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A pair of coupled dots with one electron in each dot can provide improvements in spin coherence, particularly at an electrical bias called the "sweet spot," but few measurements have been performed on self-assembled dots in this regime. Here, we directly measure the T_{2}^{*} coherence time of the singlet-triplet states in this system as a function of bias and magnetic field, obtaining a maximum T_{2}^{*} of 60 ns, more than an order of magnitude higher than an electron spin in a single quantum dot. Our results uncover two main dephasing mechanisms: electrical noise away from the sweet spot, and a magnetic field dependent interaction with nuclear spins due to a difference in g factors.

InAs quantum dots at telecommunication wavelengths are desired as single-photon sources, but a growth technique that enables wide control over quantum dot size, density, and morphology is needed. Droplet epitaxy is well suited for this purpose, but InAs nanostructures tend to form as rings on (001) InGaAs, InAlAs, and InP surfaces. In this work, we investigate how surface diffusion can be manipulated to grow quantum dots by molecular beam epitaxy without using high-index substrates or metamorphic buffers. First, surface diffusion characteristics of In on In0.52Al0.48As are compared to In and Ga on In0.53Ga0.47As. Then, a two-step arsenic exposure protocol is applied to modify the droplet crystallization step, resulting in a series of different nanostructure morphologies that have narrow-linewidth emission between 1200 and 1520 nm at 4 K. Ultimately, we show that controlling surface diffusion of the group-III species during growth is critical for achieving quantum dots appropriate for single-photon sources at telecommunication wavelengths.

We use optical transitions of positively charged excitons (trions) with one hole in an excited orbital to solve a long-standing challenge for quantum dots of having both fast optical spin control and efficient spin readout.

We leverage local strain tuning and coherent excitation and transmission to control the quantum optics of a dot-waveguide system, demonstrating building blocks for a scalable integrated quantum-optics network.

Optical spin rotations and cycling transitions for measurement are normally incompatible in quantum dots, presenting a fundamental problem for quantum information applications. Here we show that for a hole spin this problem can be addressed using a trion with one hole in an excited orbital, where strong spin-orbit interaction tilts the spin. Then, a particular trion triplet forms a double $\Lambda$ system, even in a Faraday magnetic field, which we use to demonstrate fast hole spin initialization and coherent population trapping. The lowest trion transitions still strongly preserve spin, thus combining fast optical spin control with cycling transitions for spin readout., Comment: 30 pages, 13 figures

A pair of tunnel-coupled quantum dots has advantages over single dots in tunability and spin coherence, but far less work has been done to measure and understand the quantum optics of this system. In particular, the two-electron singlet-triplet system with one electron in each dot generates an eight-level system in which two-level and four-level double-\ensuremath{\Lambda} systems are coupled only through the hyperfine interaction and other spin relaxation processes. We first measure the emission spectrum under resonant driving with a continuous-wave laser and then perform time-correlated Hanbury Brown-Twiss (HBT) and Hong-Ou-Mandel (HOM) interferometric measurements to examine nonclassical photon properties of spin-flip Raman emission and resonance fluorescence. The shapes of second-order correlation functions $[{g}^{(2)}(\ensuremath{\tau})]$ from HBT are strikingly different between resonance fluorescence and spin-flip Raman emission, reflecting the spin dynamics of the singlet-triplet system. In addition, our two-photon HOM measurements demonstrate a high raw visibility of 0.96, with a coherence time exceeding the radiative lifetime.

We demonstrate scalable quantum interactions between quantum dots embedded in the same waveguide. The dots are tuned into resonance using laser-patterned strain with a step size down to the homogeneous linewidth and sub-micron spatial resolution.

