Subpicosecond Relaxation Research Articles

Dynamic Ultrastrong Coupling in a 2 nm Gap Plasmonic Cavity at the Sub-Picosecond Scale.

Localized surface plasmon resonances (LSPRs) can enhance the electromagnetic fields on metallic nanostructures upon light illumination, providing an approach for manipulating light-matter interactions at the sub-wavelength scale. However, currently, there is no thorough investigation of the physical mechanism in the dynamic formation of the strongly coupled LSPRs on sub-5 nm plasmonic cavities at the sub-picosecond scale. In this work, through femtosecond broadband transient absorption spectroscopy, we reveal the dynamic ultrastrong coupling processes in a nanoparticle-in-trench (NPiT) structure containing 2 nm gap cavities, and demonstrate a coherent motional coupling between vibrating AuNPs and the nanogaps. We achieve a maximum Rabi splitting energy of ∼660 meV in the sub-picosecond hot-electron relaxation time scale under the resonant excitation of the nanogap cavity's LSPR, reaching the ultrastrong coupling regime. This leads to a change of global vibration modes for the 2 nm gap cavity, potentially related to the dynamical Casimir effect with nanogap resonators.

Quantifying the Relaxation Dynamics of Higher Electronic Excited States in Perylene.

Gating logical operations through high-lying electronic excited states presents opportunities for developing ultrafast, subnanometer computational devices. A lack of molecular systems with sufficiently long-lived higher excited states has hindered practical realization of such devices, but recent studies have reported intriguing photophysics from high-lying excited states of perylene. In this work, we use femtosecond spectroscopy supported by quantum chemical calculations to identify and quantify the relaxation dynamics of monomeric perylene's higher electronic excited states. The 21B2u state is accessed through single-photon absorption at 250 nm, while the optically dark 21Ag state is excited via the 11B3u state. Population of either state results in subpicosecond relaxation to the 11B3u state, and we quantify 21Ag and 21B2u state lifetimes of 340 and 530 fs, respectively. These lifetimes are significantly longer than the singlet fission time constant from the perylene 21B2u state, suggesting that the higher electronic states of perylene may be useful for gating logical operations.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling.

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

Ultrafast photocarrier and coherent phonon dynamics in type-II Dirac semimetal PtTe2 thin films probed by optical spectroscopy

We report the ultrafast photocarrier dynamics and coherent phonon excitation in type-II Dirac semimetal platinum ditelluride ( PtTe 2 ) thin films via femtosecond (fs) pump-probe spectroscopy at room temperature. Quantitative analysis revealed that the incoherent electronic relaxation consists of two components: a subpicosecond fast relaxation process and a slow component with a time constant of hundreds of picoseconds (ps). Furthermore, the launch of a coherent acoustic phonon (CAP) in the 20 nm film but absence in the 6.8 nm film uncovers the dominant role of temperature gradient in producing a strain wave. The sound velocity and Young’s modulus in the thick PtTe 2 are determined to be 1.736 km/s and 29.5 GPa, respectively. In addition, the coherent optical phonon (COP) with a frequency of 4.7 THz corresponding to Te atoms out-of-plane A 1 g vibration has been well resolved in all films, which is ascribed to displacive excitation of coherent phonon (DECP). The observation of a strong probe-wavelength dependent COP amplitude reveals the resonant feature of the optical excitation-induced atomic displacement in PtTe 2 . Our findings provide deep insight into the excitation and dynamics of CAP and COP as well as the photocarriers’ recovery pathway and lifetimes in PtTe 2 . Our study also demonstrates that the COP spectroscopy is a powerful tool to reveal the modulation of frequency-dependent optical constants induced by atomic vibrations, which may find applications in the fields of optoelectronics and ultrafast photonics.

Deciphering Photoinduced Charge Transfer Dynamics in a Cross-Linked Graphene-Dye Nanohybrid.

