N-Channel MOSFET Reliability Issue Induced by Visible/Near-Infrared Photons in Image Sensors.

Image sensors such as single-photon avalanched diode (SPAD) arrays typically adopt in-pixel quenching and readout circuits, and the under-illumination first-stage readout circuits often employs high-threshold input/output (I/O) or thick-oxide metal-oxide-semiconductor field-effect transistors (MOSFETs). We have observed reliability issues with high-threshold n-channel MOSFETs when they are exposed to strong visible light. The specific stress conditions have been applied to observe the drain current (Id) variations as a function of gate voltage. The experimental results indicate that photo-induced hot electrons generate interface trap states, leading to Id degradation including increased off-state current (Ioff) and decreased on-state current (Ion). The increased Ioff further activates parasitic bipolar junction transistors (BJT). This reliability issue can be avoided by forming an inversion layer in the channel under appropriate bias conditions or by reducing the incident photon energy.

Room-temperature two-dimensional plasmonic crystal semiconductor lasers.

Room-temperature plasmonic-crystal lasers have been demonstrated with a square-lattice gold nano-pillar arrays on top of InGaAs/GaAs quamtum wells on a GaAs substrate. The lasing wavelength is tunable in the range of 865-1001 nm by varying the lattice period. The lasers exhibit an extremely narrow linewidth and small divergence angle so could have great potential for various applications. An unexpected mirror cavity effect has been observed and investigated. The mirror-cavity lasers have a very low threshold and could be developed to realize electrically-driven plasmonic lasers.

Room-Temperature Macroscopic Coherence of Two Electron-Hole Plasmas in a Microcavity.

Macroscopic coherence of Bose condensates is a fundamental and practical phenomenon in many-body systems, such as the long-range correlation of exciton-polariton condensates with a dipole density typically below the exciton Mott-transition limit. Here we extend the macroscopic coherence of electron-hole-photon interacting systems to a new region in the phase diagram-the high-density plasma region, where long-range correlation is generally assumed to be broken due to the rapid dephasing. Nonetheless, a cooperative state of electron-hole plasma does emerge through the sharing of the superfluorescence field in an optical microcavity. In addition to the in situ coherence of e-h plasma, a long-range correlation is formed between two 8-µm-spaced plasma ensembles even at room temperature. Quantized and self-modulated correlation modes are generated for e-h ensembles in the plasma region. By controlling the distance between the two ensembles, multiple coupling regimes are revealed, from strong correlation to perturbative phase correlation and finally to an incoherent classical case, which has potential implications for tunable and high-temperature-compatible quantum devices.

Berezinskii-Kosterlitz-Thouless transition in an Al superconducting nanofilm grown on GaAs by molecular beam epitaxy.

We have performed extensive transport experiments on a 4 nm thick aluminum (Al) superconducting film grown on a GaAs substrate by molecular beam epitaxy (MBE). Nonlinear current-voltage (I-V) measurements on such a MBE-grown superconducting nanofilm show that V âˆ¼ I 3, which is evidence for the Berezinskii-Kosterlitz-Thouless (BKT) transition, both in the low-voltage (T BKT ≈ 1.97 K) and high-voltage regions (T BKT ≈ 2.17 K). In order to further study the two regions where the I-V curves are BKT-like, our experimental data are fitted to the temperature-induced vortices/antivortices unbinding model as well as the dynamical scaling theory. It is found that the transition temperature obtained in the high-voltage region is the correct T BKT as confirmed by fitting the data to the aforementioned models. Our experimental results unequivocally show that I-V measurements alone may not allow one to determine T BKT for superconducting transition. Therefore, one should try to fit one's results to the temperature-induced vortices/antivortices unbinding model and the dynamical scaling theory to accurately determine T BKT in a two-dimensional superconductor.

Photon-Detection-Probability Simulation Method for CMOS Single-Photon Avalanche Diodes.

Single-photon avalanche diodes (SPADs) in complementary metal-oxide-semiconductor (CMOS) technology have excellent timing resolution and are capable to detect single photons. The most important indicator for its sensitivity, photon-detection probability (PDP), defines the probability of a successful detection for a single incident photon. To optimize PDP is a cost- and time-consuming task due to the complicated and expensive CMOS process. In this work, we have developed a simulation procedure to predict the PDP without any fitting parameter. With the given process parameters, our method combines the process, the electrical, and the optical simulations in commercially available software and the calculation of breakdown trigger probability. The simulation results have been compared with the experimental data conducted in an 800-nm CMOS technology and obtained a good consistence at the wavelength longer than 600 nm. The possible reasons for the disagreement at the short wavelength have been discussed. Our work provides an effective way to optimize the PDP of a SPAD prior to its fabrication.

