Phonon-mediated and weakly size-dependent electron and hole cooling in CsPbBr3 nanocrystals revealed by atomistic simulations and ultrafast spectroscopy

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over similar to 10 ps) of "warm" electrons in the vicinity (within similar to 0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps(-1) carrier(-1), increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron-phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers

Quantum-optical influences in optoelectronics - an introduction

This focused review discusses the increasing importance of quantum optics in the physics and engineering of optoelectronic components. Two influences relating to cavity quantum electrodynamics are presented. One involves the development of low threshold lasers, when the channeling of spontaneous emission into the lasing mode becomes so efficient that the concept of lasing needs revisiting. The second involves the quieting of photon statistics to produce single-photon sources for applications such as quantum information processing. An experimental platform, consisting of quantum-dot gain media inside micro- and nanocavities, is used to illustrate these influences of the quantum mechanical aspect of radiation. An overview is also given on cavity quantum electrodynamics models that may be applied to analyze experiments or design devices.EC/FP7/615613/EU/External Quantum Control of Photonic Semiconductor Nanostructures/EXQUISIT

A Review of Semiconductor Quantum Well Devices

Quantum well devices feature very thin epitaxial layers of heterostructure III-V and II-VI semiconductor materials that are grown using Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapour Deposition (MOCVD) growth techniques. These devices are monolithically integrated with various optoelectronic devices to provide photonic integrated circuit with increased functionality .The quantum well structure can be realized with GaAs as wells and AlGaAs as barriers for wavelength about 0.8 μm and InGaAsP/InP offering longer wavelengths (0.9-1.6 μm). Quantum well devices find their applications in quantum well lasers or improved lasers, photodetectors, modulators and switches. These devices operate much faster, more economically and have led to a million increases in speed, a point of enormous importance to the telecommunication and computer industry. Keywords: Quantum well, Semiconductor, Heterostructures, Lasers, Detectors, Modulators

One-photon absorption by inorganic perovskite nanocrystals: A theoretical study

The one-photon absorption cross section of nanocrystals (NCs) of the inorganic perovskite CsPbBr$_{3}$ is studied theoretically using a multiband $\mathbf{k}\cdot\mathbf{p}$ envelope-function model combined with a treatment of intercarrier correlation by many-body perturbation theory. A confined exciton is described first within the Hartree-Fock (HF) approximation, and correlation between the electron and hole is then included in leading order by computing the first-order vertex correction to the electron-photon interaction. The vertex correction is found to give an enhancement of the near-threshold absorption cross section by a factor of up to 4 relative to the HF (mean-field) value of the cross section, for NCs with an edge length $L=9$-12 nm (regime of intermediate confinement). The vertex-correction enhancement factors are found to decrease with increasing exciton energy; the absorption cross section for photons of energy $\omega=3.1$ eV (about 0.7 eV above threshold) is enhanced by a factor of only 1.4-1.5 relative to the HF value. The $\mathbf{k}\cdot\mathbf{p}$ corrections to the absorption cross section are also significant; they are found to increase the cross section at an energy $\omega=3.1$ eV by about 30% relative to the value found in the effective-mass approximation. The theoretical absorption cross section at $\omega=3.1$ eV, assuming a Kane parameter $E_{P}=20$ eV, is found to be intermediate among the set of measured values (which vary among themselves by nearly an order of magnitude) and to obey a power-law dependence $\sigma^{(1)}(\omega)\propto L^{2.9}$ on the NC edge length $L$, in good agreement with experiment. The dominant contribution to the theoretical exponent 2.9 is shown to be the density of final-state excitons. The main theoretical uncertainty in these calculations is in the value of the Kane parameter $E_{P}$.Comment: 15 pages, 8 figure