The search for synthetic materials that mimic natural photosynthesis by converting solar energy into other more useful forms of energy is an ever-growing research endeavor. Graphene-based materials, with their exceptional electronic and optical properties, are exemplary candidates for high-efficiency solar energy harvesting devices. High photoactivity can be conveniently achieved by functionalizing graphene with small molecule organic semiconductors whose band-gaps can be tuned by structural modification, leading to interactions between the π-conjugated electronic systems in both the semiconductor and graphene. Here we investigate the ultrafast transient optical properties of a cross-linked graphene–dye (diphenyl-dithiophenediketopyrrolopyrrole) nanohybrid material, in which oligomers of the organic semiconductor dye are covalently bound to a random network of few-layer graphene flakes, and compare the results to those obtained for the reference dye monomer. Using a combination of ultrafast transient absorption and two-dimensional electronic spectroscopy, we provide substantial evidence for photoinduced charge transfer that occurs within 18 ps in the nanohybrid system. Notably, subpicosecond photoinduced torsional relaxation observed in the constituent dye monomer is absent in the cross-linked nanohybrid system. Through density functional theory calculations, we compare the competing effects of covalent bonding, increasing conjugation length, and the presence of multiple graphene flakes. We find evidence that the observed ultrafast charge transfer process occurs through a superexchange mechanism in which the oligomeric dye bridge provides virtual states enabling charge transfer between graphene–dye covalent bond sites.

Magnetic charge's relaxation propelled electricity in two-dimensional magnetic honeycomb lattice

SummaryEmerging new concepts, such as magnetic charge dynamics in two-dimensional magnetic material, can provide novel mechanism for spin-based electrical transport at macroscopic length. In artificial spin ice of single domain elements, magnetic charge's relaxation can create an efficient electrical pathway for conduction by generating fluctuations in local magnetic field that couple with conduction electron spins. In a first demonstration, we show that the electrical conductivity is propelled by more than an order of magnitude at room temperature due to magnetic charge defects sub-picosecond relaxation in artificial magnetic honeycomb lattice. The direct evidence to the proposed electrical conduction mechanism in two-dimensional frustrated magnet points to the untapped potential for spintronic applications in this system.

Electron and Hole Spin Relaxation in CdSe Colloidal Nanoplatelets.

Solution-processed quantum-confined nanocrystals are important building blocks for scalable implementation of quantum information science. Extensive studies on colloidal quantum dots (QDs) have revealed subpicosecond hole spin relaxation, whereas the electron spin dynamics remains difficult to probe. Here we study electron and hole spin dynamics in CdSe colloidal nanoplatelets (also called quantum wells) of varying thicknesses using circularly polarized transient absorption spectroscopy at room temperature. The clear spectroscopic features of transition bands associated with heavy, light, and spin-orbit split-off holes enabled separate probes of electron and hole dynamics. The hole spin-flip occurred within ∼200 fs, arising from strong spin-orbit coupling in the valence band. The electron spin lifetime decreased from 6.2 to 2.2 ps as the platelet thickness is reduced from 6 to 4 monolayers, reflecting an exchange interaction between the electron and the hole and/or surface dangling bond spins enhanced by quantum confinement.

Salt-assisted growth and ultrafast photocarrier dynamics of large-sized monolayer ReSe2

Owing to its anisotropic optical and electrical properties, rhenium diselenide (ReSe2) has garnered considerable attention recently as a candidate material for polarization-sensitive photodetectors. However, the direct and controllable synthesis of large-sized ReSe2 with a uniform thickness is still a great challenge. Herein, we have refined the synthesis method to obtain uniform monolayer ReSe2 flakes with a size of up to ~ 106 μm on sapphire via an ambient-pressure chemical vapor deposition technique using Na promoter from sodium chloride. Interestingly, optical pump-probe spectroscopy revealed a fast switching from saturable absorption (SA) to absorption enhancement (AE) in subpicosecond time scale, followed by a slower decay induced by exciton recombination. Furthermore, both AE and SA signals exhibited clear angular dependence with a periodicity of 180°, which reflected the dichroism in nonlinear absorption dynamics. In addition, the photocarrier dynamics including free-carrier transport and subpicosecond relaxation due to exciton formation or surface trapping was probed using time resolved terahertz spectroscopy. We believe that our study serves as a reference for atomically controlled synthesis of large-sized ReSe2 and provides useful insights on its optoelectronic properties for novel device applications.

