Quantum Light Generation Based on GaN Microring toward Fully On-Chip Source.

An integrated quantum light source is increasingly desirable in large-scale quantum information processing. Despite recent remarkable advances, a new material platform is constantly being explored for the fully on-chip integration of quantum light generation, active and passive manipulation, and detection. Here, for the first time, we demonstrate a gallium nitride (GaN) microring based quantum light generation in the telecom C-band, which has potential toward the monolithic integration of quantum light source. In our demonstration, the GaN microring has a free spectral range of 330 GHz and a near-zero anomalous dispersion region of over 100 nm. The generation of energy-time entangled photon pair is demonstrated with a typical raw two-photon interference visibility of 95.5±6.5%, which is further configured to generate a heralded single photon with a typical heralded second-order autocorrelation g_{H}^{(2)}(0) of 0.045±0.001. Our results pave the way for developing a chip-scale quantum photonic circuit.

Editorial of the Special Issue 'Nano-Optics and Nano-Optoelectronics: Challenges and Future Trends'.

Through nano-optics and nano-optoelectronics, we can investigate the characteristics of light at the nanometer scale and the interaction of nanometer-scale objects with light [...].

The Tunable Electronic and Optical Properties of Two-Dimensional Bismuth Oxyhalides.

Two-dimensional (2D) bismuth oxyhalides (BiOX) have attracted much attention as potential optoelectronic materials. To explore their application diversity, we herewith systematically investigate the tunable properties of 2D BiOX using first-principles calculations. Their electronic and optical properties can be modulated by changing the number of monolayers, applying strain, and/or varying the halogen composition. The band gap shrinks monotonically and approaches the bulk value, the optical absorption coefficient increases, and the absorption spectrum redshifts as the layer number of 2D BiOX increases. The carrier transport property can be improved by applying tensile strain, and the ability of photocatalytic hydrogen evolution can be obtained by applying compressive strain. General strain engineering will be effective in linearly tuning the band gap of BiOX in a wide strain range. Strain, together with halogen composition variation, can tune the optical absorption spectrum to be on demand in the range from visible to ultraviolet. This suggests that 2D BiOX materials can potentially serve as tunable novel photodetectors, can be used to improve clean energy techniques, and have potential in the field of flexible optoelectronics.

Very high-frequency, gate-tunable CrPS4 nanomechanical resonator with single mode.

Two-dimensional (2D) antiferromagnetic semiconductor chromium thiophosphate (CrPS4) has gradually become a major candidate material for low-dimensional nanoelectromechanical devices due to its remarkable structural, photoelectric characteristics and potentially magnetic properties. Here, we report the experimental study of a new few-layer CrPS4 nanomechanical resonator demonstrating excellent vibration characteristics through the laser interferometry system, including the uniqueness of resonant mode, the ability to work at the very high frequency, and gate tuning. In addition, we demonstrate that the magnetic phase transition of CrPS4 strips can be effectively detected by temperature-regulated resonant frequencies, which proves the coupling between magnetic phase and mechanical vibration. We believe that our findings will promote the further research and applications of the resonator for 2D magnetic materials in the field of optical/mechanical signal sensing and precision measurement.

Structural, Electronic and Optical Properties of Some New Trilayer Van de Waals Heterostructures.

Constructing two-dimensional (2D) van der Waals (vdW) heterostructures is an effective strategy for tuning and improving the characters of 2D-material-based devices. Four trilayer vdW heterostructures, BP/BP/MoS2, BlueP/BlueP/MoS2, BP/graphene/MoS2 and BlueP/graphene/MoS2, were designed and simulated using the first-principles calculation. Structural stabilities were confirmed for all these heterostructures, indicating their feasibility in fabrication. BP/BP/MoS2 and BlueP/BlueP/MoS2 lowered the bandgaps further, making them suitable for a greater range of applications, with respect to the bilayers BP/MoS2 and BlueP/MoS2, respectively. Their absorption coefficients were remarkably improved in a wide spectrum, suggesting the better performance of photodetectors working in a wide spectrum from mid-wave (short-wave) infrared to violet. In contrast, the bandgaps in BP/graphene/MoS2 and BlueP/graphene/MoS2 were mostly enlarged, with a specific opening of the graphene bandgap in BP/graphene/MoS2, 0.051 eV, which is much larger than usual and beneficial for optoelectronic applications. Accompanying these bandgap increases, BP/graphene/MoS2 and BlueP/graphene/MoS2 exhibit absorption enhancement in the whole infrared, visible to deep ultraviolet or solar blind ultraviolet ranges, implying that these asymmetrically graphene-sandwiched heterostructures are more suitable as graphene-based 2D optoelectronic devices. The proposed 2D trilayer vdW heterostructures are prospective new optoelectronic devices, possessing higher performance than currently available devices.

Gate-tunable bolometer based on strongly coupled graphene mechanical resonators.

