High Excitation Densities Research Articles

Open quantum dynamics of strongly coupled oscillators with multi-configuration time-dependent Hartree propagation and Markovian quantum jumps.

Modeling the non-equilibrium dissipative dynamics of strongly interacting quantized degrees of freedom is a fundamental problem in several branches of physics and chemistry. We implement a quantum state trajectory scheme for solving Lindblad quantum master equations that describe coherent and dissipative processes for a set of strongly coupled quantized oscillators. The scheme involves a sequence of stochastic quantum jumps with transition probabilities determined by the system state and the system-reservoir dynamics. Between consecutive jumps, the wave function is propagated in a coordinate space using the multi-configuration time-dependent Hartree method. We compare this hybrid propagation methodology with exact Liouville space solutions for physical systems of interest in cavity quantum electrodynamics, demonstrating accurate results for experimentally relevant observables using a tractable number of quantum trajectories. We show the potential for solving the dissipative dynamics of finite size arrays of strongly interacting quantized oscillators with high excitation densities, a scenario that is challenging for conventional density matrix propagators due to the large dimensionality of the underlying Hilbert space.

Nanoscale heterogeneity of ultrafast many-body carrier dynamics in triple cation perovskites

In high fluence applications of lead halide perovskites for light-emitting diodes and lasers, multi-polaron interactions and associated Auger recombination limit the device performance. However, the relationship of the ultrafast and strongly lattice coupled carrier dynamics to nanoscale heterogeneities has remained elusive. Here, in ultrafast visible-pump infrared-probe nano-imaging of the photoinduced carrier dynamics in triple cation perovskite films, a ~20 % variation in sub-ns relaxation dynamics with spatial disorder on tens to hundreds of nanometer is resolved. We attribute the non-uniform relaxation dynamics to the heterogeneous evolution of polaron delocalization and increasing scattering time. The initial high-density excitation results in faster relaxation due to strong many-body interactions, followed by extended carrier lifetimes at lower densities. These results point towards the missing link between the optoelectronic heterogeneity and associated carrier dynamics to guide synthesis and device engineering for improved perovskites device performance.

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films

As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for copper iodide thin films by a synergistic approach employing a combination of analytical techniques. Steady-state photoluminescence spectra reveal that the emission at ~420 nm arises from the recombination of electrons with neutral copper vacancies. The photogenerated carrier density dependent ultrafast physical processes are elucidated with using the femtosecond transient absorption spectroscopy. Both the effects of hot-phonon bottleneck and the Auger heating significantly slow down the cooling rate of hot-carriers in the case of high excitation density. The effect of defects on the carrier recombination and the two-photon induced ultrafast carrier dynamics are also investigated. These findings are crucial to the optoelectronic applications of copper iodide.

Dependence of the radiative lifetime on the type-II band offset in GaAsxSb1−x/GaAs quantum dots including effects of photoexcited carriers

In quantum dot (QD) heterostructures that have a type-II band alignment, either the electron or the hole is confined inside the QD. Due to smaller electron–hole overlap in such structures, relatively long radiative lifetimes can be realized, which is beneficial for devices such as intermediate-band solar cells. The use of GaAsxSb1−x/GaAs QDs allows us to control the energy level of the confined state by changing the type-II conduction-band offset (CBO) without the need of changing the QD size. However, the dependence of the radiative lifetime τr on the CBO needs to be considered to achieve optimum device performance. In this work, GaAsxSb1−x/GaAs QDs were grown by molecular beam epitaxy. The amount of deposition was controlled to obtain QDs with approximately the same size even for different values of As composition x, and the carrier lifetime was determined by time-resolved photoluminescence measurements. Since the CBO becomes smaller for larger values of x, a simple model would predict a larger electron–hole overlap for larger x values, and thus, the lifetime should decrease monotonically. However, the experimentally obtained lifetime does not decrease monotonically, which has interesting implications for applications. We explain the observed trend by the effect of photoexcited carriers; a triangular potential well is formed around the QDs in the case of high excitation densities, and thus, electrons are localized near the QDs. We also calculated τr considering the effect of photoexcited carriers to confirm our model, and a similar tendency was obtained.

