Increasing Excitation Density Research Articles

Many-Exciton Quantum Dynamics in a Ruddlesden-Popper Tin Iodide.

We present a study on the many-body exciton interactions in a Ruddlesden-Popper tin halide, namely, (PEA)2SnI4 (PEA = phenylethylammonium), using coherent two-dimensional electronic spectroscopy. The optical dephasing times of the third-order polarization observed in these systems are determined by exciton many-body interactions and lattice fluctuations. We investigate the excitation-induced dephasing (EID) and observe a significant reduction of the dephasing time with increasing excitation density as compared to its lead counterpart (PEA)2PbI4, which we have previously reported in a separate publication [J. Chem. Phys.2020, 153, 164706]. Surprisingly, we find that the EID interaction parameter is four orders of magnitude higher in (PEA)2SnI4 than that in (PEA)2PbI4. This increase in the EID rate may be due to exciton localization arising from a more statically disordered lattice in the tin derivative. This is supported by the observation of multiple closely spaced exciton states and the broadening of the linewidth with increasing population time (spectral diffusion), which suggests a static disordered structure relative to the highly dynamic lead-halide. Additionally, we find that the exciton nonlinear coherent lineshape shows evidence of a biexcitonic state with low binding energy (<10 meV) not observed in the lead system. We model the lineshapes based on a stochastic scattering theory that accounts for the interaction with a nonstationary population of dark background excitations. Our study provides evidence of differences in the exciton quantum dynamics between tin- and lead-based Ruddlesden-Popper metal halides (RPMHs) and links them to the exciton-exciton interaction strength and the static disorder aspect of the crystalline structure.

Photo‐Induced Tellurium Segregation in MoTe2

Herein, coherent phonon spectroscopy of polymorphic single crystals using a femtosecond‐pulsed laser is used to investigate the relationship between structural phase transitions and photo‐thermal effects induced by high‐density laser excitation. Even when a femtosecond‐pulsed laser is used, which generally induces fewer heat accumulation effects than the case for a continuous‐wave (CW) laser, tellurium segregation is observed in coherent phonon spectra with increasing excitation density, while no structural phase transition among the polymorphs of is observed. The use of higher photon‐energy excitation, however, is found to suppress tellurium segregation.

Influence of the excitation density and temperature on the optical properties of type I InAs/GaAsSb quantum dots

In this study, the optical properties of InAs quantum dots (QDs) were characterized using photoluminescence (PL) measurements. The QDs, capped with GaAs and GaAs1−xSbx (x = 6%) strain-reducing layer (SRL), were grown by Molecular Beam Epitaxy. Temperature-dependent photoluminescence (TDPL) of both ground state (GS) and first excited state (ES) was carried out through the analysis of the PL peak position as well as the integrated PL intensity. The temperature dependence of the integrated PL intensity shows the carrier trapping in the potential barrier at the interface between the capping layer and QDs in both samples at low temperatures for the GS and ES. The Excitation density dependent photoluminescence (EDPL) showed a redshift of the GS and ES PL peak energies with increasing excitation density. We attribute this variation to the bandgap renormalization (BGR) effect. The potential barrier reduction for the GaAsSb-capped QDs increases carrier injection efficiency inside the QDs, giving rise to a larger BGR effect compared to the QDs capped with GaAs. With increasing temperature, BGR redshift varies considerably for the GaAs- capped QDs but less for the Q with GaAsSb SRL. This effect was explained using the population rate of carriers inside the QDs while taking into account the nonradiative recombination process for the two samples. Furthermore, the variation of the integrated intensity with the excitation power density grows superlinearly for the two samples for a temperature range from 10 K to 220 K. This behavior was explained by the random capture of carriers in the dots. The sample with GaAsSb SRL having a smaller potential barrier has reduced superlinear dependence at low temperatures. Losses mechanism has a significant impact on increasing the superlinear dependence at high temperatures. The ES showed a stronger superlinearity compared to the GS. This study helps to understand the optical mechanisms in some devices, such as QD lasers.

