Watt-level continuous-wave antimonide laser diodes with high carrier-confined active region above 2.5 µm.

Thanks to high performance above room temperature, antimonide laser diodes have shown great potential for broad application in the mid-infrared spectral region. However, the laser`s performance noticeably deteriorates due to the reduction of carrier confinement with increased emission wavelength. In this paper, a novel active region with higher carrier confinements both of electron and hole, by the usage of an indirect bandgap material of Al0.5GaAs0.04Sb as the quantum barrier, was put up to address the poor carrier confinement of GaSb-based type-I multi-quantum-well (MQW) diode lasers emission wavelength above 2.5 µm. The carrier confinement and the differential gain in the designed active region are enhanced as a result of the first proposed usage of an indirect-gap semiconductor as the quantum barrier with larger band offsets in conduction and valence bands, leading to high internal quantum efficiency and low threshold current density of our lasers. More importantly, the watt-level output optical power is obtained at a low injection current compared to the state of the art. Our work demonstrates a direct and cost-effective solution to address the poor carrier confinement of the GaSb-based MQW lasers, thereby achieving high-power mid-infrared lasers.

Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation.

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS2, MoSe2, and WSe2 van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe2/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Robust design of mid-infrared GaSb-based single-mode laser diode fabricated by standard photolithography with socketed ridge-waveguide modulation.

In this paper, we put up a robust design of a stable single-mode-operated GaSb-based laser diode emitting around 1950nm. This novel design structure with socketed ridge-waveguide enables a simple fabrication and batch production of mid-infrared laser diodes on account of the mere usage of standard photolithography. By introducing micron-level index perturbations distributed along the ridge waveguide, the threshold gains of different FP modes are modulated. Four geometrical parameters of the perturbations are systematically optimized by analyzing the reflection spectrum to get a robust single-mode characteristic. Based on the optimized geometrical parameters, 1-mm long uncoated lasers are carried out and exhibit a stable single longitudinal mode from 10 °C to 40 °C with a maximum output power of more than 10 mW. Thus, we prove the feasibility of the standard photolithography to manufacture the monolithic single-mode infrared laser source without regrowth process or nanoscale lithography.

Development of a cryogen-free sub-3 K low-temperature scanning probe microscope by remote liquefaction scheme.

We developed a new scheme for cryogen-free cooling down to sub-3 K temperature range and ultra-low vibration level. An ultra-high-vacuum cryogen-free scanning probe microscope (SPM) system was built based on the new scheme. Instead of mounting a below-decoupled cryocooler directly onto the system, the new design was realized by integrating a Gifford-McMahon cryocooler into a separate liquefying chamber, providing two-stage heat exchangers in a remote way. About 10 L of helium gas inside the gas handling system was cooled, liquefied in the liquefying chamber, and then transferred to a continuous-flow cryostat on the SPM chamber through an ∼2 m flexible helium transfer line. The exhausted helium gas from the continuous-flow cryostat was then returned to the liquefying chamber for reliquefaction. A base temperature of ∼2.84 K at the scanner sample stage and a temperature fluctuation of almost within ±0.1 mK at 4 K were achieved. The cooling curves, tunneling current noise, variable-temperature test, scanning tunneling microscopy and non-contact atomic force microscopy imaging, and first and second derivatives of I(V) spectra are characterized to verify that the performance of our cryogen-free SPM system is comparable to the bath cryostat-based low-temperature SPM system. This remote liquefaction close-cycle scheme shows conveniency to upgrade the existing bath cryostat-based SPM system, upgradeability of realizing even lower temperature down to sub-1 K range, and great compatibility of other physical environments, such as high magnetic field and optical accesses. We believe that the new scheme could also pave a way for other cryogenic applications requiring low temperature but sensitive to vibration.

Annealing-Modulated Surface Reconstruction for Self-Assembly of High-Density Uniform InAs/GaAs Quantum Dots on Large Wafers Substrate.

