Advances in colloidal quantum dots
The confinement found in colloidal semiconductor quantum dots enables the design of materials with tunable properties. García de Arquer
et al
. review the recent ...advances in methods for synthesis and surface functionalization of quantum dots that enable fine tuning of their optical, chemical, and electrical properties. These important developments have driven the commercialization of display and lighting applications and provide promising developments in the related fields of lasing and sensing. —MSL
A review highlights advances in the synthesis of colloidal quantum dots that have enabled numerous applications.
BACKGROUND
Semiconductor materials feature optical and electronic properties that can be engineered through their composition and crystal structure. The use of semiconductors such as silicon gallium arsenide sparked technologies from computers and mobile phones to lasers and satellites. Semiconductor quantum dots (QDs) offer an additional lever: Because their size is reduced to the nanometer scale in all three dimensions, the restricted electron motion leads to a discrete atom-like electronic structure and size-dependent energy levels. This enables the design of nanomaterials with widely tunable light absorption, bright emission of pure colors, control over electronic transport, and a wide tuning of chemical and physical functions because of their large surface-to-volume ratio.
ADVANCES
The bright and narrowband light emission of semiconductor QDs, tunable across the visible and near-infrared spectrum, is attractive to realize more efficient displays with purer colors. QDs are engineered compositionally and structurally to manipulate energy states and charge interactions, leading to optical gain and lasing, relevant to light emission across visible and infrared wavelengths and fiberoptic communication. Their tunable surface chemistry allows application as optical labels in bio-imaging, made possible by tethering QDs with proteins and antibodies. The manipulation of QD surfaces with capping molecules that have different chemical and physical functions can be tailored to program their assembly into semiconducting solids, increasing conductivity and enabling the transduction of photonic and chemical stimuli into electrical signals. Optoelectronic devices such as transistors and photodetectors lead to cameras sensitive to visible and infrared light. Highly crystalline QDs can be grown epitaxially on judiciously chosen substrates by using high-temperature and vacuum conditions, and their use has led to commercially viable high-performance lasers. The advent of colloidal QDs, which can be fabricated and processed in solution at mild conditions, enabled large-area manufacturing and widened the scope of QD application to markets such as consumer electronics and photovoltaics.
OUTLOOK
From a chemistry perspective, further advances in QD fabrication are needed to sustain and improve desired chemical and optoelectronic properties and to do so with high reproducibility. This entails the use of inexpensive synthesis methods and precursors that are able to retain laboratory-scale QD properties to market-relevant volumes. A better understanding of the yet-incomplete picture of QD surfaces, atomic arrangement, and metastable character is needed to drive further progress. From a regulatory perspective, added attention is needed to achieve high-quality materials that do not rely on heavy metals such as Cd, Pb, and Hg. The role of nanostructuring in toxicity and life cycle analysis for each application is increasingly important. From a materials and photophysics perspective, exciting opportunities remain in the understanding and harnessing of electrons in highly confined materials, bridging the gap between mature epitaxial QDs and still-up-and-coming colloidal QDs. The yet-imperfect quality of the latter—a price paid today in exchange for their ease of manufacture—remains a central challenge and must be addressed to achieve further-increased performance in devices. From a device perspective, colloidal QD manufacturing must advance to translate from laboratory-scale to large-area applications such as roll-to-roll and inkjet printing. Photocatalysis, in which light is used to drive chemical transformations, is an emerging field in which QDs are of interest. Quantum information technologies, which rely on the transduction of coherent light and electrons, bring new challenges and opportunities to exploit quantum confinement effects. Moving forward, opportunities remain in the design of QD-enabled new device architectures.
Semiconductor quantum dot technologies.
Quantum dots feature widely tunable and distinctive optical, electrical, chemical, and physical properties. They span energy harvesting, illumination, displays, cameras, sensors, communication and information technology, biology, and medicine, among others. These have been exploited to realize efficient lasers, displays, biotags, and solar harvesting devices available in the market and are emerging in photovoltaics, sensing, and quantum information.
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.
Abstract
Topological edge states exist at the interfaces between two topologically distinct materials. The presence and number of such modes are deterministically predicted from the bulk band ...topologies, known as the bulk-edge correspondence. This principle is highly useful for predictably controlling optical modes in resonators made of photonic crystals (PhCs), leading to the recent demonstrations of microscale topological lasers. Meanwhile, zero-dimensional topological trapped states in the nanoscale remained unexplored, despite its importance for enhancing light–matter interactions and for wide applications including single-mode nanolasers. Here, we report a topological PhC nanocavity with a near-diffraction-limited mode volume and its application to single-mode lasing. The topological origin of the nanocavity, formed at the interface between two topologically distinct PhCs, guarantees the existence of only one mode within its photonic bandgap. The observed lasing accompanies a high spontaneous emission coupling factor stemming from the nanoscale confinement. These results encompass a way to greatly downscale topological photonics.
