Coherent learning control of vibrational motion in room temperature molecular gases

Abstract

An evolutionary learning algorithm in conjunction with an ultrafast optical pulse shaper was used to control vibrational motion in molecular gases at room temperature and high pressures. We demonstrate mode suppression and enhancement in sulfur hexa¯uoride and mode selective excitation in carbon dioxide. Analysis of optimized pulses discovered by the algorithm has allowed for an understanding of the control mechanism. Ó 2001 Elsevier Science B.V. All rights reserved.Controlling atoms and molecules with coherent optical ®elds has been a longstanding goal in chemical physics [1]. Recently, several experiments in this ®eld have demonstrated successful control of molecular ionization and dissociation [2,3], atomic and molecular¯uorescence [4,5], the shape of electronic and molecular wave packets [6±9], and currents in semiconductors [10,11]. Many of these experiments have employed learning control loops and programmable optical pulse shapers to discover optimal pulse shapes for control [12]. There has been increasing interest in not only achieving control in quantum systems, but also in understanding the control mechanism, which has proven to be a challenging task. Here we demonstrate two experimental advances in coherent control of quantum systems. First, we demonstrate selective control over molecular motion in gases at STP using very short, shaped excitation pulses. Coherent vibrational excitation corresponding to thermal temperatures over 2000 K is achieved. This is important in order to reach the goal of laser selective chemistry [13] on a macroscopic scale, because it extends cryogenic and molecular beam experiments to high temperatures and densities. Second, using a special cost function [14] incorporated into an evolutionary learning algorithm [15], we obtain a clear interpretation of the control mechanism, which is based on impulsive stimulated Raman scattering [16]. This serves as a demonstration of a general technique for systematically gaining insight from solutions found by learning algorithms. Furthermore, as there is no molecular resonance exploited in this excitation scheme, and the

Searching for quantum optimal controls under severe constraints

“…Successful optimal control experiments (OCEs) have included selective control of molecular vibrational [10][11][12][13][14][15][16][17] and electronic states [18][19][20][21][22][23][24][25][26][27], preservation of quantum coherence [28,29], control of photoisomerization reactions [30][31][32][33][34][35], selective manipulation of chemical bonds [36][37][38][39][40][41][42][43][44], high-harmonic generation and coherent manipulation of the resulting soft X-rays [45][46][47][48][49][50][51], and control of energy flow in biomolecular complexes [52][53][54][55]. Optimal control theory (OCT) [7,9,…”

Theory of selective excitation in stimulated Raman scattering

“…Series of successful experiments implementing this technique offer proof that it is possible to exercise quantum coherent control over a range of processes. Two such experiments employ stimulated Raman scattering in liquid methanol [2] and gas phase carbon dioxide [3]. In the former case a liquid medium is excited with a 100 fs pulse, which is equivalent to non-impulsive Raman excitation and, therefore, makes propagation effects important.…”

Maximum relative excitation of a specific vibrational mode via optimum laser-pulse duration

“…For a quarter century there has been considerable interest in optimizing the vibrational response of molecules and materials to ultrafast laser pulses. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] This problem is directly relevant to the broader issue of coherent control in physical, chemical, [16][17][18][19] and biological 20-23 systems.Here we consider excitation via impulsive stimulated Raman scattering and related techniques using femtosecondscale optical pulses. We find that the optimum full-width-athalf-maximum ͑FWHM͒ pulse duration for exciting a specific vibrational mode with angular frequency 2 / T is given by Ϸ 0.42T.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] This problem is directly relevant to the broader issue of coherent control in physical, chemical, [16][17][18][19] and biological [20][21][22][23] systems.…”