Coherent Interaction of a Few-Electron Quantum Dot with a Terahertz Optical Resonator.

We have investigated light-matter hybrid excitations in a quantum dot (QD) THz resonator coupled system. We fabricate a gate-defined QD near a THz split-ring resonator (SRR) by using a AlGaAs/GaAs two-dimensional electron system. By illuminating the system with THz radiation, the QD shows a current change whose spectrum exhibits coherent coupling between the electrons in the QD and the SRR as well as coupling between the two-dimensional electron system and the SRR. The latter coupling enters the ultrastrong coupling regime and the electron excitation in the QD also exhibits coherent coupling with the SRR with the remarkably large coupling constant, despite the fact that only a few electrons reside in the QD.

Hybrid distributed Bragg reflector laser on Si with a transfer printed InAs/GaAs quantum dot amplifier.

We demonstrate a hybrid integrated laser by transfer printing an InAs/GaAs quantum dot (QD) amplifier on a Si waveguide with distributed Bragg reflectors (DBRs). The QD waveguide amplifier of 1.6 mm long was patterned in the form of an airbridge with the help of a spin-on-glass sacrificial layer and precisely integrated on the silicon-on-insulator (SOI) waveguide by pick-and-place assembly using an elastomer stamp. Laser oscillation was observed around the wavelength of 1250 nm with a threshold current of 47 mA at room temperature and stable operation up to 80°C. Transfer printing of the long QD amplifiers will enable the development of various hybrid integrated laser devices that leverage superior properties of QDs as laser gain medium.

Electrical Detection of Ultrastrong Coherent Interaction between Terahertz Fields and Electrons Using Quantum Point Contacts.

Light-matter interaction in the ultrastrong coupling regime is attracting considerable attention owing to its applications to coherent control of material properties by a vacuum fluctuation field. However, electrical access to such an ultrastrongly coupled system is very challenging. In this work, we have fabricated a gate-defined quantum point contact (QPC) near the gap of a terahertz (THz) split-ring resonator (SRR) fabricated on a GaAs two-dimensional (2D) electron system. By illuminating the system with external THz radiation, the QPC shows a photocurrent spectrum which exhibits significant anticrossing that arises from coupling between the cyclotron resonance of the 2D electrons and the SRR. The observed photocurrent signal can be explained by energy-selective transmission/reflection of the quantum Hall edge channels at the QPC. Furthermore, at the same gate voltage and magnetic field conditions under which the anticrossing signal was observed, the QPC exhibits anomalous conductance modulation even in the dark environment.

Enhanced temperature stability of threshold current of InAs/GaAs quantum dot lasers by AlGaAs lateral potential barrier layers.

We have investigated the incorporation of an AlGaAs lateral potential barrier layer (LPBL) as a novel approach to improve the temperature stability of the threshold current in InAs/GaAs quantum dot (QD) lasers. This layer serves to increase the energy separation (ΔE) between the ground and excited states of the QD while maintaining efficient vertical carrier injection. Theoretical calculations confirm that the LPBL is effective in increasing ΔE. The LPBLs were successfully formed using the preferential growth properties of AlGaAs induced by the non-uniform distribution of strain effects on the QD surface during molecular beam epitaxy growth. To confirm the usefulness of the LPBLs, we fabricated an InAs/GaAs QD laser incorporating AlGaAs LPBLs, demonstrating that the threshold current at 150°C was significantly reduced by 48% compared to a QD laser without LPBLs. The temperature stabilization achieved by incorporating the LPBLs provides a promising way for establishing high reliability and low power operation of QD lasers in high-temperature environments.

InAs/InGaAs quantum dot lasers on multi-functional metamorphic buffer layers: erratum.

We present an erratum to correct inadvertent errors in our paper [Opt. Express29, 29378 (2021)10.1364/OE.433030]. The corrections do not affect the main conclusion.

Unidirectional output from a quantum-dot single-photon source hybrid integrated on silicon.

