Femtosecond Time-resolved Pump-probe Spectroscopy Research Articles

Optical Properties of V2O5 Thin Films on Different Substrates and Femtosecond Laser-Induced Phase Transition Studied by Pump-Probe Method.

Vanadium pentoxide can undergo a reversible phase transition by heating above 260 °C; its non-thermal phase transition, as well as ultrafast dynamical processes, is still not known. Here, femtosecond laser-induced phase transition properties in V2O5 thin films were first explored using femtosecond time-resolved pump–probe spectroscopy. The results show that the phase transient processes occur on a 10−15–10−13 temporal scale. The phase transition and recovery properties are dependent on both the substrates and pump laser energy densities. We propose the oxygen vacancies theory to explain the results, and we provide valuable insights into V2O5 films for potential applications.

Coherent vibrational dynamics of [Au25(SR)18]- nanoclusters

Coherent vibrational dynamics can be observed in atomically precise gold nanoclusters using femtosecond time-resolved pump-probe spectroscopy. It can not only reveal the coupling between electrons and vibrations, but also reflect the mechanical and electronic properties of metal nanoclusters, which holds potential applications in biological sensing and mass detection. Here, we investigated the coherent vibrational dynamics of [Au25(SR)18]− nanoclusters by ultrafast spectroscopy and revealed the origins of these coherent vibrations by analyzing their frequency, phase and probe wavelength distributions. Strong coherent oscillations with frequency of 40 cm−1 and 80 cm−1 can be reproduced in the excited state dynamics of [Au25(SR)18]−, which should originate from acoustic vibrations of the Au13 metal core. Phase analysis on the oscillations indicates that the 80 cm−1 mode should arise from the frequency modulation of the electronic states while the 40 cm−1 mode should originate from the amplitude modulation of the dynamic spectrum. Moreover, it is found that the vibration frequencies of [Au25(SR)18]− obtained in pump-probe measurements are independent of the surface ligands so that they are intrinsic properties of the metal core. These results are of great value to understand the electron-vibration coupling of metal nanoclusters.

Possible orbital crossover in the ferromagnetic Kondo lattice compound CeAgSb2

Utilizing femtosecond time-resolved pump probe spectroscopy, we investigated the ultrafast quasiparticle dynamics of the ferromagnetic Kondo lattice compound ${\mathrm{CeAgSb}}_{2}$. The photoinduced reflectivity demonstrates a positive relaxation process connected with simple metal physics at room temperature, while an additional negative decay channel associated with Kondo screening effect can be recognized immediately with temperature decreasing. The temperature-dependent amplitude of the latter exhibits a kink at ${T}^{*}\ensuremath{\approx}$ 45 K, which is close to the energy scale of the first excited crystal field splitting Ce $4f$ level. We propose that an orbital crossover is responsible for this intriguing feature. The dominating $4f$ orbitals that participated in the Kodno $c\ensuremath{-}f$ hybridization varies from the ground state below ${T}^{*}$ to multiple crystal field splitting levels. In the meantime, the hybridization gap changes from 12.6 to 220.4 K correspondingly, which is also in agreement with the fluence-dependent measurements.

(Invited) Ultrafast Spectroscopy of Doped Carbon Nanotubes

We will report on an investigation of exciton and trion dynamics in chemically- or electrochemically-doped semiconducting single-wall carbon nanotubes using VIS and NIR femtosecond time-resolved pump-probe spectroscopy.

13 nm Exciton Size in (6,5) Single-Wall Carbon Nanotubes.

Electron-hole correlation lengths, also termed exciton size, for (6,5) single-wall carbon nanotubes (SWNTs) are determined using femtosecond time-resolved pump-probe spectroscopy. The phase space filling model is used to obtain the sizes of the first subband exciton in samples of isolated and of bundled SWNTs. The experiments indicate that the exciton size of (13 ± 3) nm is a factor of 6 higher than previous experimental estimates and theoretical predictions for vacuum suspended SWNTs. This surprising result may be attributed at least in part to the effect of the dielectric environment on exciton sizes and supports recent theoretical findings predicting that screening in SWNTs may enhance rather than reduce electron-hole interactions for separations larger than the tube diameter. Thereby, the work also points to the unique nature of screening and electronic correlations in one-dimensional semiconductors.

