Excitation Fluence Research Articles

Low temperature exciton lifetime enhancement of MAPbBr3 studied by broadband terahertz spectroscopy

Over the past few years, lead halide perovskites have demonstrated significant potential in the field of solar cells, light-emitting diodes, and field-effect transistors. We investigated the transient photoconductivity and temperature-induced phase transitions in MAPbBr3 thin films, using ultrabroadband optical pump-terahertz probe spectroscopy with varying excitation fluences and temperature conditions. As the temperature drops from 293 to 123 K, MAPbBr3 undergoes two phase transitions, resulting in a significant extension of exciton lifetime in the tetragonal phase near 150 K, where MAPbBr3 exhibits a longer exciton lifetime. Our research contributes to a better understanding of the temperature-induced phase transition mechanism of MAPbBr3, enhancing its potential applications in optoelectronic devices.

High-sensitivity pump-probe spectroscopy with a dual-comb laser and a PM-Andi supercontinuum.

Amplifier-based pump-probe systems, while versatile, often suffer from complexity and low measurement speeds, especially when probing samples require low excitation fluences. To address these limitations, we introduce a pump-probe system that leverages a 60-MHz single-cavity dual-comb oscillator and an ultra-low noise supercontinuum. The setup can operate in equivalent time sampling or in programmable optical delay generation modes. We employ this system to study the wavelength-dependent excited-state dynamics of the non-fullerene electron acceptor Y6, a compound of interest in solar cell development, with excitation fluences as low as 1 nJ/cm2, well below the onset of nonlinear exciton annihilation effects. Our measurements reach a shot-noise limited sensitivity in differential transmission of 3.4·10-7. The results demonstrate the system's potential to advance the field of ultrafast spectroscopy.

Upconversion random lasing in Er3+/Yb3+ co-doped germanate-lead-magnesium glass-ceramics

Upconversion Random Laser (UC-RL) based on Yb3+ and Er3+ co-doped germanate-lead-magnesium (GPM) glass-ceramic powder is reported for the first time. Following the glass fabrication by a conventional melt-quenching technique, the samples were devitrified at 815 °C for 4 and 8 h, respectively. The samples were grained and excited using an optical parametric oscillator (pulses of 7 ns) tuned at 980 nm, in resonance with the Yb3+ transition 2F7/2 → 2F5/2 and with the Er3+ transition 4I15/2 → 4I11/2. The input-output curves revealed RL emission at ∼547 nm for an energy fluence excitation threshold (EFE)th of about 0.6 mJ/mm2 for GPM sample and ∼0.45 and ∼0.20 mJ/mm2 for samples heat-treated for 4 and 8h, respectively. For excitation above the (EFE)th, a fast pulse in the nanosecond range is observed, corroborating the lasing process. The RL feedback mechanism is due to the light reflections in the particles-air interfaces and to reflections between the glass-crystallites interfaces. Additionally, the Replica Symmetry Breaking (RSB) phenomenon was observed for all samples excited above the (EFE)th. Pearson correlation coefficients (PCC) maps were also obtained and showed that RL modes were increasingly correlated above (EFE)th. No reduction of the emission linewidth was observed above (EFE)th, due to the inhomogeneous broadening that is very large in vitreous matrices. This is the first time that RSB and PCC maps are reported for an UC-RL.

Fluence-Dependent Photoinduced Charge Transfer Dynamics in Polymer-Wrapped Semiconducting Single-Walled Carbon Nanotubes.

