Physics Education

We demonstrate major advancements in the energy efficiency of single mode, oxide-confined, vertical-cavity surface-emitting lasers at 940 nm wavelength, being promising for optical interconnects. We were able to show error-free operation (Bit Error Rate of 10exp -12) at 50 Gb/s and at 60 Gb/s, for Non-Return-to-Zero modulation across 100 m of multimode optical fiber with an energy efficiency of 98/168 fJ/bit, respectively. These are presently the most energy-efficient directly modulated light sources available for data transmission.

One-dimensional structures such as nanowires (NWs) show great promise in tailoring the rates of hot carrier thermalization in semiconductors with important implications for the design of efficient hot carrier absorbers. However, the fabrication of defect-free crystal structures and control of their intrinsic electronic properties can be challenging, raising concerns about the role of competing radiative and non-radiative recombination mechanisms that govern hot carrier effects. Here, we elucidate the impact of crystal purity and altered electronic properties on the hot carrier properties by comparing two classes of III–V semiconductor NW arrays with similar bandgap energies and geometries, yet different crystal quality: one composed of GaAsSb NWs, which host antisite point defects but are free of planar stacking defects, and the other InGaAs NWs with a very high density of stacking defects. Photoluminescence spectroscopy demonstrates distinct hot carrier effects in both NW arrays; however, the InGaAs NWs exhibit stronger hot carrier effects, as evidenced by increased carrier temperature under identical photo-absorptivity. This difference arises from higher rates of Auger recombination in the InGaAs NWs due to their increased n-type conductivity, as confirmed by excitation power-dependent measurements. Our findings suggest that while enhancing material properties is crucial for improving the performance of hot carrier absorbers, optimizing conditions to increase the rates of Auger recombination will further boost the efficiency of these devices.

An efficient approach to achieve single-polarization selectivity in single-mode double microring lasers (photonic molecule lasers) is presented in this study. The fabricated microring lasers achieved polarization extinction ratios of 11.3 dB for TM polarization and −12.8 dB for TE polarization. The fabrication process was conducted by direct laser writing on SU-8 photoresist doped with Rhodamine B dye. In order to obtain an optimum single-mode operation with a specified polarization capability, the study employs Vernier effect double microring lasers and investigates the effects of geometric characteristics, surface scattering loss, and coupling efficiency on polarization performance. The finite-difference time-domain method was utilized to conduct simulations, and the results were verified experimentally. The proposed single-polarized microring lasers provide open possibilities for advanced integrated photonic systems with potential applications in quantum photonics, nonlinear optics, and optical sensors.

The effects of the radio frequency (RF) bias power of inductively coupled plasma etching on the electrical properties of n-type Ohmic contact have been investigated. By reducing the RF bias power, a high-quality n-type Ohmic contact has been achieved on n-Al0.70Ga0.30N, with a specific contact resistivity of 1.2 × 10−4 Ω cm2. It is confirmed that low-power etching introduces fewer acceptor-state defects on the etched surface, which not only reduces the compensation for electrons but also reduces the degree of oxidation on the etched surface, providing favorable conditions for improving the electrical properties of metal–semiconductor contacts.

Charge density waves (CDWs), valley-contrasting physics, and topological bands have been observed in 1T, 1H, and 1T′ transition metal dichalcogenides (TMDs), respectively, but rarely observed together in a single TMD due to the completely different physical origins. This study discovers 1T-MoS2 monolayers can possess the valley-contrasting physics. Furthermore, the topological bands, Dirac cones, diamond-chain, and zigzag CDW can be induced by biaxial strain, indicating the exotic roles of the strain. Our findings not only broaden the way of searching the valley semiconductors but also open a door to study the topological bands and Dirac cone of CDW phases.

Elastic properties of soft tissues are important indicators for disease progression. Previous studies have utilized mechanical resonance spectroscopy to infer elastic properties of soft tissues by extracting their resonance frequencies. However, the method to accurately obtain the elastic modulus from the resonance frequencies remains inconclusive. In this study, we report a method based on a multi-degree-of-freedom (MDOF) model to determine the Young's modulus of soft tissue samples from the measured resonance spectroscopy. Resonance frequencies of agar tissue phantoms with different elastic properties were obtained, and Young's modulus was calculated using the MDOF-based method. The result was validated by mechanical compression tests and finite element method simulations. The results show that the multi-degree-of-freedom (MDOF)-based method is capable of determining Young's modulus of soft tissue samples with various elasticities and dimensions. This study provides an opportunity to accurately assess the elastic properties of small-sized soft tissue samples.

