Alignment Revivals Research Articles

Non-Markovian collisional dynamics probed with laser-aligned molecules

The Markov, as well as the secular, approximations are key assumptions that have been widely used to model decoherence in a large variety of open quantum systems, but, as far as intermolecular collisions are considered, very little has been done in the time domain. In order to probe the limits of both approximations, we here study the influence of pressure on the alignment revivals (echoes) created in properly chosen gas mixtures (HCl and ${\mathrm{CO}}_{2}$, pure and diluted in He) by one (two) intense and short laser pulse(s). Experiments and direct predictions using molecular-dynamics simulations consistently demonstrate, through analyses at very short times $(<15 \mathrm{ps})$ after the laser kick(s), the breakdown of these approximations in some of the selected systems. We show that the nonadiabatic laser-induced molecular alignment technique and model used in this paper directly provide detailed information on the physical mechanisms involved in the collisional dissipation. Besides this ``fundamental'' interest, our findings also have potential practical applications for radiative heat transfer in planetary atmospheres and climate studies. Indeed, short time delays in the dipole autocorrelation function monitoring the light absorption spectrum correspond to large detunings from the optical resonances in the frequency domain, thus influencing the atmospheric transparency windows. Furthermore, the fact that the approach tested here for linear rotors can potentially be applied to almost any gas mixture (including, for instance, nonlinear and/or reacting molecules) further strengthens and broadens the perspectives that it opens.

Investigation of the limits of the secular and Markov approximations in the dissipative dynamics of laser-aligned molecules

We point out the breakdown of two approximations widely used to describe decoherence in open quantum systems, the secular and Markov approximations. We probe their limits by studying the influence of pressure on the alignment revivals (echoes) created in properly chosen gas mixtures (HCl and CO2, pure and diluted in He) by one (two) intense and short laser pulse(s). Experiments, as well as predictions using molecular dynamics simulations, consistently demonstrate in some of the aforementioned systems the break-down of these approximations at very short times (<15 ps) after the laser kick(s).

All-optical measurement of high-order fractional molecular echoes by high-order harmonic generation.

An all-optical measurement of high-order fractional molecular echoes is demonstrated by using high-order harmonic generation (HHG). Excited by a pair of time-delayed short laser pulses, the signatures of full and high order fractional (1/2 and 1/3) alignment echoes are observed in the HHG signals measured from CO 2 molecules at various time delays of the probe pulse. By increasing the time delay of the pump pulses, much higher order fractional (1/4) alignment echo is also observed in N 2O molecules. With an analytic model based on the impulsive approximation, the spatiotemporal dynamics of the echo process are retrieved from the experiment. Compared to the typical molecular alignment revivals, high-order fractional molecular echoes are demonstrated to dephase more rapidly, which will open a new route towards the ultrashort-time measurement. The proposed HHG method paves an efficient way for accessing the high-order fractional echoes in molecules.

Development of a plasma shutter applicable to 100-mJ-class, 10-ns laser pulses and the characterization of its performance.

For the purpose of preparing a sample of aligned and oriented molecules in the laser-field-free condition, we developed a plasma shutter, which enables laser pulses with 100-mJ-class, 10-ns pulse durations to be rapidly turned off within ∼150 fs. Inthis work, the residual field intensity after the rapid turn off is carefully examined by applying the shaped laser pulse to OCS molecules in the rotational ground state. Based on the comparison between the observation of alignment revivals of the OCS molecules and the results of numerical simulations, we demonstrate that the residual field intensity is actually negligible (below 0.4% of the peak intensity) and, if any, does not influence the alignment and orientation dynamics at all.

Collisional dissipation of the laser-induced alignment of ethane gas: A requantized classical model.

We present the first theoretical study of collisional dissipation of the alignment of a symmetric-top molecule (ethane gas) impulsively induced by a linearly polarized non-resonant laser field. For this, Classical Molecular Dynamics Simulations (CMDSs) are carried out for an ensemble of C2H6 molecules based on knowledge of the laser-pulse characteristics and on an input intermolecular potential. These provide, for a given gas pressure and initial temperature, the orientations of all molecules at all times from which the alignment factor is directly obtained. Comparisons with measurements show that these CMDSs well predict the permanent alignment induced by the laser pulse and its decay with time but, as expected, fail in generating alignment revivals. However, it is shown that introducing a simple requantization procedure in the CMDS "creates" these revivals and that their predicted dissipation decay agrees very well with measured values. The calculations also confirm that, as for linear molecules, the permanent alignment of ethane decays more slowly than the transient revivals. The influence of the intermolecular potential is studied as well as that of the degree of freedom associated with the molecular rotation around the symmetry axis. This reveals that ethane practically behaves as a linear molecule because the intermolecular potential is only weakly sensitive to rotation around the C-C axis.

