Excitation Fluence Research Articles

Nonequilibrium Lattice Dynamics in Photoexcited 2D Perovskites.

Interplay between structural and photophysical properties of metal halide perovskites is critical to their utility in optoelectronics, but there is limited understanding of lattice response upon photoexcitation. Here, 2D perovskites butylammonium lead iodide, (BA)2 PbI4 , and phenethylammonium lead iodide, (PEA)2 PbI4 , are investigated using ultrafast transient X-ray diffraction as a function of optical excitation fluence to discern structural dynamics. Both powder X-ray diffraction and time-resolved photoluminescence linewidths narrow over 1 ns following optical excitation for the fluence range studied, concurrent with slight redshifting of the optical bandgaps. These observations are attributed to transient relaxation and ordering of distorted lead iodide octahedra stimulated mainly by electron-hole pair creation. The c axis expands up to 0.37% over hundreds of picoseconds; reflections sampling the a and b axes undergo one tenth of this expansion with the same timescale. Post-photoexcitation appearance of the (110) reflection in (BA)2 PbI4 would suggest a transient phase transition,however, through new single-crystal XRD, reflections are found that violate glide plane conditions in the reported Pbca structure. The static structure space group is reassigned as P21 21 21 . With this, a nonequilibrium phase transition is ruled out. These findings offer increased understanding of remarkable lattice response in 2D perovskites upon excitation.

Laser excitation effects in lifetime-based high-speed phosphor thermometry

With high-speed phosphor thermometry becoming more established follows the need for an improved understanding of laser excitation induced measurement errors. With this aim in mind, the impact of kHz laser excitation on the lifetime method was investigated for the phosphors Mg3FGeO6:Mn (MFG) and YVO4:Tm. The results showed that significant measurement errors can occur for both phosphors. The Maximum Entropy Method (MEM) was utilized to observe changes in the distribution of decay time with changes in laser excitation fluence and repetition rate. In the light of this analysis, it was concluded that the changes in measured decay time and therefore measured temperature cannot be primarily attributed to laser heating for either phosphor. The dominant source of changes in the measured decay time is ascribed to changes in the distributions of decay time with laser fluence for YVO4:Tm and laser irradiance for MFG.

Disentangling Many-Body Effects in the Coherent Optical Response of 2D Semiconductors.

In single-layer (1L) transition metal dichalcogenides, the reduced Coulomb screening results in strongly bound excitons which dominate the linear and the nonlinear optical response. Despite the large number of studies, a clear understanding on how many-body and Coulomb correlation effects affect the excitonic resonances on a femtosecond time scale is still lacking. Here, we use ultrashort laser pulses to measure the transient optical response of 1L-WS2. In order to disentangle many-body effects, we perform exciton line-shape analysis, and we study its temporal dynamics as a function of the excitation photon energy and fluence. We find that resonant photoexcitation produces a blue shift of the A exciton, while for above-resonance photoexcitation the transient response at the optical bandgap is largely determined by a reduction of the exciton oscillator strength. Microscopic calculations based on excitonic Heisenberg equations of motion quantitatively reproduce the nonlinear absorption of the material and its dependence on excitation conditions.

Gain-switching in CsPbBr3 microwire lasers

All-inorganic perovskite microwire lasers, which have intrinsic high material gain and short cavity, especially favor the generation of ultrashort optical pulses via gain switching for various potential applications. Particularly, the ultrashort gain-switched pulses may extend perovskite microwires to previously inaccessible areas, such as ultrafast switches, and chipscale microcombs pumping souces in photonic integrated circuits. Here, we show 13.6-ps ultrashort single-mode green pulses from the gain-switched CsPbBr3 microwire lasers under femtosecond optical pumping. The gain-switching dynamics is experimentally investigated by a streak camera system. The excitation fluence dependences of pulse width, delay time and rise time of the output pulses show good agreements with the rate equation simulations with taking gain nonlinearities and carrier recombination ABC model into account. Our results reveal that perovskite microwire lasers have potential for ultrashort pulse generation, while the low transient saturated gain, which may result from the high transient carrier temperature under femtosecond pumping is a significant limitation for further pulse shortening.