The quest for an integrated quantum optics platform has motivated the field of semiconductor quantum dot research for two decades. Demonstrations of quantum light sources, single photon switches, transistors, and spin-photon interfaces have become very advanced. Yet the fundamental problem that every quantum dot is different prevents integration and scaling beyond a few quantum dots. Here, we address this challenge by patterning strain via local phase transitions to selectively tune individual quantum dots that are embedded in a photonic architecture. The patterning is implemented with in operando laser crystallization of a thin HfO$_{2}$ film "sheath" on the surface of a GaAs waveguide. Using this approach, we tune InAs quantum dot emission energies over the full inhomogeneous distribution with a step size down to the homogeneous linewidth and a spatial resolution better than 1 $\mu $m. Using these capabilities, we tune multiple quantum dots into resonance within the same waveguide and demonstrate a quantum interaction via superradiant emission from three quantum dots.

Solution-processed inorganic and organic materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of solution synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, respectively, giving rise to stimulated emission from biexcitons at room temperature. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm(-2), sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities.

Developing a comprehensive understanding of the processes that govern the scintillation behavior of inorganic scintillators provides a pathway to optimize current scintillators and allows for the science-driven search for new scintillator materials. Recent experimental data on the excitation density dependence of the light yield of inorganic scintillators presents an opportunity to incorporate parameterized interactions between excitations in scintillation models and thus enable more realistic simulations of the nonproportionality of inorganic scintillators. Therefore, a kinetic Monte Carlo (KMC) model of elementary scintillation processes in NaI(Tl) is developed in this paper to simulate the kinetics of scintillation for a range of temperatures and Tl concentrations as well as the scintillation efficiency as a function of excitation density. The ability of the KMC model to reproduce available experimental data allows for elucidating the elementary processes that give rise to the kinetics and efficiency of scintillation observed experimentally for a range of conditions.

Nonlinear quenching of electron–hole pairs in the denser regions of ionization tracks created by γ-ray and high-energy electrons is a likely cause of the light yield non-proportionality of many inorganic scintillators. Therefore, kinetic Monte Carlo (KMC) simulations were carried out to investigate the scintillation properties of pure and thallium-doped CsI as a function of electron–hole pair density. The availability of recent experimental data on the excitation density dependence of the light yield of CsI following ultraviolet excitation allowed for an improved parameterization of the interactions between self-trapped excitons (STE) in the KMC model via dipole–dipole Forster transfer. The KMC simulations reveal that nonlinear quenching occurs very rapidly (within a few picoseconds) in the early stages of the scintillation process. In addition, the simulations predict that the concentration of thallium activators can affect the extent of nonlinear quenching as it has a direct influence on the STE density through STE dissociation and electron scavenging. This improved model will enable more realistic simulations of the non-proportional γ-ray and electron response of inorganic scintillators.

The spectral inhomogeneity of semiconductor quantum dots (QDs) is a major challenge preventing the development of scalable QD quantum networks. Using laser annealing, three QDs are tuned into resonance, paving the way for on-chip quantum networks.

Numerical modeling and comparison to experiment in the materials for which suitable parameters have been measured confirm that three of the most important material parameters for predicting proportionality and the related host-dependent light yield (LY) of scintillators are (i) the carrier diffusion coefficients (including hole self-trapping if present, and hot-electron diffusion if unthermalized), (ii) the kinetic order and associated rate constant of nonlinear quenching, and (iii) deep-trapping probability. Thermalized carrier diffusion appears sufficient to describe the main trends in oxides and semiconductors. For heavier halide hosts, it appears necessary to take account of hot-electron diffusion to explain several important host-dependent trends.

Recent years have seen significant advances in both theoretically understanding and mathematically modeling the underlying causes of scintillator non-proportionality. The core cause is that the interaction of radiation with matter invariably leads to a non-uniform ionization density in the scintillator, coupled with the fact that the light yield depends on the ionization density. The mechanisms that lead to the luminescence dependence on ionization density are incompletely understood, but several important features have been identified, notably Auger-like processes (where two carriers of excitation interact with each other, causing one to de-excite non-radiatively), the inability of excitation carriers to recombine (caused either by trapping or physical separation), and the carrier mobility. This paper reviews the present understanding of the fundamental origins of scintillator non-proportionality, specifically the various theories that have been used to explain non-proportionality.

