Laser control domains

The theory of laser control of chemical reactions may be classified into two different domains Laser control by continuous wave (CW) lasers and by laser pulses. The former includes, for example, the strategies of (i) vibra-tionally mediated chemistry [1] and (ii) coherent superpositions of independent excitation routes [2] for experimental demonstrations see (i) Ref. 3 and... 

One of the most widely used theoretical laser optimization routines for molecular laser control is OCT due to its relative ease in implementation and monotonic convergence, see recent reviews [12-14] and the many references therein. Optimization of the laser pulse occurs in the time-domain. Beyond the initial implementation [61], further investigations developed important features such as constraints on the frequency spectrum[62-64] and optimization of the laser pulse duration[65-67], both of which are required to produce laser pulses comparable to those obtained experimentally. Within OCT an objective function, J, is maximized according to constraints on the required excitation, constraints on the laser pulse field and it must also satisfy the TDSE. These constraints are represented by each term in the objective function, respectively. 

It has been shown in Chapter 5, the fluorescence quenching of the DPA moiety by MV2 + is very efficient in an alkaline solution [60]. On the other hand, Delaire et al. [124] showed that the quenching in an acidic solution (pH 1.5-3.0) was less effective (kq = 2.5 x 109 M 1 s 1) i.e., it was slower than the diffusion-controlled limit. They interpreted this finding as due to the reduced accessibility of the quencher to the DPA group located in the hydrophobic domain of protonated PMA at acidic pH. An important observation is that, in a basic medium, laser excitation of the PMAvDPA-MV2 + system yielded no transient absorption. This implies that a rapid back ET occurs after very efficient fluorescence quenching. 

References 29-33 introduce the notion of coherence spectroscopy in the context of two-pathway excitation coherent control. Within the energy domain, two-pathway approach to coherent control [25, 34—36], a material system is simultaneously subjected to two laser fields of equal energy and controllable relative phase, to produce a degenerate continuum state in which the relative phase of the laser fields is imprinted. The probability of the continuum state to evolve into a given product, labeled S, is readily shown (vide infra) to vary sinusoidally with the relative phase of the two laser fields < ),... 

In this chapter we explore several aspects of interferometric nonlinear microscopy. Our discussion is limited to methods that employ narrowband laser excitation i.e., interferences in the spectral domain are beyond the scope of this chapter. Phase-controlled spectral interferometry has been used extensively in broadband CARS microspectroscopy (Cui et al. 2006 Dudovich et al. 2002 Kee et al. 2006 Lim et al. 2005 Marks and Boppart 2004 Oron et al. 2003 Vacano et al. 2006), in addition to several applications in SHG (Tang et al. 2006) and two-photon excited fluorescence microscopy (Ando et al. 2002 Chuntonov et al. 2008 Dudovich et al. 2001 Tang et al. 2006). Here, we focus on interferences in the temporal and spatial domains for the purpose of generating new contrast mechanisms in the nonlinear imaging microscope. Special emphasis is given to the CARS technique, because it is sensitive to the phase response of the sample caused by the presence of spectroscopic resonances. 

In the following, we describe two prominent types of spectral phase modulation, each of which plays an important role in coherent control. Both types, namely sinusoidal (Section 6.2.1) and quadratic (Section 6.2.2) spectral phase modulation, are relevant for the experiments and simulations presented in this contribution. We provide analytic expressions for the modulated laser fields in the time domain and briefly discuss the main characteristics of both classes of pulse shapes. 

Since the development of titanium-sapphire (Ti Sa) femtosecond laser source, the domain of research fields or development covered by the use of ultrafast lasers is in continuous expansion. In femtochemistry, it was realized that, when in a photochemical reaction different pathways lead to a given final state, the presence of a well-controlled frequency pulse chirp might greatly enhance the probability with which this final state is reached [1,2]. Chirp control is needed and mastering the phase of the laser pulse is the key point. 

As far as the external action is concerned, several external stimuli can be conveniently used to achieve rotation of one ring into the other some success has been achieved by chemical means like addition of protons [60, 89, 90], demetalation of catenates [90, 91] or change of solvent [85], but by far the most convenient system for its ease of control is a redox action which can be either chemically, electro-chemically, or photochemically induced. The latter method, based on light-induced electron transfer processes, could take advantage of the development of laser technology and grant a complete control both in the space and in the time domain. 

---