Alexander Kessel

Field propagation-induced directionality of carrier-envelope phase-controlled photoemission from nanospheres

Near-fields of non-resonantly laser-excited nanostructures enable strong localization of ultrashort light fields and have opened novel routes to fundamentally modify and control electronic strong-field processes. Harnessing spatiotemporally tunable near-fields for the steering of sub-cycle electron dynamics may enable ultrafast optoelectronic devices and unprecedented control in the generation of attosecond electron and photon pulses. Here we utilize unsupported sub-wavelength dielectric nanospheres to generate near-fields with adjustable structure and study the resulting strong-field dynamics via photoelectron imaging. We demonstrate field propagation-induced tunability of the emission direction of fast recollision electrons up to a regime, where nonlinear charge interaction effects become dominant in the acceleration process. Our analysis supports that the timing of the recollision process remains controllable with attosecond resolution by the carrier-envelope phase, indicating the possibility to expand near-field-mediated control far into the realm of high-field phenomena.

Attosecond nanoscale near-field sampling

The promise of ultrafast light-field-driven electronic nanocircuits has stimulated the development of the new research field of attosecond nanophysics. An essential prerequisite for advancing this new area is the ability to characterize optical near fields from light interaction with nanostructures, with sub-cycle resolution. Here we experimentally demonstrate attosecond near-field retrieval for a tapered gold nanowire. By comparison of the results to those obtained from noble gas experiments and trajectory simulations, the spectral response of the nanotaper near field arising from laser excitation can be extracted.

Intensity dependence of the attosecond control of the dissociative ionization of D2

Light-field driven electron localization in deuterium molecules in intense near single-cycle laser fields is studied as a function of the laser intensity. The emission of D+ ions from the dissociative ionization of D2 is interrogated with single-shot carrier–envelope phase (CEP)-tagged velocity map imaging. We explore the reaction for an intensity range of (1.0–2.8) × 1014 W cm−2, where laser-driven electron recollision leads to the population of excited states of D2+. Within this range we find the onset of dissociation from 3σ states of D2+ by comparing the experimental data to quantum dynamical simulations including the first eight states of D2+. We find that dissociation from the 3σ states yields D+ ions with kinetic energies above 8 eV. Electron localization in the dissociating molecule is identified through an asymmetry in the emission of D+ ions with respect to the laser polarization axis. The observed CEP-dependent asymmetry indicates two mechanisms for the population of 3σ states: (1) excitation by electron recollision to the lower excited states, followed by laser-field excitation to the 3σ states, dominating at low intensities, and (2) direct excitation to the 3σ states by electron recollision, playing a role at higher intensities.

Carrier-envelope phase dependence of the directional fragmentation and hydrogen migration in toluene in few-cycle laser fields.

The dissociative ionization of toluene initiated by a few-cycle laser pulse as a function of the carrier envelope phase (CEP) is investigated using single-shot velocity map imaging. Several ionic fragments, CH3+, H2+, and H3+, originating from multiply charged toluene ions present a CEP-dependent directional emission. The formation of H2+ and H3+ involves breaking C-H bonds and forming new bonds between the hydrogen atoms within the transient structure of the multiply charged precursor. We observe appreciable intensity-dependent CEP-offsets. The experimental data are interpreted with a mechanism that involves laser-induced coupling of vibrational states, which has been found to play a role in the CEP-control of molecular processes in hydrocarbon molecules, and appears to be of general importance for such complex molecules.

Generation of multi-octave spanning high-energy pulses by cascaded nonlinear processes in BBO

We present the generation of optical pulses with a spectral range of 500-2400 nm and energies up to 10 µJ at 1 kHz repetition rate by cascaded second-order nonlinear interaction of few-cycle pulses in beta-barium borate (BBO). Numerical simulations with a 1D+time split-step model are performed to explain the experimental findings. The large bandwidth and smooth spectral amplitude of the resulting pulses make them an ideal seed for ultra-broadband optical parametric chirped pulse amplification and an attractive source for spectroscopic applications.

Intensity dependence of the dissociative ionization of DCl in few-cycle laser fields

We have studied the dissociative ionization of DCl in 4 fs laser fields at 720 nm central wavelength using intensities in the range (1.3-3.1) x 10(14) W cm(-2). By employing the phase-tagged velocity-map imaging technique, information about the angular distribution of deuterium ions as a function of their kinetic energy and the carrier-envelope phase is obtained. On the basis of the experimental data and semi-classical simulations, three regions are distinguished for the resulting D+ ions with different kinetic energies. The one with the lowest kinetic energy, around 5-7 eV, is from dissociation involving the X-state of DCl+, populated through direct ionization with the laser field. The second region, around 7-11 eV, originates from rescattering induced dissociative ionization. Above 2 x 10(14) W cm(-2) D+ ions with kinetic energies exceeding 15 eV are obtained, which we ascribe to double ionization induced by rescattered electrons.

Quenching of material dependence in few-cycle driven electron acceleration from nanoparticles under many-particle charge interaction

Abstract The excitation of nanoscale near-fields with ultrashort and intense laser pulses of well-defined waveform enables strongly spatially and temporally localized electron emission, opening up the possibility for the generation of attosecond electron pulses. Here, we investigate the electron photoemission from isolated nanoparticles of different materials in few-cycle laser fields at intensities where the Coulomb field of the ionized electrons and residual ions significantly contribute to the electron acceleration process. The dependences of the electron cut-off energy on the material’s dielectric properties and electron binding energy are investigated systematically in both experiments and semi-classical simulations. We find that for sufficiently high near-field intensities the material dependence of the acceleration in the enhanced near-fields is quenched by many-particle charge-interaction.

The Petawatt Field Synthesizer System

The PFS system consists of four main sections as shown in the schematic overview in Fig. 3.1. The Ti:Sa-based frontend from which all optical pulses of the system are derived. The Yb-based amplifier chain that provides high-energy pump pulses for parametric amplification. The broadband seed generation section.The OPCPA chain where pump and seed pulses are combined.

Seed Generation Schemes

In the previous chapter it was discussed that the strong spectral modulations in the seed pulses generated by spectral broadening in cascaded HCFs constitute a serious problem for the parametric amplification and temporal compression of these pulses. In the present chapter we will describe our efforts to overcome this problem and discuss the advantages and disadvantages of three alternative seed generation schemes we developed.

OPCPA Experiments with Two OPA Stages

In the following we report on the first series of OPCPA measurements which we conducted on two LBO stages in vacuum (cf. Fig. 3.9). The presented data have been taken together with Christoph Skrobol and have partially already been described in his thesis [1]. For the measurement campaign, the Tube served as the last pump amplifier and the pump compressor was still located in air.