Bernd Schütte

Continuously tunable laser emission from a wedge-shaped organic microcavity

We present an organic microcavity laser with wide tunability in the range of 595–650nm, having a threshold as low as 2nJ/pulse. The active medium consists of the organic composite tris(8-hydroxy quinoline) aluminum (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) embedded in between two distributed Bragg reflectors. The inhomogeneously broadened emission from DCM is exploited for lasing by means of a tunable Fabry–Perot-type interferometer. Continuous tuning is achieved by varying the thickness of the active layer. The device shows a high photostability under direct excitation in the absorption band of the guest molecule, thus, allowing to monitor the optical gain spectrum of the active medium.

Electron wave packet sampling with laser-generated extreme ultraviolet and terahertz fields

We report on transferring the concept of light-field streaking with intense terahertz fields from free-electron lasers to the laboratory scale. Utilizing a commercial laser system, synchronized 300 μm terahertz and 13 nm extreme ultraviolet pulses are generated by optical rectification and high harmonic generation, respectively. The terahertz fields are sufficiently strong to support electron wave packet sampling with a few fs resolution. The capability of this approach is demonstrated by measuring the duration of electron pulses formed by direct photoemission from a neon gas target.

Observation of correlated electronic decay in expanding clusters triggered by near-infrared fields

When an excited atom is embedded into an environment, novel relaxation pathways can emerge that are absent for isolated atoms. A well-known example is interatomic Coulombic decay, where an excited atom relaxes by transferring its excess energy to another atom in the environment, leading to its ionization. Such processes have been observed in clusters ionized by extreme-ultraviolet and X-ray lasers. Here, we report on a correlated electronic decay process that occurs following nanoplasma formation and Rydberg atom generation in the ionization of clusters by intense, non-resonant infrared laser fields. Relaxation of the Rydberg states and transfer of the available electronic energy to adjacent electrons in Rydberg states or quasifree electrons in the expanding nanoplasma leaves a distinct signature in the electron kinetic energy spectrum. These so far unobserved electron-correlation-driven energy transfer processes may play a significant role in the response of any nano-scale system to intense laser light.

Ionization avalanching in clusters ignited by extreme-ultraviolet driven seed electrons

We study the ionization dynamics of Ar clusters exposed to ultrashort near-infrared (NIR) laser pulses for intensities well below the threshold at which tunnel ionization ignites nanoplasma formation. We find that the emission of highly charged ions up to Ar^{8+} can be switched on with unit contrast by generating only a few seed electrons with an ultrashort extreme-ultraviolet (XUV) pulse prior to the NIR field. Molecular dynamics simulations can explain the experimental observations and predict a generic scenario where efficient heating via inverse bremsstrahlung and NIR avalanching is followed by resonant collective nanoplasma heating. The temporally and spatially well-controlled injection of the XUV seed electrons opens new routes for controlling avalanching and heating phenomena in nanostructures and solids, with implications for both fundamental and applied laser-matter science.

Bright attosecond soft X-ray pulse trains by transient phase-matching in two-color high-order harmonic generation.

We study two-color high-order harmonic generation in Neon with 790 nm and 1300 nm driving laser fields and observe an extreme-ultraviolet continuum that extends to photon energies of 160 eV. Using a 6-mm-long, high pressure gas cell, we optimize the HHG yield at high photon energies and investigate the effect of ionization and propagation under phase-matching conditions that allow us to control the temporal structure of the XUV emission. Numerical simulations that include the 3D propagation of the two-color laser pulse show that a bright isolated attosecond pulse with exceptionally high photon energies can be generated in our experimental conditions due to an efficient hybrid optical and phase-matching gating mechanism.

