Control with ultrashort pulses

With the concept of molecular coherence well established, we wrote a review making the point that with ultrashort pulses we should be able to control... 

Control of chemical reactions with ultrashort pulses... 

NONADIABATIC ROTATIONAL CONTROL WITH TRAINS OF ULTRASHORT LASER PULSES... 

S. Zhdanovich, A. A. Milner, C. Bloomquist, J. Floss, 1. Sh. Averbukh, J. W. Hepburn, and V. Milner. Control of molecular rotation with a chiral train of ultrashort pulses. Phys. Rev. Lett., 107(24) 243004 (2011). 

Organic molecules have useful optical and electronic functions that can be easily controlled by the structure, substituent, or external fields. Molecular interactions and organized molecular assemblies also can afford much higher functions than isolated or randomly distributed molecules. Photons have many superior properties such as wavelength, polarization, phase, ultrashort pulse, or parallel processability. Through interactions of molecules or molecular assemblies with photons, many properties of photons can be directly converted to changes in physical properties of materials such as fluorescence, absorption,... 

Coherent control Control of the motion of a microscopic object by using the coherent properties of an electromagnetic held. Coherent phase control uses a pair of lasers with long pulse durations and a well-defined relative phase to excite the target by two independent paths. Wave packet control uses tailored ultrashort pulses to prepare a wave packet at a desired position at a given time. 

Selective excitation of wavepackets with ultrashort broadband laser pulses is of fundamental importance for a variety of processes, such as the coherent control of photochemical reactions [36-39] or isotope separation [40--42]. It can also be used to actively control the molecular dynamics in a dissipative environment if the excitation process is much faster than relaxation. For practical applications it is desirable to establish an efficient method that allows one to increase the target product yield by using short laser pulses of moderate intensity before relaxation occurs [38]. 

Most of the pump control experiments carried out so far have used diatomic molecules, because in such simple systems, with only one vibrational degree of freedom, the dynamics can be controlled relatively easily. In larger molecular systems with three or more vibrational degrees of freedom, the situation becomes much more complicated and it is an interesting question whether the concept of controlled molecular dynamics can still be realized. Here, it is shown that different vibrational modes of the sodium trimer can be selectively excited during an electronic excitation with ultrashort laser pulses. For this reason, it should in future be possible to control subsequent reactions. The relevant control parameter in these investigations is the duration of the pump pulse. 

V. Blanche , M.A. Bouchene, and B. Girard, Coherent Control and Molecular Interferometry with Ultrashort Laser Pulses in CS2 in Femtochemistry Ultrafast Chemical and Physical Processes in Molecular Systems, M. Chergui (ed.) (World Scientific, Singapore, 1996). 

Two main approaches to the control of molecules using wave interference in quantum systems have been proposed and developed in different languages . The first approach (Tannor and Rice 1985 Tannor et al. 1986) uses pairs of ultrashort coherent pulses to manipulate quantum mechanical wave packets in excited electronic states of molecules. These laser pulses are shorter than the coherence lifetime and the inverse rate of the vibrational-energy redistribution in molecules. An ultrashort pulse excites vibrational wave packets, which evolve freely until the desired spacing of the excited molecular bond is reached at some specified instant of time on a subpicosecond timescale. The second approach is based on the wave properties of molecules as quantum systems and uses quantum interference between various photoexcitation pathways (Brumer and Shapiro 1986). Shaped laser pulses can be used to control this interference with a view to achieving the necessary final quantum state of the molecule. The probability of production of the necessary excited quantum state and the required final product depends, for example, on the phase difference between two CW lasers. Both these methods are based on the existence of multiple interfering pathways from the initial... 

The modification of the electronic potentials due to the interaction with the electric field of the laser pulse has another important aspect pertaining to molecules as the nuclear motion can be significantly altered in light-induced potentials. Experimental examples for modifying the course of reactions of neutral molecules after an initial excitation via altering the potential surfaces can be found in Refs 56, 57, where the amount of initial excitation on the molecular potential can be set via Rabi-type oscillations [58]. Nonresonant interaction with an excited vibrational wavepacket can in addition change the population of the vibrational states [59]. Note that this nonresonant Stark control acts on the timescale of the intensity envelope of an ultrashort laser pulse [60]. 

In the following, we will discuss two basic - and in a sense complementary [44] - physical mechanisms to exert efficient control on the strong-field-induced coherent electron dynamics. In the first scenario, SPODS is implemented by a sequence of ultrashort laser pulses (discrete temporal phase jumps), whereas the second scenario utilizes a single chirped pulse (continuous phase variations) to exert control on the dressed state populations. Both mechanisms have distinct properties with respect to multistate excitations such as those discussed in Section 6.3.3. 

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