Bolometers based on graphene have demonstrated outstanding performance with high sensitivity and short response time. In situ adjustment of bolometers is very important in various applications, but it is still difficult to implement in many systems. Here we propose a gate-tunable bolometer based on two strongly coupled graphene nanomechanical resonators. Both resonators are exposed to the same light field, and we can measure the properties of one bolometer by directly tracking the resonance frequency shifts, and indirectly measure the other bolometer through mechanical coupling. We find that the sensitivity and the response bandwidth of both bolometers can be independently adjusted by tuning the corresponding gate voltages. Moreover, the properties of the indirectly measured bolometer show a dependence on the coupling between the two resonators, with other parameters being fixed. Our method has the potential to optimize the design of large-scale bolometer arrays, and open new horizons in infrared/terahertz astronomy and communication systems.

Electronic and Optical Properties of BP, InSe Monolayer and BP/InSe Heterojunction with Promising Photoelectronic Performance.

Two-dimensional (2D) materials provide a new strategy for developing photodetectors at the nanoscale. The electronic and optical properties of black phosphorus (BP), indium selenide (InSe) monolayer and BP/InSe heterojunction were investigated via first-principles calculations. The geometric characteristic shows that the BP, InSe monolayer and BP/InSe heterojunction have high structural symmetry, and the band gap values are 1.592, 2.139, and 1.136 eV, respectively. The results of band offset, band decomposed charge and electrostatic potential imply that the heterojunction structure can effectively inhibit the recombination of electron--hole pairs, which is beneficial for carrier mobility of photoelectric devices. Moreover, the optical properties, including refractive index, reflectivity, electron energy loss, extinction coefficient, absorption coefficient and photon optical conductivity, show excellent performance. These findings reveal the optimistic application potential for future photoelectric devices. The results of the present study provide new insight into challenges related to the peculiar behavior of the aforementioned materials with applications.

Tuning the nonlinearity of graphene mechanical resonators by Joule heating.

As an inherent property of the device itself, nonlinearity in micro-/nano- electromechanical resonators is difficult to eliminate, and it has shown a wide range of applications in basic research, sensing and other fields. While many application scenarios require tunability of the nonlinearity, inherent nonlinearity of a mechanical resonator is difficult to be changed. Here, we report the experimental observation of a Joule heating induced tuning effect on the nonlinearity of graphene mechanical resonators. We fabricated multiple graphene mechanical resonators and detected their resonant properties by an optical interference method. The mechanical vibration of the resonators will enter from the linear to the nonlinear intervals if we enhance the external driving power to a certain value. We found that at a fixed drive power, the nonlinearity of a mechanical resonator can be tuned by applying a dc bias current on the resonator itself. The tuning mechanism could be explained by the nonlinear amplitude-frequency dependence theory. Our results may provide a research platform for the study of mechanical nonlinearity by using atomic-thin layer materials.

Two Dimensional MOene: From Superconductors to Direct Semiconductors and Weyl Fermions.

The number of semiconducting MXenes with direct band gaps is extremely low; thus, it is highly desirable to broaden the MXene family beyond carbides and nitrides to expand the palette of desired chemical and physical properties. Here, we theoretically report the existence of the single-layer (SL) dititanium oxide 2H-Ti2O MOene (MXene-like 2D transition oxides), showing an Ising superconducting feature. Moreover, SL halogenated 2H- and 1T-Ti2O monolayers display tunable semiconducting features and strong light-harvesting ability. In addition, the external strains can induce Weyl fermions via quantum phase transition in 2H-Ti2OF2 and Ti2OCl2 monolayers. Specifically, 2H- and 1T-Ti2OF2 are direct semiconductors with band gaps of 0.82 and 1.18 eV, respectively. Furthermore, the carrier lifetimes of SL 2H- and 1T-Ti2OF2 are evaluated to be 0.39 and 2.8 ns, respectively. This study extends emerging phenomena in a rich family of 2D MXene-like MOene materials, which provides a novel platform for next-generation optoelectronic and photovoltaic fields.

Single-Layer Zirconium Dihalides ZrX2 (X = Cl, Br, and I) with Abnormal Ferroelastic Behavior and Strong Anisotropic Light Absorption Ability.

Recently, two-dimensional (2D) metal halides have brought out an intensive interest for their unique mechanical, electronic, magnetic, and topological properties. Here, we theoretically report the existence of the single-layer (SL) zirconium dihalide materials ZrX2 (X = Cl, Br, and I) using first-principles calculations. SL ZrX2, which can be obtained from its bulk phase through simple mechanical exfoliation, shows the dynamic, thermodynamic, and mechanical stability. Halogen atoms can effectively tune the electronic structure, dipole moment transition, band alignment, and light absorption. Specifically, ZrX2 monolayers intrinsically exhibit a ferroelasticity with an abnormal 120° orientation rotation, possessing a moderate switching barrier of 24-39 meV/atom. Importantly, we observe superior anisotropic light absorption responses on SL ZrX2 in the visible region. Besides, a series of ZrX2-based excitonic solar cells have been proposed, which hold a large power conversion efficiency limit of 12.4-18.7%.