Because an individual single-walled carbon nanotube (SWNT) can absorb multiple photons, the exciton density within a single tube depends upon excitation conditions. In SWNT-based energy conversion systems, interactions between excitons and charges make it possible for multiple types of charge transfer reactions. We exploit a SWNT-molecular donor-acceptor hybrid system (R-PBN(b)-Ph6-PDI-[(6,5) SWNT]) that fixes spatial organization and stoichiometry of perylene diimide (PDI) electron acceptors on the nanotube surface, to elucidate how excitation fluence affects ultrafast charge separation (CS) and the nature of charge recombination (CR) dynamics triggered upon SWNT near-infrared excitation. Pump-probe data characterizing these photoinduced CS and thermal CR reactions were acquired over excitation fluences that produce ∼5-125 excitons per 700 nm long nanotube. These experiments show that optical excitation gives rise to CS states in which PDI radical anions (PDI-•) and SWNT hole polarons (SWNT•+) have geminate and nongeminate spatial relationships. Under low excitation fluences, the observed dynamics reflect CR reactions of these geminate and nongeminate CS states. As excitation fluence increases, persistent excitons, which have not undergone CS, undergo reaction with ([SWNT(•+)n]-(PDI-•)n) CS states to produce lower-energy CS states that are characterized by hole (SWNT•+) and electron (SWNT•-) polarons. When nongeminate SWNT•+ and SWNT•- charge carriers are generated, CR dynamics depend on the time scale required for these oppositely charged solvated SWNT polarons to encounter each other. Because SWNT excitons have substantial excited-state reduction (1E-/*) and excited-state oxidation (1E*/+) potentials, they can drive additional charge transfer reactions involving initially prepared CS states under experimental conditions where excess excitons are present.

Nonequilibrium bandgap modification in porphyrin-based metal-organic frameworks revealed by transient absorption spectroscopy

Modifying the equilibrium bandgap has proven to be an effective strategy for optimizing photocarrier properties in metal-organic frameworks (MOFs). In this work, we have investigated the nonequilibrium bandgap modification in cobalt porphyrin-based MOF (Co-TCPP MOF) nanofilms through transient absorption spectroscopy. Our results reveal a captivating redshift–blueshift crossover in the nonequilibrium bandgap of Co-TCPP MOFs, with a staggering maximum shifting value of approximately 170 meV, achieved with an excitation fluence of 96 μJ/cm2. This phenomenon sets the stage for further investigations into harnessing nonequilibrium bandgap modification as a powerful tool for tailoring photocarrier properties. Another key facet of our research is the revelation that the bandgap modification effect observed in Co-TCPP MOFs is strongly dependent on the excitation fluence and is absent in disordered porphyrin molecules. This observation suggests a correlation between the bandgap modification and the amplified many-body interactions present within the ordered MOF structure, thus offering valuable insights into the intricate relationship between bandgap modification, excitation fluence, and ordered MOF structures.

Hot Electron Cooling in n-Doped Colloidal Nanoplatelets Following Near-Infrared Intersubband Excitation.

Intersubband transition was recently discovered in colloidal nanoplatelets, but the associated intersubband carrier relaxation dynamics remains poorly understood. In particular, it is crucial to selectively excite the intersubband transition and to follow the hot electron dynamics in the absence of valence-band holes. This is achieved herein by exciting the predoped electrons in CdSe/ZnS nanoplatelets using near-infrared femtosecond pulses and monitoring nonequilibrium electron dynamics using broad-band visible pulses. We find that the n = 2 electrons relax to the n = 1 subband and establish a Fermi-Dirac distribution within 200 fs, and finally reach an equilibrium with the lattice within a few ps. The cooling dynamics depend mainly on the excitation fluence but weakly on the doping density and the lattice temperature. These characteristics are well captured by our numerical simulation that explicitly accounts for the state occupation effect and optical phonon scattering.

Competition among recombination pathways in single FAPbBr3 nanocrystals.