Scintillator of Polycrystalline Perovskites for High‐Sensitivity Detection of Charged‐Particle Radiations

AbstractHerein, the first study on the scintillation properties of CsCu2X3 and Cs3Cu2X5 (where X: Cl−, Br−, I−) is presented, describing their charged particle‐induced luminescence involving electrons, protons, α‐particles, and heavy ions, as well as revealing their capabilities on the timing and spectroscopic evaluation of single‐particle events. The thin layers are prepared with a simple and cost‐effective deposition procedure, without the incorporation of external dopants, exploiting the intrinsic radiative recombination observed in low‐dimensional perovskites. The combined effect of the high binding energy and localized stability of self‐trapped excitons, large Stokes shift, defect tolerance, and the high excitation density along the particle track leads to the emergence of boosted scintillation pulses. The observations demonstrate the first use of inorganic thin‐film scintillators with optical pulse characteristics and light yield competitive with doped single crystal scintillators, while also providing improved structural and functional stability under extreme environmental conditions.

High Power Irradiance Dependence of Charge Species Dynamics in Hybrid Perovskites and Kinetic Evidence for Transient Vibrational Stark Effect in Formamidinium.

Hybrid halide perovskites materials have the potential for both photovoltaic and light-emitting devices. Relatively little has been reported on the kinetics of charge relaxation upon intense excitation. In order to evaluate the illumination power density dependence on the charge recombination mechanism, we have applied a femtosecond transient mid-IR absorption spectroscopy with strong excitation to directly measure the charge kinetics via electron absorption. The irradiance-dependent relaxation processes of the excited, photo-generated charge pairs were quantified in polycrystalline MAPbI3, MAPbBr3, and (FAPbI3)0.97(MAPbBr3)0.03 thin films that contain either methylamonium (MA) or formamidinium (FA). This report identifies the laser-generated charge species and provides the kinetics of Auger, bimolecular and excitonic decay components. The inter-band electron-hole (bimolecular) recombination was found to dominate over Auger recombination at very high pump irradiances, up to the damage threshold. The kinetic analysis further provides direct evidence for the carrier field origin of the vibrational Stark effect in a formamidinium containing perovskite material. The results suggest that radiative excitonic and bimolecular recombination in MAPbI3 at high excitation densities could support light-emitting applications.

Excitation density dependent carrier dynamics in a monolayer MoS2: Exciton dissociation, formation and bottlenecking

Research on Molybdenum Disulfide (MoS2) monolayer at high excitation densities is critical due to its rising demand in optoelectronic applications such as lasers, compact optical parametric amplifiers, and high power detectors. We show that at high excitation densities, measurements on a single monolayer MoS2 reveal a transient response rich in physical processes. Despite the fact that carriers are excited straight to the A-excitonic state, the time and fluence dependence of the observed optical response indicates exciton dissociation followed by hot-carrier generation. In the picosecond time scale, these hot-carriers form excitons, which have a longer lifetime at higher excitation densities, implying a bottlenecking process. For the use of MoS2 at high-excitation densities, the fluence dependency of the relaxation mechanisms detailed here is essential.

A comparative study of ultrafast carrier dynamics near A, B, and C-excitons in a monolayer MoS2 at high excitation densities

With the growing demand for monolayer MoS2 in diverse optoelectronic applications, it is more important than ever to understand carrier behavior under varied excitation conditions and at different excitonic levels. In this article, we show that band structure, excitation wavelength, and excitation density all have a significant impact on the carrier dynamics in a monolayer MoS2. At the A and B excitation levels, an initial bandgap renormalization is detected, while band bleaching is found to play a stronger role at the C-excitonic state. The exciton dissociation at the band edge near A and B-exciton causes the bandgap renormalization. On the other hand, band bleaching, which occurs at the C-exciton, is caused by the exciton formation due to the high availability of states. At this excitation energy, carrier relaxation and thermalization occur through lattice interaction by releasing a high number of hot phonons, which creates a bottleneck effect. Due to the hot phonon bottlenecking effect, carrier relaxation time is prolonged.

