Pump-probe Time Delay Research Articles

Adaptive noise canceling for transient absorption microscopy.

.Significance: Ultrafast fiber lasers are an attractive alternative to bulk lasers for nonlinear optical microscopy for their compactness and low cost. The high relative intensity noise (RIN) of these lasers poses a challenge for pump-probe measurements such as transient absorption and stimulated Raman scattering, along with modalities that provide label-free contrast from the vibrational and electronic structure of molecules.Aim: Digital adaptive filtering was applied to determine the applicability for canceling laser RIN in a transient absorption microscope with an ultrafast fiber laser source.Approach: Digitized signals from the transmitted probe and reference photodetectors were fed to an adaptive filter in MATLAB, running in a noise canceling configuration. This result was then fed to a software lock-in algorithm to demodulate the pump-probe signal. Images were built up one line scan at a time with a 3.5-kHz resonant scanner, with averaging. The imaging target was , which exhibits nondegenerate two-photon absorption at the pump/probe wavelengths used (530-nm pump and 490-nm probe).Results: Without adaptive noise cancellation, the lock-in output primarily passes the laser RIN within its detection bandwidth, resulting in images that closely follow the linear transmissivity and lack sensitivity to pump-probe time delay. With adaptive noise cancellation in front of the lock-in, the RIN rejection is enough to restore the z-sectioning and sensitivity to pump-probe delay, as expected for transient absorption. Results were limited primarily by noise from the photodetector and analog-to-digital converter.Conclusions: Digital adaptive noise cancellation, even when limited by electronics noise, can recover pump-probe signals by removal of laser RIN, under conditions where averaging alone fails.

Jitter of pump-probe time delays in XFEL experiments: How to eliminate it?

Jitter of XFEL signals due to the disorder of pump-probe time delay is explored theoretically. Two basically different methods are applied: statistical mechanics and dimensional analysis. Although completely different, these two approaches suggest the same method to suppress jitter generated perturbations. The method which is proposed here is easily applicable and requires no computational effort.

Optical spectroscopy and ultrafast pump-probe study on Bi2Rh3Se2 : Evidence for charge density wave order formation

The parkerite-type ternary chalcogenide Bi$_2$Rh$_3$Se$_2$ was discovered to be a charge density wave (CDW) superconductor. However, there was a debate on whether the observed phase transition at 240 K could be attributed to the formation of CDW order. To address the issue, we performed optical spectroscopy and ultrafast pump-probe measurements on single crystal samples of Bi$_2$Rh$_3$Se$_2$. Our optical conductivity measurement reveals clearly the formation of an energy gap with associated spectral change only at low energies, yielding strong evidence for a CDW phase transition at 240 K. Time resolved pump-probe measurement provides further support for the CDW phase transition. The amplitude and relaxation time of quasiparticles extracted from the photoinduced reflectivity show strong enhancement near transition temperature, yielding further evidence for the CDW energy gap formation. Additionally, a collective mode is identified from the oscillations in the pump-probe time delay at low temperature. This mode, whose frequency decreases gradually at elevated temperature, could be naturally attributed to the amplitude mode of CDW state.

Attosecond resolution from free running interferometric measurements.

Attosecond measurements reveal new physical insights in photoionization dynamics from atoms, molecules and condensed matter. However, on such time scales even a small timing jitter can significantly reduce the time resolution in pump-probe measurements. Here, we propose a novel technique to retrieve attosecond delays from a well-established attosecond interferometric technique, referred to as reconstruction of attosecond beating by interference of two-photon transition (RABBITT), which is unaffected by timing jitter and significantly improves the precision of state-of-the-art experiments. We refer to this new technique as the timing-jitter unaffected rabbitt time delay extraction method, in short TURTLE. Using this TURTLE technique we could measure the attosecond ionization time delay between argon and neon in full agreement with prior measurements. The TURTLE technique allows for attosecond time resolution without pump-probe time delay stabilization and without attosecond pulses because only a stable XUV frequency comb is required as a pump. This will more easily enable attosecond measurements at FELs, for example, and thus provide a valuable tool for attosecond science. Here we also make a MATLAB code available for the TURTLE fit with appropriate citation in return.

The longer timescale excited state dynamics of isolated benzene.