Time-dependent radial diffusion and drift are modeled in the high carrier concentration gradient characteristic of electron tracks in scintillators and other radiation detector materials. As expected, the lower mobility carrier (typically the hole) controls the ambipolar diffusion. Carrier separation when electron and hole mobilities are unequal produces a built-in radial electric field near the track analogous to an n-intrinsic semiconductor junction. The diffusion is shown to have significant effects on both the low dE / dx and high dE / dx ends of electron light-yield curves and their respective contributions to nonproportionality. In CsI:Tl, it is shown that electron confinement toward the end of the track accentuates high-order quenching such as Auger recombination or dipole–dipole transfer, while in HPGe extremely rapid ( dE / dx .

Information on quenching as a function of electron–hole density through the range of 10 19 to 2×10 20 e–h/cm 3 typically deposited towards the end of an electron track has been acquired using 0.5 ps pulses of 5.9 eV light to excite in the band-to-band or high-exciton region of CsI and CsI:Tl. A streak camera records partially quenched luminescence from self-trapped excitons (STE) and excited activators (Tl +⁎ ). Both the Tl +⁎ and STE luminescence exhibit decreasing light yield versus excitation density N max , but it is only the 302 nm STE luminescence that exhibits decay time quenching dependent on N max . Fitting the STE decay time data to a model of dipole–dipole quenching yields the time-dependent bimolecular rate constant for quenching of STEs (and Tl +⁎ light yield) in CsI at room temperature: k 2 ( t )=2.4×10 −15 cm 3 s −1/2 ( t −1/2 ).

Stopping of an energetic electron produces a track of high excitation density, especially near its end, and consequent high radial concentration gradient. The effect of high excitation density in promoting nonlinear quenching is generally understood to be a root cause of nonproportionality in scintillators. However, quantitative data on the kinetic rates of nonlinear quenching processes in scintillators are scarce. We report experimental measurements of second-order dipole–dipole rate constants governing the main nonlinear quenching channel in CsI, CsI:Tl, NaI, and NaI:Tl. We also show that the second of the extreme conditions in a track, i.e., radial concentration gradient, gives rise to fast (≤picoseconds) diffusion phenomena which act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent and paired carriers, where the branching ratio changes with dE/dx along the primary electron track. To investigate the interplay of these phenomena in determining nonproportionality of light yield, we use experimentally measured rate constants and mobilities in CsI and NaI to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical Gaussian in this version of the model.

We report picosecond time-resolved measurements of optical absorption induced by a sub-picosecond pulse of light producing two-photon bandgap excitation of Csl and CsI:Tl at room temperature. The transient spectrum of undoped Csl reveals for the first time strong infrared absorption rising through the 0.8-eV limit of present measurements. We suggest that this infrared band is due to transitions of the bound electron in the off-center self-trapped exciton (STE), implying that there should be a band deeper in the infrared associated with the known on-center STE in Csl. Previously reported visible and ultraviolet transient absorption bands at 1.7, 2.5, and 3.4 eV are confirmed in these measurements as attributable to hole excitations of STE. In 0.3% thallium doped Csl, infrared absorption possibly attributable to STEs is observed for approximately the first 5 ps after excitation at room temperature, but decays quickly. The absorption bands of Tl0 (electron trapped at Ti+ activator) and of self-trapped holes are the main species seen at longer times after excitation, during which most of a scintillation pulse would occur. This is in accord with a recently published report of nanosecond induced absorption in CsI:Tl.

Review: emphasis on recent advances in the development of heavy metal-free nanocrystals; 289 refs.