Tracking Ultrafast Change of Multiterahertz Broadband Response Functions in a Photoexcited Dirac Semimetal Cd3As2 Thin Film.

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd3As2 by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 μm), which covers the crossover between the inter- and intraband response. We resolve dynamics of the photoexcited nonthermal electrons, which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics.

Ultrafast carrier dynamics in a monolayer MoS2 at carrier densities well above Mott density

Due to the growing interest in monolayer (ML) molybdenum disulfide (MoS2) in several optoelectronic applications like lasers, detectors, sensors, it is important to understand the ultrafast behavior of the excited carriers in this material. In this article, a comprehensive study of the charge carrier dynamics of a monolayer MoS2 flake has been studied using transient transmission technique near A-exciton under high excitation densities well above the Mott density. Fluence dependent studies has been carried out to understand the origin of the processes which modifies its optical response under excitation. The dissociation of excitons leads to an observed fast bandgap renormalization. At later times when large number of carriers relax the remaining carriers forms excitons leading to a bleaching effect.

System of localized excitons on oxygen complexes in CdS

Intense CdS luminescence in the blue and green spectral regions is widely used in all areas of optoelectronics. In this spectrum band are working on lasers CdS. This paper presents the results of a study of the exciton region of the CdS spectrum based on the theory of anti-intersecting bands (bands anticrossing theory-BAC) with the involvement of broader initial data for the analysis of optical properties. Depending on the growth conditions, the presence and change of oxygen concentration and intrinsic point defects determining the composition of crystals are taken into account. The concept of the nonuniform distribution of isoelectronic oxygen centers in the bulk of CdS due to their predominant segregation on compensating stacking faults is introduced. Cathodoluminescence (CL) spectra were studied using various recording methods, excitation intensity and temperature, as well as pulsed CL at high excitation intensities. In a scanning electron microscope from local registration and a high excitation density, the emission of the edge luminescence components of CdS was detected at 300 K. To analyze the optical data, we used the capabilities of the method for constructing band models based on the BAC theory, which collects extensive and multilateral information about specific samples. A model of a CdS · O multizone with stacking faults, which determines the spectrum of edge emission is presented. An explanation of the nature of the green edge emission of cadmium sulfide as excitons localized on oxygen-containing complexes in SF layers has been obtained for the first time. It was found that the system of levels of localized excitons at stacking faults does not change either with temperature up to 300 K, or with a change in the oxygen solubility in the crystal to the limiting one. It is shown that the presence of isoelectronic oxygen centers appear itself in the electro-physical properties of crystals. Recommendations are given for the diagnostics of crystals suitable for the creation of luminescent systems or lasers that are stable in operation. Keywords: isoelectronic centers, bands anticrossing theory, localized exciton, stacking faults, point defects, stimulated radiation.

Tuning Spin-Polarized Lifetime in Two-Dimensional Metal-Halide Perovskite through Exciton Binding Energy.

Metal-halide perovskite semiconductors have attracted attention for opto-spintronic applications where the manipulation of charge and spin degrees of freedom have the potential to lower power consumption and achieve faster switching times for electronic devices. Lower-dimensional perovskites are of particular interest since the lower degree of symmetry of the metal-halide connected octahedra and the large spin-orbit coupling can potentially lift the spin degeneracy. To achieve their full application potential, long spin-polarized lifetimes and an understanding of spin-relaxation in these systems are needed. Here, we report an intriguing spin-selective excitation of excitons in a series of 2D lead iodide perovskite (n = 1) single crystals by using time- and polarization-resolved transient reflection spectroscopy. Exciton spin relaxation times as long as ∼26 ps at low excitation densities and at room temperature were achieved for a system with small binding energy, 2D EOA2PbI4 (EOA = ethanolamine). By tuning the excitation density and the exciton binding energy, we identify the dominant mechanism as the D'yakonov-Perel (DP) mechanism at low exciton densities and the Bir-Aronov-Pikus (BAP) mechanism at high excitation densities. Together, these results provide new design principles to achieve long spin lifetimes in metal-halide perovskite semiconductors.