The excited state photodynamics of isolated benzene have been studied in the nanosecond range by two-step photoionization through various vibrations of the lowest singlet state, with imaging photoelectron spectroscopy detection. Photoelectron spectra are measured as a function of pump-probe delay time, and their time evolution is successfully compared to a biexponential decay function without regard to a particular kinetic model. The only reasonable kinetic model with only two exponentials is the one that involves an intersystem crossing from S1 to T1, although that model has previously been called into question by high-resolution studies that failed to find any singlet-triplet perturbations in Zeeman studies of the S1 spectrum. That contradiction remains unresolved.

Long-term stabilization of carrier envelope phases of mid-infrared pulses for the precise detection of phase-sensitive responses to electromagnetic waves

We report a high performance mid-infrared pump visible probe measurement system, which can measure phase-sensitive responses to a mid-infrared pulse along the oscillating electromagnetic field. In this system, the pump light is a phase-locked mid-infrared pulse with a temporal width of 100 fs, which is produced via difference frequency generation (DFG) from two idler pulses of two optical parametric amplifiers (OPAs) that are excited by the same Ti:sapphire regenerative amplifier. The probe pulse is a visible pulse with a temporal width of 9 fs and is generated from a custom-built non-collinear OPA. By measuring the electric-field waveforms of mid-infrared pump pulses with electro-optic sampling and evaluating their carrier envelope phase (CEP) and the temporal positions of their envelopes relative to ultrashort visible probe pulses, we are able to perform double feedback corrections that eliminate both the following sources of drift. The CEP drift in mid-infrared pulses originating from fluctuations in the difference of optical-path lengths of the two idler pulses before the DFG is corrected by inserting a wedge plate in one idler path, and the drift in pump–probe delay times due to fluctuations in the difference of the overall optical-path lengths of the pump and probe pulses is corrected with mechanical delay lines. In this double feedback system, the absolute carrier phase of mid-infrared pulses can be fixed within 200 mrad and errors in the measurement of phase-sensitive responses can be reduced to within 1 fs over a few tens of hours.

Intersystem crossing of 2-Methlypyrazine studied by femtosecond photoelectron imaging

The ultrafast nonadibatic relaxation dynamics of the excited state of 2-methylpyrazine has been studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S<sub>1</sub> of 2-methylpyrazine was excited by 323 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The lifetime of S<sub>1</sub> state 98 ps is obtained by time-resolved mass spectroscopy. The intersystem crossing from the S<sub>1</sub> state to the T<sub>1</sub> state is observed on real time. The relaxation dynamics of S<sub>1</sub> state of 2-methlypyrazine is different from that of pyrazine, the results show that the intersystem crossing process between S<sub>1</sub> and T<sub>1</sub> is the main relaxation channel of S<sub>1</sub> state of 2-methlypyrazine, but the internal conversion process between S<sub>1</sub> and S<sub>0</sub> is also a main relaxation channel of S<sub>1</sub> state. By using the advantages of femtosecond time-resolved photoelectron imaging, the photoelectron angular distribution at different pump-probe time delay was obtained experimentally. From the photoelectron angle distribution combined with photoelectron kinetic energy distributions, we tried to observe the field-free nonadiabatic alignment. However, due to the fact that the molecular symmetry of 2-methylpyrazine is lower than that of pyrazine, it is more challenging to observe the phenomenon of molecular nonadiabatic alignment with lower symmetry. Therefore, it is fail to observe nonadiabatic alignment feature of 2-methylpyrazine in this experiment. This work provides a clearer physical picture for S<sub>1</sub> state nonadibatic relaxation dynamics of 2-methylpyrazine.

Spectral attenuation of a 400-nm laser pulse propagating through a plasma filament induced by an intense femtosecond laser pulse**Project supported by the National Natural Science Foundation of China (Grant Nos. U1932133, 51733004, 51525303, and 21702085) and the Fundamental Research Funds for the Central Universities, China (Grant Nos. lzujbky-2016-35 and lzujbky-2018-it36).

The spectral attenuation of a 400-nm probe laser propagating through a femtosecond plasma in air is studied. Defocusing effect of the low-density plasma is an obvious effect by examining the far-field patterns of the 400-nm pulse. Besides, the energy of 400-nm pulse drops after interaction with the plasma, which is found to be another effect leading to the attenuation. To reveal the physical origin behind the energy loss, we measure fluorescence emissions of the interaction area. The fluorescence is hardly detected with the weak 400-nm laser pulse, and the line spectra from the plasma filament induced by the 800-nm pump pulse are clearly shown. However, when the 400-nm pulse propagates through the plasma filament, the fluorescence at 391 nm from the first negative band system of is enhanced, while that from the second positive band of neutral N2 at 337 nm remains constant. Efficient near-resonant absorption of the 400-nm pulse by the first negative band system occurs inside the plasma, which results in the enhanced fluorescence. Furthermore, the spectral attenuation of the 400-nm probe laser is measured as a function of the pump–probe time delay as well as the pump-pulse energy.

