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  • Optimal Pulse Control Deciphered

Photonics Spectra
Mar 2003
Gary Boas

Ultrafast laser pulses can control reactive pathways in chemicals. In a proposed strategy known as adaptive optimal laser pulse control, an algorithm iteratively determines the combination of laser parameters that will best be able to achieve the desired reactive pathway. Now, a group of researchers at Université Louis Pasteur in Strasbourg, France, and at Freie Universität Berlin has developed a means of decoding the information about the dynamics of a system that is contained in its optimal pulses and, thereby, of controlling the dynamics.

Using a combination of femtosecond high-resolution pump-probe experiments, ab initio quantum calculations and simulations of wave packet dynamics, researchers have identified the mechanisms underlying adaptive optimal laser pulse control.

The researchers originally set out to achieve a particular reactive pathway in the model system CpMn(CO)3. But they found that the optimal pulse they obtained experimentally consisted of two subpulses that could be assigned as a pump-probe process and thus used to investigate the mechanisms of control.

"Pump-probe experiments using unshaped pulses provided valuable insights into the molecular dynamics of the system," said Leticia González of Freie Universität Berlin. "Once the general dynamics of the system were understood, we could proceed to decipher the mechanism of the optimal field."

Using the information obtained with the experiments, the researchers explored the dynamics of the molecule under the influence of selected laser pulses. They were able to discern the timing and frequency structure necessary for the control laser pulse to produce the desired reactive pathway.

They employed two laser systems in the study. For the optimization experiment, the system consisted of a Spectra-Physics Ti:sapphire oscillator driven by a CW diode-pumped Nd:YVO4, with the oscillator serving as a seed laser for a Quantronix regenerative multipass amplifier. For the pump-probe experiment, they replaced the oscillator with a homebuilt Ti:sapphire oscillator, and a different Quantronix multipass amplifier delivered sub-50-fs laser pulses. They analyzed the laser pulses with a Jobin Yvon spectrometer and with a second-harmonic-generation frequency-resolved optical gating system that used a Kappa Opto-Electronics CCD camera.

The most likely immediate applications of the technique are in the pharmacological and chemical industries, where it can be used to control chemical reactions, to enhance desired products or to eliminate those that are not wanted. The major obstacle to such application is the yield of the processes.

"The number of molecules in a molecular beam is still [too] small to be relevant to industry," González said. "It is very important to rationalize how such experiments work to be able to 'propose' further improvements, which will in a few years be used in industry."

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