Ultrafast dynamics in the Lifshitz-type 5d pyrochlore antiferromagnet Cd2Os2O7

We investigate the ultrafast dynamics of Cd$_2$Os$_2$O$_7$, a prototype material showing a Lifshitz-type transition as a function of temperature. In the paramagnetic metallic state, the photo-reflectivity shows a sub-picosecond relaxation, followed by a featureless small offset. In the antiferromagnetic state slightly below $T_N$, however, the photo-reflectivity resurges over hundreds of picoseconds, which goes beyond the usual realm of the effective-temperature model. Our observations are consistent with the Lifshitz phase transition of Cd$_2$Os$_2$O$_7$ driven by the evolution of the local magnetic moment.

Temperature-Dependent Intraband Relaxation of Hybrid Perovskites.

Intraband relaxation in polycrystalline films of hybrid perovskites methylammonium lead tribromide and methylammonium lead triiodide are studied by transient absorption spectroscopy from 80 K to >350 K. This temperature range spans the transitions of these materials from the high-temperature cubic phases, intermediate tetragonal phases, and low-temperature orthorhombic phases. The organic cation undergoes a distinct transition from an ordered lattice in the orthorhombic phase to a plastic crystal in cubic and tetragonal phases, which reportedly influences many optoelectronic properties. The much larger exciton binding energy of orthorhombic MAPbI3 (compared to cubic or tetragonal phases) or MAPbBr3 substantially changes the transient spectral responses of the materials by reducing the number of free carriers. However, for these measurements at low fluences, both MAPbBr3 and MAPbI3 exhibit subpicosecond intraband relaxation over the entire temperature range studied. Intraband relaxation becomes somewhat faster at higher temperatures, but freezing of organic cations are not accompanied by a discontinuity of the intraband relaxation time. These results suggest that configurational freedom of organic cations does not screen carriers from electron-phonon coupling.

Analysis of Defect Recovery in Reduced Graphene Oxide and Its Application as a Heater for Self-Healing Polymers.

Reduced graphene oxide (RGO) obtained from graphene oxide has received much attention because of its simple and cost-effective manufacturing process. Previous studies have demonstrated the scalable production of RGO with relatively high quality; however, irreducible defects on RGO deteriorate the unique intrinsic physical properties of graphene, such as high-mobility electrical charge transport, limiting its potential applicability. Using the enhanced chemical reactivity of such defects, atomic layer deposition (ALD) can be a useful method to selectively passivate the defect sites. Herein, we analyzed the selective formation of Pt by ALD on the defect sites of RGO and investigated the effect of Pt formation on the electrical properties of RGO by using ultrafast terahertz (THz) laser spectroscopy. Time-resolved THz measurements directly corroborated that the degree of the defect-recovering property of ALD Pt-treated RGO appearing as Auger-type sub-picosecond relaxation, which is otherwise absent in pristine RGO. In addition, the conductivity improvement of Pt-recovered RGO was theoretically explained by density functional theory calculations. The ALD Pt-passivated RGO yielded a superior platform for the fabrication of a highly conductive and transparent graphene heater. By using the ALD Pt/RGO heater embedded underneath scratched self-healing polymer materials, we also demonstrated the effective recovery property of self-healing polymers with high-performance heating capability. Our work is expected to result in significant advances toward practical applications for RGO-based flexible and transparent electronics.

Ultrafast excited-state dynamics in chlorosomes isolated from the photosynthetic filamentous green bacterium Chloroflexus aurantiacus.