Ultrafast dynamics of the low frequency shear phonon in 1T′-MoTe2

We report on the dynamics of coherent phonons in semimetal 1T′-MoTe2 using femtosecond pump-probe spectroscopy. On an ultrafast sub-picosecond timescale at room temperature, a low frequency and long-lifetime shear phonon mode was observed at 0.39 THz, which was previously reported in the form of a characteristic phonon only in the low temperature Td−MoTe2 phase. Unlike the other optical phonon modes, the shear phonon mode was found to strongly couple with photoexcited carriers. Moreover, the amplitude of the shear mode surprisingly decreased with increasing excitation density, a phenomenon that can be attributed to be a consequence of the lattice temperature increasing after excitation. These results provide useful physical information on ultrafast lattice symmetry switching between the normal semimetal 1T′ and the lattice inversion symmetry breaking Type-II Weyl semimetal Td phases.

Lasing from reduced dimensional perovskite microplatelets: Fabry-Pérot or whispering-gallery-mode?

Cesium lead bromide (CsPbBr3) perovskite has attracted great attention recently for its potentials for next-generation green-color lasing devices owing to the relatively high structural stability and the high emission efficiency among the perovskite family. Herein, we explore the origins of cavity modes in CsPbBr3 microplatelets (MPs) lasers by using angle-resolved microphotoluminescence Fourier imaging technique, which is still controversial so far. In-plane Fabry-Pérot (F-P) mode lasing transition to whispering-gallery-mode (WGM) lasing is verified at room temperature, which mostly occurs in large MPs with edge length (L) over 13 μm. The F-P lasing is suppressed upon decreasing L or increasing excitation density, and the WGM lasing is predominant for all MPs at high excitation density. Furthermore, the parity and symmetry of in-plane F-P modes are classified. These results advance the fundamental understanding of lasing modes in planar microcavities as well as their applications in on-chip interconnection and quantum optics.

Field dependent ultrafast carrier dynamics in InGaN/GaN p-i(MQW)-n structure

Abstract Field dependent ultrafast carrier dynamics in InGaN/GaN multiple quantum wells (MQW) embedded within a p-i-n structure (InGaN/GaNp-i (MQW)-n) was systematically investigated by photoluminescence (PL), time-resolved PL and transient differential reflectivity measurements. We observed two PL peaks (centered at 450 nm and 500 nm) originated from MQW and localized defect states. Excitation density dependent PL spectra shows that the amplified spontaneous emission participates in the quantum well emission. Surprisingly, time-resolved PL clearly exhibits that quantum well emission has an extremely long decay time, which is much slower than the lower energy peak from defect states. We infer that the built-in PN field affects dominantly the carrier recombination more than the piezoelectric polarization field. The distinct excitation-dependent decay kinetic allows us to identify the dynamic interplay of screening and descreening effect by photo-generated carriers. Additionally transient reflectivity measurements also exhibit that carrier thermalization process becomes faster with increasing excitation density, which could be due to the amplified spontaneous emission.

Optical excitation density dependence of spin dynamics in bulk cubic GaN

The influence of the optical excitation density on the electron spin dynamics is experimentally investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. The nanosecond spin relaxation times in moderately n-doped β-GaN decrease with increasing excitation density, though the effective lifetimes of the optically excited carriers are almost two orders of magnitude shorter than the spin relaxation times. This counterintuitive finding is explained by the heating of the electron system due to the excitation process. The spin relaxation times in degenerately n-doped β-GaN are found to be independent of excitation density as the very high electron Fermi temperature completely dominates over carrier heating processes in this case.

Carrier Transfer of Deep‐Level Localized States in Type‐II InxGa1−xAs/GaNyAs1−y Short‐Period Superlattice

Optical properties of InGaAs/GaNAs superlattice are investigated by the photoluminescence and reflectance measurements. A quantum well (QW) emission PM (centered at ≈1.2 eV) and a broad low‐energy emission (LEE) band, which can be resolved to three peaks PA′, PA, and PB (0.77, 0.83, and 0.92 eV) are observed. The peak positions of the LEE exhibit S‐shaped behavior, whereas the QW emission shows a red shift with increasing temperature. The LEE bands are attributed to N‐related deep‐level localized state. The photoluminescence (PL) peak of QW exhibits a significant blue shift and saturation effect with increasing excitation density, which is interpreted in terms of the band‐bending model in the InGaAs/GaNAs short‐period superlattice (SPSL) with a type II band alignment. Furthermore, carrier transfer among these N‐related localized states is elucidated by examining the temperature‐dependent photoluminescence. Such carrier transfer is also demonstrated by the thermal effect under high excitation density.

