Pump-probe Time Delay Research Articles

Characterization of longitudinal acoustic phonons in InGaAsP multiple quantum wells by asynchronous optical sampling

We performed time-resolved reflectivity measurements and characterized the time-varying oscillations of longitudinal acoustic phonons in InGaAsP multiple quantum wells (MQWs) by asynchronous optical sampling with Er-doped fiber lasers. Oscillations of longitudinal acoustic phonons ranging from a few tens to a few hundred gigahertz were observed, which were induced by forward and backward Raman scattering in Brillouin and first zone-folded acoustic phonon modes. To investigate the dynamics of each phonon mode for various pump-probe time delays, we analyzed the experimental data with a continuous wavelet transformation (CWT) and a subsequent modified inverse CWT. For high-frequency oscillations, we observed the oscillatory profile with beats and identified the contribution of each phonon mode. For low-frequency oscillations, a series of dissipations of phonon wave packets from each QW to the InP substrate was deduced from the analyzed data. These results suggest that our analysis method is useful for investigating the dynamics of acoustic phonons.

In Situ Measurement of Exciton Dynamics During Thin-Film Formation Using Single-Shot Transient Absorption.

The exciton dynamics of pseudoisocyanine (PIC) is reported during the formation of a thin film dropcast from solution. Tilted pump pulses are used to spatially encode a pump-probe time delay, enabling the collection of a transient in a single shot. We demonstrate that a spatially encoded delay can be used to accurately measure exciton dynamics in thin-film samples, with a signal-to-noise ratio above 20 attained in 2 s. We report in situ linear absorption, fluorescence, and transient absorption measurements during the molecular aggregation of PIC. These measurements reveal a highly fluorescent intermediate stage during thin-film formation that we ascribe to J-aggregates, in contrast to the final, less fluorescent, dry thin film that exhibits photophysics indicative of disordered J-aggregates. The ability to measure exciton dynamics in situ during materials formation will provide a deeper understanding of how functional materials properties evolve, and will enable direct feedback for rational materials design.

Strong-Field Fourier Transform Vibrational Spectroscopy of D_{2}^{+} Using Few-Cycle Near-Infrared Laser Pulses.

The photoionization of D_{2} and dissociation of the resultant D_{2}^{+} are monitored by pump-probe measurements using intense near-infrared few-cycle laser pulses. The yields of D_{2}^{+} and D^{+} recorded up to the pump-probe delay time of 527ps exhibit oscillatory structures reflecting the motion of the created vibrational wave packet of D_{2}^{+}, and the Fourier transform of the data in time domain reveals the vibrational level separations with uncertainties of 0.0002-0.0097 cm^{-1}, showing a potential application of the strong-field pump-probe measurements to high-resolution spectroscopy of molecular ions.

Single-shot transient absorption spectroscopy of an organic film

ABSTRACTWe report single-shot transient absorption (SSTA) measurements of an organic film of 3,3’-Diethyloxatricarbocyanine iodide (DOTCI). In SSTA, the pump-probe time delay is spatially encoded by using a tilted pump pulse. Translation of the sample during SSTA measurements averages over any spatial heterogeneity in the film. We demonstrate that exciton dynamics measured with the single-shot technique agrees with traditional transient absorption measurements of the same film. A signal-to-noise ratio of ∼40 is achieved in 10 s. The ability to measure exciton dynamics in organic films will enable future SSTA measurements of exciton dynamics during the molecular aggregation events that result in film formation.

XFEL experiments: jitter of pump-probe time delays and pulse intensities.

Jitter of XFEL signals due to fluctuations in shot-to-shot time delays and intensities are explored in the frame of a statistical theory of X-ray diffraction from liquids. Deformed signals are calculated at different levels of pump-probe jitter. A new method is proposed to eliminate these distortions.

Dual-Frequency Comb Transient Absorption: Broad Dynamic Range Measurement of Femtosecond to Nanosecond Relaxation Processes.

