Femtosecond Time-resolved Pump Probe Research Articles

Determination of the refractive-index change in the excited state based on transient absorption microscopy

Photoinduced excited-state carriers can affect both the absorption coefficient and refractive index of materials and influence the performance of photoelectric devices. Femtosecond time-resolved pump–probe transient absorption (TA) spectroscopy is usually used to detect carrier dynamics and excited-state absorption coefficients; however, measurements of transient refractive-index change are still difficult. We propose a method for determining the excited-state refractive-index change using TA microscopy. In TA measurements, a Fabry–Pérot cavity formed by the front and back surfaces of the sample could lead to interference of the probe light. As the wavelength of standing waves in the Fabry–Pérot cavity is closely related to the refractive index, the carrier-induced excited-state refractive-index change was obtained by comparing the transmission probe spectra between the ground and excited states. The proposed method was used to study the dynamics of excited-state refractive-index change in a perovskite film.

Photocarrier relaxation and saturable absorption in defect-rich, direct chemical vapor deposition-grown graphene glass

Direct growth of large-area uniform graphene films on insulating materials could facilitate the applications of graphene in optoelectronic devices. The ultrafast photocarrier relaxation and saturable absorption of direct chemical vapor deposition-grown graphene glasses, with lots of defects, are studied by using femtosecond time-resolved pump–probe and Z-scan techniques at 800 nm. We find that both the relaxation times associated with hot carrier cooling and the hot phonon effect are greatly suppressed in these defect-rich graphene glasses, which further leads to the increase of both saturation intensity and anisotropy in transient optical response for graphene glasses as compared with defect-free graphene. And, both the suppression effect and saturation intensity increase with the thickness of graphene film. The dominance of defect-assisted carrier-acoustic phonon scattering (i.e., supercollision, the collision of a carrier with both an acoustic phonon and defects) in the cooling process of hot carriers is responsible for the suppression of relaxation time.

Unveiling Ultrafast Carrier Extraction in Highly Efficient 2D/3D Bilayer Perovskite Solar Cells

The development of multidimensional heterostructure (2D/3D) lead halide perovskites has emerged as an effective approach to enhancing the efficiency and long-term stability of perovskite solar cells (PSCs). However, a fundamental understanding of the working mechanisms, such as carrier extraction, and carrier transfer dynamics in the multidimensional perovskites heterostructures remains elusive. Here, we observe the ultrafast carrier extraction in highly efficient 2D/3D bilayer PSCs (power conversion efficiency of 21.12%) via femtosecond time-resolved pump–probe transient absorption spectroscopy (TAS). Notably, the formation of quasi-equilibrium states resulting in a subband absorption feature with an ultrafast lifetime of 440 fs was observed, and this feature is found only in 2D/3D perovskite heterostructure. The short-lived feature gives rise to the local electric-field-induced electroabsorption, resulting in an enhanced power conversion efficiency in 2D/3D PSCs. These findings can help comprehend the advanced working mechanism of highly efficient solar cells and other 2D/3D bilayer perovskite-based optoelectronic devices.

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.

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.

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.

Ultrafast Acoustic Vibrations of Bimetallic Nanoparticles

Investigations of the ultrafast acoustic response of metal nanosystems yield important information on the validity of continuous elastic mechanics at the nanoscale and also provide an optical way to probe nanoobject morphologies. In this context, we used femtosecond time-resolved pump–probe spectroscopy to study two classes of bimetallic nanoparticles: chemically synthesized AuAg nanospheres in water in the 20–45 nm size range, both with alloyed and segregated core–shell morphologies, and mass-selected glass-embedded PtAu core–shell nanospheres in the very small size range (2.3–2.5 nm), synthesized by physical methods. The analysis of the corresponding breathing mode periods demonstrates validity of the predictions of the continuous elastic model for bimetallic nanoobjects with the investigated sizes, morphologies and composition. Moreover, discrimination of nanoparticles internal structure (alloy or core–shell) by measurement of their acoustic response is also demonstrated.