Single particle level microscopy of immobilized FAPbBr3 nanocrystals (NCs) has elucidated the involvement of different processes in their photoluminescence (PL) intermittency. Four different blinking patterns are observed in the data from more than 100 NCs. The dependence of PL decays on PL intensities brought out in fluorescence lifetime intensity distribution (FLID) plots is rationalized by the interplay of exciton- and trion-mediated recombinations along with hot carrier (HC) trapping. The high intensity-long lifetime component is attributed to neutral exciton recombination, the low intensity-short lifetime component is attributed to trion assisted recombination, and the low intensity-long lifetime component is attributed to hot carrier recombination. Change-point analysis (CPA) of the PL blinking data reveals the involvement of multiple intermediate states. Truncated power law distribution is found to be more appropriate than power law and lognormal distribution for on and off events. Probability distributions of PL trajectories of single NCs are obtained for two different excitation fluences and wavelengths (λex = 400, 440nm). Trapping rate (kT) prevails at higher power densities for both excitation wavelengths. From a careful analysis of the FLID and probability distributions, it is concluded that there is competition between the HC and trion assisted blinking pathways and that the contribution of these mechanisms varies with excitation wavelength as well as fluence.

Ultrafast Optical Control of Exciton Diffusion in WSe2/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy.

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion in a monolayer WSe2 stacked on graphene. The diffusion dynamics can be optically tuned within a few picoseconds by altering the photoexcited carrier density in graphene. The effective diffusion constant in initial picoseconds in the WSe2/graphene heterostructure is (40.3 ± 4.5) cm2 s-1, representing a substantial improvement over (2.1 ± 0.8) cm2 s-1, typical for an isolated WSe2 monolayer. This enhancement can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe2 by photoexcited charge carriers in graphene. Furthermore, diffusion within WSe2 is affected by interlayer interactions, such as charge transfer, varying with the incident excitation fluence. These findings underscore the dynamical nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide insights for future applications of these systems in ultrafast optoelectronic devices.

Momentum-Space Observation of Optically Excited Nonthermal Electrons in Graphene with Persistent Pseudospin Polarization.

The unique optical properties of graphene, with broadband absorption and ultrafast response, make it a critical component of optoelectronic and spintronic devices. Using time-resolved momentum microscopy with high data rate and high dynamic range, we report momentum-space measurements of electrons promoted to the graphene conduction band with visible light and their subsequent relaxation. We observe a pronounced nonthermal distribution of nascent photoexcited electrons with lattice pseudospin polarization in remarkable agreement with results of simple tight-binding theory. By varying the excitation fluence, we vary the relative importance of electron-electron vs electron-phonon scattering in the relaxation of the initial distribution. Increasing the excitation fluence results in increased noncollinear electron-electron scattering and reduced pseudospin polarization, although up-scattered electrons retain a degree of polarization. These detailed momentum-resolved electron dynamics in graphene demonstrate the capabilities of high-performance time-resolved momentum microscopy in the study of 2D materials and can inform the design of graphene devices.

Integrating multimodal Raman and photoluminescence microscopy with enhanced insights through multivariate analysis

This paper introduces a novel multimodal optical microscope, integrating Raman and laser-induced photoluminescence (PL) spectroscopy for the analysis of micro-samples relevant in Heritage Science. Micro-samples extracted from artworks, such as paintings, exhibit intricate material compositions characterized by high complexity and spatial heterogeneity, featuring multiple layers of paint that may be also affected by degradation phenomena. Employing a multimodal strategy becomes imperative for a comprehensive understanding of their material composition and condition. The effectiveness of the proposed setup derives from synergistically harnessing the distinct strengths of Raman and laser-induced PL spectroscopy. The capacity to identify various chemical species through the latter technique is enhanced by using multiple excitation wavelengths and two distinct excitation fluence regimes. The combination of the two complementary techniques allows the setup to effectively achieve comprehensive chemical mapping of sample through a raster scanning approach. To attain a competitive overall measurement time, we employ a short integration time for each measurement point. We further propose an analysis protocol rooted in a multivariate approach. Specifically, we employ non-negative matrix factorization as the spectral decomposition method. This enables the identification of spectral endmembers, effectively correlated with specific chemical compounds present in samples. To demonstrate its efficacy in Heritage Science, we present examples involving pigment powder dispersions and stratigraphic micro-samples from paintings. Through these examples, we show how the multimodal approach reinforces material identification and, more importantly, facilitates the extraction of complementary information. This is pivotal as the two optical techniques exhibit sensitivity to different materials. Looking ahead, our method holds potential applications in diverse research fields, including material science and biology.