High-Operation-Temperature Plasmonic Nanolasers on Single-Crystalline Aluminum.

The recent development of plasmonics has overcome the optical diffraction limit and fostered the development of several important components including nanolasers, low-operation-power modulators, and high-speed detectors. In particular, the advent of surface-plasmon-polariton (SPP) nanolasers has enabled the development of coherent emitters approaching the nanoscale. SPP nanolasers widely adopted metal-insulator-semiconductor structures because the presence of an insulator can prevent large metal loss. However, the insulator is not necessary if permittivity combination of laser structures is properly designed. Here, we experimentally demonstrate a SPP nanolaser with a ZnO nanowire on the as-grown single-crystalline aluminum. The average lasing threshold of this simple structure is 20 MW/cm(2), which is four-times lower than that of structures with additional insulator layers. Furthermore, single-mode laser operation can be sustained at temperatures up to 353 K. Our study represents a major step toward the practical realization of SPP nanolasers.

Method to evaluate afterpulsing probability in single-photon avalanche diodes.

We propose and demonstrate a new method for evaluating the afterpulsing effect in single-photon avalanche photodiodes (SPADs). By analyzing the statistical property of dark count rate, we can quantitatively characterize afterpulsing probability (APP) of a SPAD. In experiment, the temperature-dependent low dark count rate (DCR) distribution becomes non-Poissonian at lower temperature and has higher excess bias as the afterpulsing effect becomes significant. Our work provides a flexible way to examine APP in either single-device or circuit level.

Polarized emission of quantum dots in microcavity and anisotropic Purcell factors.

We study the polarization properties of quantum dot (QD) emission coupled with the fundamental cavity modes. A rotation of polarization axis and a change of polarization degree are observed as the coupling is varied. To explain this observation, we derive an analytical model considering the polarization misalignment between QD dipole and cavity mode field. Our model also provides a new approach to extract the anisotropic Purcell factors by analyzing the polarization of detected quantum dot emission coupled to the cavity mode, which paves the way to develop high-efficiency polarized single photon sources.

Two-dimensional photo-mapping on CMOS single-photon avalanche diodes.

Two-dimensional (2-D) photo-count mapping on CMOS single photon avalanche diodes (SPADs) has been demonstrated. Together with the varied incident wavelengths, the depth-dependent electric field distribution in active region has been investigated on two SPADs with different structures. Clear but different non-uniformity of photo-response have been observed for the two studied devices. With the help of simulation tool, the non-uniform photo-counts arising from the electric field non-uniformity have been well explained. As the quasi-3D distribution of electric field in the active region can be mapped, our method is useful for engineering the device structure to improve the photo-response of SPADs.

High-quality planar light emitting diode formed by induced two-dimensional electron and hole gases.

A high-quality planar two-dimensional p-i-n light emitting diode in an entirely undoped GaAs/AlGaAs quantum well has been fabricated by using conventional lithography process. With twin gate design, two-dimensional electron and hold gases can be placed closely on demand. The electroluminescence of the device exhibit high stability and clear transition peaks so it is promising for applications on electrically-driven single photon sources.

Role of incoherent pumping in a strongly coupled system of exchange-split excitons and pillar-microcavities.

We present an experimental study of the influence of incoherent pumping upon the strong coupling between quantum dot excitons and a micropillar-cavity. For two exchange-split excitons, the inherent difference in the detuning with respect to the cavity modes leads to an unequal reduction of Rabi splitting for two orthogonal polarizations as excitation power is increased, which could destroy the indistinguishable decay paths built by the coupled states. This work prompts a careful implementation of optical pumping for the pursuit of polarization-entangled photon pairs in the strong coupling regime.

Low-noise single-photon avalanche diodes in 0.25 µm high-voltage CMOS technology.

By using 0.25 µm high-voltage CMOS technology, we have designed and fabricated a structure of single-photon detectors. The new single-photon avalanche diode (SPAD) has (to our knowledge) the lowest dark count rate per unit area at room temperature without any technology customization. Our design is promising for realizing low-cost and high-performance SPAD arrays for imaging applications.

Design and demonstration of high quality-factor H1-cavity in two-dimensional photonic crystal.

We introduce a method for designing an H1 photonic crystal cavity to enhance its quality factor (Q factor). The highest theoretical Q factor of 120,000 is obtained. The Fourier transformation of field distribution shows that the enhancement arises from the component reduction of a leaky mode. The Q-factor improvement has also been demonstrated experimentally with the highest value of 11,700. Our design could be useful for studying light-matter interaction in an H1 cavity as the mode volume only increases slightly.

Long-wavelength mid-infrared reflectors using guided-mode resonance.

