C. Sarpe-Tudoran

Control of ionization processes in high band gap materials via tailored femtosecond pulses

Control of two basic ionization processes in dielectrics i.e. photo ionization and electron-electron impact ionization on intrinsic time and intensity scales is investigated experimentally and theoretically. Temporally asymmetric femtosecond pulses of identical fluence, spectrum and pulse duration result in different final free electron densities. We found that an asymmetric pulse and its time reversed counterpart address two ionization processes in a different fashion. This results in the observation of different thresholds for surface material modification in sapphire and fused silica. We conclude that control of ionization processes with tailored femtosecond pulses is suitable for robust manipulation of breakdown and thus control of the initial steps of laser processing of high band gap materials.

Compact, robust, and flexible setup for femtosecond pulse shaping

We present an improved design and adjustment concept for femtosecond pulse shaping. The concept results in a compact and robust pulse shaping setup. A systematic adjustment procedure, high reproducibility and stability, as well as easy adaptability to different femtosecond laser sources are the key features of the presented design. The constructed prototype pulse shaper was tested in an open loop and feedback-controlled adaptive pulse shaping on two different femtosecond laser sources.

Strong field quantum control by selective population of dressed states

We study the dynamics of potassium atoms in intense laser fields using femtosecond phase-locked pulse pairs in order to extract physical mechanisms of strong field quantum control. The structure of the Autler–Townes (AT) doublet in the photoelectron spectra is measured to analyse transient processes. The analysis shows that the physical mechanism is based on the selective population of dressed states (SPODS). Experimental results of closed loop optimization of SPODS are presented in addition. Applications to decoherence measurements with implications for quantum information are also proposed.

Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse

We present a joint experimental and theoretical study on strong- field photo-ionization of sodium atoms using chirped femtosecond laser pulses. By tuning the chirp parameter, selectivity among the population in the highly excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways enabling control are identified by simultaneous ionization and measurement of photoelectron angular distributions employing the velocity map imaging technique. Free electron wave packets at an energy of around 1eV are observed. These photoelectrons originate from two channels. The predominant 2+1+1 resonance enhanced multi-photon ionization (REMPI) proceeds via the strongly driven two-photon transition 4s 3s, and subsequent ionization from the states 5p, 6p and 7p whereas the second pathway involves 3+1 REMPI via the states 5f and 6f. In addition, electron wave packets from two-photon ionization of the non-resonant transiently populated state 3p are observed close to the ionization threshold. A mainly qualitative five-state model for the predominant excitation channel is studied theoretically to provide insights into the physical mechanisms at play. Our analysis shows that by tuning the chirp parameter the dynamics is effectively controlled by dynamic Stark shifts and level crossings. In particular, we show that under the experimental conditions the passage through

Quantum control and quantum control landscapes using intense shaped femtosecond pulses

A hitherto not considered physical mechanism of quantum control with intense shaped femtosecond laser pulses is investigated. Phase modulated pulses are used to exert control on the strong-field ionization of potassium atoms. We use a sinusoidal phase modulation function to manipulate the intensity of the Autler–Townes (AT) components in the photoelectron spectrum. The effect of all sine parameters is studied systematically. In addition, controllability is investigated using parameterized pulse shapes to generate a two-dimensional quantum control landscape. Our results show that the selective population of dressed states is the underlying strong-field physical mechanism. Due to its robustness with respect to the laser parameters, the selective dressed state population is an important general control mechanism.

Filling a spectral hole via self-phase modulation

The effect of spectral amplitude modulation on self-phase modulation is studied. To that end we remove a small interval of frequency components from the broad spectrum of a femtosecond laser pulse. We investigate the regeneration of these missing frequency components via self-phase modulation. A water jet serves as a transparent sample. A physical model is given which explains the observation that the removed frequency components are not only replenished by self-phase modulation but can even overshoot their adjacent frequencies in power spectral density. In addition, we suggest possible applications in the field of nonlinear microscopy.

Coherent control of electrons, atoms and molecules with intense shaped light pulses

We report on a physical mechanism of coherent control with intense shaped femtosecond laser pulses. We study photoelectron spectra from multi-photon ionization of potassium atoms and dimers using tailored femtosecond laser pulses. Our results are interpreted in terms of Selective Population of Dressed States (SPODS). Two realizations of SPODS by Photon Locking (PL) via pulse sequences and Rapid Adiabatic Passage (RAP) via chirped pulses are discussed. New physical mechanisms arise, when both PL and RAP are at play simultaneously. Control by the combined efiect of PL and RAP is studied by mapping out a two-parameter Quantum Control Landscape (QCL) for selective population of dressed states.

Femtosecond Pulse Tailoring For Nanoscale Laser Processing Of Wide‐Bandgap Materials: Temporal Asymmetric Pulses Versus Frequency Sweeps

Control of two basic ionization processes in dielectrics i.e. photo ionization and electron‐electron impact ionization on intrinsic time and intensity scales is investigated experimentally. In our experiment, we use a modified microscope set up to irradiate the surface of a fused silica sample with a single shaped pulse resulting in nanoscale ablation structures. The topology of the laser generated structures is measured by Atomic Force Microscopy (AFM). Structure parameters are investigated as a function of the pulse energy and the modulation parameters. We find different thresholds for surface material modification with respect to an asymmetric temporal pulse and its time reversed counterpart both showing a constant instantaneous frequency. However, we do not observe pronounced differences between up‐ and down‐chirped radiation (i.e. symmetric temporal pulse envelope but asymmetric instantaneous frequency) in the measured structure diameters and thresholds.

Ultrafast Switching of Coherent Electronic Excitation: Great Promise for Reaction Control on the Femtosecond Time Scale

We report on a physical mechanism of coherent control with shaped intense femtosecond laser pulses. To this end we study photoelectron spectra from the multi-photon ionization of potassium atoms using tailored femtosecond laser pulses. Our results are interpreted in terms of Selective Population of dressed states (SPODS). Two realizations of SPODS by Photon Locking and Rapid Adiabatic Passage are discussed. A physical picture of SPODS based on the interplay of the laser electric field and the atomic wave function is presented. In addition, coherent control of a larger molecule (isopropyl alcohol) by shaped femtosecond laser pulses is demonstrated experimentally.

Principles of femtosecond pulse tailoring for advanced material processing

The Principles of femtosecond pulse tailoring for advanced material processing are reviewed. In our experiments we study laser control of two basic ionization processes in dielectrics on intrinsic time and intensity scales with temporally asymmetric pulse trains. We create robust structures one order below the diffraction limit.