We report a quantum-dot single-photon source (QD SPS) hybrid integrated on a silicon waveguide embedding a photonic crystal mirror, which reflects photons and enables efficient unidirectional output from the waveguide. The silicon waveguide is constituted of a subwavelength grating so as to maintain the high efficiency even under the presence of stacking misalignment accompanied by hybrid integration processes. Experimentally, we assembled the hybrid photonic structure by transfer printing and demonstrated single-photon generation from a QD and its unidirectional output from the waveguide. These results point out a promising approach toward scalable integration of SPSs on silicon quantum photonics platforms.

InAs/InGaAs Quantum Dot Lasers on Multi-Functional Metamorphic Buffer Layers.

With the development of dry fiber over the past two decades, the E-band has become a new telecommunication wavelength. However, owing to material constraints, an effective high-performance semiconductor light source has not yet been realized. InAs quantum dot (QD) lasers on GaAs substrates are in the spotlight as O-band light sources because of their excellent thermal properties and high efficiency. The introduction of a very thick InGaAs metamorphic buffer layer is essential for realizing an E-band InAs QD laser, but it can cause degradation in laser performance. In this study, we fabricate an E-band InAs/GaAs QD laser on a GaAs substrate with an AlInGaAs multifunctional metamorphic buffer layer that realizes the function of the bottom cladding layer of normal thickness in addition to the functions of a metamorphic buffer layer and a dislocation filter layer. The lasing oscillation at a wavelength of 1428 nm is demonstrated at room temperature under continuous-wave operation. This result paves the way toward the realization of highly efficient light sources suitable for E-band telecommunications.

Semiconductor quantum dots: Technological progress and future challenges.

In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information.

Experimental demonstration of topological slow light waveguides in valley photonic crystals.

We experimentally demonstrate topological slow light waveguides in valley photonic crystals (VPhCs). We employed a bearded interface formed between two topologically-distinct VPhCs patterned in an air-bridged silicon slab. The interface supports both topological and non-topological slow light modes below the light line. By means of optical microscopy, we observed light propagation in the topological mode in the slow light regime with a group index ng over 30. Furthermore, we confirmed light transmission via the slow light mode even under the presence of sharp waveguide bends. In comparison between the topological and non-topological modes, we found that the topological mode exhibits much more efficient waveguiding than the trivial one, demonstrating topological protection in the slow light regime. This work paves the way for exploring topological slow-light devices compatible with existing photonics technologies.

Single-photon emission from isolated monolayer islands of InGaN.

We identify and characterize a novel type of quantum emitter formed from InGaN monolayer islands grown using molecular beam epitaxy and further isolated via the fabrication of an array of nanopillar structures. Detailed optical analysis of the characteristic emission spectrum from the monolayer islands is performed, and the main transmission is shown to act as a bright, stable, and fast single-photon emitter with a wavelength of ~400 nm.

Slow light waveguides in topological valley photonic crystals.

Valley photonic crystals (VPhCs) are an attractive platform for the implementation of topologically protected optical waveguides in photonic integrated circuits (PICs). The realization of slow light modes in the topological waveguides may lead to further miniaturization and functionalization of the PICs. In this Letter, we report an approach to realize topological slow light waveguides in semiconductor-slab-based VPhCs. We show that a bearded interface of two topologically distinct VPhCs can support topological kink modes with large group indices over 100 within the topological bandgap. We numerically demonstrate robust light propagation in the topological slow light waveguide with large group indices of ∼60, even under the presence of sharp bends. Our work opens a novel route to implement topological slow light waveguides in a way compatible with current PIC technology.

Spin-dependent directional emission from a quantum dot ensemble embedded in an asymmetric waveguide.

