Femtosecond strong-field quantum control with sinusoidally phase-modulated pulses

Abstract

The quantum control of the ionization of potassium atoms using shaped intense femtosecond laser pulses is investigated. We use sinusoidal phase modulation as a prototype for complex shaped pulses to investigate the physical mechanism of the strong-field quantum control by shaped femtosecond light fields. The influence of all parameters characterizing the sinusoidal phase modulation on strong-field-induced dynamics is studied systematically in experiment and theory. Our results are interpreted in terms of the selective population of dressed states ͑SPODS͒ which gives a natural physical picture of the dynamics in intense laser fields. We show that modulated femtosecond pulses in combination with photoelectron spectroscopy are a versatile tool to prepare and to probe SPODS. The decomposition of the excitation and ionization process induced by shaped pulses into elementary physically transparent steps is discussed. INTRODUCTIONOne of the intriguing aspects of quantum control is the ability to manipulate quantum systems with suitably tailored laser light fields almost at will. For instance, shaped pulses are employed to guide a quantum system from an initial state to a preselected final state with high efficiency. In recent years, numerous quantum control schemes have been proposed and successfully demonstrated which are reported in recent monographs and reviews ͓1-5͔. Besides quantum control schemes based on the detailed knowledge of the potentials and the use of simple pulse sequences, adaptive pulse manipulation using complex pulse shapes has opened new perspectives for quantum control ͓6͔. The combination of pulse-shaping techniques ͓7͔ with closed loop adaptive feedback learning algorithms allows us to optimize virtually any conceivable observable ͓8-18͔ with applications ranging from laser science, quantum optics, atomic and molecular physics, solid state physics, photochemistry to biophysics, or quantum computing. However, it is not always possible to deduce the underlying physical mechanism from the electrical fields obtained by this procedure. Therefore, the need to bridge the gap between the efficient "black box" closed loop optimal control methods and detailed understanding of the physical processes-especially in strong laser fields-is quite evident.Generally, weak-field control schemes are not applicable for intense laser fields where perturbation theory is no longer valid as exemplified on a generic example of ultrafast strongfield coherent control ͓19͔. However, some of the principles of weak-field excitation are applicable to nonperturbative control employing so-called real pulses ͓20͔. In this context real or complex pulses are defined by the properties of their temporal phase which determines whether the pulse envelope is real or complex valued. For example, in this context, linearly chirped pulses are complex whereas third order dispersion ͑TOD͒ produces real pulses. The implications of real or complex pulses with respect to the manipulation of the symmetry of photoelectron spectra was recently discu...

Enhancing strong-field-induced molecular vibration with femtosecond pulse shaping

“…Sinusoidal phase modulation represents one of the most popular methods of periodic shaping [32,33,36,37]. In this case,…”

“…Unlike the case of a frequency chirp, all three vibrational bands are now excited with the maximum possible amplitude (set by the available bandwidth) if the period T is a multiple integer of the vibrational period. The modulation amplitude A determines the number of pulses in the pulse train, as well as the line width of the spectral peaks in the two-photon spectrum [36]. Similarly to the parabolic phase shaping described above, we tune the value of A for the best coverage of the rotationally broadened Raman transitions in iodine.…”

Optimal control theory – closing the gap between theory and experiment

“…To reduce the complexity of the shaped laser fields in optimal control experiments (OCE), the experimentalists started to use analytic, parameterized phase functions such as sinusoidal phase modulation to control a quantum system. [19][20][21][22][23][24][25][26][27] But this reduction of the search space does not lead to sufficient understanding of the mechanisms that steer a reaction.…”

Coherent phase control of internal conversion in pyrazine

“…A variety of phase functions were applied to the AOM to produce different shaped UV laser pulses. These included a sine phase to generate sequences of evenly spaced TL pulses, [29][30][31][32] a V-shaped phase to produce a pulse pair, and a quadratic phase to produce chirped pulses.…”