Monday, February 25, 2013

Physics Colloquium - Fri., Mar. 1, 4:30, PS 161: Controlling strong-field dynamics with synthetic CEP-stabilized pulses

Controlling strong-field dynamics with synthetic CEP-stabilized pulses

Wendell T. Hill, III
University of Maryland, College Park, Maryland, 20742 USA

Strong-field dynamics has been shown to be sensitive to the carrier-envelop phase (CEP).  Experiments with CEP-stabilized pulses, however, have been limited to few-cycle pulses ( 2 or 3 cycles).  This is because the more cycles a pulse has, the less sensitive experiments are to variations of the CEP.  Recently, we introduced a new approach we call synthetic CEP stabilization.  Synthetic CEP stabilization is possible with pulses containing a pair of peaks, but the relative phase and temporal separation must be controllable independently.  As we will show, the first peak induces coherence in an incoherent, isolated quantum system allowing the second peak to interact with a coherent, isolated quantum system.  Even though neither peak is CEP stabilized – the phases of the peaks vary randomly from shot to shot – we are able to observe phase effects even with pulses as long as 50 fs (20+ cycles) because there is always a well-defined phase between the two peaks – synthetic CEP stabilization.  To demonstrate the power and utility of our approach, we have investigated strong-field optimal control of molecular dynamics.  Three distinct processes concomitant with strong-field enhanced ionization were studied as the CEP was varied:  (i) the propensity for large amplitude bending vibration of a nominally linear 3-atom system, (ii) the alteration of the explosion strength and (iii) the modification of the branching ratios into degenerate channels. We will present evidence suggesting the relative phase, which has garnered little attention in optimal control literature, plays a much more fundamental role in optimal control than previously thought.  Our results shed light on why closed-loop optimal control searches (e.g., genetic algorithms) lead to multiple solutions with nearly the same efficiency.  Specifics of our results and their ramification will be presented.

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