We experimentally demonstrate a dual-frequency comb-based transient absorption (DFC-TA) technique, which has a 12 fs time resolution and an ultrafast scan rate. Here, the fast scan rate is achieved by employing asynchronous optical sampling (ASOPS), which utilizes two independent mode-locked lasers with a slightly detuned repetition rates. The ASOPS approach is advantageous because photodegradation damage of optical sample during TA measurements can be minimized by a gated sampling. We show that the vibrational and electronic population relaxations of near-IR dye molecules in solution that occur in the time range from femtoseconds to nanoseconds can be resolved even with a single time scan measurement. The phase coherent nature of our dual-frequency comb lasers is shown to be the key for successful coherent averaging with femtosecond time resolution preserved over many data acquisitions. We anticipate that the present DFC-TA method without using any pump-probe time delay devices could be of use in developing ultrafast TA-based microscopy and time-resolved coherent multidimensional spectroscopy.

Bias-induced modulation of ultrafast carrier dynamics in metallic single-walled carbon nanotubes

The gate bias dependence of excited-state relaxation dynamics in metallic single-walled carbon nanotubes (MCNTs) was investigated using pump-probe transient absorption spectroscopy coupled with electrochemical doping through an ionic liquid. The transient transmittance decayed exponentially with the pump-probe delay time, whose value could be tuned via the Fermi-level modulation of Dirac electrons under a bias voltage. The obtained relaxation time was the shortest when the Fermi level was at the Dirac point of the MCNTs, and exhibited a U-shaped dependence on the bias voltage. Because optical dipole transitions between the Dirac bands are forbidden in MCNTs, the observed dynamics were attributed to carrier relaxation from the ${E}_{11}$ band to the Dirac band. Using a model that considers the suppression of electron-electron scattering (impact ionization) due to Pauli blocking, we could qualitatively explain the obtained bias dependence of the relaxation time.

Spatiotemporal dynamics of Raman coherence in hollow-core fibers for a pump-probe setup

We present an experimental and theoretical study of the stimulated Raman emission in hollow-core kagome waveguides in a pump-probe arrangement. We perform an experimental investigation of the power of the Stokes signal from the probe, which is below the stimulated Raman scattering threshold, as a function the pump-probe delay time. The results show the Stokes power to increase with pump-probe delay, reaching a maximum at 10 ns, and to decrease afterward. In view of a coherence decay time of only 0.25 ns, we demonstrate a surprisingly slow reduction of Stokes signal with the characteristic time much longer than the coherence decay time by a factor of up to 40. The numerical investigations explain the observed phenomenon as a result of the spatiotemporal dynamics of the probe pulse and Raman coherence. We show that the increase of the characteristic time can be related to the spatial position of the intense sideband generation event and its dependence on the pump-probe delay.

Single-shot transient absorption spectroscopy with a 45 ps pump-probe time delay range.

We report a single-shot transient absorption apparatus that successfully uses a tilted pump pulse to spatially encode a 45ps pump-probe time delay. The time delay range is significantly improved over other reported instruments by using a spatial light modulator to flatten the intensity of the excitation field at the sample position. The full time delay range of the instrument is demonstrated by measuring a long-lived dye. A signal-to-noise ratio of >35 is attained in 8s. This advance will enable the measurement of excited state dynamics of systems that are not at structural equilibrium.

Watching proton transfer in real time: Ultrafast photoionization-induced proton transfer in phenol-ammonia complex cation.

In this paper, we give a full account of our previous work [C. C. Shen et al., J. Chem. Phys. 141, 171103 (2014)] on the study of an ultrafast photoionization-induced proton transfer (PT) reaction in the phenol-ammonia (PhOH-NH3) complex using ultrafast time-resolved ion photofragmentation spectroscopy implemented by the photoionization-photofragmentation pump-probe detection scheme. Neutral PhOH-NH3 complexes prepared in a free jet are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via the S1 state. The evolving cations are then probed by delayed pulses that result in ion fragmentation, and the ionic dynamics is followed by measuring the parent-ion depletion as a function of the pump-probe delay time. By comparing with systems in which PT is not feasible and the steady-state ion photofragmentation spectra, we concluded that the observed temporal evolutions of the transient ion photofragmentation spectra are consistent with an intracomplex PT reaction after photoionization from the initial non-PT to the final PT structures. Our experiments revealed that PT in [PhOH-NH3]+ cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ∼70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the time scale to complete the reaction can be much slower and is determined by the rate of energy dissipation into other modes.