Field-free molecular alignment of carbon dioxide molecules measured with above-threshold ionization yields

We present measurements of above-threshold ionization (ATI) of CO2 in a pump-probe experiment where the pump pulse creates a rotational wave packet and the linearly polarized probe pulse generates the ATI spectrum as a function of the pump-probe delay time, which sweeps over the revival time of field-free alignment and its quarter fractions. The observed alignment signals, which are the electron yields of ATI by a probe pulse as a function of a delay between the pump and probe pulse, are compared with the calculated alignment parameter [Formula: see text]. The results are explained by nuclear spin statistics and the wave packet evolution of the CO2 molecule in terms of the highest occupied molecular orbital.

Theory of coherent-oscillations generation in terahertz pump-probe spectroscopy: From phonons to electronic collective modes

Time-resolved spectroscopies using intense THz pulses appear as a promising tool to address collective electronic excitations in condensed matter. In particular recent experiments showed the possibility to selectively excite collective modes emerging across a phase transition, as it is the case for superconducting and charge-density-wave (CDW) systems. One possible signature of these excitations is the emergence of coherent oscillations of the differential probe field in pump-probe protocols. While the analogy with the case of phonon modes suggests that the basic underlying mechanism should be a sum-frequency stimulated Raman process, a general theoretical scheme able to describe the experiments and to define the relevant optical quantity is still lacking. Here we provide this scheme by showing that coherent oscillations as a function of the pump-probe time delay can be linked to the convolution in the frequency domain between the squared pump field and a Raman-like non-linear optical kernel. This approach is applied and discussed in few paradigmatic examples: ordinary phonons in an insulator, and collective charge and Higgs fluctuations across a superconducting and a CDW transition. Our results not only account very well for the existing experimental data in a wide variety of systems, but they also offer an useful perspective to design future experiments in emerging materials.

Solvated electron formation from the conduction band of liquid methanol: Transformation from a shallow to deep trap state

We report solvated electron (esolv -) formation dynamics from the conduction band of liquid methanol studied using femtosecond time-resolved photoelectron spectroscopy. Liquid methanol is excited with vacuum UV (9.3 eV) pump pulses, and the subsequent electron dynamics are probed with UV pulses. The photoelectron signal exhibits a short-lived component (τ = 85 fs) without spectral evolution followed by a long-lived component with continuous spectral evolution over tens of picoseconds. We ascribe the former to a superexcited state, most likely the Wannier exciton, and the latter to the ground electronic state of esolv -. In order to extract accurate energetics from the observed photoelectron spectra, we employ a spectral retrieval method to account for spectral broadening and shifting due to inelastic scattering of photoelectrons in the liquid. The electron binding energy (eBE) of the initial trap state of an electron is determined to be about 1.5 eV, and its biexponential increase up to 3.4 eV is observed with time constants of 2 and 31 ps, which are greater than 0.27 and 13 ps observed for esolv - created by the charge-transfer-to-solvent reaction from CH3O- to liquid methanol. The solvation dynamics of esolv - created by the electron trapping exhibit a pseudoisosbestic point at a pump-probe delay time of around 15 ps, and the peak energy of the eBE distribution rapidly changes around that time. These results indicate that there exist two trap states, both of which exhibit increasing eBE with time; however, the eBE of the shallow trap state increases only up to 2.1 eV, and transformation to a deep trap state at 25 ps occurs to reach an eBE of 3.4 eV.

State-to-State Rate Coefficients for NH3-NH3 Collisions from Pump-Probe Chirped Pulse Experiments.

The kinetics of rotational inelastic NH3–NH3 collisions are recorded using pump–probe experiments, carried out with a K-band waveguide chirped pulse Fourier transform microwave spectrometer, in which the population of one inversion doublet is altered by the pump pulse. Due to self-collisions, the resulting deviation from equilibrium propagates to other states and, thus, can be interrogated by probe pulses as a function of the pump–probe delay time. A clear hierarchy of the state-to-state collision processes is found and subsequently translated into propensity rules. State-to-state rate coefficients are estimated, first via an analysis of the kinetics, and then more robustly and accurately derived from the pressure-dependent measurements using a global fitting procedure.