In this work, we developed pre-grown annealing to form ß2 reconstruction sites among ß or α (2 × 4) reconstruction phase to promote nucleation for high-density, size/wafer-uniform, photoluminescence (PL)-optimal InAs quantum dot (QD) growth on a large GaAs wafer. Using this, the QD density reached 580 (860) µm-2 at a room-temperature (T) spectral FWHM of 34 (41) meV at the wafer center (and surrounding) (high-rate low-T growth). The smallest FWHM reached 23.6 (24.9) meV at a density of 190 (260) µm-2 (low-rate high-T). The mediate rate formed uniform QDs in the traditional ß phase, at a density of 320 (400) µm-2 and a spectral FWHM of 28 (34) meV, while size-diverse QDs formed in ß2 at a spectral FWHM of 92 (68) meV and a density of 370 (440) µm-2. From atomic-force-microscope QD height distribution and T-dependent PL spectroscopy, it is found that compared to the dense QDs grown in ß phase (mediate rate, 320 µm-2) with the most large dots (240 µm-2), the dense QDs grown in ß2 phase (580 µm-2) show many small dots with inter-dot coupling in favor of unsaturated filling and high injection to large dots for PL. The controllable annealing (T, duration) forms ß2 or ß2-mixed α or ß phase in favor of a wafer-uniform dot island and the faster T change enables optimal T for QD growth.

Controllable spin-resolved photon emission enhanced by a slow-light mode in photonic crystal waveguides on a chip.

We report the slow-light enhanced spin-resolved in-plane emission from a single quantum dot (QD) in a photonic crystal waveguide (PCW). The slow light dispersions in PCWs are designed to match the emission wavelengths of single QDs. The resonance between two spin states emitted from a single QD and a slow light mode of a waveguide is investigated under a magnetic field with Faraday configuration. Two spin states of a single QD experience different degrees of enhancement as their emission wavelengths are shifted by combining diamagnetic and Zeeman effects with an optical excitation power control. A circular polarization degree up to 0.81 is achieved by changing the off-resonant excitation power. Strongly polarized photon emission enhanced by a slow light mode shows great potential to attain controllable spin-resolved photon sources for integrated optical quantum networks on chip.

Bright semiconductor single-photon sources pumped by heterogeneously integrated micropillar lasers with electrical injections.

The emerging hybrid integrated quantum photonics combines the advantages of different functional components into a single chip to meet the stringent requirements for quantum information processing. Despite the tremendous progress in hybrid integrations of III-V quantum emitters with silicon-based photonic circuits and superconducting single-photon detectors, on-chip optical excitations of quantum emitters via miniaturized lasers towards single-photon sources (SPSs) with low power consumptions, small device footprints, and excellent coherence properties is highly desirable yet illusive. In this work, we present realizations of bright semiconductor SPSs heterogeneously integrated with on-chip electrically-injected microlasers. Different from previous one-by-one transfer printing technique implemented in hybrid quantum dot (QD) photonic devices, multiple deterministically coupled QD-circular Bragg Grating (CBG) SPSs were integrated with electrically-injected micropillar lasers at one time via a potentially scalable transfer printing process assisted by the wide-field photoluminescence (PL) imaging technique. Optically pumped by electrically-injected microlasers, pure single photons are generated with a high-brightness of a count rate of 3.8 M/s and an extraction efficiency of 25.44%. Such a high-brightness is due to the enhancement by the cavity mode of the CBG, which is confirmed by a Purcell factor of 2.5. Our work provides a powerful tool for advancing hybrid integrated quantum photonics in general and boosts the developments for realizing highly-compact, energy-efficient and coherent SPSs in particular.

Promotion of Specific Single-Transverse-Mode Beam Characteristics for GaSb-Based Narrow Ridge Waveguide Lasers via Customized Parameter Design.

GaSb-based single-transverse-mode narrow ridge waveguide (RW) lasers with high power and simultaneous good beam quality have broad application prospects in the mid-infrared wavelength region. Yet its design and formation have not been investigated systematically, while the beam characteristics that affect their suitability for specific applications remain rarely analyzed and optimized. The present work addresses these issues by theoretically establishing a waveguide parameter domain that generalizes the overall possible combinations of ridge widths and etch depths that support single-transverse-mode operation for GaSb-based RW lasers. These results are applied to develop two distinct and representative waveguide designs derived from two proposed major optimization routes of model gain expansion and index-guiding enhancement. The designs were evaluated experimentally based on prototype 1-mm cavity-length RW lasers in the 1950 nm wavelength range, which were fabricated with waveguides having perpendicular ridge and smooth side-walls realized through optimized dry etching conditions. The model gain expanded RW laser design with a relatively shallow-etched (i.e., 1.55 [Formula: see text]m) and wide ridge (i.e., 7 [Formula: see text]m) yielded the highest single-transverse-mode power to date of 258 mW with a narrow lateral divergence angle of 11.1[Formula: see text] full width at half maximum at 800 mA under room-temperature continuous-wave operation, which offers promising prospects in pumping and coupling applications. Meanwhile, the index-guiding enhanced RW laser design with a relatively deeply etched (i.e., 2.05 [Formula: see text]m) and narrow ridge (i.e., 4 [Formula: see text]m) provided a highly stable and nearly astigmatism-free fundamental mode emission with an excellent beam quality of M[Formula: see text] factor around 1.5 over the entire operating current range, which is preferable for seeding external cavity applications and complex optical systems.