Hot-carrier generation and extraction in InAs/GaAs quantum dot superlattice solar cells

We demonstrate hot-carrier (HC) generation and extraction processes in InAs/GaAs quantum dot superlattice (QDSL) solar cells with the excitation energy tuned below the GaAs band edge at 15 K. The short-circuit current density and the open-circuit voltage originated from the HC extraction are enhanced by increasing the number of the QDSL layers owing to the increased absorptivity in the QDSLs. The short-circuit current density demonstrates an almost linear dependence with increasing excitation density and increases with increasing excitation energy, which reflects the absorption coefficient in the QDSLs. Conversely, the open-circuit voltage tends to saturate to the same value independently from the excitation energy, indicating that the hot electrons extracted to the conduction band edge of GaAs are retrapped at the open-circuit condition due to a low built-in electric field of ∼1.3 kV cm−1.

A polariton graph simulator

We discuss polariton graphs as a new platform for simulating the classical XY and Kuramoto models. Polariton condensates can be imprinted into any two-dimensional graph by spatial modulation of the pumping laser. Polariton simulators have the potential to reach the global minimum of the XY Hamiltonian in a bottom-up approach by gradually increasing excitation density to threshold or to study large scale synchronization phenomena and dynamical phase transitions when operating above the threshold. We consider the modelling of polariton graphs using the complex Ginzburg–Landau model and derive analytical solutions for a single condensate, the XY model, two-mode model and the Kuramoto model establishing the relationships between them.

A Yellow Polariton Condensate in a Dye Filled Microcavity

Polariton condensation in the yellow part of the visible spectrum from a planar organic semiconductor microcavity containing the molecular dye bromine‐substituted boron‐dipyrromethene is observed. This study provides experimental fingerprint of polariton condensation under nonresonant optical excitation, including the nonlinear dependence of the emission intensity and wavelength blueshift with increasing excitation density, single excitation pulse dispersion imaging, and real space interferometry. The latter two allow to visualize the collapse of the energy distribution and the long‐range coherence of the polariton condensate.

Observation of Exciton Redshift-Blueshift Crossover in Monolayer WS2.

We report a rare atom-like interaction between excitons in monolayer WS2, measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction-repulsion crossover of the exciton-exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to interparticle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials.

Zinc vacancy-related complex and its abnormal photoluminescence in Zn+-implanted ZnO single crystals

Zinc vacancy (VZn) plays key roles in the optical and electrical properties of ZnO, but its behaviors are not fully understood. Here we report the formation and abnormal photoluminescence (PL) of VZn-related complex in Zn+-implanted ZnO single crystals. With increasing excitation density, we observed a new and gradually increased broad emission around 550nm. The 550nm emission is unexpectedly invisible under low power level excitation, indicating the lower priority of recombination of the related defect centers. Electron paramagnetic resonance (EPR) results suggest the formation of VZn-Zni complex after implantation, which is responsible for the abnormal PL properties.

Impact of Radiative and Nonradiative Recombination Processes on the Efficiency-Droop Phenomenon in InxGa1−xN Single Quantum Wells Studied by Scanning Near-Field Optical Microscopy

Reduced emission efficiency (``efficiency droop'') at high current is a serious problem for solid-state lighting based on In${}_{x}$Ga${}_{1-x}$N LEDs. Auger recombination may be the culprit, yet fluctuations in alloy composition also seem to matter. The authors use a scanning near-field optical microscope to study the photoluminescence of blue- and green-emitting In-Ga-N quantum wells, and find that increasing excitation density causes carrier overflow from potential minima. The mechanism behind droop depends on emission color (indium content), and carrier-activated nonradiative recombination is key in green emitters.

Acceptor and Excitation Density Dependence of the Ultrafast Polaron Absorption Signal in Donor–Acceptor Organic Solar Cell Blends

Transient absorption spectroscopy on organic semiconductor blends for solar cells typically shows efficient charge generation within ∼100 fs, accounting for the majority of the charge carriers. In this Letter, we show using transient absorption spectroscopy on blends containing a broad range of acceptor content (0.01-50% by weight) that the rise of the polaron signal is dependent on the acceptor concentration. For low acceptor content (<10% by weight), the polaron signal rises gradually over ∼1 ps with most polarons generated after 200 fs, while for higher acceptor concentrations (>10%) most polarons are generated within 200 fs. The rise time in blends with low acceptor content was also found to be sensitive to the pump fluence, decreasing with increasing excitation density. These results indicate that the sub-100 fs rise of the polaron signal is a natural consequence of both the high acceptor concentrations in many donor-acceptor blends and the high excitation densities needed for transient absorption spectroscopy, which results in a short average distance between the exciton and the donor-acceptor interface.