Coherent control of optical phonons in bismuth

We have conducted degenerate pump-pump-probe experiments on semimetal bismuth with femtosecond time resolution, varying both pump-probe and interpump time delays. The observed phonon dynamics, including the amplitude, damping, frequency, and phase, can be modeled by the interference of two chirped damped oscillators. No lattice anharmonicity along the trigonal axis is observed. We also find evidence of phonon-mediated relaxation of the energy density at the sample surface.

Frequency-Dependent Terahertz Transient Photoconductivity of Mesoporous SnO2 Films

The transient photoconductive properties of tin(IV) oxide (SnO2) mesoporous films have been studied by time-resolved terahertz (THz) spectroscopy. We gain insight into carrier dynamics by measuring overall injection and trapping lifetimes using optical pump–THz probe spectroscopy, as well as the frequency-dependent complex conductivity at various pump–probe delay times. It is found that the method of charge generation, either direct above band gap excitation (at 267 nm) or dye-sensitized electron injection (at 400 nm), has a dramatic effect on the overall injection and trapping dynamics of mobile charge carriers on the picosecond to nanosecond time scale. In the presence of aqueous electrolyte, direct band gap excitation of nonsensitized SnO2 films results in instrument response limited subpicosecond injection lifetimes, while dye-sensitized films require tens of picoseconds for interfacial electron transfer to complete. On the other hand, the rate for trapping of mobile charges is at least 2 orders of ma...

Large anisotropic spin relaxation time of exciton bound to donor states in triple quantum wells

We have studied the spin dynamics of a dense two-dimensional electron gas confined in a GaAs/AlGaAs triple quantum well by using time-resolved Kerr rotation and resonant spin amplification. Strong anisotropy of the spin relaxation time up to a factor of 10 was found between the electron spins oriented in-plane and out-of-plane when the excitation energy is tuned to an exciton bound to neutral donor transition. We model this anisotropy using an internal magnetic field and the inhomogeneity of the electron g-factor. The data analysis allows us to determine the direction and magnitude of this internal field in the range of a few mT for our studied structure, which decreases with the sample temperature and optical power. The dependence of the anisotropic spin relaxation was directly measured as a function of several experimental parameters: excitation wavelength, sample temperature, pump-probe time delay, and pump power.

X-ray Pump–Probe Investigation of Charge and Dissociation Dynamics in Methyl Iodine Molecule

Molecular dynamics is of fundamental interest in natural science research. The capability of investigating molecular dynamics is one of the various motivations for ultrafast optics. We present our investigation of photoionization and nuclear dynamics in methyl iodine (CH3I) molecule with an X-ray pump X-ray probe scheme. The pump–probe experiment was realized with a two-mirror X-ray split and delay apparatus. Time-of-flight mass spectra at various pump–probe delay times were recorded to obtain the time profile for the creation of high charge states via sequential ionization and for molecular dissociation. We observed high charge states of atomic iodine up to 29+, and visualized the evolution of creating these high atomic ion charge states, including their population suppression and enhancement as the arrival time of the second X-ray pulse was varied. We also show the evolution of the kinetics of the high charge states upon the timing of their creation during the ionization-dissociation coupled dynamics. We demonstrate the implementation of X-ray pump–probe methodology for investigating X-ray induced molecular dynamics with femtosecond temporal resolution. The results indicate the footprints of ionization that lead to high charge states, probing the long-range potential curves of the high charge states.

Laser control over the ultrafast Coulomb explosion of N22+ after Auger decay: A quantum-dynamics investigation

By theoretical calculation, we demonstrate the possibility to control and partially suppress the Coulomb explosion of ${\mathrm{N}}_{2}$ molecules after core-level photoionization by an x-ray laser and subsequent Auger decay. This is achieved by means of a femtosecond infrared laser pulse interacting with the ${\mathrm{N}}_{2}{}^{2+}$ dication produced by the x-ray pulse. Suppression of molecular fragmentation requires few-femtosecond IR pulses interacting with the system either during or shortly after the arrival of the x-ray pulse. The IR pulse suppresses fragmentation mostly by optically coupling the electronic routes to ultrafast molecular dissociation with electronic channels able to support long-lived vibrational resonances. The effect is strongly dependent on the orientation of the molecule with respect to the polarization axis of the IR field. Our calculations are motivated by x-ray pump--IR probe experiments performed at an x-ray free-electron laser [J. M. Glownia et al., Opt. Express 18, 17620 (2010)], where only enhancement of ${\mathrm{N}}_{2}{}^{2+}$ fragmentation as a function of the pump-probe delay time was reported. The opposite effect reported here becomes apparent when the various electronic channels are considered separately. In practice, this corresponds to a coincident measurement of the energy of the ejected Auger electron.