In this study, we examine a photonic wire waveguide embedded with an ensemble of quantum dots (QDs) that directionally emits into the waveguide depending on the spin state of the ensemble. The directional emission is facilitated by the spin-orbit interaction of light. The waveguide has a two-step stair-like cross section and QDs are embedded only in the upper step, such that the circular polarization of emission from the spin-polarized QDs controls the direction of the radiation. We numerically verify that more than 70% of the radiation from the ensemble emitter is toward a specific direction in the waveguide. We also examine a microdisk resonator with a stair-like edge, which supports selective coupling of the QD ensemble radiation into a whispering gallery mode that rotates unidirectionally. Our study provides a foundation for spin-dependent optoelectronic devices.

High-temperature continuous-wave operation of directly grown InAs/GaAs quantum dot lasers on on-axis Si (001).

Laser devices for silicon photonics are expected to be implemented in an integrated environment to complement CMOS devices. For this reason, quantum dot (QD) lasers with excellent thermal properties have been considered as strong candidates for Si photonics light sources. The direct growth of QD lasers on Si (001) on-axis substrates has been garnering attention owing to the possibility of monolithic integration on a CMOS-compatible wafer. In this paper, we report on the high-temperature (over 100°C) continuous-wave operation of an InAs/GaAs QD laser directly grown on on-axis Si (001) substrates through the use of only molecular beam epitaxy.

Scheme for media conversion between electronic spin and photonic orbital angular momentum based on photonic nanocavity.

Light with nonzero orbital angular momentum (OAM) or twisted light is promising for quantum communication applications such as OAM-entangled photonic qubits. Methods and devices for the conversion of the photonic OAM to photonic spin angular momentum (SAM), as well as for the photonic SAM to electronic SAM transformation are known but the direct conversion between the photonic OAM and electronic SAM is not available within a single device. Here, we propose a scheme which converts photonic OAM to electronic SAM and vice versa within a single nanophotonic device. We employed a photonic crystal nanocavity with an embedded quantum dot (QD) which confines an electron spin as a stationary qubit. The confined spin-polarized electrons could recombine with holes to give circularly polarized emission, which could drive the rotation of the nanocavity modes via the strong optical spin-orbit interaction. The rotating modes then radiate light with nonzero OAM, allowing this device to serve as a transmitter. As this can be a unitary process, the time-reversed case enables the device to function as a receiver. This scheme could be generalized to other systems with a resonator and quantum emitters such as a microdisk and defects in diamond for example. Our scheme shows the potential for realizing an (ultra)compact electronic SAM-photonic OAM interface to accommodate OAM as an additional degree of freedom for quantum information purposes.

All MBE grown InAs/GaAs quantum dot lasers on on-axis Si (001).

Directly grown III-V quantum dot (QD) laser on on-axis Si (001) is a good candidate for achieving monolithically integrated Si photonics light source. Nowadays, laser structures containing high quality InAs / GaAs QD are generally grown by molecular beam epitaxy (MBE). However, the buffer layer between the on-axis Si (001) substrate and the laser structure are usually grown by metal-organic chemical vapor deposition (MOCVD). In this paper, we demonstrate all MBE grown high-quality InAs/GaAs QD lasers on on-axis Si (001) substrates without using patterning and intermediate layers of foreign material.

Thresholdless quantum dot nanolaser.

Thresholdless lasing is an outstanding challenge in laser science and is achievable only in devices having near unity quantum efficiency even when not lasing. Such lasers are expected to exhibit featureless linear light output curves. However, such thresholdless behavior hinders identification of the laser transition, triggering a long-lasting argument on how to identify the lasing. Here, we demonstrate thresholdless lasing in a semiconductor quantum dot nanolaser with a photonic crystal nanocavity. We employ cavity resonant excitation for enabling the thresholdless operation via focused carrier injection into high cavity field regions. Under conventional (above bandgap) excitation, the same nanolaser exhibits a typical thresholded lasing transition, thereby facilitating a systematic comparison between the thresholdless and thresholded laser transitions in the single device. Our approach enables a clear verification of the thresholdless lasing and reveals core elements for its realization using quantum dots, paving the way to the development of ultimately energy-efficient nanolasers.