High-performance infrared photodetectors based on InAs/InAsSb/AlAsSb superlattice for 3.5†µm cutoff wavelength spectra.

High-performance infrared p-i-n photodetectors based on InAs/InAsSb/AlAsSb superlattices on GaSb substrate have been demonstrated at 300K. These photodetectors exhibit 50% and 100% cut-off wavelength of ∼3.2 µm and ∼3.5 µm, respectively. Under -130 mV bias voltage, the device exhibits a peak responsivity of 0.56 A/W, corresponding to a quantum efficiency (QE) of 28%. The dark current density at 0 mV and -130 mV bias voltage are 8.17 × 10-2 A/cm2 and 5.02 × 10-1 A/cm2, respectively. The device exhibits a saturated dark current shot noise limited specific detectivity (D*) of 3.43 × 109 cm·Hz1/2/W (at a peak responsivity of 2.5 µm) under -130 mV of applied bias.

Terahertz cavity optomechanics using a topological nanophononic superlattice.

Cavity optomechanical systems operating at the quantum ground state provide a novel way for the ultrasensitive measurement of mass and displacement and provide a new toolbox for emerging quantum information technologies. The high-frequency optomechanical devices could reach the quantum ground state at a high temperature because the access to high frequency is favorable for the cavity optomechanical devices to decouple from the thermal environment. However, reaching ultra-high frequency (THz) is extremely difficult due to the structure of cavity optomechanical devices and properties of materials. In this paper, by introducing acoustic topological interface states, we designed a THz mechanical frequency semiconductor pillar microcavity optomechanical device based on a GaAs/AlAs nanophononic superlattice. In the optomechanical system, multi-optical cavity modes are obtained and the frequency separation between adjacent optical modes is equal to the frequency of the mechanical mode (optomechanical frequency matching). By detuning the laser pump to a lower (higher) energy-resolved sideband to make a spontaneously scattering photon doubly resonate with optical cavity modes at an anti-Stokes (Stokes) frequency and pump frequency, we can achieve an anti-Stokes (Stokes) scattering efficiency 2600 (1800) times larger than that of Stokes (anti-Stokes) scattering, which provides potential for laser cooling and low threshold phonon lasing in the optomechanical system.

Fine modulation of the energy band strategy to control the carrier confinement capability of digital alloys.

In this paper, a strategy to finely modulate the energy band structure to control the carrier confinement capability of digital alloys (DA) is proposed. Strain analysis shows that As and Sb atoms are exchanged within the AlAsSb DA. The bottom of the corrected potential well is low on the left and high on the right in the growth direction, resulting in a higher band offset of the AlSb potential barrier layer on the left side of the potential well than on the right side. The modulation of the band leads to a higher probability of electron tunneling in DA under the action of an electric field opposite to the growth direction. Conversely, it is difficult for the electrons to tunnel into the lower energy level potential wells. TheI-Vcurve of DA shows that the current value under positive bias is significantly smaller than the value under negative bias when the voltage is higher. The measured results correspond perfectly with the modified energy band model, which verifies the feasibility of energy band modulation. This is important for the structural design of DA and the reduction of dark current in optoelectronic devices.

Single- and Twin-Photons Emitted from Fiber-Coupled Quantum Dots in a Distributed Bragg Reflector Cavity.

In this work, we develop single-mode fiber devices of an InAs/GaAs quantum dot (QD) by bonding a fiber array with large smooth facet, small core, and small numerical aperture to QDs in a distributed Bragg reflector planar cavity with vertical light extraction that prove mode overlap and efficient output for plug-and-play stable use and extensive study. Modulated Si doping as electron reservoir builds electric field and level tunnel coupling to reduce fine-structure splitting (FSS) and populate dominant XX and higher excitons XX+ and XXX. Epoxy package thermal stress induces light hole (lh) with various behaviors related to the donor field: lh h1 confined with more anisotropy shows an additional XZ line (its space to the traditional X lines reflects the field intensity) and larger FSS; lh h2 delocalized to wetting layer shows a fast h2-h1 decay; lh h2 confined shows D3h symmetric higher excitons with slow h2-h1 decay and more confined h1 to raise h1-h1 Coulomb interaction.

Tailoring solid-state single-photon sources with stimulated emissions.