Slow Hot-Exciton Cooling and Enhanced Interparticle Excitonic Coupling in HgTe Quantum Dots.

Rapid hot-carrier/exciton cooling constitutes a major loss channel for photovoltaic efficiency. How to decelerate the hot-carrier/exciton relaxation remains a crux for achieving high-performance photovoltaic devices. Here, we demonstrate slow hot-exciton cooling that can be extended to hundreds of picoseconds in colloidal HgTe quantum dots (QDs). The energy loss rate is 1 order of magnitude smaller than bulk inorganic semiconductors, mediated by phonon bottleneck and interband biexciton Auger recombination (BAR) effects, which are both augmented at reduced QD sizes. The two effects are competitive with the emergence of multiple exciton generation. Intriguingly, BAR dominates even under low excitation fluences with a decrease in interparticle distance. Both experimental evidence and numerical evidence reveal that such efficient BAR derives from the tunneling-mediated interparticle excitonic coupling induced by wave function overlap between neighboring HgTe QDs in films. Thus, our study unveils the potential for realizing efficient hot-carrier/exciton solar cells based on HgTe QDs. Fundamentally, we reveal that the delocalized nature of quantum-confined wave function intensifies BAR. The interparticle excitonic coupling may cast light on the development of next-generation photoelectronic materials, which can retain the size-tunable confinement of colloidal semiconductor QDs while simultaneously maintaining high mobilities and conductivities typical for bulk semiconductor materials.

Dynamics of phase transformations in Si and Ge upon strong excitation with UV femtosecond laser pulses

Femtosecond laser processing of semiconductors has evolved into a mature, high-precision fabrication technique, enabling a wide range of applications. While initially most studies have employed pulses at near infrared wavelengths, the interest in using UV laser pulses is constantly increasing due to the different excitation conditions as a consequence of the much shorter optical penetration depth, leading to an improved resolution. In this context, fundamental studies on the temporal dynamics of phase transformations triggered by such pulses are necessary in order to comprehend and eventually control the complex phase transformation pathways. Here, we report a detailed time-resolved study on the phase transformation dynamics of crystalline silicon and germanium upon irradiation with single 400 nm, 100 fs laser pulses in the moderate and high excitation regime. To this end, we have employed fs-resolved optical microscopy with a probe wavelength of 800 nm to study the reflectivity evolution of the irradiated surface over a temporal window ranging from 100 fs up to 20 ns. At moderate excitation fluence, the data reveals the entire sequence of laser-induced processes, starting from the generation of a free-electron plasma, non-thermal melting, ablation onset and expansion of a semi-transparent ablation layer with sharp interfaces. At excitation with peak fluences more than 30 times the ablation threshold, an anomalous transient high-reflectivity state is observed, which might be indicative of a recoil pressure-induced liquid–liquid phase transition. Moreover, 70 nm-thick amorphous surface layers are formed in both materials after irradiation at moderate fluences. Overall, our results provide relevant information on both, transformation dynamics and final state of both materials for fs-pulse excitation in the near-UV wavelength range.

Ultrafast Shift Current in SnS2 Single Crystals: Structure Considerations, Modeling, and THz Emission Spectroscopy

AbstractAbove‐band gap optical excitation of non‐centrosymmetric semiconductors can lead to the spatial shift of the center of electron charge in a process known as shift current. Shift current is investigated in single‐crystal SnS2, a layered semiconductor with the band gap of ≈2.3 eV, by THz emission spectroscopy and first principles density functional theory (DFT). It is observed that normal incidence excitation with above gap (400 nm; 3.1 eV) pulses results in THz emission from 2H SnS2 () polytype, where such emission is nominally forbidden by symmetry. It is argued that the underlying symmetry breaking arises due to the presence of stacking faults that are known to be ubiquitous in SnS2 single crystals and construct a possible structural model of a stacking fault with symmetry properties consistent with the experimental observations. In addition to shift current, it is observed THz emission by optical rectification excited by below band gap (800 nm; 1.55 eV) pulses but it requires excitation fluence more than two orders of magnitude higher to produce same signal amplitude. These results suggest that ultrafast shift current in which can be excited with visible light in blue–green portion of the spectrum makes SnS2 a promising source material for THz photonics.