Correlated electronic decay in expanding clusters triggered by intense XUV pulses from a Free-Electron-Laser

Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states. The dynamics of intense laser-cluster interaction is encoded in electron kinetic energy spectra, which contain signatures of direct photoelectron emission as well as emission of thermalized nanoplasma electrons. In this work we report on a so far not observed spectrally narrow bound state signature in the electron kinetic energy spectra from mixed Xe core - Ar shell clusters ionized by intense extreme-ultraviolet (XUV) pulses from a free-electron-laser. This signature is attributed to the correlated electronic decay (CED) process, in which an excited atom relaxes and the excess energy is used to ionize the same or another excited atom or a nanoplasma electron. By applying the terahertz field streaking principle we demonstrate that CED-electrons are emitted at least a few picoseconds after the ionizing XUV pulse has ended. Following the recent finding of CED in clusters ionized by intense near-infrared laser pulses, our observation of CED in the XUV range suggests that this process is of general relevance for the relaxation dynamics in laser produced nanoplasmas.

Tracing transient charges in expanding clusters

We study transient charges formed in methane clusters following ionization by intense nearinfrared laser pulses. Cluster ionization by 400 fs (I = 1 × 10 W/cm) pulses is highly efficient, resulting in the observation of a dominant C ion contribution. The C ion yield is very small, but is strongly enhanced by applying a time-delayed weak near-infrared pulse. We conclude that most of the valence electrons are removed from their atoms during the laser-cluster interaction, and that electrons from the nanoplasma recombine with ions and populate Rydberg states when the cluster expands, leading to a decrease of the average charge state of individual ions. Furthermore, we find clear bound-state signatures in the electron kinetic energy spectrum, which we attribute to Auger decay taking place in expanding clusters. Such nonradiative processes lead to an increase of the final average ion charge state that is measured in experiments. Our results suggest that it is crucial to include both recombination and nonradiative decay processes for the understanding of recorded ion charge spectra.

Oblique angle lasing in a periodically pumped organic microcavity

We investigate a planar organic microcavity under spatially periodic optical excitation. The host:guest system of Alq3:DCM is the emitting layer embedded in between two dielectric mirrors. Excitation by an interference field of two femtosecond laser pulses generates an array of lasers spaced by few microns. The far field of the cavity response shows conventional stimulated emission at k=0 and, in addition, two stripes of laser emission at oblique angles. The excitation pattern generates a periodic modification of the optical properties of the cavity, a dynamic diffraction grating with a period of few microns. This enhances the spontaneous emission in the direction of the Bragg angle, which depends on the distance of the interference stripes. Via the angle of incidence of the excitation beams, we can optically tune output angle and the wavelength of lasing. Measurements are confirmed by simulations of the mode dynamics inside a lossy cavity with small excitation spot sizes, where the local gain exceeds the total mirror and absorptive losses. We find that adjacent cavity quasimodes couple out of phase at certain separation distances, which critically depend on the quasimode radius and, thus, on the residual absorption. Thus, we gain insight into the development of coherence and mode-locking in microcavities.

Lower limit of the lasing threshold in an organic microcavity

The application of organic materials as solid state lasers critically relies on a low lasing threshold. We investigate the characteristics of emission from an organic vertical cavity surface emitting laser. The microcavity studied here consists of two highly reflective distributed Bragg reflectors enclosing a wedge-shaped active layer of Alq3:DCM. Lasing of the DCM molecules is induced via two different pump regimes, either exciting Alq3 at a wavelength of 400 nm or pumping directly into the absorption band of DCM at 532 nm. By a variation of the pump beam position with respect to the microcavity surface, we demonstrate a continuous wavelength tuning in the organic microcavities in a range of 55 nm. The continuously variable cavity thickness allows us to study the thickness dependence of the input-output characteristics in a single sample. These data are obtained at a certain emission wavelength, λ, close to the maximum of the gain spectrum, for a number of cavity thicknesses, which correspond to different multiples of λ/2. For a decreasing thickness of the active layer, one-dimensional optical confinement is expected to result in an increased spontaneous emission factor. On the other hand, the loss rate through the mirrors increases with decreasing thickness resulting in a minimum threshold value for an active layer thickness of approximately 3/2 λ. This lower threshold limit is set by nonradiative losses as well as residual absorption.