Resonant anisotropic emission in two-photon interferometric spectroscopy

We theoretically explore a variant of RABBITT spectroscopy in which the attosecond-pulse train comprises isolated pairs of consecutive harmonics of the fundamental infrared probe frequency. In this scheme, one-photon and two-photon amplitudes interfere resulting in an asymmetric photoelectron emission. This interferometric principle has the potential of giving access to the time-resolved ionization of systems that exhibit autoionizing states, since it imprints the group delay of both one-photon and two-photon resonant transitions in the energy-resolved photoelectron anisotropy as a function of the pump-probe time delay. To bring to the fore the connection between the pump-probe ionization process and its perturbative analysis, on the the one side, and the underlying field-free scattering observables as well as the radiative couplings in the target system, on the other side, we test this scheme with an exactly solvable analytical one-dimensional model that supports both bound states and shape-resonances. The asymmetric photoelectron emission near a resonance is computed using perturbation theory as well as solving the time-dependent Sch\"odinger equation; the results are in excellent agreement with the field-free resonant scattering properties of the model.

High-order harmonic generation from molecular ions in the coherent electronic states

The high-order harmonic generation from the tritium molecular ions in the coherent state has been numerically simulated by solving the three-state time-dependent Schrödinger equation based on the non-Born-Oppenheimer approximation. Two resonant pump pulses prepare the system in the coherent superposition of the and the states. A long-wavelength probe pulse is selected to generate the intense harmonics. By increasing the population of the state, the dependence of the harmonic spectra on pump-probe delay time tends to exhibit obvious redshift. In this work, not only is the physical image of harmonic emission from coherent wave packets established, but also a new physical mechanism of the redshift in long-wavelength region is revealed. Moreover, we present a scheme of manipulating the redshift to further verify the underlying mechanism.

Photoconductivity, carrier lifetime and mobility evaluation of GaAs films on Si (100) using optical pump terahertz probe measurements

The carrier lifetimes and electron mobility values were estimated for 2 μm thick GaAs films grown on Si (100) substrates by means of optical pump terahertz probe (OPTP) technique. The GaAs/Si films measured were epitaxial grown at different substrate temperatures (TS = 520 °C or TS = 630 °C). From x-ray diffraction measurements and Raman spectroscopy, the GaAs/Si films were shown to experience minimum strain at room temperature, and crystal misorientation in the (111) or (110) direction. With no measureable photoluminescence at room temperature, carrier lifetimes were measured via OPTP and found to be ∼20 and ∼35 ps for a fluence of ∼4 μJ cm−2, which is in the same order of magnitude as a reference bulk GaAs grown on SI–GaAs (TS = 630 °C) having a lifetime of ∼70 ps. From OPTP photoconductivity measurements, the estimated GaAs/Si films’ electron mobility are ∼2900 cm2 V−1 s−1 (TS = 520 °C) and ∼3500 cm2 V−1 s−1 (TS = 630 °C) at a pump-probe delay time of Δt = 50 ps, in which the bulk GaAs electron mobility is ∼5200 cm2 V−1 s−1.

Strong-field Fourier transform vibrational spectroscopy of methanol cation and its isotopologues using few-cycle near-infrared laser pulses

ABSTRACTPump-probe measurements of methanol and its isotopologues (CH3OH, CH3OD and CD3OH) were performed using near-infrared few-cycle intense laser pulses. The yields of the parent ion and the fragment ions oscillate as a function of the pump-probe time delay, reflecting the time evolution of the vibrational wave packet created in methanol and methanol cation by the pump pulse. By the Fourier transform (FT) of the time-domain data, the vibrational mode frequencies of methanol and methanol cations were determined on the basis of the assignment of the peak profiles in the FT spectra made using their relative phases and the theoretical vibrational frequencies and potential energy distributions.