Nonresonant optical control of a spinor polariton condensate

We investigate the spin dynamics of polariton condensates spatially separated from and effectively confined by the pumping exciton reservoir. We obtain a strong correlation between the ellipticity of the non-resonant optical pump and the degree of circular polarisation (DCP) of the condensate at the onset of condensation. With increasing excitation density we observe a reversal of the DCP. The spin dynamics of the trapped condensate are described within the framework of the spinor complex Ginzburg-Landau equations in the Josephson regime, where the dynamics of the system are reduced to a current-driven Josephson junction. We show that the observed spin reversal is due to the interplay between an internal Josephson coupling effect and the detuning of the two projections of the spinor condensate via transition from a synchronised to a desynchronised regime. These results suggest that spinor polariton condensates can be controlled by tuning the non-resonant excitation density offering applications in electrically pumped polariton spin switches.

Quasimomentum-Space Image for Ultrafast Melting of Silicon.

By exciting electron-hole pairs that survive for picoseconds strong femtosecond lasers may transiently influence the bonding properties of semiconductors, causing structure changes, in particular, ultrafast melting. In order to determine the energy flow during this process in silicon we performed abinitio molecular dynamics simulations and an analysis in quasimomentum space. We found that energy flows very differently as a function of increasing excitation density, namely, mainly through long wavelength, L-point, or X-point lattice vibrations, respectively.

Negative differential gain in quantum dot systems: Interplay of structural properties and many-body effects

The saturation behaviour of optical gain with increasing excitation density is an important factor for laser device performance. For active materials based on self-organized InGaAs/GaAs quantum dots, we study the interplay between structural properties of the quantum dots and many-body effects of excited carriers in the optical properties via a combination of tight-binding and quantum-kinetic calculations. We identify regimes where either phase-space filling or excitation-induced dephasing dominates the saturation behavior of the optical gain. The latter can lead to the emergence of a negative differential material gain.

Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals.

We simultaneously determined the charge carrier mobility and picosecond to nanosecond carrier dynamics of isolated silicon nanowires (Si NWs) and nanocrystals (Si NCs) using time-resolved terahertz spectroscopy. We then compared these results to data measured on bulk c-Si as a function of excitation fluence. We find >1 ns carrier lifetimes in Si NWs that are dominated by surface recombination with surface recombination velocities (SRV) between ∼1100-1700 cm s(-1) depending on process conditions. The Si NCs have markedly different decay dynamics. Initially, free-carriers are produced, but relax within ∼1.5 ps to form bound excitons. Subsequently, the excitons decay with lifetimes >7 ns, similar to free carriers produced in bulk Si. The isolated Si NWs exhibit bulk-like mobilities that decrease with increasing excitation density, while the hot-carrier mobilities in the Si NCs are lower than bulk mobilities and could only be measured within the initial 1.5 ps decay. We discuss the implications of our measurements on the utilization of Si NWs and NCs in macroscopic optoelectronic applications.

Spin polarization, dephasing, and photoinduced spin diffusion in (110)-grown two-dimensional electron systems

We study the optically induced spin polarization, spin dephasing, and diffusion in several high-mobility two-dimensional electron systems, which are embedded in GaAs quantum wells grown on (110)-oriented substrates. The experimental techniques comprise a two-beam magneto-optical spectroscopy system and polarization-resolved photoluminescence. Under weak excitation conditions at liquid-helium temperatures, we observe spin lifetimes above 100 ns in one of our samples, which are reduced with increasing excitation density due to additional, hole-mediated, spin dephasing. The spin dynamic is strongly influenced by the carrier density and the ionization of remote donors, which can be controlled by temperature and above-barrier illumination. The absolute value of the average electron spin polarization in the samples is directly observable in the circular polarization of photoluminescence collected under circularly polarized excitation and reaches values of about 5%. Spin diffusion is studied by varying the distance between pump and probe beams in microspectroscopy experiments. We observe diffusion lengths above 100 $\ensuremath{\mu}$m and, at high excitation intensity, a nonmonotonic dependence of the spin polarization on the pump-probe distance.