Ultrafast Photoinduced Phase Change in SnSe.

Time-resolved multiterahertz (THz) spectroscopy is used to observe an ultrafast, nonthermal electronic phase change in SnSe driven by interband photoexcitation with 1.55eV pump photons. The transient THz photoconductivity spectrum is found to be Lorentzian-like, indicating charge localization and phase segregation. The rise of photoconductivity is bimodal in nature, with both a fast and slow component due to excitation into multiple bands and subsequent intervalley scattering. The THz conductivity magnitude, dynamics, and spectra show a drastic change in character at a critical excitation fluence of approximately 6 mJ/cm^{2} due to a photoinduced phase segregation and a macroscopic collapse of the band gap.

Frequency Response of MAPbI3 Perovskites for Photodetection Application

AbstractThe frequency response of the photocurrent (Iph) generated in perovskite photodetectors is a crucial issue in imaging or telecommunication applications, although it is briefly discussed in the literature. The present work obtains for the first time the complete expression of the Iph generated on MAPbI3 (MA:methylamoniumI) perovskite polycrystalline thin films. A conditioning circuit is used to extract Iph at 1 V of bias voltage under a square‐modulated laser excitation with a sensitivity smaller than 1 nW and a linear dynamic range LDR > 200 dB; It allows an accurate determination of the module of Iph together with the phase, which is not usually reported in photodetector systems. The frequency domain analysis reveals that Iph can be modeled by two fractional poles located at low (10 kHz) and high (39–250 kHz) cut‐off frequencies. Optimum geometrical parameters and excitation fluence are found for the wider response, resulting for the best device on a cut off frequency up to 250 kHz, and the reproduction of square‐modulated optical waves up to 100 kHz. These results represent an important strategy toward the electrical analysis of MAPbI3 (or other perovskite materials) for the design of posterior electronic stages, optimization of devices and determination of their figures of merit.

Cross-examination of photoinitiated carrier and structural dynamics of black phosphorus at elevated fluences.

Revived attention in black phosphorus (bP) has been tremendous in the past decade. While many photoinitiated experiments have been conducted, a cross-examination of bP's photocarrier and structural dynamics is still lacking. In this article, we provide such analysis by examining time-resolved data acquired using optical transient reflectivity and reflection ultrafast electron diffraction, two complementary methods under the same experimental conditions. At elevated excitation fluences, we find that more than 90% of the photoinjected carriers are annihilated within the first picosecond (ps) and transfer their energy to phonons in a nonthermal, anisotropic fashion. Electronically, the remaining carrier density around the band edges induces a significant interaction that leads to an interlayer lattice contraction in a few ps but soon diminishes as a result of the continuing loss of carriers. Structurally, phonon-phonon scattering redistributes the energy in the lattice and results in the generation of out-of-plane coherent acoustic phonons and thermal lattice expansion. Their onset times at ∼6ps are found to be in good agreement. Later, a thermalized quasi-equilibrium state is reached following a period of about 40-50ps. Hence, we propose a picture with five temporal regimes for bP's photodynamics.

Strong Light-Matter Coupling in Lead Halide Perovskite Quantum Dot Solids.