This study investigates the steepness of the heterostructure interface between the p-side optical-waveguide and electron blocking layer (EBL) in ultraviolet-B (UV-B) laser diodes (LDs), focusing on the impact of growth temperature. The results revealed that lowering the growth temperature significantly reduced the thickness of the “unintended compositionally graded layer” a diffusion layer formed at the interface through solid-phase diffusion. However, a bottleneck also existed in LDs with extremely steep interfaces, where the diode characteristics could not be obtained due to the device's high resistance. This study highlights the trade-off between the steepness of the interfaces in the AlGaN heterostructure and diode performance, indicating the need for further optimization to achieve high-performance UV-B LDs. Specifically, future efforts should focus on refining growth conditions to reduce impurity concentrations resulting from low-temperature growth and controlling the thickness of individual layers, such as the EBL, to address high resistance and achieve high-performance UV-B LDs.

This paper highlights the critical role of solid-state quantum dot (QD) light sources in both classical and quantum applications, with an emphasis on their integration with silicon photonics to advance future optical networks and quantum technologies. Quantum dot lasers, renowned for their low threshold currents, temperature stability, low-noise optical amplification, and enhanced coherence, are highlighted as essential components for scalable quantum systems. These features contribute to improved chip architectures, reduced module sizes, and increased channel density. The paper also explores the synergy between quantum dot lasers and silicon photonics in the generation of frequency combs, optimizing efficiency and scalability in optical networks. Furthermore, it delves into the impact of quantum dot-based single-photon sources, particularly their ability to generate entangled and polarized photons, in driving advancements across quantum technologies.

A method for measuring the electron temperature in the inversion layer of Si metal-oxide-semiconductor structures is presented. This technique utilizes a nano-transistor as a thermometer, placed in close proximity to the inversion layer under investigation, enabling measurements of the electron temperature for values above approximately 10 K. When applied to Joule-heating experiments, this method reveals a notable discrepancy between the measurement results and predictions made by the conventional theory based on the deformation-potential coupling with low-energy acoustic phonons. Specifically, the injected-power dependence of the electron temperature is much weaker than expected. The results strongly suggest that another mechanism causing a significant electron energy loss plays a role.

Optical metasurfaces have enabled high-speed, low-power image processing within a compact footprint. However, reconfigurable imaging in such flat devices remains a critical challenge for fully harnessing their potential in practical applications. Here, we propose and demonstrate phase-change metasurfaces capable of dynamically switching between edge-detection and bright-field imaging in the visible spectrum. This reconfigurability is achieved through engineering angular dispersion at electric and magnetic Mie-type resonances. The customized metasurface exhibits an angle-dependent transmittance profile in the amorphous state of Sb2S3 meta-atoms for efficient isotropic edge detection, and an angle-independent profile in the crystalline state for uniform bright-field imaging. The nanostructured Sb2S3-based reconfigurable image processing metasurfaces hold significant potential for applications in computer vision for autonomous driving systems.

Two-dimensional (2D) van der Waals (vdW) materials offer vast potential for designing ferroelectrics with desired properties through simple layer stacking. Here, based on first principles, we demonstrate that the vdW layered crystals RuX2 (X = Cl, Br, and I) are a class of 2D multiferroic sliding ferroelectrics. The stacking of two magnetic RuX2 monolayers with the same orientation breaks the spatial inversion symmetry, resulting in a stable vertical polarization. In addition, the direction of polarization can be reversed through slight interlayer sliding, in which it only needs to overcome the small energy barrier of 7.16 meV. Among these layered crystals, the bilayer RuI2 not only possesses a remarkable sliding ferroelectricity of 0.49 pC/m but also exhibits stable long-range magnetic order due to its large magnetic anisotropy energy. When the RuI2 stack is increased to trilayers, the polarization significantly increases to 1.03 pC/m, which is much larger than that of its bilayer structure. Furthermore, the application of compressive strain results in a substantial increase in vertical polarization. This work provides an efficient method for designing 2D multiferroic sliding ferroelectric materials by stack engineering.