Intermolecular exciton–exciton annihilation in phospholipid vesicles doped with [Ru(bpy)2dppz]2+

The ultrafast photophysics of [Ru(bpy)2dppz]2+ (dppz=dipyrido[3,2-a:2′,3′-c]-phenazine) embedded into the walls of phospholipid vesicles has been studied by femtosecond time-resolved pump–probe spectroscopy. While [Ru(bpy)2dppz]2+ has been studied intensively with respect to its intramolecular charge transfer processes, which are associated with the well known light-switch effect, this study focuses on intermolecular energy transfer processes taking place upon dense packing of the complexes into a phospholipid membrane composed of dipalmitoyl-l-α-phosphatidylglycerol, which can be thought of as a simplistic model of a cellular membrane. The data indicate additional quenching of excited [Ru(bpy)2dppz]2+ upon increasing the pump–pulse intensity. Hence, the observed photophysics, which is assigned to the presence of intermolecular exciton–exciton annihilation at high pump-intensities, might be related to the ultrafast photophysics of [Ru(bpy)2dppz]2+ when used as a chromophore to stain cells, an effect that may be taken into account during the employment of novel cellular markers based on Ru polypyridine complexes.

Gap-Dependent Quasiparticle Dynamics and Coherent Acoustic Phonons in CaFe2As2 across Spin Density Wave Phase Transition

We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165 K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T_SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW, whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of ~100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.

Ultrafast quasiparticle relaxation dynamics in superconducting iron pnictide Ca(Fe0.944Co0.056)2As2

Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting Ca(Fe0.944Co0.056)2As2 single crystals by measuring transient reflectivity changes using femtosecond time-resolved pump-probe spectroscopy. Large changes in the temperature-dependent differential reflectivity values in the vicinity of the spin density wave (TSDW) and superconducting (TSC) transition temperatures of the sample have been inferred to have charge gap opening at those temperatures. We have estimated the zero-temperature charge gap value in the superconducting state to be ∼1.8kBTSC and an electron–phonon coupling constant λ of ∼0.1 in the normal state that signifies the weak coupling in iron pnictides. From the peculiar temperature-dependence of the quasiparticle dynamics in the intermediate temperature region between TSC and TSDW we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures.

Temperature dependence of coherent phonons in TbVO4 crystal probed by ultrafast optical spectroscopy

Coherent optical phonons in terbium vanadate (TbVO4) are investigated by using femtosecond time-resolved pump-probe spectroscopy at temperatures from 20 to 300 K. Combined with the Raman spectrum, the coherent phonon mode is attributed to an optical phonon mode of B1g symmetry. The main generation mechanism of the coherent optical phonons is revealed to be the impulsive stimulated Raman scattering. The temperature dependence of the dephasing time reveals that the main mechanism of the coherent phonon population decay is anharmonic phonon–phonon coupling, which causes a redshift of the coherent phonon frequency with increasing temperature.

Ultrafast nonlinear optical response of photoexcited Ge/SiGe quantum wells: Evidence for a femtosecond transient population inversion

Femtosecond time-resolved pump-probe spectroscopy is used to investigate the ultrafast carrier dynamics of Ge/SiGe quantum wells grown on a Si substrate. Pronounced nonequilibrium effects in the relaxation dynamics of the optically injected carrier distributions are observed and analyzed using a microscopic many-body theory. Transient population inversion and optical gain is obtained on a femtosecond time scale for excitation at energies slightly above the lowest direct quantum-well transition.

Femtosecond Dynamics of Isolated Phenylcarbenes

Understanding the primary photophysical processes in molecules is essential for interpreting their photochemistry, because molecules rarely react from the initially excited electronic state. In this study the ultrafast excited-state dynamics of chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), two species that are considered as models for carbene dynamics, were investigated by femtosecond time-resolved pump probe spectroscopy in the gas phase. Their dynamics was followed in real time by time-resolved photoionization and photoelectron imaging. CPC was excited at 265 nm into the 3 1A' state, corresponding to excitation from a pi-orbital of the aromatic ring into the LUMO. The LUMO contains a contribution of the p-orbital at the carbene center. Three time constants are apparent in the photoelectron images: A fast decay process with tau1 approximately 40 fs, a second time constant of tau2 approximatley 350 fs, and an additional time constant of tau3 approximately 1 ps. The third time constant is only visible in the time-dependence of low kinetic energy electrons. Due to the dense manifold of excited states between 3.9 and 5 eV, known from ab initio calculations, the recorded time-resolved electron images show broad and unstructured bands. A clear population transfer between the states thus can not directly be observed. The fast deactivation process is linked to either a population transfer between the strongly coupled excited states between 3.9 and 5.0 eV or the movement of the produced wave packet out of the Franck-Condon region. Since the third long time constant is only visible for photoelectrons at low kinetic energy, evidence is given that this time constant corresponds to the lifetime of the lowest excited A 1A' state. The remaining time constant reflects a deactivation of the manifold of states in the range 3.9-5.0 eV down to the A 1A' state.