Anomalous Contribution to the Nematic Electronic States from the Structural Transition in FeSe Revealed by Time- and Angle-Resolved Photoemission Spectroscopy.

High-resolution time- and angle-resolved photoemission measurements were made on FeSe superconductors. With ultrafast photoexcitation, two critical excitation fluences that correspond to two ultrafast electronic phase transitions were found only in the d_{yz}-orbit-derived band near the Brillouin-zone center within our time and energy resolution. Upon comparison to the detailed temperature dependent measurements, we conclude that there are two equilibrium electronic phase transitions (at approximately 90 and 120K) above the superconducting transition temperature, and an anomalous contribution on the scale of 10meV to the nematic states from the structural transition is experimentally determined. Our observations strongly suggest that the electronic phase transition at 120K must be taken into account in the energy band development of FeSe, and, furthermore, the contribution of the structural transition plays an important role in the nematic phase of iron-based high-temperature superconductors.

Programmable chalcogenide-based all-optical deep neural networks

Abstract We demonstrate a passive all-chalcogenide all-optical perceptron scheme. The network’s nonlinear activation function (NLAF) relies on the nonlinear response of Ge2Sb2Te5 to femtosecond laser pulses. We measured the sub-picosecond time-resolved optical constants of Ge2Sb2Te5 at a wavelength of 1500 nm and used them to design a high-speed Ge2Sb2Te5-tuned microring resonator all-optical NLAF. The NLAF had a sigmoidal response when subjected to different laser fluence excitation and had a dynamic range of −9.7 dB. The perceptron’s waveguide material was AlN because it allowed efficient heat dissipation during laser switching. A two-temperature analysis revealed that the operating speed of the NLAF is ≤ 1 $\le 1$ ns. The percepton’s nonvolatile weights were set using low-loss Sb2S3-tuned Mach Zehnder interferometers (MZIs). A three-layer deep neural network model was used to test the feasibility of the network scheme and a maximum training accuracy of 94.5% was obtained. We conclude that combining Sb2S3-programmed MZI weights with the nonlinear response of Ge2Sb2Te5 to femtosecond pulses is sufficient to perform energy-efficient all-optical neural classifications at rates greater than 1 GHz.

Determination of sub-ps lattice dynamics in FeRh thin films

Understanding the ultrashort time scale structural dynamics of the FeRh metamagnetic phase transition is a key element in developing a complete explanation of the mechanism driving the evolution from an antiferromagnetic to ferromagnetic state. Using an X-ray free electron laser we determine, with sub-ps time resolution, the time evolution of the (–101) lattice diffraction peak following excitation using a 35 fs laser pulse. The dynamics at higher laser fluence indicates the existence of a transient lattice state distinct from the high temperature ferromagnetic phase. By extracting the lattice temperature and comparing it with values obtained in a quasi-static diffraction measurement, we estimate the electron–phonon coupling in FeRh thin films as a function of laser excitation fluence. A model is presented which demonstrates that the transient state is paramagnetic and can be reached by a subset of the phonon bands. A complete description of the FeRh structural dynamics requires consideration of coupling strength variation across the phonon frequencies.

Intrinsic Trapping and Recombination Dynamics in Low‐Dimensional Bismuth Sulfide Nanocrystals

AbstractBismuth sulfide (Bi2S3) possesses bandgap of 1.30–1.45 eV, perfectly suitable for light‐harvesting materials in solar cells. Carrier dynamics in the Bi2S3 absorber material are determinant for energy conversion performance of solar cells, yet the carrier dynamics and associated recombination mechanism are rarely studied. Taking advantage of spectroscopic study on Bi2S3 nanocrystals, here new findings are uncovered inherent from the unique crystal structure that causes the energy loss in Bi2S3 materials. One is the trap state emission at near‐infrared region, the other is the ultrafast intrinsic trapping in picosecond scale without saturation at 1019 cm−3 carrier density and various decay rates by adjusting excitation fluence. These observations indicate that photoinduced carriers are likely to be intrinsically trapped due to lattice distortion and display different recombination pathways. This study provides fundamental understanding on the detailed carriers dynamics of Bi2S3 nanocrystals and new insights into the optimization of Bi2S3‐based photovoltaic devices.