Efficient optical Kerr gate of tellurite glass for acquiring ultrafast fluorescence

We investigated the ultrafast optical Kerr effect of tellurite glass (Te glass) by the femtosecond time-resolved pump–probe technique. The Kerr-gated spectra from a chirped supercontinuum (SC) obtained by a Te glass optical Kerr gate (OKG) were of a narrow bandwidth and wide range of transmittance spectra. Time-resolved violet fluorescence of a ZnO thin film with high temporal resolution was measured using the Te glass OKG. Experimental results showed that Te glass was a good candidate for an OKG medium due to its ultrafast response, large transmittance window, and large nonlinear refractive index.

Ultrafast zone-center coherent lattice dynamics in ferroelectric lithium tantalate

Femtosecond time-resolved pump–probe experiments were carried out to study ultrafast lattice dynamics of ferroelectric lithium tantalate. Both the fully symmetric (A1 mode) and doubly degenerate (E mode) coherent phonons at the center of the Brillouin zone were excited via impulsive stimulated Raman scattering, as confirmed by the excitation intensity dependence.

Femtosecond pulse x-ray imaging with a large field of view

Femtosecond pulse x-ray imaging is demonstrated in a sample-multiplexed Fourier transform holography scheme. Parallel imaging of multiple samples over an extended field of view is achieved by exploiting the coherence properties of the free-electron laser (FEL) source and the large profile of the unfocused x-ray pulse. The resulting photon flux density per pulse allows for damage-free single-pulse imaging with moderate image resolution. We envision the application of the method for femtosecond time-resolved pump–probe experiments with the feasibility of recording multiple steps in time with a single pulse. Furthermore, the scheme presented allows for a characterization of FEL radiation pulse parameters.

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.

Dynamics of plasma formation and permanent structural transformation in ZBLAN excited by tightly focused femtosecond laser pulses

Time-resolved dynamics of plasma formation and bulk refractive-index modification in fluoride glass (ZBLAN) excited by a tightly focused femtosecond (130 fs) Ti:sapphire laser ( λ p=790 nm) was observed in situ. The femtosecond time-resolved pump–probe measurement with perpendicularly linear polarized beams was used to study the dynamics of both plasma formation and induced permanent structural transformation with refractive-index change. In the refractive-index domain, the lifetime of induced plasma formation is ∼35 ps and structural transition time for forming the refractive-index change is ∼80 ps. In the optical damage domain, however, the lifetime of induced plasma formation is ∼40 ps and structural transition time for forming the optical damage is ∼140 ps. We found that the process of refractive-index bulk modification is significantly different from that of optical cracks. From the diffraction efficiency of Kogelnik's coupled mode theory, the maximum value of refractive-index change (Δ n) was estimated to be 1.3×10 −2. By the scanning of fluoride glass on the optical X–Y–Z stages, the fabrication of internal grating with refractive-index modification was demonstrated in fluoride glass using tightly focused femtosecond laser.

Ultrafast excitation relaxation in titanylphthalocyanine thin film

Abstract Ultrafast excitation relaxation in the whole Q band of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond time-resolved pump–probe technique. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the laser beam and consists of three quite different processes: an ultrafast process with a lifetime of 0.5–5 ps, a fast and a long-lived processes with lifetimes of about 5–10 ps and longer than 100 ps, respectively. The initial ultrafast decay appearing to be excitation intensity dependent is suggested to represent a bimolecular exciton–exciton annihilation process with a t −1/2 time dependence of the excited-state population, assigned to a one-dimensional exciton diffusion. The exciton–exciton annihilation is observed in the pump intensity as low as 0.27 GW/cm 2 .