Dicke Superradiance in Solids

Recent advances in optical studies of condensed matter have led to the emergence of phenomena that have conventionally been studied in the realm of quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body correlations inherent in optical processes in condensed matter systems. This article is concerned with superradiance (SR), a profound quantum optical process predicted by Dicke in 1954. The basic concept of SR applies to a general $N$-body system where constituent oscillating dipoles couple together through interaction with a common light field and accelerate the radiative decay of the system. In the most fascinating manifestation of SR, known as superfluorescence (SF), an incoherently prepared system of $N$ inverted atoms spontaneously develops macroscopic coherence from vacuum fluctuations and produces a delayed pulse of coherent light whose peak intensity $\propto N^2$. Such SF pulses have been observed in atomic and molecular gases, and their intriguing quantum nature has been unambiguously demonstrated. Here, we focus on the rapidly developing field of research on SR in solids, where not only photon-mediated coupling but also strong Coulomb interactions and ultrafast scattering exist. We describe SR and SF in molecular centers in solids, molecular aggregates and crystals, quantum dots, and quantum wells. In particular, we will summarize a series of studies we have recently performed on quantum wells in strong magnetic fields. These studies show that cooperative effects in solid-state systems are not merely small corrections that require exotic conditions to be observed; rather, they can dominate the nonequilibrium dynamics and light emission processes of the entire system of interacting electrons.Comment: 23 pages, 26 figure

Advancing nanoelectronic device modeling through peta-scale computing and deployment on nanoHUB

Recent improvements to existing HPC codes NEMO 3-D and OMEN, combined with access to peta-scale computing resources, have enabled realistic device engineering simulations that were previously infeasible. NEMO 3-D can now simulate 1 billion atom systems, and, using 3D spatial decomposition, scale to 32768 cores. Simulation time for the band structure of an experimental P doped Si quantum computing device fell from 40 minutes to I minute. OMEN can perform fully quantum mechanical transport calculations for real-word UTB FETs on 147,456 cores in roughly 5 minutes. Both of these tools power simulation engines on the nanoHUB, giving the community access to previously unavailable research capabilities

Investigations of low-dimensional emitter system by dynamic strain platform

Two-dimensional transitional metal dichalcogenides (2D TMDs) and zero-dimensional quantum dots (QDs) are among the most representative low-dimensional emitter systems, with one or three dimensions on nano-scale. Both of them exhibit potential for (quantum) optical applications. Analog to the electric field and magnetic field, strain is a powerful probe to detect the physics of the emitter systems. The reduced dimension renders strain tuning more applicable to deepen the understanding and tune their properties. Previous researches demonstrate that strain can change the distance of particles or/and the symmetry. Based on this, we conduct some investigations: first, we detect the responses of monolayer WSe2 to biaxial in-plane strain. Generally, all the helicities of excitons and trions are related to the scattering process. In our observation, the decreases of exciton circular helicities in WSe2 and MoSe2 are associated with their e-h exchange interactions. The helicity of trion in MoSe2 is almost intact, and a phenomenological rate equation model is developed to describe the decrease of trions in WSe2, which agrees with our observation well. Our findings provide a new strategy to tune the read-in/read-out in TMDs-based memory devices. Second, we focus on the responses of WSe2 to uniaxial strain. We identify fine structures of neutral exciton in polarization-dependent photoluminescence spectroscopy. The nonlinear evolutions, in terms of amplitude and phase, with an active uniaxial strain are interpreted by the interaction of wavefunction with strain. Though these two bulk strain-tuning platforms hold the potential for sophisticated emitter systems, a more versatile strain-tuning platform is needed. In the last section of this work, a 2-leg MEMS strain-tuning platform is fabricated and then integrated with a QDs-embedded membrane. We resolve the position-dependent anisotropic strain on the strain-tuning platform and compare the opposite responses of positive and negative trions to the same strain. Our observation agrees well with the previous pseudo-potential/configuration interaction calculations. Notably, the 2-leg strain platform applies to 2D TMDs. These findings act as some helpful attempts to deepen the understanding of low-dimensional emitter systems. In some ongoing work, we get a prototype as a more versatile strain-tuning platform. We envision this platform can add a degree of freedom for the integrated photonic circuits