Several outstanding questions, including why complex halide scintillator host structures allow higher light yield and flatter electron energy response than simple monovalent metal halides, have remained unanswered by current models of luminescence in dense ionization tracks. Our measurements of nonlinear quenching kinetic order, recent literature on hot-electron transport in scintillators, and calculations presented here of hot-electron velocity from band structure of SrI2 and NaI, lead us to expand our previously described diffusion and nonlinear quenching model to include hot-electron transport. Trends in multivalent versus monovalent metal halides, heavier versus lighter halides, and halides versus oxides versus semiconductors can be predicted based on optical phonon frequency, thermalized band edge mobilities, velocity in the upper conduction bands, and hole self-trapping. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Core-shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that makes them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties, poses a major challenge. This work demonstrates how core-shell CdSe/CdS quantum dot-in-rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot-in-rods. Films made from water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a similar threshold (130 microJ/cm2) as the pristine material (115 microJ/cm2). To demonstrate feasibility for lasing applications, self-assembled micro lasers are fabricated via the coffee-ring effect that display single-mode operation and a very low threshold of around 10 microJ/cm2. The performance of these micro lasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot-in-rods., 20 pages, 4 figures, published paper with supporting information

This chapter discusses the electron-hole recombination processes that occur in the high excitation densities and strong radial gradients of particle tracks in scintillator detectors of radiation. The particle tracks are commonly those of high-energy Compton- or photo-electrons produced in energy-resolving gamma-ray detectors, but could also include those of heavier charged particles such as those following interaction with neutrons. In energy-resolving radiation detectors, intrinsic proportionality of light yield to gamma ray energy or electron energy is an important concern. This chapter gives special emphasis to understanding the physical basis for nonproportionality, while reviewing recent results on fundamental physics of nonlinear quenching, cooling and capture of hot electrons, co-evolving free-carrier and exciton populations, and diffusion in the dense and highly structured excitation landscape of electron tracks. Particular attention is paid to short-pulse laser experiments at Wake Forest University giving data and insight on the above phenomena complementary to more traditional scintillator experiments using gamma-ray or electron excitation. Numerical modeling of diffusion, nonlinear quenching (NLQ), exciton formation, and linear capture processes serves to test and establish links between the laser excitation and particle excitation measurements.

Models of nonproportional response in scintillators have highlighted the importance of parameters such as branching ratios, carrier thermalization times, diffusion, kinetic order of quenching, associated rate constants, and radius of the electron track. For example, the fraction ηeh of excitations that are free carriers versus excitons was shown by Payne and coworkers to have strong correlation with the shape of electron energy response curves from Compton-coincidence studies. Rate constants for nonlinear quenching are implicit in almost all models of nonproportionality, and some assumption about track radius must invariably be made if one is to relate linear energy deposition dE/dx to volume-based excitation density n (eh/cm3) in terms of which the rates are defined. Diffusion, affecting time-dependent track radius and thus density of excitations, has been implicated as an important factor in nonlinear light yield. Several groups have recently highlighted diffusion of hot electrons in addition to thermalized carriers and excitons in scintillators. However, experimental determination of many of these parameters in the insulating crystals used as scintillators has seemed difficult. Subpicosecond laser techniques including interband z scan light yield, fluence-dependent decay time, and transient optical absorption are now yielding experimental values for some of the missing rates and ratios needed for modeling scintillator response. First principles calculations and Monte Carlo simulations can fill in additional parameters still unavailable from experiment. As a result, quantitative modeling of scintillator electron energy response from independently determined material parameters is becoming possible on an increasingly firmer data base. This paper describes recent laser experiments, calculations, and numerical modeling of scintillator response.

The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval τ when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (Deffτ)1/2 where Deff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of Deff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K2 and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r0 from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K2/Deff and does not depend on r0 in lowest order; however, the absolute rates of both processes decrease with increasing r0.

