Molecular Alignment Revivals Research Articles

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.

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.

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.

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.

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.

Control of femtosecond filamentation by field-free revivals of molecular alignment

We show that the filamentation dynamics of a femtosecond laser probe pulse can be readily controlled by properly matching it to the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation with an advancing ultrashort pump pulse. Several features of the filamentation process including supercontinuum generation, the length of the plasma channel generated in the wake of the filament, the associated secondary radiations and the multiple filamentation pattern are all easily modified by tuning the cross phase modulation induced by the field-free revivals of molecular alignment, through the delay between the pump and the probe pulses. We show that molecular alignment can also be used to generate conical waves with extremely short intensity spike called shocked X-waves and to further tune the frequency of a few-cycle laser pulse in the wake of a self-guided intense filament.

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.

Quantum logic in coarse grained control of wavepackets

We describe ‘coarse-grained’ control of wavepackets with finite resolution in phase space and show how this control can be approached as quantum information processing. To this end, we introduce the wavepacket analogues of qubits, designed to represent the information recorded within a wavepacket, and show how they can be addressed within a single wavepacket. The control is implemented both for non-dispersive wavepacket propagation using linear optical elements, illustrated for the classical waves, and for the dispersive evolution using fractional wavepacket revivals and AC Stark shifts as control tools, illustrated for the revivals of molecular alignment.