Arne Senftleben

Spatio-temporal resolution studies on a highly compact ultrafast electron diffractometer

Time-resolved diffraction with femtosecond electron pulses has become a promising technique to directly provide insights into photo induced primary dynamics at the atomic level in molecules and solids. Ultrashort pulse duration as well as extensive spatial coherence are desired, however, space charge effects complicate the bunching of multiple electrons in a single pulse. We experimentally investigate the interplay between spatial and temporal aspects of resolution limits in ultrafast electron diffraction (UED) on our highly compact transmission electron diffractometer. To that end, the initial source size and charge density of electron bunches are systematically manipulated and the resulting bunch properties at the sample position are fully characterized in terms of lateral coherence, temporal width and diffracted intensity. We obtain a so far not reported measured overall temporal resolution of 130 fs (full width at half maximum) corresponding to 60 fs (root mean square) and transversal coherence lengths up to 20 nm. Instrumental impacts on the effective signal yield in diffraction and electron pulse brightness are discussed as well. The performance of our compact UED setup at selected electron pulse conditions is finally demonstrated in a time-resolved study of lattice heating in multilayer graphene after optical excitation.

Formation of very-low-energy states crossing the ionization threshold of argon atoms in strong mid-infrared fields

Atomic ionization by intense mid-infrared (mid-IR) pulses produces low electron energy features that the strong-field approximation, which is expected to be valid in the tunneling ionization regime characterized by small Keldysh parameters (

Temporal Airy pulses for controlled high aspect ratio nanomachining of dielectrics

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Complete analysis of a transmission electron diffraction pattern of a MoS2–graphite heterostructure

), cannot describe. These features include the low-energy structure (LES), the very-low-energy structure (VLES), and the more recently found zero-energy structure (ZES). They result from the interplay between the laser electric field and the atomic Coulomb field which controls the low-energy spectrum also for small

Temporal Airy pulses control cell poration

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Molecular structure from aligned polyatomic molecules using laser-induced electron diffraction

. In the present joint experimental and theoretical study we investigate the vectorial momentum spectrum at very low energies. Using a reaction microscope optimized for the detection of very low energy electrons, we have performed a thorough study of the three-dimensional momentum spectrum well below 1 eV. Our measurements are complemented by quantum and classical simulations, which allow for an interpretation of the LES, VLES and of the newly identified ZES in terms of two-dimensional Coulomb focusing and recapture into Rydberg states, respectively.

Origins of Very-Low and Zero-Energy Electron Structures in Strong-Field Ionization with Intense Mid-IR Pulses

Understanding the interplay between optical pulse parameters and ultrafast material response is critical in achieving efficient and controlled laser nanomachining. In general, the key to initiate material processing is the deposition of a sufficient energy density within the electronic system. In dielectrics this critical energy density corresponds typically to a plasma frequency in the near-IR spectral region. Creating this density instantaneously with ultrashort laser pulses of a few tens of femtoseconds pulse duration in the same spectral region, the penetration depth into the material will strongly decrease with increasing electron density. Consequently, staying below this critical density will allow deep penetration depths. This calls for delayed ionization processes to deposit the energy for processing, thus introducing the temporal structure of the laser pulses as a control parameter. In this contribution we demonstrate this concept experimentally and substantiate the physical picture with numerical calculations. Bandwidth-limited pulses of 30 fs pulse duration are stretched up to 1.5 ps either temporally symmetrically or temporally asymmetrically. The interplay between pulse structure and material response is optimally exploited by the asymmetrically structured temporal Airy pulses leading to the inherently efficient creation of high aspect ratio nanochannels. Depths in the range of several micrometers and diameters around 250 nm are created within a single laser shot and without making use of self-focusing and filamentation processes. In addition to the machining of nanophotonic devices in dielectrics, the technique has the potential to enhance laser-based nanocell surgery and cell poration techniques.

Material Processing of Dielectrics via Temporally Shaped Femtosecond Laser Pulses as Direct Patterning Method for Nanophotonic Applications

The combination of various 2D layered materials in multilayer heterostructures arises great interest in the current science. Due to the large variety of electronic properties of the group of 2D layered materials the combination opens a new pathway towards ultrasmall electronic devices. In this contribution we present a full mathematical description of multilayer heterostructure samples and their diffraction patterns including a proposal of a consistent assignment of the superstructure diffraction spots. A 27nm thick MoS2-graphite heterostructure was produced and fully analysed with the methods presented in this paper.

Modelling, design and fabrication of dielectric photonic crystal structures using temporally asymmetric shaped femtosecond laser pulses

We show that spectral phase shaping of fs-laser pulses can be used to optimize laser-cell membrane interactions in water environment. The energy and peak intensity thresholds required for cell poration with single pulse in the nJ range can be significantly reduced (25% reduction in energy and 88% reduction in peak intensity) by using temporal Airy pulses, controlled by positive third order dispersion, as compared to bandwidth limited pulses. Temporal Airy pulses are also effective to control the morphology of the induced pores, with prospective applications from cellular to tissue opto-surgery and transfection.

Temporally shaped femtosecond laser pulses as direct patterning method for dielectric materials in nanophotonic applications

We report the simultaneous retrieval of multiple bond lengths of the polyatomic molecule C₂H₂ by coherent “self-electron” wave packets generated by the interaction of the molecular target with intense mid-IR laser pulses [1].