Double-stage guided-mode converter for pure TM-mode guiding in pillar photonic-crystal waveguide devices.

We propose a double-stage guided-mode converter for pillar photonic-crystal (PhC) waveguide devices. The converter consists of a pillar-to-wire waveguide coupler and a transverse-magnetic-mode-selective spot-size converter. The former secures high-efficiency wide-band optical coupling of a pillar-PhC waveguide to a wire waveguide. The latter improves the coupling efficiency of the wire waveguide and an outside waveguide such as an optical fiber and also the signal-to-noise ratio of light guided in the pillar-PhC waveguide. The transmission band of a fabricated pillar-PhC waveguide having the converters on both ends was 88 nm in wavelength. The cutoff at the band edge was steep and deep with an extinction ration of 40 dB in a 4-nm wavelength range.

Ultraclean Single Photon Emission from a GaN Quantum Dot.

Wide bandgap III-nitride quantum dots (QDs) are promising materials for the realization of solid-state single-photon sources, especially operating at room temperature. However, so far a large degree of inhomogeneous broadening induced by spectral diffusion has compromised their use. Here, we demonstrate the ultraclean emission from single GaN QDs formed at macrostep edges in a GaN/AlGaN quantum well. As a likely consequence of the high growth temperature and hence a reduced defect density, spectral diffusion is heavily suppressed to levels at least 1 order of magnitude lower than conventional GaN QDs. A record narrow line width of as small as 87 µeV is obtained, while the low inhomogeneous broadening enables us to assess an upper limit of homogeneous broadening in the QDs (27 µeV). Furthermore, the uncontaminated emission facilitates the generation of ultraviolet single-photons with unprecedented purity (g(2)(0) = 0.02). The realization of high-quality GaN QDs will enable exploration of optoelectronic properties of III-nitrides, opening up the possibility of realizing single-photon quantum information systems operating at room temperature.

Direct modulation of 1.3 µm quantum dot lasers on silicon at 60 °C.

We demonstrate direct modulation of an InAs/GaAs quantum dot (QD) laser on Si. A Fabry-Pérot QD laser was integrated on Si by an ultraviolet-activated direct bonding method, and a cavity was formed using cleaved facets without HR/AR coatings. The bonded laser was operated under continuous-wave pumping at room temperature with a threshold current of 41 mA and a maximum output power of 30 mW (single facet). Even with such a simple device structure and fabrication process, our bonded laser is directly modulated using a 10 Gbps non-return-to-zero signal with an extinction ratio of 1.9 dB at room temperature. Furthermore, 6 Gbps modulation with an extinction ratio of 4.5 dB is achieved at temperatures up to 60 °C without any current or voltage adjustment. These results of device performances indicate an encouraging demonstration on III-V QD lasers on Si for the applications of the photonic integrated circuits.

A Nanowire-Based Plasmonic Quantum Dot Laser.

Quantum dots enable strong carrier confinement and exhibit a delta-function like density of states, offering significant improvements to laser performance and high-temperature stability when used as a gain medium. However, quantum dot lasers have been limited to photonic cavities that are diffraction-limited and further miniaturization to meet the demands of nanophotonic-electronic integration applications is challenging based on existing designs. Here we introduce the first quantum dot-based plasmonic laser to reduce the cross-sectional area of nanowire quantum dot lasers below the cutoff limit of photonic modes while maintaining the length in the order of the lasing wavelength. Metal organic chemical vapor deposition grown GaAs-AlGaAs core-shell nanowires containing InGaAs quantum dot stacks are placed directly on a silver film, and lasing was observed from single nanowires originating from the InGaAs quantum dot emission into the low-loss higher order plasmonic mode. Lasing threshold pump fluences as low as ∼120 µJ/cm(2) was observed at 7 K, and lasing was observed up to 125 K. Temperature stability from the quantum dot gain, leading to a high characteristic temperature was demonstrated. These results indicate that high-performance, miniaturized quantum dot lasers can be realized with plasmonics.