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We show that a dramatic field-free molecular alignment can be achieved after exciting molecules with proper trains of strong ultrashort laser pulses. Optimal two- and three-pulse excitation schemes are defined, providing an efficient and robust molecular alignment. This opens new prospects for various applications requiring macroscopic ensembles of highly aligned molecules.

We introduce a new scheme for controlling the sense of molecular rotation. By varying the polarization and the delay between two ultrashort laser pulses, we induce unidirectional molecular rotation, thereby forcing the molecules to rotate clockwise/counterclockwise under field-free conditions. We show that unidirectionally rotating molecules are confined to the plane defined by the two polarization vectors of the pulses, which leads to a permanent anisotropy in the molecular angular distribution. The latter may be useful for controlling collisional cross-sections and optical and kinetic processes in molecular gases. We discuss the application of this control scheme to individual components within a molecular mixture in a selective manner.

When a wave is reflected from a moving object, its frequency is Doppler shifted 1 . Similarly, when circularly polarized light is scattered from a rotating object, a rotational Doppler frequency shift may be observed 2,3 , with manifestations ranging from the quantum world (fluorescence spectroscopy, rotational Raman scattering and so on 3,4 ) to satellite-based global positioning systems 5 . Here, we observe for the first time the Doppler frequency shift phenomenon for a circularly polarized light wave propagating through a gas of synchronously spinning molecules. An ensemble of such spinning molecules was produced by double-pulse laser excitation, with the first pulse aligning the molecules and the second (linearly polarized at a 458 8 8 8 8 angle) causing a concerted unidirectional rotation of the 'molecular propellers' 6,7 . We observed the resulting rotating birefringence of the gas by detecting a Doppler-shifted wave that is circularly polarized in a sense opposite to that of the incident probe.In his famous 1905 paper on special relativity 8 , Einstein derived the frequency shift Dv for linearly polarized light of frequency v reflected from a mirror moving with speed n and showed that in the non-relativistic limit Dv ¼+2kv, where k ¼ v/c, c is the speed of light, and the sign depends on the relative direction of motion. When an anisotropic polarizable object rotates with angular velocity V, a rotational Doppler frequency shift may be observed in the scattering of a circularly polarized (CP) electromagnetic wave. The scattered field consists of a CP component with the same frequency and handedness as the incident one and a CP wave of the opposite handedness 9-13 with a frequency shift of Dv ¼+2V, where the sign depends on the relative sense of rotation. To date, table-top observations of this phenomenon have made use of mechanical rotation of anisotropic optical elements 10,11,14 and electro-optic effects in a nonlinear crystal subject to a rotating microwave electric field 15 . In the present study, we observe the rotational Doppler frequency shift (RDS) from molecules rotating unidirectionally at terahertz frequencies, which is many orders of magnitude larger than that observed in mechanically rotated systems.We induce molecular unidirectional rotation (UDR) by applying two time-delayed, ultrashort, linearly polarized laser pulses (Fig. 1). This two-pulse technique was demonstrated by Kitano et al. 7 and subsequently generalized [16][17][18] . Although UDR persists for as long as the molecules do not collide, the field-free anisotropy of the angular distribution gradually disappears because of angular velocity dispersion. The cigar-like shape of the angular distribution reappears periodically because of quantum revival of the rotational wave packets 19-21 with a revival period of T rev ¼ 1/(2Bc), where B is the rotational constant. Substantial anisotropy of the angular distribution is also observed at fractions of T rev , especially near T rev /2. Probing the ensemble at full or fractional revival enables...

Spectroscopy aims at extracting information about matter through its interaction with light. However, when performed on gas and liquid phases as well as solid phases lacking long‐range order, the extracted spectroscopic features are in fact averaged over the molecular isotropic angular distributions. The reason is that light–matter processes depend on the angle between the transitional molecular dipole and the polarization of the light interacting with it. This understanding gave birth to the constantly expanding field of “laser‐induced molecular alignment”. In this paper, we attempt to guide the readers through our involvement (both experimental and theoretical) in this field in the last few years. We start with the basic phenomenon of molecular alignment induced by a single pulse, continue with selective alignment of close molecular species and unidirectional molecular rotation induced by two time‐delayed pulses, and lead up to novel schemes for manipulating the spatial distributions of molecular samples through rotationally controlled scattering off inhomogeneous fields and surfaces.

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Trains of ultrashort laser pulses separated by the time of rotational revival (typically, tens of picoseconds) have been exploited for creating ensembles of aligned molecules. In this work we introduce a chiral pulse train--a sequence of linearly polarized pulses with the polarization direction rotating from pulse to pulse by a controllable angle. The chirality of such a train, expressed through the period and direction of its polarization rotation, is used as a new control parameter for achieving selectivity and directionality of laser-induced rotational excitation. The method employs chiral trains with a large number of pulses separated on the time scale much shorter than the rotational revival (a few hundred femtosecond), enabling the use of conventional pulse shapers.

Semiclassical catastrophes in the dynamics of a quantum rotor (molecule) driven by a strong time-varying field are considered. We show that for strong enough fields, a sharp peak in the rotor angular distribution can be achieved via a time-domain focusing phenomenon, followed by the formation of rainbowlike angular structures. A strategy leading to the enhanced angular squeezing is proposed that uses a specially designed sequence of pulses. The predicted effects can be observed in many processes, ranging from molecular alignment (orientation) by laser fields to heavy-ion collisions, and the trapping of cold atoms by a standing light wave.