The coherent interaction of electromagnetic fields with solid-state two-level systems can yield deterministic quantum light sources for photonic quantum technologies. To date, the performance of semiconductor single-photon sources based on three-level systems is limited mainly due to a lack of high photon indistinguishability. Here we tailor the cavity-enhanced spontaneous emission from a ladder-type three-level system in a single epitaxial quantum dot through stimulated emission. After populating the biexciton (XX) of the quantum dot through two-photon resonant excitation, we use another laser pulse to selectively depopulate the XX state into an exciton (X) state with a predefined polarization. The stimulated XX-X emission modifies the X decay dynamics and improves the characteristics of a polarized single-photon source, such as a source brightness of 0.030(2), a single-photon purity of 0.998(1) and an indistinguishability of 0.926(4). Our method can be readily applied to existing quantum dot single-photon sources and expands the capabilities of three-level systems for advanced quantum photonic functionalities.

High spectral purity GaSb-based blazed grating external cavity laser with tunable single-mode operation around 1940nm.

In this article, we present a tunable GaSb-based blazed grating external cavity laser (BG-ECL) with high spectral purity and high output power single-mode operation around 1940nm. The drastic increase in spectral selectivity and optical power results from the employment of a single-transverse-mode operating narrow ridge waveguide laser diode with an optimized AR coating on the front facet. The stable fundamental spatial mode output beam from the laser diode enables efficient collimation and high coupling efficiency with the blazed grating, leading to stronger wavelength-selective feedback. The AR coating with proper low reflectivity on the straight waveguide effectively suppresses the internal cavity mode lasing without causing extra optical loss. As a result, the BG-ECL device exhibits excellent comprehensive performance with a side mode suppression ratio (SMSR) over 50 dB with optical power exceeding 30 mW within a 70 nm tuning range. A maximum SMSR of 56.26 dB with 35.12 mW output power was observed in continuous-wave operation. By increasing the working temperature of the diode laser, the tuning range can be further extended to over 100 nm without noticeable degradation in spectral and output power performance.

Optimization and robustness of the topological corner state in second-order topological photonic crystals.

The second-order topological photonic crystal with the 0D corner state provides a new way to investigate cavity quantum electrodynamics and develop topological nanophotonic devices with diverse functionalities. Here, we report on the optimization and robustness of the topological corner state in the second-order topological photonic crystal both in theory and in experiment. The topological nanocavity is formed based on the 2D generalized Su-Schrieffer-Heeger model. The quality factor of the corner state is optimized theoretically and experimentally by changing the gap between two photonic crystals or just modulating the position or size of the airholes surrounding the corner. The fabricated quality factors are further optimized by the surface passivation treatment which reduces surface absorption. A maximum quality factor of the fabricated devices is about 6000, which is the highest value ever reported for the active topological corner state. Furthermore, we demonstrate the robustness of the corner state against strong disorders including the bulk defect, edge defect, and even corner defect. Our results lay a solid foundation for further investigations and applications of the topological corner state, such as the investigation of a strong coupling regime and the development of optical devices for topological nanophotonic circuitry.

Insights into Growth-Oriented Interfacial Modulation within Semiconductor Multilayers.

Interfacial engineering plays a crucial role in regulating the quality and property of heterogeneous structures, especially for nanometer-scaled devices. However, traditional methods for interfacial modulation (IFM) generally treat all the interfaces uniformly, neglecting the inherent disparities of interfaces like their growth sequence. Herein, it is found that the growth-oriented characteristic of IFM strongly determines the main regions where the modulation takes effect. Specifically, in a semiconductor quantum well structure, the arsenic atoms modulated at the well-on-barrier (WoB) interface tend to diffuse into and thus affect the next-grown well layer. In contrast, the arsenic atoms introduced at the barrier-on-well (BoW) interface mainly take effect within the next-grown barrier layer. According to theoretical simulations and electron holography (EH) experiments, the depth of quantum wells and the height of potential barriers are extended by introducing arsenic atoms at WoB and BoW interfaces, respectively. Resultantly, while modulating at the BoW interface has little impact on the photoluminescence (PL) spectrum, applying IFM at the WoB interface could dramatically improve the luminescent intensity (about 30%), which demonstrates the impact of the growth-oriented characteristic. Furthermore, in situ bias EH results indicate that IFM at the WoB interface helps to suppress the quantum-confined Stark effect.

High-Performance Anodic Vulcanization-Pretreated Gated P+-π-M-N+ InAs/GaSb Superlattice Long-Wavelength Infrared Detector.

