Fs Probe Pulse Research Articles

Rabi Oscillations and Coherence Dynamics in Terahertz Streaking-Assisted Photoelectron SpectrumSupported by the National Natural Science Foundation of China (Grant Nos. 11827806, 11874368 and 61675213).

We present an approach, a Terahertz streaking-assisted photoelectron spectrum (THz SAPS), to achieve direct observations of ultrafast coherence dynamics with timescales beyond the pulse duration. Using a 24 fs probe pulse, the THz SAPS enables us to well visualize Rabi oscillations of 11.76 fs and quantum beats of 2.62 fs between the 5S 1/2 and 5P 3/2 in rubidium atoms. The numerical results show that the THz SAPS can simultaneously achieve high resolution in both frequency and time domains without the limitation of Heisenberg uncertainty of the probe pulse. The long probe pulse promises sufficiently high frequency resolution in photoelectron spectroscopy allowing to observe Autler–Townes splittings, whereas the streaking THz field enhances temporal resolution for not only Rabi oscillations but also quantum beats between the ground and excited states. The THz SAPS demonstrates a potential applicability for observation and manipulation of ultrafast coherence processes in frequency and time domains.

Femtosecond pump-probe studies of atomic hydrogen superfluorescence in flames

We investigate the excited-state dynamics of hydrogen (H) atoms in flames by using a femtosecond (fs) pump-probe scheme and measuring directional emission signals. An approximately 100-fs pump pulse at 205.1 nm excites H atoms through a two-photon process (n = 3 ←← n = 1), which is followed by detection of the forward emission signals induced by a broadband fs probe pulse near 656 nm. Above a certain threshold, we observe a quadratic dependence of the emission signal on the pump laser energy. Moreover, the linewidth of the forward emission signal varies with the probe delay and the probe laser energy. This behavior can be explained in terms of superradiance. We perform a theoretical analysis and compare the experimental results with the theory, and conclude that, within the conditions of our experiment, the behavior of the atomic system involves atomic coherence, which is produced non-adiabatically and corresponds to a superradiant process. Variations in the duration of the gain time window and lifetime of the excited-state H atoms in flames are explored at different flame conditions (i.e., equivalence ratio and heights above the burner). The fs pump-probe technique demonstrated here can also be extended to characterize the time-resolved population dynamics and the corresponding collisional energy transfer rates for energy levels involved in laser-induced fluorescence detection of H atoms in flames relevant to practical combustion applications.

Laser-induced alignment dynamics of gas phase CS2 dimers.

Rotational dynamics of gas phase carbon disulfide (CS2) dimers were induced by a moderately intense, circularly polarized alignment laser pulse and measured as a function of time by Coulomb explosion imaging with an intense fs probe pulse. For the alignment pulse, two different temporal intensity profiles were used: a truncated pulse with a 150 ps turn-on and a 8 ps turn-off, or a 'kick' pulse with a duration of 1.3 ps. For both types of pulse, rich rotational dynamics with characteristic full and fractional revivals were recorded, showing that the intermolecular carbon-carbon axis periodically aligns along the propagation direction of the laser pulses. The truncated pulse gave the strongest alignment, which we rationalize as being due to a flat relative phase between the components in the rotational wave packet generated. Fourier analysis of the alignment dynamics gave well-spaced peaks which were fit to determine the rotational constant, B, and the centrifugal constant, DJ, for the ground state of the dimer. Our results agree with values from high-resolution IR spectroscopy. Numerical simulations of the alignment accurately reproduced the experimental dynamics when the truncated pulse or a low intensity kick pulse was used, but failed to reproduce the dynamics induced by a high intensity kick pulse. We posit that the discrepancy is due to excitation of the intermolecular torsional motion by the kick pulse.