Reflection high-energy electron diffraction (RHEED) has wide application because it allows in situ observation of the sample surface behavior during molecular beam epitaxy growth. In particular, the ...RHEED pattern has been used as a milestone for growth condition calibration because it dynamically changes depending on the sample temperature, material supply rate, and supply ratio. However, RHEED pattern analysis depends on the accumulated know-how of the operator and has a time limitation; thus, its application to real-time feedback control is difficult. Moreover, with the conventional computerization method, it is difficult to correctly reflect and recognize the changes in RHEED due to changes in the observation conditions. On the other hand, the machine learning method using the convolutional neural network (CNN) recognizes feature points in the input database and is suitable for the classification of images with variability. In this study, we propose a measurement method for identifying the RHEED pattern of GaAs substrates during continuous rotation and build a data set of the growth conditions. A classification model is established by training the deep learning model using CNN, and is found to be more than 99% accurate. This is expected to be useful in the field of highquality III–V growth on GaAs.
Topological photonics emerged as a novel route to engineer the flow of light. Topologically protected photonic edge modes, which are supported at the perimeters of topologically nontrivial insulating ...bulk structures, are of particular interest as they may enable low-loss optical waveguides immune to structural disorder. Very recently, there has been a sharp rise of interest in introducing gain materials into such topological photonic structures, primarily aiming at revolutionizing semiconductor lasers with the aid of physical mechanisms existing in topological physics. Examples of remarkable realizations are topological lasers with unidirectional light output under time-reversal symmetry breaking and topologically protected polariton and micro/nanocavity lasers. Moreover, the introduction of gain and loss provides a fascinating playground to explore novel topological phases, which are in close relevance to non-Hermitian and parity-time symmetric quantum physics and are, in general, difficult to access using fermionic condensed matter systems. Here, we review the cutting-edge research on active topological photonics, in which optical gain plays a pivotal role. We discuss recent realizations of topological lasers of various kinds, together with the underlying physics explaining the emergence of topological edge modes. In such demonstrations, the optical modes of the topological lasers are determined by the dielectric structures and support lasing oscillation with the help of optical gain. We also address recent research on topological photonic systems in which gain and loss, themselves, essentially influence topological properties of the bulk systems. We believe that active topological photonics provides powerful means to advance micro/nanophotonics systems for diverse applications and topological physics, itself, as well.
Topological photonics provides a novel route for designing and realizing optical devices with unprecedented functionalities. Topological edge states, which are supported at the boundary of two ...photonic systems with different band topologies, enable robust light transport immune to structural imperfections and/or sharp bends in waveguides. Furthermore, the topological edge states are expected to revolutionize cavity-based optical devices such as lasers. Optical devices with built-in topological protection with a small footprint are fascinating as on-chip optical devices for low-loss and functional photonic integrated circuits. Semiconductor photonic crystals are promising platforms enabling the miniaturization of topological optical devices. Herein, we review the recent realizations of semiconductor topological photonic crystals. In particular, we discuss topological waveguides in valley photonic crystals, which have received increasing attention because of their simple realization. In addition, we provide recent demonstrations of topological nanocavities, which are another key component of topological nanophotonics. Progress in semiconductor topological photonic crystals will propel the use of topological photonic devices in various applications as well as deepen the understanding of topological photonic phenomena at the wavelength scale.
The device characteristics of semiconductor lasers have been improved with progress in active layer structures. Carrier confinement dimension plays an important role especially in temperature ...sensitivity as well as slope efficiency. Three-dimensional carrier confinement to nano-scale semiconductor crystal, known as "quantum dots (QDs)" had been predicted to show ultimately superior device performances. Self-assembly formed InAs QDs grown on GaAs had been intensively promoted in order to achieve QD lasers with superior device performances. Now high-density, high-optical quality QDs have been realized through improved molecular beam epitaxy growths and QD lasers with better temperature characteristics are in the stage of mass-production for a data-com market. Fabry-Perot type, as well as distributed feedback type QD lasers show quite improved laser characteristics. Also, the unique device characteristics of QD lasers opened new application fields such as the use for resource searching by utilizing high-temperature operation such as lasing at higher than 200 °C. For silicon-photonics, QD lasers are used as an optical source for athermal operation. In this paper, the evolution of QDs, as well as improved device performances for novel application fields are discussed.
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.
Spontaneous two-photon emission from a solid-state single quantum emitter is observed. We investigated photoluminescence from the neutral biexciton in a single semiconductor quantum dot coupled with ...a high Q photonic crystal nanocavity. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity inspires the biexciton to simultaneously emit two photons into the mode, resulting in clear emission enhancement of the mode. Meanwhile, the suppression of other single photon emission from the biexciton was observed, as the two-photon emission process becomes faster than the others at the resonance.
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