Franziska Sauer

Coherent control with shaped femtosecond laser pulses applied to ultracold molecules

We report on coherent control of excitation processes of translationally ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond laser pulses. Evolution strategies are applied in a feedback loop in order to optimize the photoexcitation of the Rb{sub 2} molecules, which subsequently undergo ionization or fragmentation. A superior performance of the resulting pulses compared to unshaped pulses of the same pulse energy is obtained by distributing the energy among specific spectral components. The demonstration of coherent control to ultracold ensembles opens a path to actively influence fundamental photoinduced processes in molecular quantum gases.

Isotope selective photoionization of NaK by optimal control: theory and experiment.

We present a joint theoretical and experimental study of the maximization of the isotopomer ratio (23)Na(39)K(23)Na(41)K using tailored phase-only as well as amplitude and phase modulated femtosecond laser fields obtained in the framework of optimal control theory and closed loop learning (CLL) technique. A good agreement between theoretically and experimentally optimized pulse shapes is achieved which allows to assign the optimized processes directly to the pulse shapes obtained by the experimental isotopomer selective CLL approach. By analyzing the dynamics induced by the optimized pulses we show that the mechanism involving the dephasing of the wave packets between the isotopomers (23)Na (39)K and (23)Na (41)K on the first excited state is responsible for high isotope selective ionization. Amplitude and phase modulated pulses, moreover, allow to establish the connection between the spectral components of the pulse and corresponding occupied vibronic states. It will be also shown that the leading features of the theoretically shaped pulses are independent from the initial conditions. Since the underlying processes can be assigned to the individual features of the shaped pulses, we show that optimal control can be used as a tool for analysis.

Characteristic oscillations in the coherent transients of ultracold rubidium molecules using red and blue detuned pulses for photoassociation

We investigate the interaction of femtosecond laser pulses with an ensemble of ultracold rubidium atoms by applying shaped excitation pulses with two different types of spectral filtering. Although the pulses, which are frequency filtered with a high pass, have no spectral overlap with molecular states, we observe coherent molecular transients. Similar transients obtained with nearly transform-limited pulses, where only the atomic resonance is removed, reveal two differing oscillatory components. The resulting transients are compared among themselves and supported with quantum dynamical simulations which indicate a photoassociation process. The effect is due to the strong field interaction of the pulse with the colliding atom pair.

Multi-objective optimization on alkali dimers

We present experimental results of a multi-objective evolutionary algorithm that controls the ionization of the NaK molecule and the fragmentation/ionization of potassium dimers. The two physically contradictive goals are dimer ionization and fragmentation into the atomic isotopes. In the NaK case, we use an approach to simplify the usually complex results of feedback-loop coherent control experiments and implement the ‘pulse cleaning’ method. It defines the objective function in a way that unnecessary spectral features are reduced and the most important vibronic transitions are enhanced. This method will be developed in terms of a classical multi-objective optimization and we present some corresponding Pareto-optimal solutions.

Controlling the dynamical pathways in the ionization processes of NaK dimers

This chapter analyzes the control of the dynamical pathways in the ionization processes of NaK dimers. Active control by means of feedback optimization using tailored femtosecond laser pulses is carried out in a resonant 3-photonic ionization process for optimizing the 23 Na 39 K isotopomer. The acquired optimized pulses are analyzed regarding their nuclear dynamics, and a simple model that is based on full quantum-mechanical calculations at similar conditions is proposed. Novel coherent control methods such as parametric optimization and control pulse cleaning are applied to receive information about the involved oscillations and the vibronic transitions within the optimized ionization path. When optimizing the isotopomer ratio, considerable optimization factors are obtained and intrinsic properties of the photo-induced process emerges in the resulting pulse shapes. This latter experiment reveals how the algorithm precisely addresses the vibrational wavefunctions both spectrally and temporally by coherent interactions with the shaped light field.

Coherent Control of Isotope Selective Ionization and Fragmentation of Potassium

By applying evolution strategies we performed isotope selctive ionization and fragmentation of potassium dimers. The optimized pulse structures revealed the ionization process. For optimizing the fragmentation of potassium dimers to atoms we performed a new two-criterium optimization.