Bacteriochlorophyll (BChl) c pigments in the aggregated state are responsible for efficient light harvesting in chlorosomes of the filamentous anoxygenic photosynthetic bacterium, Chloroflexus (Cfx.) aurantiacus. Absorption of light creates excited states in the BChl c aggregates. After subpicosecond intrachlorosomal energy transfer, redistribution and relaxation, the excitation is transferred to the BChl a complexes and further to reaction centers on the picosecond time scale. In this work, the femtosecond excited state dynamics within BChl c oligomers of isolated Cfx. aurantiacus chlorosomes was studied by double difference pump-probe spectroscopy at room temperature. Difference (Alight - Adark ) spectra corresponding to excitation at 725 nm (blue side of the BChl c absorption band) were compared with those corresponding to excitation at 750 nm (red side of the BChl c absorption band). A very fast (time constant 70 ± 10fs) rise kinetic component was found in the stimulated emission (SE) upon excitation at 725 nm. This component was absent at 750-nm excitation. These data were explained by the dynamical red shift of the SE due to excited state relaxation. The nature and mechanisms of the ultrafast excited state dynamics in chlorosomal BChl c aggregates are discussed.

Polarimetry Using Graphene-Integrated Anisotropic Metasurfaces

Polarization is one of the important properties of light, and its detection is of significant interest for various fundamental and practical applications. We demonstrate a mid-infrared polarimetry device using a gate-tunable graphene-integrated anisotropic metasurface. The Stokes parameters of the incident light are extracted by sweeping the gate voltage applied to the device and subsequent fitting of the measured reflected intensities. Considering subpicosecond carrier relaxation times in graphene, the polarization measurement rate of our device is governed only by the speed of the gate voltage sweep. Thus, our work serves as a proof-of-principle demonstration for high-speed microscale polarimetric devices.

Intersystem Crossing as a Key Component of the Nonadiabatic Relaxation Dynamics of Bithiophene and Terthiophene.

We present a nonadiabatic dynamics study concerning the subpicosecond relaxation of excited states in dimeric and trimeric thiophene chains. The influence of the triplet states in the overall process is, for the first time, taken into account by explicitly including spin-orbit couplings and hence allowing intersystem crossing phenomena. We observe the fundamental role of the triplet state manifold in driving the full relaxation process. In particular we evidence the effect of both, inter-ring rotation and ring-opening, in the process, as compared to the monomer, where the ring-opening process appears as the dominant one. In addition, the evolution of the open structures allows for trans to cis isomerization in the dimer and trimer. The overall relaxation process slows down with chain elongation. The complex decay mechanism characterized by the presence of different competing channels, due to the presence of a quasi degenerate manifold, is explained allowing the rationalization of oligothiophenes photophysics.

Disentangling the photochemistry of benzocyclobutenedione.

The ultrafast photophysics and photochemistry of benzocyclobutenedione (BCBD) dissolved in dichloromethane is investigated by transient absorption spectroscopy in both the IR and the UV/Vis regime. The molecule is excited at 300 nm to the S3 (ππ*) state and a time scale from roughly 100 fs to several nanoseconds is covered. The initially excited S3 deactivates quickly to the lower-lying S1 (nπ*) state. Three parallel photochemical reaction pathways starting in the S1 state that compete with deactivation to S0 are identified in the transient IR spectra, two of them consisting of a sequence of steps. DFT/TDDFT calculations of the normal modes of the reactant and various photoproducts support the analysis of the transient spectra. The rapid internal conversion (IC) to the S1 state of BCBD is followed by a sub-picosecond vibrational relaxation (VR) to S1 (ν = 0). In parallel BCBD loses one carbonyl group and forms benzocyclopropenone, which subsequently rearranges to cyclopentadienylidene ketene. Ring opening in the S1 (ν = 0) state produces vibrationally hot bisketene, which cools within 22 ps. This reaction competes with the intramolecular rearrangement to singlet oxacarbene, which subsequently converts into the triplet carbene via intersystem crossing (ISC). The late-time product identified in the transient UV/Vis spectra is probably due to dimerization of the carbene. Molecular dynamics (MD) simulations of the early-time photochemistry of BCBD successfully reproduce the formation of the three main photoproducts.