Probing two-electron dynamics of helium in time domain via fluorescence channel

Abstract We theoretically study two-electron dynamics in a wave packet consisting of low-lying doubly excited states of helium, using an attosecond single-color XUV pump–XUV probe scheme. By solving the time-dependent Schrodinger equation in the hyperspherical coordinates, we analyze an ionization plus excitation process followed by a fluorescence emission. Pump–probe delay time dependence of the transition probabilities, and hence the fluorescence signal, is proved to unravel the bending vibrational motion of the two electrons with the nucleus at the center. Such a motion having an analog to that of a floppy triatomic molecule is the true feature of the electron correlation during temporal evolution of an intrashell two-electron wave packet.

Unexpectedly broad photoelectron spectrum as a signature of ultrafast electronic relaxation of Rydberg states of carbon dioxide

The dynamics of ${\mathrm{CO}}_{2}$ excited into Rydberg states lying 0.2 eV below the ionization threshold is studied by means of time resolved photoelectron imaging. Over 3 eV broad photoelectron spectra are measured for all pump-probe delay times. Quantum mechanical calculations demonstrate that the spectral broadening is due to ultrafast electronic relaxation of Rydberg states and identify the likely relaxation pathways. Experiment and theory bracket the relaxation time between 15 and 65 fs. A weak time independent ionization signal is attributed to ${\mathrm{CO}}_{2}$ trapped in near-threshold triplet states.

Chirality Specific Triplet Exciton Dynamics in Highly Enriched (6,5) and (7,5) Carbon Nanotube Networks

Single-walled carbon nanotubes (SWNTs) show high aspect ratio, thermal and chemical stability as well as charge mobility, and therefore appear ideally suited for improving charge extraction in organic photovoltaic (OPV) devices. Since typical charge extraction times in OPV devices are in the microsecond range, the interplay of the desired charged states with long-lived neutral states in SWNTs such as triplet excitons becomes important. Triplet excitons have recently been investigated in (6,5) SWNTs with an optical yield close to 32%. Here, we present transient absorption (TA) dynamics of (6,5)- and (7,5)-rich SWNT networks from the femtosecond to microsecond time scale. Comparing our TA spectra to results from a recent spectroelectrochemical study allows us to distinguish between contributions from charged and neutral photoexcitations. For long pump–probe delay times we identify an excess photobleach which we use as a chirality specific probe for the density of triplet excitons. We show that triplet energ...

Quantum state-resolved, bulk gas energetics: Comparison of theory and experiment.

Until very recently, the computational model of state-to-state energy transfer in large gas mixtures, introduced by the author and co-workers, has had little experimental data with which to assess the accuracy of its predictions. In a novel experiment, Alghazi et al. [Chem. Phys. 448, 76 (2015)] followed the equilibration of highly vibrationally excited CsH(D) in baths of H2(D2) with simultaneous time- and quantum state-resolution. Modal temperatures of vibration, rotation, and translation for CsH(D) were obtained and presented as a function of pump-probe delay time. Here the data from this study are used as a test of the accuracy of the computational method, and in addition, the consequent changes in bath gas modal temperatures, not obtainable in the experiment, are predicted. Despite large discrepancies between initial CsH(D) vibrational states in the experiment and those available using the computational model, the quality of agreement is sufficient to conclude that the model's predictions constitute at least a very good representation of the overall equilibration that, for some measurements, is very accurate.

Identification of the cationic excited state of cyclopentanone via time-resolved Ion yield measurements

Abstract We report the experimental evidence of the one-photon resonance in the cationic excited state of cyclopentanone using the femtosecond time-resolved pump-probe method. The transients of the parent ion and the C 2 H 4 + fragment exhibit constant depletion in the pump-probe delay time up to 7 ps, however, that of the C 4 H 8 + /C 3 H 4 O + fragment presents formation behavior. By recording dependence of the ion yields as the probe laser intensity, we demonstrate one 400-nm photon resonance in the cationic excited state, which is assigned as the D 4 ( 2 A) cationic state based on the theoretical calculations. Possible dissociation mechanism of the D 4 state is also discussed.