Adaptive pulse shaping

To date, similar experiments on dissociative ionization of molecules by adaptively shaped ultrashort laser pulses have been reported, where specific ion ratios were targeted during iterative control using GA and SA [14,15]. However, less detailed comparisons between adaptive control and open-loop control experiments regarding the laser pulse width, pulse train, and peak intensity exist. Moore et al. claimed that not only accelerated specific bondbreaking, but even new bond formation, can specified by adaptive pulse shape control [14]. Because ethanol molecules have a relatively simple main struc-... 

Ohno K, Tanabe T, Kannari F (2002) Adaptive pulse shaping of phase and amplitude of an amplified femtosecond pulse laser by direct reference to frequency-resolved optical gating traces. J. Opt. Soc. Amer. B19 2781-2790... 

Zeidler, D., Homung, T., Proch, D., and Motzkus, M. 2000. Adaptive compression of tunable pulses from a non-coUinear type OPA to below 20fs by feedback-controlled pulse shaping. Appl. Phys. B 70 8125-8131. 

Fig. 2. Overview of pulse shaping results, (a) and (b) depict measured SHG FROG traces before and after adaptive phase correction, respectively. Corresponding retrieved traces are displayed in (c) and (d). (e) Shows fundamental spectrum (shaded contour) measured at the crystal location in the FROG apparatus and the spectrum recovered by the FROG retrieval algorithm (open circles). Dash-dotted curve represents spectral phase prior to adaptive shaping, whereas dashed curve shows the optimized phase, (f) Initial (solid curve) and optimized (shaded contour) temporal intensity profiles. Dashed curve depicts temporal phase of the optimized pulse.

Figure 13.1 Schematic of acousto-optic pulse-shaping feedback apparatus used in adaptive feedback experiments. (Taken from Fig. 1, Ref. [41].)...

Figure 13.9 Aspects of optimum laser pulses in adaptive feedback control of products pi laser excitation of CpFe(CO)2Cl, Autocorrelation G2(t) is shown for three different cases described in text. Pulse shape differences in these three pulses is evident. (Taken from Fig. 4 Ref. [43].)...

Fig. 2.3.3 Current (top) and gradient (bottom) pulse shapes, (a) Without preemphasis, (b) With preemphasis. Adapted fiom [Krel] with permission from Publicis MCD.

Figure 19.1 Diagram showing the arrangement for closed-loop learning control. Following a femtosecond laser pulse, the products of the photochemical process are detected and compared with the user-defined objectives stored on the computer. A learning algorithm then calculates the modified electric fields required to shape the laser pulse and further optimize the yield of the desired product. Cycling through the loop many times gives the optimum pulse shape and best product yield. Adapted from Brixner et o/, Chem. Phys. Chem., 2003, 4 418, with permission of John Wiley Sons Ltd...

It is important to emphasize that, in the above examples, knowledge of the PES was not required for the optimization process. The adaptive-control learning algorithm explores the available phase space and optimizes the evolution of the wave packet on the excited state PES without any prior knowledge of the surface. Thus, the intrinsic information about the excited-state dynamics of these polyatomic systems remains concealed in the detailed shape and phase of the optimized pulse. Inevitably, however, scientific curiosity, together with a desire to imder-stand how chemical reactions can be controlled, has led to pioneering studies that aim to identify the underlying rules and rationale that lead to a particular pulse shape or phase relationship that produces the optimum yield. 

The fact that such an experimental window for coherent control in liquids does actually exist was verified in experiments on the selective multiphoton excitation of two distinct electronically and structurally complex dye molecules in solution (Brixner et al. 2001(b)). In these experiments, despite the failure of single-parameter variation (wavelength, intensity or linear chirp control), adaptive femtosecond pulse shaping revealed that complex laser fields could achieve chemically selective molecular excitation. These results prove, first, that the phase coherence of complex molecules persists for more than 100 fs in a solvent environment. Second, this is direct proof that it is the nontrivial coherent manipulation of the excited state and not of the frequency-dependent two-photon cross sections that is responsible for the coherent control of the population of the excited molecular state. 

Meshulach, D., Yelin, D., and Silberberg, Y. 1997. Adaptive ultrashort pulse compression and shaping. Opt. Comm. 138(4-6) 345 8. 

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