Lennart Seiffert

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.

Trapping field assisted backscattering in strong-field photoemission from dielectric nanospheres

We study strong-field ionization of dielectric nanospheres and focus on the enhancement of the cut-off energies for backscattering electrons resulting from charge interaction. Though recent studies clearly demonstrated the decisive impact of a surface trapping field on the electron backscattering process, a clear picture of the underlying mechanism is lacking. Here, we provide this picture and present a simple and transparent extension of the famous three-step model of strong-field science by adding a triangular surface trapping potential. We justify this model for the case of dielectric nanospheres based on high-level transport simulations. The analysis of the trapping field assisted backscattering provides a universal scaling of the maximal recollision and backscattering energies as 9 and 14.5, respectively, where is the local ponderomotive potential. The universal nature of the enhancement over the conventional three-step model is of particular interest for the generation of attosecond electron bunches via near-field induced photoemission and high harmonic generation at nanostructures.

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.

High-order above-threshold photoemission from nanotips controlled with two-color laser fields

We investigate the process of phase-controlled high-order above-threshold photoemission from sharp metallic nanotips under bichromatic laser fields. Experimental photoelectron spectra resulting from two-color excitation with a moderately intense near-infrared fundamental field (1560 nm) and its weak second harmonic show a strong sensitivity on the relative phase and clear indications for a plateau-like structure that is attributed to elastic backscattering. To explore the relevant control mechanisms, characteristic features, and particular signatures from near-field inhomogeneity, we performed systematic quantum simulations employing a one-dimensional nanotip model. Besides rich phase-dependent structures in the simulated above-threshold ionization (ATI) photoelectron spectra we find ponderomotive shifts as well as substantial modifications of the rescattering cutoff as function of the decay length of the near-field. To explore the quantum or classical nature of the observed features and to discriminate the two-color effects stemming from electron propagation and from the ionization rate we compare the quantum results to classical trajectory simulations. We show that signatures from direct electrons as well as the modulations in the plateau region mainly stem from control of the ionization probability, while the modulation in the cutoff region can only be explained by the impact of the two-color field on the electron trajectory. Despite the complexity of the phase-dependent features that render two-color strong-field photoemission from nanotips intriguing for sub-cycle strong-field control, our findings support that the recollision features in the cutoff region provide a robust and reliable method to calibrate the relative two-color phase.

Strong near-field induced molecular processes on nanoparticles

Strong-field induced processes in molecules such as ionization and dissociation have been subject to theoretical and experimental investigations for many decades [1]. These processes include, e.g., above threshold ionization, high harmonic generation with the generation of attosecond pulses, and laser induced electron diffraction. Since these effects strongly rely on the exact spatial and temporal evolution of the electric fields they are also influenced and controlled by the presence of enhanced near-fields in the proximity of a nanostructure [2]. Recent theoretical and experimental work focused mainly on either atomic processes in strong near-fields or near-field driven photoemission from nanostructures. Here, we go beyond these studies and investigate the near-field control of one of the most fundamental molecular strong-field processes, namely multiple ionization leading to Coulomb explosion.

All-optical spatio-temporal control of electron emission from isolated dielectric nanospheres with two-color laser pulses

Field localization by nanostructures illuminated with laser pulses of well-defined waveform enables spatial-temporal tailoring of the near-fields for sub-cycle control of electron dynamics at the nanoscale [1]. Transition from the ponderomotive to sub-cycle acceleration regime of the near-field-driven electron emission was shown to be determined by the ratio of the near-field decay length and amplitude of the electron quiver motion [2]. Field-propagation-induced shaping of the near-field in turn has been utilized for size-dependent directional tuning of the electron emission from isolated nanospheres [3]. Here, we go beyond these studies and use intense 30 fs two-color laser pulses (780 nm/390 nm) to tailor the spatio-temporal near-fields of SiO2 nanoparticles, enabling the all-optical control of the electron emission direction.

Photoemission from Nanomaterials in Strong Few-Cycle Laser Fields

The application of ultra-short waveform-controlled laser fields to nanostructured materials enables the generation of localized near-fields with well-defined spatiotemporal field evolution. The optical fields that can be tailored on sub-wavelength spatial and attosecond temporal scales have a high potential for the control of ultrafast processes at the nanoscale, with important implications for laser-driven electron acceleration, extreme ultraviolet (XUV) light generation, and nanoscale electronics operating at optical frequencies.