Phonon lasing with an atomic thin membrane resonator at room temperature.

Graphene has been considered as one of the best materials to implement mechanical resonators due to their excellent properties such as low mass, high quality factors and tunable resonant frequencies. Here we report the observation of phonon lasing induced by the photonthermal pressure in a few-layer graphene resonator at room temperature, where the graphene resonator and the silicon substrate form an optical cavity. A marked threshold in the oscillation amplitude and a narrowing linewidth of the vibration mode are observed, which confirms a phonon lasing process in the graphene resonator. Our findings will stimulate the studies on phononic phenomena, help to establish new functional devices based on graphene mechanical resonators, and might find potential applications in classical and quantum sensing fields, as well as in information processing.

Group 11 Transition-Metal Halide Monolayers: High Promises for Photocatalysis and Quantum Cutting.

Recently, two-dimensional (2D) metal halides have triggered an enormous interest for their tunable mechanical, electronic, magnetic, and topological properties, greatly enriching the family of 2D materials. Here, based on first-principles calculations, we report a systematic study of group 11 transition-metal halide MX (M = Cu, Ag, Au; X = Cl, Br, I) monolayers. Among them, CuBr, CuI, AgBr, and AgI monolayers exhibit high thermodynamic, dynamic, and mechanic stability. The four stable monolayers have a direct band gap of ∼3.12-3.36 eV and possess high carrier mobility (∼103 cm2 V-1 s-1), suggestive of future photocatalysts for water splitting applications. What is more, the simulations of optical properties confirm that the stable MX monolayers hold the potential for further applications in ultraviolet optical devices and quantum cutting solar materials.

Promotion on Acetone Sensing of Single SnO2 Nanobelt by Eu Doping.

SnO2 nanobelts (NBs) have unique structural and functional properties which attract great attention in gas detecting. In this work, Eu doping is adopted to improve the gas sensitivity of pure SnO2, especially to enhance the response to one single gas. The Eu-doped SnO2 NBs, pure-SnO2 NBs, and their single NB devices are fabricated by simple techniques. The sensing properties of the two sensors have been experimentally investigated. It is found that the two sensors possess long-term stability with rapid response performance, and Eu doping improves the electronic performance and the gas-sensing response, particularly to acetone. In addition, the effects aroused by Eu have been theoretically calculated, which indicates that Eu doping enhances the sensing performance of SnO2. Consequently, Eu-doped SnO2 NBs show great potential applications in the detection of acetone.

InGaAsP/InP Nanocavity for Single-Photon Source at 1.55-µm Telecommunication Band.

A new structure of 1.55-µm pillar cavity is proposed. Consisting of InP-air-aperture and InGaAsP layers, this cavity can be fabricated by using a monolithic process, which was difficult for previous 1.55-µm pillar cavities. Owing to the air apertures and tapered distributed Bragg reflectors, such a pillar cavity with nanometer-scaled diameters can give a quality factor of 104-105 at 1.55 µm. Capable of weakly and strongly coupling a single quantum dot with an optical mode, this nanocavity could be a prospective candidate for quantum-dot single-photon sources at 1.55-µm telecommunication band.

High quality-factor Si/SiO(2)-InP hybrid micropillar cavities with submicrometer diameter for 1.55-µm telecommunication band.

We theoretically demonstrate high quality(Q)-factor micropillar cavities at 1.55-µm wavelength based on Si/SiO(2)-InP hybrid structure. An adiabatic design in distributed Bragg reflectors (DBRs) improves Q-factor for upto 3 orders of magnitude, while reducing the diameter to sub-micrometer. A moderate Q-factor of ~3000 and a Purcell factor of ~200 are realized by only 2 taper segments and fewer conventional DBR pairs, enabling single photon generation at GHz rate. As the taper segment number is increased, Q-factor can be boosted to ~10(5)-10(6), enabling coherent exchange between the emitter and the optical mode at 1.55 µm, which is applicable in quantum information networks.

Design of Si/SiO2 micropillar cavities for Purcell-enhanced single photon emission at 1.55 µm from InAs/InP quantum dots.

Numerical simulations were carried out on micropillar cavities consisting of Si/SiO2 distributed Bragg reflectors (DBRs) with an InP spacer layer. Owing to a large refractive index contrast of ~2 in DBRs, cavities with just 4/6.5 top/bottom DBR pairs that give a low pillar height (~4.5 µm), have noticeable Purcell-enhancement effect in the 1.55-µm band. With careful designs on cavities with diameters of ~2.30 µm, a quality factor of up to 3300, a nominal Purcell factor of up to 110, and an output efficiency of ~60% are obtainable. These results ensure improvement of operation frequency and enhancement of photon indistinguishability for 1.55-µm single photon sources based on InAs/InP quantum dots.