The InAs/GaSb superlattice infrared detector has been developed with tremendous effort. However, the performance of it, especially long-wavelength infrared detectors (LWIR), is still limited by the electrical performance and optical quantum efficiency (QE). Forcing the active region to be p-type through proper doping can highly improve QE, and the gating technique can be employed to greatly enhance electrical performance. However, the saturation bias voltage is too high. Reducing the saturation bias voltage has broad prospects for the future application of gate voltage control devices. In this paper, we report that the gated P+-π-M-N+ InAs/GaSb superlattice long-wavelength infrared detectors exhibit different π region doping levels that have a reduced minimum saturation bias at - 10 V with a 200-nm SiO2 layer after a simple and effective anodic vulcanization pretreatment. The saturation gate bias voltage is much lower than - 40 V that reported with the same thickness of a 200-nm SiO2 passivation layer and similar structure. The optical and electrical characterization indicates that the electrical and optical performance of the device would be weakened by excessive doping concentration. At 77 K, the 50% cutoff wavelength of the device is about 8 µm, the 100% cutoff wavelength is 10 µm, the maximum quantum efficiency is 62.4%, the maximum of responsivity is 2.26 A/W at 5 µm, and the maximum RA of the device is 1259.4 Ω cm2. Besides, the specific detectivity of Be 780 °C-doped detector without gate electrode exhibits a peak of 5.6 × 1010 cm Hz1/2/W at 5 µm with a 70-mv reverse bias voltage, which is more than three times that of Be 820 °C-doped detector. Moreover, the peak specific detectivity could be further increased to 1.3 × 1011 cm Hz1/2/W at 5 µm with a 10-mv reserve bias voltage that has the bias of - 10 V at the gate electrode.

Wet-Etched Microlens Array for 200 nm Spatial Isolation of Epitaxial Single QDs and 80 nm Broadband Enhancement of Their Quantum Light Extraction.

Uniform arrays of three shapes (gauss, hat, and peak) of GaAs microlenses (MLs) by wet-etching are demonstrated, ∼200 nm spatial isolation of epitaxial single QDs embedded (λ: 890-990 nm) and broadband (Δλ∼80 nm) enhancement of their quantum light extraction are obtained, which is also suitable for telecom-band epitaxial QDs. Combined with the bottom distributed Bragg reflector, the hat-shaped ML forms a cavity and achieves the best enhancement: extraction efficiency of 26%, Purcell factor of 2 and single-photon count rate of 7×106 counts per second at the first lens; while the gauss-shaped ML shows a broader band (e.g., longer λ) enhancement. In the MLs, single QDs with featured exciton emissions are observed, whose time correlations prove single-photon emission with multi-photon probability g(2)(0)=0.02; some QDs show both biexciton XX and exciton X emissions and exhibit a perfect cascade feature. This work could pave a step towards a scalable array of QD single-photon sources and the application of QD photon-pair emission for entanglement experiments.

Self-Induced Dark States in Two-Dimensional Excitons.

We have obtained an ultralong lifetime exciton emission in InAs/GaAs single quantum dots (QDs) when the QD films are transferred onto the Si substrate covered by Ag nanoparticles. It is found that when the separation distance from the QD layer (also the wetting layer) to the Ag nanoparticles is around 19 nm, the QD emission lifetime changes from approximately 1 to 2000 ns. A classical dipole oscillator model is used to quantitatively calculate the spontaneous radiation decay rate of the excitons in the wetting layer (WL), and the simulated calculation result is in good agreement with the experimental one, revealing that the long lifetime exciton emission is due to the existence of the dark state in the WL. The self-induced dark state stems from the destructive interference between the exciton emission field and the induced dipole field of the Ag nanoparticles.

Boost of single-photon emission by perfect coupling of InAs/GaAs quantum dot and micropillar cavity mode.

We proposed a precise calibration process of Al 0.9Ga0.1As/GaAs DBR micropillar cavity to match the single InAs/GaAs quantum dot (QD) exciton emission and achieve cavity mode resonance and a great enhancement of QD photoluminescence (PL) intensity. Light-matter interaction of single QD in DBR micropillar cavity (Q ∼ 3800) under weak coupling regime was investigated by temperature-tuned PL spectra; a pronounced enhancement (14.6-fold) of QD exciton emission was observed on resonance. The second-order autocorrelation measurement shows g(2)(0)=0.070, and the estimated net count rate before the first objective lens reaches 1.6×107 counts/s under continuous wave excitation, indicating highly pure single-photon emission at high count rates.