Magnetic Configuration Driven Femtosecond Spin Dynamics in Synthetic Antiferromagnets.

Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt]n-1/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

Molecular diffusion and aggregate formation of photoproducts from ketoprofen in aqueous solutions

We investigated photochemical reaction of deprotonated ketoprofen (KP−) in aqueous solutions at neutral pH by transient grating (TG) method. At low excitation fluences or KP− concentrations, the TG signal showed molecular diffusion of KP− and its photoproducts such as hydrogen carbonate and 3-ethylbenzophenone (3-EBP). On the other hand, at high fluences or high KP− concentrations, the aggregation dynamics of 3-EBP was observed. The hydrodynamic radii of the formed aggregates reached beyond 5 nm, and their formation was driven in water-rich solutions.

Superfluorescence of Sub-Band States in C-Plane In0.1Ga0.9N/GaN Multiple-QWs.

Superfluorescence is a collective emission from quantum coherent emitters due to quantum fluctuations. This is characterized by the existence of the delay time ( for the emitters coupling and phase-synchronizing to each other spontaneously. Here we report the observation of superfluorescence in c-plane In0.1Ga0.9N/GaN multiple-quantum wells by time-integrated and time-resolved photoluminescence spectroscopy under higher excitation fluences of the 267 nm laser and at room temperature, showing a characteristic from 79 ps to 62 ps and the ultrafast radiative decay (7.5 ps) after a burst of photons. Time-resolved traces present a small quantum oscillation from coupled In0.1Ga0.9N/GaN multiple-quantum wells. The superfluorescence is attributed to the radiative recombination of coherent emitters distributing on strongly localized subband states, Ee1→Ehh1 or Ee1→Elh1 in 3nm width multiple-quantum wells. Our work paves the way for deepening the understanding of the emission mechanism in the In0.1Ga0.9N/GaN quantum well at a higher injected carrier density.

Nanoscale-Resolved Surface-to-Bulk Electron Transport in CsPbBr3 Perovskite.

Describing the nanoscale charge carrier transport at surfaces and interfaces is fundamental for designing high-performance optoelectronic devices. To achieve this, we employ time- and angle-resolved photoelectron spectroscopy with ultraviolet pump and extreme ultraviolet probe pulses. The resulting high surface sensitivity reveals an ultrafast carrier population decay associated with surface-to-bulk transport, which was tracked with a sub-nanometer spatial resolution normal to the surface, and on a femtosecond time scale, in the case of the inorganic CsPbBr3 lead halide perovskite. The decay time exhibits a pronounced carrier density dependence, which is attributed via modeling to enhanced diffusive transport and concurrent recombination. The transport is found to approach an ordinary diffusive regime, limited by electron–hole scattering, at the highest excitation fluences. This approach constitutes an important milestone in our capability to probe hot-carrier transport at solid interfaces with sub-nanometer resolution in a theoretically and experimentally challenging, yet technologically relevant, high-carrier-density regime.

Electron-hole plasma Fabry-Perot lasing in a Ga-incorporated ZnO microbelt via Ag nanoparticle deposition.