The elementary steps of the photodissociation and recombination reactions of iodine molecules enclosed in cages and channels of zeolite crystals: A femtosecond time-resolved study of the geometry effect

We present femtosecond time-resolved pump-probe experiments on iodine molecules enclosed into well-defined cages and channels of different crystalline SiO2 modifications of zeolites. The new experimental results obtained from iodine in TON (Silica-ZSM-22), FER (Silica-Ferrierit), and MFI (Silicalit-1) porosils are compared with data published earlier on the iodine/DDR (Decadodecasil 3R) porosil system [Flachenecker et al., Phys. Chem. Chem. Phys. 5, 865 (2003)]. A summary of all findings is given. The processes analyzed by means of the ultrafast spectroscopy are the vibrational relaxation as well as the dissociation and recombination reactions, which are caused by the interaction of the photo-excited iodine molecules with the cavity walls of the porosils. A clear dependence of the observed dynamics on the geometry of the surrounding lattice structure can be seen. These measurements are supported by temperature-dependent experiments. Making use of a theoretical model which is based on the classical Langevin equation, an analysis of the geometry-reaction relation is performed. The Brownian dynamics simulations show that in contrast to the vibrational relaxation the predissociation dynamics are independent of the frequency of collisions with the surroundings. From the results obtained in the different surroundings, we conclude that mainly local fields are responsible for the crossing from the bound B state to the repulsive a/a' states of the iodine molecules.

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.

Two-photon absorption spectrum of all- trans retinal

The nondegenerate two-photon absorption spectrum of all- trans retinal in a nonpolar solvent at room temperature has been measured for the first time. The multichannel detection utilizing the technique of femtosecond time-resolved pump–probe absorption spectroscopy has enabled us to obtain precise two-photon absorption spectra. It was found that the two-photon absorption maximum of all- trans retinal is located at 376 nm, which is 7 nm longer than the one-photon absorption peak. This peak wavelength difference indicates the 500-cm −1 energy gap between the S 3 (B u) state and the S 2 (A g) state. On the basis of the obtained two-photon absorption data and the available fluorescence data, we determined the 0–0 position of the S 2 state to be 2.47 × 10 4 cm −1.

Photodissociation and recombination dynamics of I2 in DDR (decadodecasil 3R): Dependence on the geometry of the host matrix monitored by femtosecond time-resolved pump–probe experiments

We present polarization dependent femtosecond time-resolved pump–probe experiments on iodine molecules embedded in well-defined cages of the crystalline microporous SiO2 modification decadodecasil 3R (DDR). Inside the DDR porosils the I2 molecules reside isolated from each other in hexagonal cages of ≈350 A3. After pump excitation of the B 0+u (3Π) state, the collision induced dissociation takes place via different crossing repulsive potential curves. The surrounding cages cause the atoms to recombine. The resulting iodine molecules are prepared in their A, A′, and X states where vibrational relaxation takes place due to the collisions with the surroundings. For a better understanding of the complex dynamics, a theoretical model was developed based on the classical Langevin equation (Brownian dynamics). The simulations of the experimental data obtained from powder samples are compared with earlier calculations by Ermoshin et al. (V. A. Ermoshin, G. Flachenecker, A. Materny and V. Engel, J. Chem. Phys., 2001, 114, 8132) based on molecular dynamics modeling. Brownian dynamics simulations are also applied to polarization dependent pump–probe experiments on single crystalline I2-DDR samples. It is demonstrated, that by switching the polarization of the lasers relative to the crystal axis the predissociation process of the excited B state of I2 can be influenced. This effect can be simulated using the theoretical model.

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.

Femtosecond visible absorption study of excited-state dynamics of 9-cis retinal

Femtosecond time-resolved pump–probe absorption spectroscopy has been used to study the photophysics and photochemistry of 9-cis retinal in aerated hexane. Three excited singlet states (S 3, S 2, S 1) and the lowest triplet state (T 1) have been identified by the SVD analysis of the observed time-resolved absorption spectra. No evidence has been obtained for a 9-cis→all-trans isomerization reaction pathway that completes in the excited singlet manifold.