Time‐domain measurement of pure rotational Raman collisional linewidths of ethane C2H6

AbstractThe pure rotational Raman linewidths of the symmetric‐top molecule C2H6 are determined from a time‐resolved femtosecond experiment at room temperature and moderate pressure in a pump‐probe scheme. After excitation by a strong 100 fs‐duration nonresonant near infrared pump pulse, the molecular sample exhibits periodic post‐pulse alignment revivals that produce an anisotropic angular distribution of the molecular axis. Such molecular motion induces in turn a time‐dependent optical birefringence, which can be probed using a weak time‐delayed probe pulse. The Fourier transform of the temporal signal, recorded over a wide time delay range (several hundred picoseconds) for high frequency resolution, is directly related to the rotational Raman spectrum of ethane molecules. The Raman lines are analyzed with a frequency lineshape taking into account the collisional relaxation and the apparatus function. The linewidths are measured for rotational quantum numbers J=3 to J=30 in the pressure range 0.69–1.06 bar. A decrease of the collisional broadening coefficient is observed as J increases. These first measurements of pure rotational Raman linewidths of ethane are compared with previously reported infrared and far‐infrared values.

Manifestations of induced molecular alignment in the interaction of ultrafast laser pulses with gases

The dynamics of rotational wave packets of laser-aligned linear molecules was studied with femtosecond laser pulses in a pump-probe configuration. The alignment produced with a pump beam creates a transiently anisotropic medium, which exhibits birefringence and also a dependence of ionization on the mutual orientation of the polarizations of the probe and pump beams. The induced molecular alignment was observed via variations of the yields of photoelectrons in strong field ionization. In addition, the induced transient birefringence due to alignment revivals altered the nonlinear interaction of the probe beam with the gas, resulting in delay dependent modulation of the probe beam. The revival signatures measured for N2, CO and C2H2 gas molecules were in good agreement with the calculated molecular alignments taking into account the effects of electronic structure and molecular symmetry.

Rotational Dynamics of Quantum State-Selected Symmetric-Top Molecules in Nonresonant Femtosecond Laser Fields.

Rotational dynamics of quantum state selected and unselected CH3I molecules in intense femtosecond laser fields has been studied. The orientation and alignment evolutions are derived from a pump-probe measurement and in good agreement with the numerical results from the time-dependent Schrödinger equation (TDSE) calculation. The different rotational transitions through nonresonant Raman process have been assigned from the Fourier analysis of the orientation and alignment revivals. These revivals are derived from a pump-probe measurement and in good agreement with the numerical results from the TDSE calculation. For the molecules in rotational state |1, ±1, ∓1⟩, the transitions can be assigned to ΔJ = ±1, ±2, while for thermally populated molecules, the transitions are ΔJ = ±2. Our results illustrate that the orientation and alignment revivals of the rotational quantum-state-selected molecules give a deep insight into the rotational excitation pathways for the transition of different rotational states of molecules in ultrafast laser fields.

Spectral modulation of third-harmonic generation by molecular alignment and preformed plasma**Project supported by the National Key Scientific Instrument Project, China (Grant No. 2012YQ150092), the National Basic Research Program of China (Grant No. 2011CB808105), the National Natural Science Foundation of China (Grant No. 11434005), China Postdoctoral Science Foundation (Grant No. 2014M560348), the National Natural Science Foundation of China (Grant No. 11504237),

We demonstrate spectral modulation of third-harmonic generation from molecular alignment effects. The third harmonic spectrum is broadened or narrowed under different influences of cross-phase modulations originating from various molecular alignment revivals. Furthermore, the spectrum and spatial distribution of the generated third harmonic pulse change dramatically in the presence of a preformed plasma. Under the influence of a preformed plasma, a narrower third harmonic spectrum is observed, and the conical third-harmonic pulse increases while the axial part decreases. The investigation provides an effective method to modulate the spectral characteristic and spatial distribution of third-harmonic generation from intense femtosecond filament.

Field free alignment of hexapole state selected methyliodide

SynopsisIn this contribution we will discuss the field free laser induced spatial alignment of hexapole quantum state selected methyl iodide. We will demonstrate that we have control over the spatial alignment and can chose ‹P2(cos θ)› between 0.7 and -0.1 coupled to ‹P4(cos θ)› between 0.3 and 0.0. The effect of the relative orientation between a static electric field and a strong laser field on the structure of the field free rotational alignment revivals will be discussed.

Molecular alignment induced ultraviolet femtosecond pulse modulation

Ultrashort ultraviolet pulses at 267 nm were generated through four-wave mixing processes in dual-color femtosecond filaments. The generated ultrashort ultraviolet pulses experienced intensity modulation due to molecular-alignment-induced spatial (de)focusing effects, and the output beam profiles were shown to vary observably at different molecular alignment revivals, facilitating all-optical field-free and ultrafast spatial shaping and profile optimization of the ultraviolet pulses.