In this work, individual ZnO via Ga-doped (ZnO:Ga) microbelts with excellent crystallinity and smooth facets can enable the realization of lateral microresonator Fabry-Perot (F-P) microlasers, and the F-P lasing action originates from excitonic state. Interestingly, introducing Ag nanoparticles (AgNPs) deposited on the microbelt can increase F-P lasing characteristics containing a lower threshold and enhanced lasing output. Especially for the large size AgNPs (the diameter d is approximately 200 nm), the lasing features also exhibit a significant redshift of each lasing peak and an observable broadening of the spectral line width with an increase of the excitation fluence. And the remarkable lasing characteristics are belonging to the electron-hole plasma (EHP) luminescence. The behavior and dynamics of the stimulated radiation in an AgNPs@ZnO:Ga microbelt are studied, suggesting the Mott-transition from the excitonic state to EHP state that is responsible for the F-P lasing. These features can be attributed to the working mechanism that the hot electrons created by the large size AgNPs through nonradiative decay can fill the conduction band of nearby ZnO:Ga, leading to a downward shift of the conduction band edge. This novel filling influence can facilitate bandgap renormalization and result in EHP emission. The results provide a comprehensive understanding of the transition between excitonic and EHP states in the stimulated emission process. More importantly, it also can provide new scheme to developing high efficiency and ultra-low threshold microlasing diodes.

Enhancement of biexciton generation via resonance energy transfer in nanoclusters of colloidal CdTe quantum dots for optical gain applications

Colloidal CdTe quantum dots (QDs) of two different sizes (2.7 nm and 3.7 nm diameter) were self-assembled into mixed quantum dot donor-acceptor nanoclusters (QDANs) in aqueous solution. The ratio of small QDs (SQDs) to large QDs (LQDs) was 10:1, and the assembly was mediated by electrostatic attraction between oppositely-charged ligands (thioglycolic acid, TGA, and mercaptoethylamine, MA) on the QD surfaces. Under low excitation fluence (well below the threshold for biexciton formation), photoluminescence (PL) emission spectra and time-resolved decays in the QDANs revealed quenching in the SQDs and enhanced emission from the LQDs, providing evidence for resonance energy transfer (RET) from the wider band gap SQDs to the narrower band gap LQDs. The observed RET rate constant, determined from comparison of decays of free SQDs and those in the QDANs, was (3.4 ns)−1. Under these low-fluence conditions, the PL emission lifetimes of the LQDs in QDANs were the same as the free LQDs. Under higher excitation intensity, the decay of the LQDs in the QDAN showed a new, fast (sub-nanosecond), fluence-dependent component, suggesting formation of biexcitons and multiexcitons on the LQDs. The free LQDs required significantly (approximately 4-fold) higher excitation intensity than those in the QDANs to show similar evidence of biexciton formation through lifetime shortening. Since the QDANs contained a 10-fold excess of SQDs, the excitation energy that was initially distributed over the full population of QDs was funneled and concentrated into the small number of LQDs. These results support the idea that optical gain through biexciton formation can be enhanced by using RET, even when the rate constant for RET is much lower than that for biexciton decay.

Evolution of laser-induced strain in a Ge crystal for the [111] and [100] directions probed by time-resolved X-ray diffraction

Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.

Slow Auger Recombination of Trapped Excitons Enables Efficient Multiple Electron Transfer in CdS-Pt Nanorod Heterostructures.

Solar-to-fuel conversion reaction often requires multiple proton-coupled electron transfer (PCET) processes powered by the energetic electrons and/or holes generated by the absorption of multiple photons. The effective coupling of multiple electron transfer from the light absorber with the multiple PCET reactions at the catalytic center is one of the key challenges in efficient and selective conversion of solar energy to chemical fuels. In this paper, we examine the dynamics of multiple electron transfer in quantum confined CdS nanorods with a Pt tip, in which the CdS rod functions as the light absorber and the Pt tip the catalytic center. By excitation-fluence-dependent transient absorption spectroscopic measurements, we show that the multiexciton Auger recombination rate in CdS rods follows a carrier-collision model, knA = n2(n - 1)/4k2A, with a biexciton lifetime (1/k2A) of 2.0 ± 0.2 ns. In CdS-Pt nanorods, electron transfer kinetics from the CdS conduction band edge to the Pt show negligible dependence on the excitation fluence, occurring with a half-life time of 5.6 ± 0.6 ps. The efficiency of multiple exciton dissociation by multiple electron transfer to Pt decreases from 100% in biexciton states to ∼41% at 22 exciton state due to the competition with Auger recombination. The half-lifetime of the n-charge separated state recombination (with n electrons in the Pt and n holes in the CdS) decreases from 10 μs in the single charge separated state to 42 ns in nine charge separated states. Our findings suggest the possibility of driving multielectron photocatalytic reactions under intense illumination and controlling product selectivity through multielectron transfer.

