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.

We experimentally investigate the effect of quantum resonance in the rotational excitation of the simplest quantum rotor--a diatomic molecule. Using the techniques of high-resolution femtosecond pulse shaping and rotational state-resolved detection, we measure directly the amount of energy absorbed by molecules interacting with a periodic train of laser pulses, and study their dependence on the train period. We show that the energy transfer is significantly enhanced at quantum resonance, and use this effect to demonstrate selective rotational excitation of two nitrogen isotopologs, (14)N(2) and (15)N(2). Moreover, by tuning the period of the pulse train in the vicinity of a fractional quantum resonance, we achieve selective rotational excitation of para- and ortho-isomers of (15)N(2).

We provide a detailed theoretical analysis of molecular rotational excitation by a chiral pulse train -- a sequence of linearly polarised pulses with the polarisation direction rotating from pulse to pulse by a controllable angle. Molecular rotation with a preferential rotational sense (clockwise or counter-clockwise) can be excited by this scheme. We show that the directionality of the rotation is caused by quantum interference of different excitation pathways. The chiral pulse train is capable of selective excitation of molecular isotopologues and nuclear spin isomers in a mixture. We demonstrate this using 14N2 and 15N2 as examples for isotopologues, and para- and ortho-nitrogen as examples for nuclear spin isomers

We show, both classically and quantum mechanically, enantioselective orientation of gas phase chiral molecules excited by laser fields with twisted polarization. Counterintuitively, the induced orientation, whose direction is laser controllable, does not disappear after the excitation, but stays approximately constant long after the end of the laser pulses, behavior unique to chiral systems. We computationally demonstrate this long-lasting orientation using propylene oxide molecules (CH3CHCH2O, or PPO) as an example, and consider two kinds of fields with twisted polarization: a pair of delayed cross-polarized pulses, and an optical centrifuge. This novel chiral effect opens new avenues for detecting molecular chirality, measuring enantiomeric excess and separating enantiomers with the help of inhomogeneous external fields.

Molecular chirality is an omnipresent phenomenon of fundamental significance in physics, chemistry and biology. For this reason, search for novel techniques for enantioselective control, detection and separation of chiral molecules is of particular importance. It has been recently predicted that laser fields with twisted polarization may induce persistent enantioselective field-free orientation of chiral molecules. Here we report the first experimental observation of this phenomenon using propylene oxide molecules (CH 3 CHCH 2 O, or PPO) spun by an optical centrifuge -a laser pulse, whose linear polarization undergoes an accelerated rotation around its propagation direction. We show that PPO molecules remain oriented on a time scale exceeding the duration of the centrifuge pulse by several orders of magnitude. The demonstrated long-time field-free enantioselective orientation opens new avenues for optical manipulation, discrimination, and, potentially, separation of molecular enantiomers.

The paper explores the prospects of observing the phenomenon of dynamical Anderson localisation via non-resonant Raman-type rotational excitation of molecules by periodic trains of short laser pulses. We define conditions for such an experiment, and show that current femtosecond technology used for non-adiabatic laser alignment of linear molecules is sufficient for this task. Several observables which can serve as indicator for Anderson localisation are suggested for measurement, and the influence of experimental limitations imposed by laser intensity noise, finite pulse duration, limited number of pulses in a train, and thermal effects is analysed.PACS numbers: 05.45. Mt, 37.10.Vz, 42.65.Re By the use of Eq. (8), one can express the one-cycle evolution (10) as (compare with [3, 8]) T (α) J u (α,M) J + J ′ W (M) J,J ′ u (α,M) J ′ (M) J,J ′ is given as C (M) J

We describe a universal behavior of linear molecules excited by a periodic train of short laser pulses under quantum resonance conditions. In a rigid rotor, the resonance causes an unlimited ballistic growth of the angular momentum. We show that the centrifugal distortion of rotating molecules eventually halts the growth, by causing Anderson localization beyond a critical value of the angular momentum--the Anderson wall. Its position solely depends on the molecular rotational constants and lies in the range of a few tens of ℏ. Below the wall, rotational excitation oscillates with the number of pulses due to a mechanism similar to Bloch oscillations in crystalline solids. We suggest optical experiments capable of observing the rotational Anderson wall and Bloch oscillations at near-ambient conditions with the help of existing laser technology.

The periodically kicked quantum rotor 1 is known for non-classical effects such as quantum localisation in angular momentum space [2][3][4] or quantum resonances in rotational excitation 2,5 . These phenomena have been studied in diverse systems mimicking the kicked rotor, such as cold atoms in optical lattices 6,7 , or coupled photonic structures [8][9][10] . Recently, it was predicted -may manifest themselves in the rotational dynamics of laser-kicked molecules. Here, we report the first observation of rotational Bloch oscillations in a gas of nitrogen molecules kicked by a periodic train of femtosecond laser pulses. A controllable detuning from the quantum resonance creates an effective accelerating potential in angular momentum space, inducing Bloch-like oscillations of the rotational excitation. These oscillations are measured via the temporal modulation of the refractive index of the gas. Our results introduce room-temperature laser-kicked molecules as a new laboratory for studies of localisation phenomena in quantum transport. Bloch oscillations present one of the most famous and intriguing quantum localisation effects in solid state physics: Electrons in crystalline solids subject to an external dc electric field exhibit oscillatory motion instead of a mere uniform acceleration as in empty space. This effect was predicted at the inception of quantum mechanics in 1929 15,16 , but it took more than 60 years to first observe Bloch oscillations in semiconductor superlattice structures 19 . Bloch oscillations were later observed in the momentum distribution of ultracold atoms driven by a constant force 20 . Recently, it was proposed to induce Bloch oscillations in the rotation of linear molecules excited by periodic trains of short nonresonant laser pulses 13 . This proposal is based on an analogy between coherent rotational excitation caused by the laser pulses and propagation of an electron in a one-dimensional periodic lattice.A non-resonant, linearly polarised short laser pulse acts as a kick to a molecule. It excites a rotational wave packet via multiple coherent Raman-type interactions 21,22 : |Ψ (t) = J C J (t)|J , where |J are the angular momentum states, and the projection quantum number M is dropped since it is not changing. Under field-free conditions, the coefficients C J (t) oscillate as exp[−iBJ(J + 1)t/ ], where B is the molecular rotational constant. The dynamics of the wave packet are determined by a single parameter, the rotational revival time t rev = π /B. The wave packet revives exactly at integer multiples of the revival time 23 when all the timedependent phase factors become equal to unity.Consider a train of laser kicks with a constant time delay τ between the pulses. The time delay is chosen to be slightly detuned from the rotational revival time: τ = (1 + δ)t rev . Due to the detuning, each component of the rotational wave packet acquires a small phase φ J = πδJ(J + 1) from pulse to pulse (integer multiples of 2π are dropped). From now on, we follow the dynamics stroboscopically, by ...