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We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located.

Fitness landscapes based on a limited number of laser pulse shape parameters can elucidate reaction pathways and can help to find the underlying control mechanism of optimal pulses determined by adaptive femtosecond quantum control. In a first experiment, we employ colored double pulses and systematically scan both the temporal subpulse separation and the relative amplitude of the two subpulses to acquire fitness landscapes. Comparison with results obtained from a closed-loop experiment demonstrates the capability of fitness landscapes for the revelation of possible control mechanisms. In a second experiment, using transient absorption spectroscopy, we investigate and compare the dependence of the excitation efficiency of the solvated dye molecule 5,5{sup '}-dichloro-11-diphenylamino-3,3{sup '}-diethyl-10,12-ethylene thiatricarbocyanine perchlorate (IR140) on selected pulse shapes in two parametrizations. The results show that very different pulse profiles can be equivalently adequate to maximize a given control objective. Fitness landscapes thus provide valuable information about different pathways along which a molecular system can be controlled with shaped laser pulses.

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.

We employ a 128-pixel liquid-crystal spatial light modulator to generate variable pulse sequences from a titanium-sapphire femtosecond laser amplifier system centered at 800 nm. By applying phase modulations based on triangular-shaped spectral phase patterns, pulse sequences can be generated whose overall spectrum is still conserved but whose subpulses differ in their spectral composition. We further use nonlinear crystals to analyze the shape of these pulses after frequency conversion. Our experiments show that it is possible to transfer these pulse sequences into the ultraviolet at central wavelengths of 400 nm and 267 nm without losing the ability to spectrally distinguish between the femtosecond subpulses. This is affirmed by measurements using cross-correlation and XFROG (cross-correlation frequency resolved optical gating) techniques with an unmodulated laser pulse. Simulations of the experiments are performed for comparison. We also discuss promising applications in spectroscopy or information encoding.

We demonstrate that the use of time-dependent light polarization opens a new level of control over quantum systems. With K2 molecules we show that polarization-shaped laser pulses increase the multiphoton-ionization yield compared to linearly-polarized laser pulses.

We demonstrate that the use of time-dependent light polarization opens a new level of control over quantum systems. With potassium dimer molecules from a supersonic molecular beam, we show that a polarization-shaped laser pulse increases the ionization yield beyond that obtained with an optimally shaped linearly polarized laser pulse. This is due to the different multiphoton ionization pathways in K2 involving dipole transitions which favor different polarization directions of the exciting laser field. This experiment is a qualitative extension of quantum control mechanisms which opens up new directions giving access to the three-dimensional temporal response of molecular systems. DOI: 10.1103/PhysRevLett.92.208301

Since the advent of quantum mechanics there has been a desire to understand in detail the behavior of quantum systems. This quest has been accompanied by the implicit dream to not only be able to observe in a passive way, but in fact also to actively control quantum-mechanical processes. The key question in quantum control is: Can one find external control parameters which guide the temporal evolution of quantum-mechanical systems in a desired way, even if this evolution is very complex?

We present an experimental concept for the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet. Polarization-shaped laser pulses are frequency-doubled in an interferometrically stable setup comprising two perpendicularly oriented nonlinear crystals. Dual-channel spectral interferometry is employed to fully characterize the electric field of the polarization-shaped ultraviolet pulses. The method is experimentally demonstrated for a central wavelength of 400 nm. Advantages and prospective applications, as well as limitations and possible alternatives, are discussed.

We experimentally demonstrate the generation and characterization of polarization-shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 400 nm. Near-infrared laser pulses are first polarization shaped and then frequency doubled in an interferometrically stable setup that employs two perpendicularly oriented nonlinear crystals. A new pulse shaper design involving volume phase holographic gratings reduces losses and hence leads to an increase in pulse energy.

We investigate the applicability of neural networks (NNs) for the automated generation of effective computer models for coherent light?matter interactions. The simulation of Autler?Townes doublets from strong-field ionization of potassium atoms is chosen as a test system that exhibits distinct quantum-mechanical effects. Shaped femtosecond laser pulses are employed for studying the response of a quantum-mechanical system to a large variety of different electric fields, and the resulting data can be used for training a NN. We show that a NN is able to approximate the investigated process in parameter regions sampled by the training data and that it can be employed for the interpolation of control landscapes.