We have proposed and fabricated a new mid-infrared reflector using the guided-mode resonance (GMR). The GMR reflector consists of subwavelength Ge grating on GaAs substrate with a low-refractive-index SiOx layer in between. With a total thickness of about 2 µm, a near-100% reflectivity at 8 µm has been obtained both theoretically and experimentally.

Imbalanced initial populations between dark and bright states in semiconductor quantum dots.

We present the observation and analysis of long-lived exciton in individual InAs quantum dots (QDs). The general model considering the interplay between dark and bright states reveals the two key factors responsible for the long decay time: the shortened spin-flip time at elevated temperature and the imbalanced initial populations between the dark and bright states. The later one plays a key role in the unusual phenomena and leads to the possibility of spin-dependent relaxation process in QDs.

Selecting detection wavelength of resonant cavity-enhanced photodetectors by guided-mode resonance reflectors.

We propose and demonstrate a novel device structure of resonant cavity-enhanced photodetector (RCE-PD). The new RCE-PD structure consists of a bottom distributed Bragg reflector (DBR), a cavity with InGaAs multiple quantum wells (MQWs) for light absorption and a top mirror of sub-wavelength grating. By changing the fill factor of the 2-D grating, the effective cavity length of RCE-PDs can be varied so the resonant wavelength can be selected post growth. Accordingly, we can fabricate an array of PDs on a single chip, on which every PD aims for a specific wavelength.

Strong coupling of different cavity modes in photonic molecules formed by two adjacent microdisk microcavities.

Strong couplings between cavity modes in photonic molecules formed by two preselected nearly identical microdisk microcavities with embedded quantum dots are investigated. By continuously tuning the refractive index of one microdisk, clear anticrossings in the resonant peak energies associated with crossings in the peak linewidths can be observed. The coupling strengths are extracted by the coupled mode theory and analyzed by the model considering the effective potential confining the electromagnetic waves in the microcavities.

Surface roughness effects on aluminium-based ultraviolet plasmonic nanolasers.

We systematically investigate the effects of surface roughness on the characteristics of ultraviolet zinc oxide plasmonic nanolasers fabricated on aluminium films with two different degrees of surface roughness. We demonstrate that the effective dielectric functions of aluminium interfaces with distinct roughness can be analysed from reflectivity measurements. By considering the scattering losses, including Rayleigh scattering, electron scattering, and grain boundary scattering, we adopt the modified Drude-Lorentz model to describe the scattering effect caused by surface roughness and obtain the effective dielectric functions of different Al samples. The sample with higher surface roughness induces more electron scattering and light scattering for SPP modes, leading to a higher threshold gain for the plasmonic nanolaser. By considering the pumping efficiency, our theoretical analysis shows that diminishing the detrimental optical losses caused by the roughness of the metallic interface could effectively lower (~33.1%) the pumping threshold of the plasmonic nanolasers, which is consistent with the experimental results.

Single-crystalline aluminum film for ultraviolet plasmonic nanolasers.

Significant advances have been made in the development of plasmonic devices in the past decade. Plasmonic nanolasers, which display interesting properties, have come to play an important role in biomedicine, chemical sensors, information technology, and optical integrated circuits. However, nanoscale plasmonic devices, particularly those operating in the ultraviolet regime, are extremely sensitive to the metal and interface quality. Thus, these factors have a significant bearing on the development of ultraviolet plasmonic devices. Here, by addressing these material-related issues, we demonstrate a low-threshold, high-characteristic-temperature metal-oxide-semiconductor ZnO nanolaser that operates at room temperature. The template for the ZnO nanowires consists of a flat single-crystalline Al film grown by molecular beam epitaxy and an ultrasmooth Al2O3 spacer layer synthesized by atomic layer deposition. By effectively reducing the surface plasmon scattering and metal intrinsic absorption losses, the high-quality metal film and the sharp interfaces formed between the layers boost the device performance. This work should pave the way for the use of ultraviolet plasmonic nanolasers and related devices in a wider range of applications.

Transient dual-energy lasing in a semiconductor microcavity.

We demonstrate sequential lasing at two well-separated energies in a highly photoexcited planar microcavity at room temperature. Two spatially overlapped lasing states with distinct polarization properties appear at energies more than 5 meV apart. Under a circularly polarized nonresonant 2 ps pulse excitation, a sub-10-ps transient circularly polarized high-energy (HE) state emerges within 10 ps after the pulse excitation. This HE state is followed by a pulsed state that lasts for 20-50 ps at a low energy (LE) state. The HE state is highly circularly polarized as a result of a spin-preserving stimulated process, while the LE state shows a significantly reduced circular polarization because of a diminishing spin imbalance.