Mechanisms of Two-Color Laser-Induced Field-Free Molecular Orientation

Two mechanisms of two-color (ω+2ω) laser-induced field-free molecular orientation, based on the hyperpolarizability and ionization depletion, are explored and compared. The CO molecule is used as a computational example. While the hyperpolarizability mechanism generates small amounts of orientation at intensities below the ionization threshold, ionization depletion quickly becomes the dominant mechanism as soon as ionizing intensities are reached. Only the ionization mechanism leads to substantial orientation (e.g., on the order of <cosθ> ≳0.1). For intensities typical of laser-induced molecular alignment and orientation experiments, the two mechanisms lead to robust, characteristic timings of the field-free orientation wave-packet revivals relative to the alignment revivals and the revival time. The revival timings can be used to detect the active orientation mechanism experimentally.

Field-free molecular alignment detection by 4f coherent imaging

Time-resolved detection of field-free molecular alignment is investigated by phase contrast. The technique based on a 4f imager operating as an inverted Zernike spatial filter makes it possible to discriminate between positive and negative molecular alignment revivals produced in a linear molecule. The measurements are performed in a way that minimizes the contamination of the signal by the plasma generated during the aligning pulse. The observations are supported by a semi-analytical model, from which the degree of alignment produced at the beam focus is estimated.

All-optical ultrafast polarization switching of terahertz radiation by impulsive molecular alignment

We experimentally demonstrate ultrafast polarization switching of terahertz (THz) radiation generated by dual-color driving pulses composed of orthogonally polarized fundamental and second-harmonic waves, which can be controlled by field-free molecular alignment in air by modulating the relative phase between the two field components as a transient dynamic wave plate. By fine-tuning the time delay to properly match the molecular alignment revivals, a significant polarization modulation of the THz radiation is observed and both linearly and elliptically polarized THz radiations can be obtained.

Nonadiabatic alignment of van der Waals--force-bound argon dimers by femtosecond laser pulses

We demonstrated that the weak van der Waals–force-bound argon dimer can be nonadiabatically aligned by nonresonant femtosecond laser pulses, showing periodic alignment and anti-alignment revivals after the extinction of the laser pulse. Based on the measured nonadiabatic alignment trace, the rotational constant of the argon dimer ground state is determined to be B0 = 0.05756 ± 0.00004 cm −1 . Noticeable alignment dependence of frustrated tunneling ionization and bond-softening induced dissociation of the argon dimer are observed.

THz generation by a two-color pulse in prealigned molecules

We demonstrate that the THz generation in air from a two-color pulse, composed of the fundamental and second-harmonic waves, can be coherently controlled by field-free molecular alignment. By tuning its time delay to properly match various molecular alignment revivals, the THz generation from the two-color pulse can be promoted or decreased due to the spatial cross-(de)focusing effect and the alignment-dependent ionization probabilities of the prealigned diatomic molecules in air. For the two-color pulse of orthogonally polarized fundamental and second-harmonic waves, the polarization of the generated THz radiation can be controlled by the field-free molecular alignment, which functions as a transient dynamic wave plate for the two-color pulse with different phase velocities for the orthogonally polarized field components. The plasma effect on the THz generation of the two-color pulse is also observed, leading to additional amplitude and polarization control of the THz radiation.

Elongation of femtosecond filament by molecular alignment in air

We study the influence of the molecular alignment on the plasma channel length of femtosecond filament at 800 nm in air. The filament length is observed to be nearly doubled when the high-energy femtosecond laser pulse is properly tuned to match the perpendicular revivals of the molecular alignment, which is different from the low-energy femtosecond laser pulse where the filament is promoted for parallel molecular alignment revivals. These are understood to be the loosened or tightened focusing condition of the propagation of the femtosecond laser pulse by the cross-(de)focusing effect from the prealigned molecules.

Interaction of two parallel femtosecond filaments at different wavelengths in air

We study the interaction of two parallel-launched femtosecond filaments at their fundamental-wave and second-harmonic frequencies in air. Besides the Kerr effect within the pulse duration, the impulsive alignment of the diatomic molecules in the fundamental-wave filament leads to controllable and field-free achievable attraction, fusion, or repulsion of the second-harmonic filament. The molecular-alignment-assisted filament interaction is further confirmed by the fluorescence intensity variation and spectral modulation of the second-harmonic filament at various molecular alignment revivals.

Spectral modulation of femtosecond laser pulse induced by molecular alignment revivals

We demonstrate experimentally and numerically that the quantum wake of molecular alignment induced by impulsive rotational Raman excitations with femtosecond pump pulses produces observable phase modulations on femtosecond probe pulses. This leads to a spectral red- or blueshift of the probe pulses when they are properly delayed around the rising or falling edge of the half-revival time of the molecular alignment, respectively.

Femtosecond filamentation and pulse compression in the wake of molecular alignment

We numerically demonstrate that femtosecond filamentation in a molecular gas can be controlled in the wake of molecular alignment following impulsive rotational Raman excitation with a weak pump pulse. The succeeding filamentation dynamics of the collinearly propagating probe pulse is significantly influenced by the spatiotemporally modulated refractive index at the revivals of molecular alignment. The length of the filament generated by the probe pulse can be efficiently increased to obtain an ultrabroad spectrum and a few-cycle pulse via self-compression by tuning its delay to match the molecular alignment revivals.