High-order harmonic generation is a cornerstone of attosecond science, with applications spanning from spectroscopy to the creation of ultrashort light pulses with temporal duration falling in the attosecond regime. In addition, light beams carrying orbital angular momentum (OAM) allow studies of light–matter interactions mediated by OAM couplings. In this work, we present an alternative approach to generating high-order harmonic vortices using elegant Laguerre–Gaussian (eLG) beams. We examine the spatiotemporal characteristics of these harmonic vortices in the far-field regime and demonstrate how the low divergence of eLG beams makes them suitable for producing extreme ultraviolet (XUV) twisted attosecond pulses. Additionally, by solving the far-field Fraunhofer integral, we analyze the influence of azimuthal and radial indices on the spatial profile of vortex beams, thereby exploring the impact of larger topological charges. This study extends the concept of harmonic vortices generated by Laguerre–Gaussian beams to applications beyond the paraxial approximation.

Internal stress in gallium nitride (GaN) induced during epitaxy growth can degrade the performance of GaN devices. This work studied the internal stress distribution and dislocation configuration around an inclusion of ∼300 μm in GaN substrates grown by hydride vapor phase epitaxy, by means of combined Raman spectroscopy, x-ray topography, and two-photon excitation photoluminescence. The inclusion-induced internal stress decreased exponentially along the radial direction. However, the internal stress, though reduced to a small magnitude, was unexpectedly maintained and propagated over long distances. A stress localization phenomenon, which was out of the prediction of classic elasticity theory, was also observed. The inclination of threading dislocations was found to be substantially influenced by the unreported distribution of internal stress. Four characteristic dislocation inclination patterns were identified: the two-short-tooth pattern, two-long-tooth pattern, gear pattern, and sun-like pattern. The dependence of internal stress on the dislocation inclination pattern was revealed. Based on this dependence, a method to predict the stress field in crystal based on dislocation pattern without corrosion was proposed.

In this work, nanoscale spin field-effect transistors (spin-FETs) based on lateral heterojunctions composed of two-dimensional (2D) ferromagnetic half-metallic Sc2CHO electrodes and nonmagnetic semiconductor Sc2CHF channel are theoretically designed. The channel lengths (Lc) for investigated nanoscale spin-FETs are shorter than 6 nm. The spin transport properties of these nanoscale spin-FETs are subsequently studied by using the nonequilibrium Green's function method in combination with density functional theory. Due to the strong electronic coupling at the interfaces between electrodes and channel, p-type Ohmic contacts are obtained for spin down. Calculations reveal that at very-low temperature, the spin injection efficiency can reach 100%, and the magnetoresistance ratio (MR) is generally larger than 109% for these nanoscale spin-FETs. Very-low subthreshold swing (SS) values below 60 mV/dec are found for spin-FETs with Lc≥ 4.05 nm, and the lowest SS value is 39 mV/dec for the spin-FET with Lc=5.75 nm. At room temperature, the values of MR exceed 106%, and the corresponding SS values are below 92 mV/dec with a minimum SS of 82 mV/dec, still demonstrating high performance for designed nanoscale spin-FETs. Our study provides valuable insights into the design of high-performance nanoscale spin-FET devices based on 2D MXenes.

Zero-dimensional/one-dimensional (0D/1D) heterojunctions have excellent potential in the field of optoelectronic devices due to the synergy effect of different dimensions. Most reported 0D/1D heterojunction photodetectors only focus on optimizing the separation efficiency of photogenerated carriers at the interface. However, the carriers within the quantum dots (QDs) cannot be transferred to the electrodes, resulting in recombination of photogenerated carriers separated at the interface. Therefore, the response speed of most 0D/1D heterojunction photodetectors is still limited to the order of seconds (s) and milliseconds (ms). In our work, we demonstrate a nanosecond (ns) scale ZnO/CuO heterojunction photodetector with efficient photoelectric conversion by engineering the type-II 0D/1D heterojunction interface. Herein, the surface defect states of ZnO QDs are deliberately introduced as “electrons storage pool” to suppress carrier recombination and further promote separation, which has been confirmed by photoluminescence (PL) and time-resolved photoluminescence (TRPL). As a result, the photodetector exhibited excellent performance with ultrafast response speed of 20 ns, responsivity of 213 A/W, and detectivity of 2.95 × 1011 Jones, respectively. This defect related interface engineering provides a feasible strategy for the development of high-performance 0D/1D heterojunction photodetectors.