Two-Channel Space Charge Transfer-Induced Thermally Activated Delayed Fluorescent Materials for Efficient OLEDs with Low Efficiency Roll-Off.

Enhancing the reverse intersystem crossing (RISC) process of thermally activated delayed fluorescent (TADF) emitters is an effective approach to realize efficient organic light-emitting diodes (OLEDs) with low efficiency roll-off. In this work, we designed two novel TADF emitters, SAT-DAC and SATX-DAC, via a spiro architecture. Efficient maximum external quantum efficiencies (EQEs) of 22.6 and 20.9% with reduced efficiency roll-off (EQEs of 17.9 and 17.0% at 1000 cd m-2) were achieved via a "two-RISC-channel" strategy. X-ray diffraction shows close donor (D)/acceptor (A) spacing and suitable D/A orientation in crystals of the two emitters favoring both intra- and intermolecular through-space charge transfer (TSCT) processes. Transient photoluminescence decay measurements show that both emitters have two RISC channels leading to kISCT exceeding 106 s-1. These results suggest that the "two-RISC-channel" design can be a novel approach for enhancing performance of TADF emitters, in particular at high excitation densities.

All-inorganic high efficiency LuAG:Ce3+ converter based on phosphor-in-glass for laser diode lighting

High-power, high-brightness laser lighting technology put forward new requirements for the service stability of color conversion materials. Phosphor-in-glass materials (PiGs) have emerged with their unique advantages of withstanding high power excitation density. High luminous efficiency (LE) and high internal quantum efficiency (IQE) LuAG:Ce PiG is still an urgent demand for high power laser lighting conversion materials. In this paper, LuAG:Ce PiGs was successfully prepared by careful design of glass matrix composition. The maximum internal quantum efficiency of LuAG:Ce PiG is 93.8%. The conversion efficiency (CE) of LuAG:Ce PiG from blue to green light is increased to 56.3%. The luminous efficiency (LE) even reaches 225 lm W−1, which is the best performance of LuAG:Ce in LD lighting so far. These results show that this work has taken a big step in improving the performance of LuAG:Ce Green converter and will greatly promote the development of LD lighting.

Photofluorochromic water-dispersible nanoparticles for single-photon-absorption upconversion cell imaging

Photofluorochromic diarylethene (DAE) molecules have been widely investigated due to their excellent fatigue resistance and thermal stability. However, the poor water solubility of DAEs limits their biological applications to some extent. Herein, we reported two kinds of water-dispersible DAE nanoparticles (DAEI-NPs and DAEB-NPs), in which DAE molecules were stabilized by the amphiphilic polymer DSPE-mPEG2000 using the nanoprecipitation approach. The fabricated nanoparticles retain well-controlled luminescence and fluorescence photoswitching properties in aqueous solution, which could be reversibly switched on and off under the alternating irradiation of ultraviolet (UV) and visible light. In addition, the closed-ring isomers of DAEB-NPs performed hot-band-absorption-based photon upconversion when excited by a 593.5 nm laser. Bearing excellent photophysical properties and low cytotoxicity, DAEB-NPs were applicable for upconversion cell imaging without high-excitation power density and free from oxygen removal. Additionally, the imaging process could be switched on by regulating the photofluorochromic nanoparticles.

Imaging Seebeck drift of excitons and trions in MoSe2 monolayers

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe2 monolayers (ML) encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS2 ML at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening attributed to a much less pronounced halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as thermal drift of trions due to a Seebeck term in the particle current. The model shows that the difference between trion and exciton profiles is simply understood in terms of the very different lifetimes of these two quasiparticles.