Study of various material particles by third harmonic generation method based on laser pulse induced plasma

Third harmonic generation (THG) from plasma can be used as probe radiation to characterize the plasma components. In this work, we present the study of the spatiotemporal profile of generated particles in various plasmas produced from bulk targets (C, Cr, In, Ni, Ag, CdS and ZnS) using pulsed laser ablation and THG using 800 nm, 40 fs probe pulses. The dependence of THG on the probe pulse intensity and the distance of the probe beam from the target position are analyzed. The influence of the ejected particle mass on THG with respect to the distance between the target and the probe pulses position was observed. The temporal evolution of THG depended on the delay between heating 6 ns pulses and driving 40 fs pulses. The variation of third harmonic intensity in the plasma components with atomic weight M on the delay (td) was consistent with the kinetic model described by relation td ∞ M0.5. THG from metal targets was mostly originated from the atomic and ionic components in plasma, whereas the clusters and heavier particles were responsible for this process in the case of ablated semiconductor materials.

Nonlinear optical characterization of copper oxide nanoellipsoids

Recently, nonspherical nanoparticles took attention due to advanced properties of these structures. We report the study of the nonlinear optical properties of copper oxide nanoellipsoids using 800 nm and 400 nm, 60 fs pulses. The optical limiting effect of copper oxide nanoellipsoids is analyzed. The influence of band gap of copper nanoparticles and copper oxide nanoellipsoids on their nonlinear optical response was studied. For the first time, the low- and high-order nonlinear optical responses of copper nanoellipsoids were studied. The magnitudes of nonlinear optical parameters of the suspension of copper oxide nanoellipsoids were measured to be γ = 1.23 × 10−15 cm2 W−1, and β = 1.0 × 10−11 cm W−1 respectively. We observed the four-fold enhancement of the nonlinear optical refraction of copper oxide nanoellipsoids at the wavelength of 400 nm, 60 fs probe pulses compared to 800 nm radiation. We also analyzed the high-order nonlinear response of CuO nanoellipsoids through generation of high-order harmonics of 800 nm, 60 fs pulses in the plasmas produced during laser ablation of the nanoellipsoid-contained targets. We demonstrated the harmonics up to the 35th order (E = 50 eV) in case of single-color pump and 24th (30 eV) in case of two-color pump.

Femtosecond Decay Dynamics of Intact Adenine and Thymine Base Pairs in a Supersonic Jet

We investigated the decay dynamics of the DNA base pairs adenine-adenine (A(2)), adenine-thymine (AT), and thymine-thymine (T(2)) produced in a supersonic jet by femtosecond (fs) time-resolved photoionization spectroscopy. The base pair was excited by a fs pump pulse at 267 nm and the population change of its excited state was monitored by non-resonant three-photon ionization using a fs probe pulse at 800 nm after a certain time delay. All of the transients recorded in the mass channel of the parent ion exhibited a tri-exponential decay, with time constants ranging from 100 fs to longer than 100 ps. Most of these time constants coincide well with the previous values deduced indirectly from the transients of protonated adenine (AH(+)) and thymine (TH(+)), which were assumed to be produced by fragmentation of the base-pair ions. Notably, for the transient of T(2), we observed a new decay component with a time constant of 2.3 ps, which was absent in the transient of TH(+). We suggest that the new decay component arises from the decay of stacked T(2) dimers that are mostly ionized to T(2)(+), whereas the decay signal recorded in the mass channel of TH(+) is merely from the relaxation of hydrogen-bonded T(2) dimers. From the amplitude of the new decay component, the population of the stacked T(2) dimers relative to the hydrogen-bonded dimers was estimated to be ∼2 % in the supersonic jet, which is about fifteen times higher than the theoretical value.

Three‐state model for femtosecond broadband stimulated Raman scattering

AbstractStimulated Raman scattering (SRS) is analyzed with a three‐state model. Using a diagrammatic density‐matrix formalism, SRS by a pair of Raman pump and probe pulses, with observation along the probe direction, is described principally by eight terms. The eight terms can be grouped into four sets, which are labeled as SRS or IRS (inverse Raman scattering): SRS(I), SRS(II), IRS(I), and IRS(II). Specializing to the case of femtosecond SRS (FSRS) by a picosecond (ps) Raman pump pulse and a femtosecond (fs) probe pulse, the spectra for the four sets of terms under off‐resonance and resonance conditions were calculated. The results obtained can explain the FSRS experimental observations from a (decaying) stationary vibrational state, such as (1) high wavenumber resolution (determined by the narrow bandwidth Raman pump pulse) and high time resolution (determined by the fs probe pulse), (2) Stokes gain vs anti‐Stokes loss in off‐resonance FSRS, and (3) dispersive lineshapes in resonance FSRS. Copyright © 2008 John Wiley & Sons, Ltd.