Analysis of nonlinear polarization rotation by an ultrashort optical pump and probe pulse in a strained semiconductor optical amplifier

In this paper, a comprehensive dynamic model is used to study the nonlinear polarization rotation (NPR) of a probe signal in the presence of a copropagating pump signal in a strained semiconductor optical amplifier (SOA). Pump and probe signals are Gaussian pulse with 200 fs pulse width, propagating in a 500 μm SOA. In the present model, the effects of carrier dynamics on the SOA gain and phase are considered and the NPR of the probe signal is measured by the ellipticity angle using the Stokes vector and Mueller matrices. The effects of several important factors on the NPR of the probe signal are investigated. These factors include pump signal energy, free carrier absorption and two-photon-absorption nonlinear effects, tensile/compressive strain, pump-probe pulse width, and pump-probe delay time. The results show that the pulse width and strain have a considerable effect on the ellipticity angle deviation (EAD); we especially see that the tensile strain increases the EAD, and the compressive strain reduces the EAD compared to the no strain structure. For a specific range of pulse width (here 0.2 ps to 1 ps), the EAD increases noticeably with the pulse width. It is also shown that, for the pump-probe delay time, the maximum EAD is obtained when the delay time is 0.1 ps.

Imaging alignment of rotational state-selected CH3I molecule**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574116, 11534004, 10704028, and 11474123) and the Natural Science Foundation of Jilin Province, China (Grant No. 20170101154JC).

We experimentally and numerically investigate CH3I molecular alignment by using a femtosecond laser and a hexapole. The hexapole provides the single rotational state condition at 4.5-kV hexapole rod voltage. Based on this single rotational state, an enhanced alignment degree of 0.73 is achieved. Our experimental results are in agreement with the simulation results. We experimentally obtain the ion velocity map images and show the influence of the initial rotational-state population. With the I+ ion images and angular distributions at different pump-probe delay time, the alignment and anti-alignment phenomena are further demonstrated. The molecules will be under field-free conditions when the laser effect disappears completely at the full revival time. Our work shows that the quantum control and spatial control on CH3I molecules can be realized and molecular coordinate frame can be obtained for further molecular experiment.

Decomposing electronic and lattice contributions in optical pump - X-ray probe transient inner-shell absorption spectroscopy of CuO.

Electronic and lattice contributions to picosecond time-resolved X-ray absorption spectra (trXAS) of CuO at the oxygen K-edge are analyzed by comparing trXAS spectra, recorded using excitation wavelengths of 355 nm and 532 nm, to steady-state, temperature-dependent XAS measurements. The trXAS spectra at pump-probe time-delays ≥150 ps are dominated by lattice heating effects. Insight into the temporal evolution of lattice temperature profiles on timescales up to 100s of nanoseconds after laser excitation are reported, on an absolute temperature scale, with a temporal sensitivity and a spatial selectivity on the order of 10s of picoseconds and 10s of nanometers, respectively, effectively establishing an "ultrafast thermometer". In particular, for the 532 nm experiment at ∼5 mJ cm-2 fluence, both the initial sample temperature and its dynamic evolution are well captured by a one-dimensional thermal energy deposition and diffusion model. The thermal conductivity k = (1.3 ± 0.4) W m-1 K-1 derived from this model is in good agreement with the literature value for CuO powder, kpowder = 1.013 W m-1 K-1. For 355 nm excitation, a quantitative analysis of the experiments is hampered by the large temperature gradients within the probed sample volume owing to the small UV penetration depth. The impact of the findings on mitigating or utilizing photoinduced lattice temperature changes in future X-ray free electron laser (XFEL) experiments is discussed.

Dinitrogen Coupling to a Terpyridine-Molybdenum Chromophore Is Switched on by Fermi Resonance

Summary The traditional view of a chemical change is inherently local and classical, and such a change relies on a mix of thermodynamic and kinetic parameters to control reactivity. Often, the thermodynamic stability of chemical bonds necessitates significant energy input for activation. One fundamental question is potentially transformative: can quantum mechanics enable selective bond activation? A possible approach involves strategic input of energy to reaction-specific vibrational levels. Toward this goal, our work describes the coupling of vibrational motions in a terpyridine-molybdenum complex hosting a nonreactive substrate—dinitrogen. Ultrafast coherence spectroscopies revealed a Fermi-resonance coupling mechanism connecting in-plane breathing motion of the light-harvesting terpyridines with the stretching motion of the spatially disparate dinitrogen bridge. Notably, the coupling is significantly enhanced in the photoexcited state. This Fermi resonance indicates an energy conduit that drives the two motions in sync and thereby amplifies vibrational energy exchange. Achieving selective bond activation by bridging vibrations could present a quantum-inspired design principle in synthetic chemistry.