Ultrafast and nonlinear optical properties of two-dimensional CdSe nanostructures prepared using MoS2 nanosheets as template

Two-dimensional (2D) materials have attracted tremendous research interest because of their unique optical, electronic, and mechanical properties which make them suitable building blocks for various electronic and optoelectronic applications. The heterostructures formed by 2D semiconductors can play an important role in modern semiconductor industry. Here, we have grown 2D-CdSe structures over MoS2 nanosheets via successive ion layer adsorption and reaction method. The dynamics of photoinduced charge carriers in 2D-CdSe nanostructures have been investigated using femtosecond transient absorption spectroscopy. Our photophysical studies infer that charge carrier recombination is monomolecular at low and bimolecular at high excitation densities. Nonetheless, femtosecond Z-scan technique was employed to demonstrate optical limiting behavior of 2D-CdSe films. This study will open up a new avenue of designing 2D nanomaterials for optoelectronic applications.

Time-resolved high energy ionoluminescence of Al2O3

We have compared the luminescence decay from intact and pre-damaged Al2O3 crystals registered during single swift heavy ion and photo- picosecond laser pulse excitation (λex. = 440 nm). The decay curves measured during 1.2 ÷ 1.6 MeV/amu C, Ar, V, Kr, Xe ion irradiation at room temperature are composed of three components – fast (τ1 < 1 ns), τ2 = 1.8 ns (F+-centers) and τ3 = 54–80 ns (E-luminescence, the nature of emission is under discussion). The measurements performed on pre-irradiated samples have demonstrated that accumulated radiation damage suppresses the E-luminescence and only τ1 and τ2 components are observed.It was found that photoexcitation at 440 nm induces strong emission of F22+- centers with a lifetime of about 8 ns which is not detected during high energy ion irradiation. This is ascribed to the quenching effect of high density excitations in a particle track.

The reduction of the thermal quenching effect in laser-excited phosphor converters using highly thermally conductive hBN particles

Phosphor converters for solid state lighting applications experience a strong thermal stress under high-excitation power densities. The recent interest in laser diode based lighting has made this issue even more severe. This research presents an effective approach to reduce the thermal quenching effect and damage of laser-excited phosphor-silicone converters using thermally conductive hexagonal boron nitride (hBN) particles. Herein, the samples are analyzed by employing phosphor thermometry based on the photoluminescence decay time, and thermo-imaging techniques. The study shows that hBN particle incorporation increases the thermal conductivity of a phosphor-silicone mixture up to 5 times. It turns out, that the addition of hBN to the Eu^{2+} doped chalcogenide-silicone converters can increase the top-limit excitation power density from 60 to 180 W cm^{-2}, thus reaching a 2.5 times higher output. Moreover, it is shown that the presence of hBN in Ce^{3+} activated garnet phosphor converters, may increase the output power by up to 1.8 times and that such converters can withstand 218 W cm^{-2} excitation. Besides, hBN particles are also found to enhance the stability of the converters chromaticity and luminous efficacy of radiation. This means that the addition of hBN particles into silicone-based phosphor converter media is applicable in a wide range of different areas, in particular, the ones requiring a high optical power output density.

Identification of Excitons and Biexcitons in Sb2Se3 under High Photoluminescence Excitation Density

AbstractThe near‐band‐edge emission of Sb2Se3 microcrystals is studied in detail under high photoluminescence excitation density using a pulsed UV laser (λ = 266 nm, pulse width 0.6 ns). Based on the peak energy positions and the excitation power density and temperature dependencies (T = 3–110 K) of the photoluminescence spectra, the emission is interpreted as a recombination of two pairs (A and B) of free excitons and biexcitons appearing because of valence band splitting. The magnitude of the valence band splitting due to the crystal field (Δcr = 20–22 meV) is estimated in the Brillouin zone center. At T = 3 K, the A and B biexciton emission at 1.302 and 1.322 eV, respectively, and the A and B free exciton emission at 1.311 and 1.333 eV, respectively, are observed. The binding energy of A and B biexcitons is 9 and 11 meV, respectively, and the binding energy of A free exciton is 6 meV. The activation energy of thermal quenching for all observed peaks is 20 meV.