One- and two-photon induced QD-based energy transfer and the influence of multiple QD excitations

Due to the increased use of quantum dots (QDs) in diverse laser microscopies, it is interesting to study the excitation pump power and excitation wavelength dependence of QD-based energy transfer (ET) processes. The ET in QD conjugates with phthalocyanines (Pcs) was studied with femtosecond time-resolved pump-probe spectroscopy upon one- and two-photon excitation. At the used excitation wavelengths only the QDs are excited and become the energy donors. Due to the matched spectral overlap of QD photoluminescence and Pc absorption, the ET occurs on a picosecond time scale. The ET process shows strong pump power dependence whereby an increase in excitation power results in multiple QD excitations and in shorter excited state lifetimes on the QDs due to Auger relaxation. As a result, high excitation pump power leads also to an accelerated ET to the acceptor molecules from the initially multiply excited states of the QDs. Excited state quenching studies as function of pump power suggest that ET occurs mainly from the lowest one-exciton state (n = 1) and only to a minor extent from the multiply excited states (n > 1). For the short-lived, multiply excited states the ET competes inefficiently with Auger recombinations and energy transfer efficiencies of phi(ET)(n=1>) approximately 20%, phi(ET)(n=2>) approximately 7%, phi(ET)(n=3>) < or = 2% were obtained. Also after two-photon excitation the ET efficiency is highest from the one-exciton state. The experimentally determined ET efficiencies were compared with theoretical ET efficiencies upon multiple excitations. In both cases the ET efficiency decreases with the increase in excitation pump power.

Electron-Hole Pair Relaxation Dynamics in Binary Copper-Based Semiconductor Quantum Dots

The relaxation dynamics of the charge carriers in copper-based semiconductor quantum dots were investigated using femtosecond time-resolved pump-probe spectroscopy. It was found that copper oxide quantum dots show significantly longer carrier lifetimes ( and a nanosecond component) compared to the copper sulfides . This is probably due to a more perfect surface structure and less trapping in the copper oxide system. Both quantum dot materials seem to be interesting for solar cell applications based on the presented study.

Photoinduced melting of the orbital order in a perovskite-type vanadate LaVO 3

The photoinduced dynamics in a perovskite-type vanadate LaVO 3 has been studied by femtosecond time-resolved pump–probe spectroscopy. By irradiation with light in the orbital-ordered phase, an anisotropic large reflectivity change appears within the time resolution of about 200 fs. It is attributable to the instantaneous melting of the orbital order. One excitation photon is found to induce the disordering of the t 2g orbitals over about 60 V-sites. The orbital order partially recovers through thermalization with the lattice vibrations in a few picoseconds.

Femtosecond Dynamics of a Simple Merocyanine Dye: Does Deprotonation Compete with Isomerization?

The primary photochemistry of the trans isomer of a simple merocyanine dye of the stilbazolium betaine type 1-methyl-4-(4‘-hydroxytyryl)pyridinium betaine (Mtrans) and its conjugate acid MH+trans in aqueous solution is studied by femtosecond time-resolved pump probe spectroscopy. The measured rate of the primary photodynamics is determined to be k = 1.1 × 1012 s-1 for Mtrans at pH 10 and 0.8 × 1012 s-1 for MH+trans at pH 6. This was assigned to either conformational changes or a simple vibrational relaxation before the actual isomerization takes place. Wavelength excitation studies give support for the former assignment. These results are discussed in terms of the recent results found for the primary processes of retinal in bacteriorhodopsin. Time-resolved transient measurements show that no excited-state deprotonation of MH+trans occurs in aqueous solutions at pH 6 or pH 0, suggesting that the deprotonation occurs on a longer time scale than the picosecond time domain. This is in agreement with present theories of intermolecular proton-transfer reactions, which require solvent reorganization as well as the time of deprotonation estimated from the pKa value of this molecule in the excited state. The results of our MO calculations on the electronic structure of these two compounds could account for the fact that, while MH+trans photoisomerizes, its deprotonated form does not.