Probing the ultrafast gain and refractive index dynamics of a VECSEL

Typically, strong gain saturation and gain dynamics play a crucial role in semiconductor laser mode-locking. While there have been several investigations of the ultrafast gain dynamics in vertical-external-cavity surface-emitting lasers (VECSELs), little is known about the associated refractive index changes. Yet, such refractive index changes do not only have a profound impact on the pulse formation process leading to self-phase modulation, which needs to be compensated by dispersion, but they are also of particular relevance for assessing the feasibility of Kerr-lens mode-locking of VECSELs. Here, we measure both refractive index as well as gain dynamics of a VECSEL chip using the ultrafast beam deflection method. We find that, in contrast to the gain dynamics, the refractive index dynamics is dominated by an instantaneous (∼100 fs) and a very slow component (∼100 ps). The time-resolved measurement of nonlinear refraction allows us to predict a pulse-length dependent, effective nonlinear refractive index n2,eff, which is shown to be negative and on the order of 10−16m2/W for short pulse lengths (∼100 fs). It becomes positive for large excitation fluences and large pulse lengths (few ps). These results agree with some previous reports of self-mode-locked VECSELs for which the cavity design and pulse properties determine sign and strength of the nonlinear refractive index when assuming Kerr-lens mode-locking.

Drude-Lorentz Model for Optical Properties of Photoexcited Transition Metals under Electron-Phonon Nonequilibrium

Evaluating the optical properties of matter under the action of ultrafast light is crucial in modeling laser–surface interaction and interpreting laser processing experiments. We report optimized coefficients for the Drude-Lorentz model describing the permittivity of several transition metals (Cr, W, Ti, Fe, Au, and Ni) under electron-phonon nonequilibrium, with electrons heated up to 30,000 K and the lattice staying cold at 300 K. A Basin-hopping algorithm is used to fit the Drude-Lorentz model to the nonequilibrium permittivity calculated using ab initio methods. The fitting coefficients are provided and can be easily inserted into any calculation requiring the optical response of the metals during ultrafast irradiation. Moreover, our results shed light on the electronic structure modifications and the relative contributions of intraband and interband optical transitions at high electron temperatures corresponding to the laser excitation fluence used for surface nanostructuring.

Dielectric Engineering for Manipulating Exciton Transport in Semiconductor Monolayers.

The dielectric screening from the disordered media surrounding atomically thin transition metal dichalcogenides (TMDs) monolayers modifies the effective defect energy levels and thereby the transport and energy dynamics of excitons. In this work, we study this effect in WSe2 monolayers for different combinations of surrounding dielectric media. Specifically, we study the source of the anomalous diffusion of excitons in the WSe2 monolayer and attribute the anomaly to the modification of the energy distribution of defect states in different disordered dielectric environments. We use this insight to manipulate exciton transport by engineering the dielectric environment using a graphene/hexagonal boron nitride (h-BN) moiré superlattice. Finally, we observe that the effect of dielectric disorder is even more significant at high excitation fluences, contributing to the nonequilibrium phonon drag effect. These results provide an important step toward achieving control over the exciton energy transport for next-generation opto-excitonic devices.

Optical damage limit of efficient spintronic THz emitters

SummaryTHz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures at the excitation wavelength. For the thickness-optimized spintronic heterostructure, the THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta)-based ferromagnetic/nonmagnetic (FM/NM) spintronic heterostructures have been studied for an optimized performance for THz generation when pumped by sub-50 fs amplified laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an efficient THz source. The optimized FM/NM spintronic heterostructure having α-phase Ta as the nonmagnetic layer shows the highest damage threshold as compared to those with Pt, irrespective of their generation efficiency. The damage threshold of the Fe/Ta heterostructure on a quartz substrate is ∼85 GW/cm2.