Feedback Optimization Strategy for Rotational Alignment Echo Spectroscopy

• Published in: Advanced photonics research Vol. 4; no. 12
• Main Authors: Bournazel, Manon, Espaignol, Antoine, Béjot, Pierre, Hertz, Edouard, Billard, Franck, Faucher, Olivier
• Format: Journal Article
• Language: English
• Published: Hoboken John Wiley & Sons, Inc 01.12.2023; Wiley; Wiley-VCH
• Subjects: Electrons; Lasers; molecular alignments; Optics; Physics; pulse shaping; rotational alignment echoes; Spectrum analysis; Temperature; ultrafast laser control
• ISSN: 2699-9293; 2699-9293
• DOI: 10.1002/adpr.202300221

The recent discovery of rotational echoes has contributed to extend the applications of field‐free molecular alignment to short time scales, namely to a temporal region preceding the first alignment revival of the molecule. Most echo measurements require adjusting the position of the echo over time, which in the case of molecular alignment echo unavoidably leads to a change in its amplitude, unless the intensity of the pulse sequence triggering the echo is properly modified. Herein, it is proposed to avoid this drawback by using an optical pulse shaper steered by a learning algorithm. It is demonstrated that a temporal shaping, whose characteristics are optimized to maintain a constant echo amplitude regardless of its creation time, can be generated by controlling the spectral phase and amplitude of the laser pulse which are then used in an open‐loop control system. As a proof of principle, the optimization strategy is applied to the observation of short‐term dissipative dynamics of laser‐aligned molecules exposed to collisions and to the measurement of associated decoherence and population decay time constants. An optimization strategy to improve molecular alignment echo spectroscopy is demonstrated. Based on femtosecond pulse shaping steered by a learning algorithm, the approach allows to disentangle the intrinsic dependence of the echo amplitude from its production time, undoing a technical lock for precise quantitative measurements. The method is applied to study the short‐time dissipation dynamic of rotationally excited molecules exposed to collisions.