Strong coupling between lead halide perovskite materials and optical resonators enables both polaritonic control of the photophysical properties of these emerging semiconductors and the observation of fundamental physical phenomena. However, the difficulty in achieving optical-quality perovskite quantum dot (PQD) films showing well-defined excitonic transitions has prevented the study of strong light-matter coupling in these materials, central to the field of optoelectronics. Herein we demonstrate the formation at room temperature of multiple cavity exciton-polaritons in metallic resonators embedding highly transparent Cesium Lead Bromide quantum dot (CsPbBr3-QD) solids, revealed by a significant reconfiguration of the absorption and emission properties of the system. Our results indicate that the effects of biexciton interaction or large polaron formation, frequently invoked to explain the properties of PQDs, are seemingly absent or compensated by other more conspicuous effects in the CsPbBr3-QD optical cavity. We observe that strong coupling enables a significant reduction of the photoemission line width, as well as the ultrafast modulation of the optical absorption, controllable by means of the excitation fluence. We find that the interplay of the polariton states with the large dark state reservoir plays a decisive role in determining the dynamics of the emission and transient absorption properties of the hybridized light-quantum dot solid system. Our results should serve as the basis for future investigations of PQD solids as polaritonic materials.

Exciton Bimolecular Annihilation Dynamics in Push-Pull Semiconductor Polymers.

Exciton-exciton annihilation is a ubiquitous nonlinear dynamic phenomenon in materials hosting Frenkel excitons. In this work, we investigate the nonlinear exciton dynamics of an electron push-pull conjugated polymer by fluence-dependent transient absorption and excitation-correlation photoluminescence spectroscopy, where we can quantitatively show the latter to be a more selective probe of the nonlinear dynamics. Simulations based on a time-independent exciton annihilation model show a decreasing trend for the extracted annihilation rates with excitation fluence. Further investigation of the fluence-dependent transients suggests that the exciton-exciton annihilation bimolecular rates are not constant in time, displaying a t-1/2 time dependence, which we rationalize as reflective of one-dimensional exciton diffusion, with a diffusion length estimated to be 9 ± 2 nm. In addition, exciton annihilation gives rise to a long-lived species that recombines on a nanosecond time scale. Our conclusions shed broad light onto nonlinear exciton dynamics in push-pull conjugated polymers.

How to Interpret Transient Absorption Data?: An Overview of Case Studies for Application to Organic Solar Cells

AbstractTransient absorption spectroscopy (TAS) is a time‐resolved spectroscopic technique that can quantitatively observe temporal changes in the absorption spectrum associated with transient species, such as excitons and charges. Therefore, it is extensively used in photophysics, photochemistry, and photobiology. TAS plays a crucial role in unveiling charge generation and recombination dynamics in organic solar cells (OSCs). However, the interpretation of TA data is sometimes difficult for those who are not familiar with TAS. In this Perspective, therefore, the utility of TAS is briefly explained in studying OSCs and highlight key considerations for interpreting TA data. Measuring TA data at appropriate excitation fluence and appropriate spectral assignment are essential to correctly interpret photophysics of OSCs. With this Perspective, it aims to offer readers sufficient advice to enable them to perform TA measurements appropriately and interpret the TA data correctly.

A coherent phonon-induced hidden quadrupolar ordered state in Ca2RuO4

Ultrafast laser excitation provides a means to transiently realize long-range ordered electronic states of matter that are hidden in thermal equilibrium. Recently, this approach has unveiled a variety of thermally inaccessible ordered states in strongly correlated materials, including charge density wave, ferroelectric, magnetic, and intertwined charge-orbital ordered states. However, more exotic hidden states exhibiting higher multipolar ordering remain elusive owing to the challenge of directly manipulating and detecting them with light. Here we demonstrate a method to induce a dynamical transition from a thermally allowed to a thermally forbidden spin-orbit entangled quadrupolar ordered state in Ca2RuO4 by coherently exciting a phonon that is strongly coupled to the order parameter. Combining probe photon energy-resolved coherent phonon spectroscopy measurements with model Hamiltonian calculations, we show that the dynamical transition is manifested through anomalies in the temperature, pump excitation fluence, and probe photon energy dependence of the strongly coupled phonon. With this procedure, we introduce a general pathway to uncover hidden multipolar ordered states and to control their re-orientation on ultrashort timescales.