Generalized GW+Boltzmann Approach for the Description of Ultrafast Electron Dynamics in Topological Insulators.

Quantum-phase transitions between trivial insulators and topological insulators differ from ordinary metal-insulator transitions in that they arise from the inversion of the bulk band structure due to strong spin–orbit coupling. Such topological phase transitions are unique in nature as they lead to the emergence of topological surface states which are characterized by a peculiar spin texture that is believed to play a central role in the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Here, we provide a generalized +Boltzmann approach for the description of ultrafast dynamics in topological insulators driven by electron–electron and electron–phonon scatterings. Taking the prototypical insulator BiTe as an example, we test the robustness of our approach by comparing the theoretical prediction to results of time- and angle-resolved photoemission experiments. From this comparison, we are able to demonstrate the crucial role of the excited spin texture in the subpicosecond relaxation of transient electrons, as well as to accurately obtain the magnitude and strength of electron–electron and electron–phonon couplings. Our approach could be used as a generalized theory for three-dimensional topological insulators in the bulk-conducting transport regime, paving the way for the realization of a unified theory of ultrafast dynamics in topological materials.

Intersubband absorption in Si‐ and Ge‐doped GaN/AlN heterostructures in self‐assembled nanowire and 2D layers

GaN/AlN is a promising material system for the development of an ultrafast intersubband technology operating at telecommunication wavelengths, thanks to the large conduction band offset and sub‐picosecond intraband relaxation time. In this paper, we explore short wavelength infrared intersubband transitions occurring in the conduction band of GaN/AlN heterostructures in both planar and nanowire (NW) geometries. We study the effect of Si and Ge doping in both quantum wells and NW heterostructures using the same active region thickness and dopant concentration. We demonstrate that both Si and Ge are suitable dopant candidates at a concentration of mid‐1019 cm−3 for the fabrication of intersubband devices based on planar and NW heterostructures.

Enhancement of electron hot spot relaxation in photoexcited plasmonic structures by thermal diffusion

We demonstrate that in confined plasmonic metal structures subject to ultra-fast laser excitation electron thermal diffusion can provide relaxation faster than the energy transfer to the lattice. This relaxation occurs due to excitation of nanometer-sized hot spots in the confined structure and the sensitivity of its optical parameters to the perturbation in these regions. Both factors become essential when the plasmonic resonance condition is met for both excitation and detection. A pump-probe experiment on plasmonic gold lattices shows sub-picosecond relaxation with the characteristic times well-described by a two-temperature model. The results suggest that dynamical optical response in plasmonic structures can be tuned by selection of the structural geometry as well as the choice of wavelength and polarization of the excitation and detection light.

Effects of single atom doping on the ultrafast electron dynamics of M1Au24(SR)18 (M = Pd, Pt) nanoclusters.

Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M1@Au24(SR)18 (M = Pd, Pt; R = CH2CH2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M1Au12 core states; (2) core to shell relaxation in a few picoseconds; and (3) relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au25(SR)18 cluster. The detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications.

Electron-relocalization dynamics in xenon clusters in intense soft-x-ray fields

Intense and temporally structured x-ray laser fields enable the controlled generation of strongly coupled nonequilibrium cluster nanoplasmas and the time-resolved investigation of their dynamics. Recent femtosecond pump-probe experiments on xenon clusters revealed subpicosecond relaxation dynamics in the cluster nanoplasma via delay-dependent charge states of emitted atomic ions [M. Krikunova et al., J. Phys. B: At. Mol. Opt. Phys. 45, 105101 (2012)]. Here we report a scheme based on local electron single-particle energy spectra that enables microscopic tracing of the underlying electron-relocalization processes in molecular dynamics simulations up to the strong-coupling regime. We find more efficient recombination in the cluster core and delay-dependent ion charge states, in good agreement with experiments. Our method is applicable to any particle-based plasma simulation and expected to offer insights into correlated relaxation processes in inhomogenous, strongly coupled plasmas.