Manipulating external degrees of freedom with intense light: Laser focusing and trapping of molecules

Scattering of adiabatically aligned molecules by nonresonant optical standing waves

We study the effect of rotational state-dependent alignment in the scattering of molecules by optical fields. CS₂ molecules in their lowest few rotational states are adiabatically aligned and transversely accelerated by a nonresonant optical standing wave. The width of the measured transverse velocity distribution increases to 160 m/s with the field intensity, while its central peak position moves from 10 to -10 m/s. These changes are well reproduced by numerical simulations based on the rotational state-dependent alignment but cannot be modeled when ignoring these effects. Moreover, the molecular scattering by an off-resonant optical field amounts to manipulating the translational motion of molecules in a rotational state-specific way. Conversely, our results demonstrate that scattering from a nonresonant optical standing wave is a viable method for rotational state selection of nonpolar molecules.

Rotational-State-Dependent Dispersion of Molecules by Pulsed Optical Standing Waves

We report on the rotational-state-dependent, transverse acceleration of CS_{2} molecules affected by pulsed optical standing waves. The steep gradient of the standing wave potential imparts far stronger dipole forces on the molecules than propagating pulses do. Moreover, large changes in the transverse velocities (i.e., up to 80  m/s) obtained with the standing waves are well reproduced in numerical simulations using the effective polarizability that depends on the molecular rotational states. Our analysis based on the rotational-state-dependent effective polarizability can therefore serve as a basis for developing a new technique of state selection for both polar and nonpolar molecules.

Predicted ordered assembly of ethylene molecules induced by polarized off-resonance laser pulses

We illustrate a new phenomenon in the dynamics of molecular ensembles subjected to moderately intense, far-off-resonance laser fields, namely, field-driven formation of perfectly ordered, defect-free assembly. Interestingly, both the arrangement of the constituting molecules within the individual assembly and the long-range order of the assembly with respect to one another are subject to control through choice of the field polarization. Relying on strong induced dipole-induced dipole interactions that are established in dense molecular media, the effect is expected to be general.

Electric deflection of rotating molecules

We provide a theory of the deflection of polar and nonpolar rotating molecules by inhomogeneous static electric field. Rainbowlike features in the angular distribution of the scattered molecules are analyzed in detail. Furthermore, we demonstrate that one may efficiently control the deflection process with the help of short and strong femtosecond laser pulses. In particular, the deflection process may be turned off by a proper excitation, and the angular dispersion of the deflected molecules can be substantially reduced. We study the problem both classically and quantum mechanically, taking into account the effects of strong deflecting field on the molecular rotations. In both treatments we arrive at the same conclusions. The suggested control scheme paves the way for many applications involving molecular focusing, guiding, and trapping by inhomogeneous fields.

Molecular focusing and alignment with plasmon fields

We show the possibility of simultaneously aligning molecules and focusing their center-of-mass motion near a metal nanoparticle in the field intensity gradient created by the surface plasmon enhancement of incident light. The rotational motion is described quantum mechanically while the translation is treated classically. The effects of the nanoparticle shape on the alignment and focusing are explored. Our results carry interesting implications to the field of molecular nanoplasmonics and suggest several potential applications in nanochemistry.

Deflection of field-free aligned molecules

We consider deflection of polarizable molecules by inhomogeneous optical fields, and analyze the role of molecular orientation and rotation in the scattering process. We show that by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. This opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.

Optimization of laser-focused deposition lines: Rydberg atoms

Optimally narrow nanoscale lines are computationally obtained for Rb Rydberg atoms deposited on surfaces. The use of optimized polychromatic fields is shown to allow lines as narrow as 1 nm in the absence of transverse velocities and shown to counter the deleterious effects of transverse velocities in laser cooled beams. Specifically, lines as narrow as 6.5 nm wide are obtained in the presence of transverse velocities associated with a temperature of 1 mK. Using this approach it is possible to deposit a single narrow line, even when the atomic beam is bigger than the period of the focusing lens, using as few as two, relatively weak, laser fields.

Ultrafast noncollinear optical parametric chirped pulse amplification in KTiOAsO4

Amplification of femtosecond pulses at 1.56 microm based on noncollinear parametric chirped pulse amplification in a potassium titanyl arsenate (KTA) crystal with pumping at 1.05 microm is reported. The 100 fs pulses of an erbium fiber laser are parametrically amplified while synchronously pumped by an amplified mode-locked Nd:YLF laser. This amplifier has a saturated gain of 65 dB with 30% conversion efficiency and has produced 160 fs pulses with peak powers of up to 0.75 GW. The system produced 380 mW before compression and can be readily scaled to the multiwatt range with bandwidths to support sub-100 fs pulses.

Separations based on the mechanical forces of light

A photon as a particle has an energy and a momentum. In a matter-photon interaction, the matter and photons may exchange their momenta observing the momentum conservation law. The consequence of the momentum transfer from a photon to a matter particle is a mechanical force exerted on the particle. Several separation methods based on this force of light are reviewed. Photophoresis separations for micron-sized particles and optical force chromatography for chemical-sized molecules are discussed.

Alignment of molecules in pulsed resonant laser fields

We investigate by numerical simulations the dynamics of alignment of linear molecules in resonant pulsed laser fields and its dependence on pulse length, field strength, and molecular parameters. We propose an analytical short-time approximation for the time-dependent wave packets. We provide a theoretical basis for the occurrence of saturation in the rotational pumping. We present a formula to predict the time at which the maximum alignment occurs. We discuss the magnitude of the laser-induced alignment and we relate it to a theoretical upper limit.

Active control of the dynamics of atoms and molecules

There has been much progress in the control of chemical reactions since methods of active control were first proposed by Brumer & Shapiro and by Tannor & Rice ten years ago. This chapter reviews both theoretical and experimental advances in the field. Control schemes based on quantum mechanical interference between competing paths and the manipulation of wave packets with tailored laser pulses are discussed. The theory of optimal control, the limitations of control theory applied to many-body dynamics, and the effects of constraints on the trajectory of the controlled observable are presented. Experimental progress in controlling the population of specific quantum states, in manipulating the dynamics of bound wave packets, and in the control of chemical reactions are reviewed, and current problems in the field are summarized.

Molecular alignment by trains of short laser pulses

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.

Angular focusing, squeezing, and rainbow formation in a strongly driven quantum rotor

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.

Coherently controlled nanoscale molecular deposition

Quantum interference effects are shown to provide a means of controlling and enhancing the focusing of a collimated neutral molecular beam onto a surface. The nature of the aperiodic pattern formed can be altered by varying laser field characteristics and the system geometry.

Molecular lens of the nonresonant dipole force

A cylindrical molecular lens is formed by focusing a nanosecond IR laser pulse. Trajectories of a CS2 molecular beam deflected by the lens are traced using the velocity map imaging technique. The characteristic lens parameters including the focal length, minimum beam width, and distance to the minimum-width position are determined. The laser intensity dependence of the parameters is in good agreement with theoretical predictions. Exciting possibilities for molecular optics and a new type of optical chromatography are opened up.