Using 0.5 ps pulses of 5.9 eV light to excite electron-hole concentrations varied up to 2x1020 e-h/cm3 corresponding to energy deposition within electron tracks, we measure dipole-dipole quenching rate constants K2 in SrI2 and CsI. We previously reported determination of K2 directly from the time dependence of quenched STE luminescence in CsI. The nonlinear quenching rate decreases rapidly within a few tens of picoseconds as the host excitation density drops below the Förster threshold. In the present work, we measure the dependence of integrated light yield on excitation density in the activated scintillators SrI2:Eu2+ and CsI:Tl+. The “z-scan” method of yield vs. irradiance is applicable to a wider range of materials, e.g. when the quenching population is not the main light-emitting population. Furthermore, because of using an integrating sphere and photomultiplier for light detection, the signal-to-noise is substantially better than the time-resolved method using a streak camera. As a result, both 2nd and 3rd orders of quenching (dipole-dipole and Auger) can be distinguished. Detailed comparison of SrI2 and CsI is of fundamental importance to help understand why SrI2 achieves substantially better proportionality than CsI in scintillator applications. The laser measurements, in contrast to scintillation, allow evaluating the rate constants of nonlinear quenching in a population which has small enough spatial gradient to suppress the effect of carrier diffusion.

We have previously described a numerical model for carrier diffusion and nonlinear quenching in the track of an electron in a scintillator. Significant inequality of electron and hole mobilities predicts a characteristic “hump” in the light yield vs gamma energy, whereas low mobility of either or both carriers accentuates the universal roll-off due to nonlinear quenching at low gamma energy (high dE/dx). The material parameter basis of the two major trends in nonproportionality of scintillators can be related to the effective diffusion coefficient of excitations and the difference of electron and hole mobilities, respectively. Activator concentration, type of activator, and effect of transport anisotropy are associated with minor trends. The predicted trends are qualitatively consistent with empirical measures of nonproportionality including electron yield curves.

The effect of high excitation density in promoting nonlinear quenching that is 2nd or 3rd order in electron-hole density is generally understood to be a root cause of nonproportionality in scintillators. We report and discuss quantitative data on just how fast these nonlinear channels are in specific cases. Kinetic rate constants for the creation of excitons from electrons and holes and for their quenching by dipole-dipole transfer have been measured in CsI and NaI. We show in addition that the strong radial concentration gradient in an electron track gives rise to fast (~ picoseconds) diffusion phenomena that act both as a competitor in reducing excitation density during the relevant time of nonlinear quenching, and as a determiner of branching between independent carriers and pairs (excitons), where the branching ratio changes along the primary electron track. We use the experimentally measured nonlinear quenching rate constants and values of electron and hole carrier mobilities to carry out quantitative modeling of diffusion, drift, and nonlinear quenching evaluated spatially and temporally within an electron track which is assumed cylindrical in this version of the model. Magnitude and inequality of electron and hole mobilities has consequences for quenching and kinetic order that vary with dE/dx along the path of an electron and therefore affect nonproportionality. It will be demonstrated that in a material with high mobilities like high-purity germanium, Auger recombination is effectively turned off by diffusive carrier dilution within < 1 fs in all parts of the track. In alkali halide scintillators like CsI and CsI:Tl, electron confinement and high-order quenching are accentuated toward the end of a particle track because of hole self-trapping, while separation of geminate carriers is accentuated toward the beginning of the track, leading to 2nd order radiative recombination and opening additional opportunities for linear trapping.

Electron-hole pairs created by the passage of a high-energy electron in a scintillator radiation detector find themselves in a very high radial concentration gradient of the primary electron track. Since nonlinear quenching that is generally regarded to be at the root of nonproportional response depends on the fourth or sixth power of the track radius in a cylindrical track model, radial diffusion of charge carriers and excitons on the ∼10 picosecond duration typical of nonlinear quenching can compete with and thereby modify that quenching. We use a numerical model of transport and nonlinear quenching to examine trends affecting local light yield versus excitation density as a function of charge carrier and exciton diffusion coefficients. Four trends are found: (1) nonlinear quenching associated with the universal “roll-off” of local light yield versus dE/dx is a function of the lesser of mobilities μe and μh or of DEXC as appropriate, spanning a broad range of scintillators and semiconductor detectors; (...