon dots (CDs) show great application potential with their unique and excellent performances. Coal and its derivatives are rich in aromatic ring structure, which is suitable for preparing CDs in microstructure. Coal-burning dust from coal-fired power plants can be utilized as a rich resource to separate and extract CDs. It is shown in the results that there are two main possible mechanisms for the formation of CDs in coal-burning dust. One is the self-assembly of polycyclic aromatic hydrocarbons contained in coal or produced by incomplete combustion of coal. The other mechanism is that the bridge bonds linking different aromatic structures in coal break, which will form CDs with different functional groups when the coals burn at high temperature. Under violet light excitation at 310–340 nm or red light at 610–640 nm, CDs extracted from coal-burning dust can emit purple fluorescence around 410 nm. The mechanism of up-conversion fluorescence emission of CDs is due to a two-photon absorption process. The recycling of CDs from coal-burning dust from coal-fired power plants are not only good to protect environment but will also be helpful for mass production of CDs.

Herein, deep learning-ghost imaging (DLGI) based on a digital micromirror device is realized to avoid the difficulties of a charge-coupled device (CCD) scientific camera being unable to obtain the sample images in extremely weak illumination conditions and to solve the problem of the inverse relationship between imaging quality and imaging time in practical applications. Deep learning for computational ghost imaging typically requires the collection of a large set of labeled experimental data to train a neural network. Herein, we demonstrate that a practically usable neural network can be prepared based on the simulation results. The acquisition results of the CCD scientific camera and the simulation results with low sampling are used as the training set (1000 observations) and we can complete the data acquisition process within one hour. The results show that the proposed DLGI method can be used to significantly improve the quality of the reconstructed images when the sampling rate is 60%. This method also reduces the imaging time and the memory usage, while simultaneously improving the imaging quality. The imaging results of the proposed DLGI method have great significance for application in clinical diagnosis.

Topological Photonic Insulators

In article number 2400023, Vien Van and Hanfa Song present the design and realization of (2 + 1)D topological photonic insulators hosting all flat bands, which exhibit novel properties such as anomalous Floquet insulator phase, ultra-wide edge mode continuum, super robustness to disorder, and photon caging in compact localized bulk states. These lattices have broadband applications in topologically-protected quantum photonics and programmable photonic circuits.

The hole accelerator is proven to benefit the hole injection for traditional light-emitting diodes (LEDs) because the induced electric field provides the holes with more kinetic energy to pass through the electron-blocking layer, enhancing the hole injection efficiency. Herein, the effect of the hole accelerator (HA) layer on the micro-LEDs by modeling the characteristics of the devices with a current density of lower than 10 A cm⁻² is investigated. The simulation results show that the appended HA layer brings a knot of the electric field in the HA layer, leading to higher internal quantum efficiency (IQE) than the device without HA under the low current density. The thickness and composition of HA, the quantum number, and the material of quantum barrier are also simulated and analyzed. The simulated radiative, Shockley–Read–Hall, and Auger recombination rates show that the IQE of the micro-LED with the HA layer is higher than that without the HA layer under the current density of lower than 10 A cm⁻².

Photoinduced Structural Modification

In article number 2300326, Ashim Dhakal and co-workers show that photo-induced metastable modification of electronic structure in graphite allows for multiphoton processes that can up-convert an O-band infrared excitation to visible-NIR band in graphite powder. Theoretically, this process can upconvert an infrared light near the wavelength of 3 μm to VIS-NIR wavelengths. It opens exciting new avenues for applications in visible light generation and low-noise imaging using infrared light excitation.

Herein, a novel approach is presented to mitigate the fluorescence interference during the detection of vibrational signal via the stimulated emission depletion (STED). STED is the mechanism commonly employed in optical imaging; however, its application should not be confined solely to this field. To explore additional possibilities, a novel application of STED in vibrational spectroscopy detection is introduced. Vibrational spectroscopy is a widely used technique for the material detection and identification, but its sensitivity is influenced by impurity signals, especially the fluorescence. The proposed method is capable of suppressing fluorescence without influencing vibrational signal. At the low concentration of fluorescent impurities, the signal-to-background ratio of vibrational spectroscopy is 2.6 times as high as that without this method. The introduction of depletion light can enhance the detection of vibrational signals, resulting in more optimal signal detection. A promising new application of STED other than super-resolution imaging is investigated.

Optically pumped threshold fluences are a widely reported metric to benchmark the performance of thin-film gain media and lasers. Estimating the threshold fluence for nonhomogeneous beams, such as a circular Gaussian excitation, is not trivial since the average fluence depends on the estimated spot diameter. Using an exemplary lead halide perovskite film, the inversion volume at different pump energies is mapped. It is shown that the peak fluence of an arbitrary spatial beam profile is more relevant at the threshold, as it provides an upper bound to the threshold fluence. Also, simple conversion factors to estimate the peak fluence using Gaussian excitation beams are provided and the methodology to arbitrary profiles is extrapolated. Furthermore, it is advocated for using flat-top or uniform stripe excitations to unambiguously extract the threshold fluence, since these excitations display minor discrepancies between the average and peak fluence, and keep the inversion volume relatively constant during the measurement.