Read and write amplitude and phase information by using high-precision molecular wave-packet interferometry

We demonstrate an experimental approach to read and write populations and relative phases of vibrational eigenstates within a wave packet created in the $B$ state of the iodine molecule by using a pair of phase-locked femtosecond laser pulses. Our highly stabilized optical interferometer provides attosecond stability and resolution in the interpulse delay. The stability and resolution have realized an exquisite tuning of the interference of two vibrational wave packets to manipulate the relative populations and the relative quantum phases among the vibrational eigenstates within the wave packets. These populations and phases have been retrieved by measuring fluorescence from the upper $E$ state induced by another nanosecond (ns) or femtosecond (fs) probe laser pulse. The bandwidth of the ns probe pulse is narrow enough to select only a small portion of the rotational progression of a particular vibrational band of the $E\ensuremath{-}B$ transition. By scanning the probe wavelength, we measure the population distribution of the vibrational eigenstates within the wave packet. The fs probe pulse is used to measure quantum beats arising from the temporal evolution of the wave packet. Combining these two complementary measurements, we can read both population and phase information written and stored in the wave packet.

Imaging and control of interfering wave packets in a dissociating molecule.

Using two identical 110 femtosecond (fs) optical pulses separated by 310 fs, we launch two dissociative wave packets in I2. We measure the square of the wave function as a function of both the internuclear separation, /Psi(R)/(2), and of the internuclear velocity, /Psi(v(R))/(2), by ionizing the dissociating molecule with an intense 20 fs probe pulse. Strong interference is observed in both /Psi(R)/(2) and in /Psi(v(R))/(2). The interference, and therefore the shape of the wave function, is controlled through the phase difference between the two dissociation pulses in good agreement with calculations.

High-field electron transport in Al xGa 1− x as diodes investigated tracing ultrafast absorption changes

The topic of this contribution is the investigation of transient, nonequilibrium high-field electron transport in GaAs. Our two key ingredients are a unique laser system, which provides fs pump and probe pulses of independently tunable wavelengths and an AlGaAs hetero p–i–n diode with sophisticated design. This combination allows us to obtain time- and spatially resolved information about the temporal evolution of a photo-generated electron ensemble from ballistic to side-valley dominated transport, without disturbing hole contributions. Fields ranging from 7 to 180 kV/cm are investigated. Results are compared successfully to Monte Carlo Simulations.

Phase Relations, Quasicontinuous Spectra and Subfemtosecond Pulses in High-Order Stimulated Raman Scattering with Short-Pulse Excitation

High-order stimulated Raman scattering for pumping by ps and sub-ps pulses is studied in the frame of a time-domain approach without the slowly varying envelope approximation. Formation of pulse trains with sub-fs durations in the ps-pump regime is demonstrated using external phase compensation. For sub-ps excitation a novel broadening mechanism of Raman lines is predicted that leads to a quasicontinuous spectrum and permits one to generate single sub-fs pulses. We predict also essential shortening of a delayed fs-probe pulse by the Raman oscillations without phase control.

Vibrational and vibronic dynamics of large molecules in solution studied on a 20 fs timescale

The ultrafast dynamics of organic dye molecules in solution are studied in temporally and spectrally resolved experiments with 30 fs pump pulses resonant to short-wavelength S 0−S n transitions and 20 fs probe pulses overlapping with both the S 0−S 1 absorption and emission bands. Around zero delay, a pronounced coherent signal occurs with sign and amplitude depending on the spectral position within the probe pulses. The coherent coupling of the type electronic transitions is due to the coherent vibrational motion induced by an impulsive Raman-type excitation in the electronic ground state. The molecules promoted to S n states give rise to a fast ground state bleaching on a 50 fs timescale that is followed by the picosecond onset of stimulated emission after accumulation of the excited molecules at the bottom of the S 1 state.