2020, Liebmann, Max, Treffer, Alexander, Bock, Martin, Wallrabe, Ulrike, Grunwald, Ruediger

Recently, the spatio-spectral propagation dynamic of ultrashort-pulsed vortex beams was demonstrated by 2D mapping of spectral moments. The rotation of characteristic anomalies, so-called "spectral eyes", was explained by wavelength-dependent Gouy phase shift. Controlling of this spectral rotation is essential for specific applications, e.g., communication and processing. Here, we report on advanced concepts for spectral rotational control and related first-proof-of-principle experiments. The speed of rotation of spectral eyes during propagation is shown to be essentially determined by angular and spectral parameters. The performance of fixed diffractive optical elements (DOE) and programmable liquid-crystal-on silicon spatial light modulators (LCoS-SLMs) that act as spiral phase gratings (SPG) or spiral phase plates (SPP) is compared. The approach is extended to radially chirped SPGs inducing axially variable angular velocity. The generation of time-dependent orbital angular momentum (self-torque) by superimposing multiple vortex pulses is proposed. © 2020 by the authors.

2017, Lübcke, Andrea, Andreev, Alexander A., Höhm, Sandra, Grunwald, Ruediger, Ehrentraut, Lutz, Schnürer, Matthias

In laser-based proton acceleration, nanostructured targets hold the promise to allow for significantly boosted proton energies due to strong increase of laser absorption. We used laser-induced periodic surface structures generated in-situ as a very fast and economic way to produce nanostructured targets capable of high-repetition rate applications. Both in experiment and theory, we investigate the impact of nanostructuring on the proton spectrum for different laser-plasma conditions. Our experimental data show that the nanostructures lead to a significant enhancement of absorption over the entire range of laser plasma conditions investigated. At conditions that do not allow for efficient laser absorption by plane targets, i.e. too steep plasma gradients, nanostructuring is found to significantly enhance the proton cutoff energy and conversion efficiency. In contrast, if the plasma gradient is optimized for laser absorption of the plane target, the nanostructure-induced absorption increase is not reflected in higher cutoff energies. Both, simulation and experiment point towards the energy transfer from the laser to the hot electrons as bottleneck.

2015, Wallrabe, Ulrike, Brunne, Jens, Treffer, Alexander, Grunwald, Ruediger, Bellouard, Yves

We report on the design, fabrication and testing of novel types of low-dispersion axicons for the adaptive shaping of ultrashort laser pulses. An overview is given on the basic geometries and operating principles of our purely reflective adaptive MEMS-type devices based on thermal or piezoelectric actuation. The flexible formation of nondiffracting beams at pulse durations down to a few oscillations of the optical field enables new applications in optical communication, pulse diagnostics, laser-matter interaction and particle manipulation. As an example, we show first promising results of adaptive autocorrelation. The combination of excellent pulse transfer, self-reconstruction properties and propagation invariance of nondiffracting beams with an adaptive approach promises to extend the field of practical applications significantly.

2020, Poyyathuruthy Bruno, Binal, Schütze, Robert, Grunwald, Ruediger, Wallrabe, Ulrike

We present the design and fabrication of a miniaturized array of piezoelectrically actuated high speed Fresnel mirrors with individual mirror control. These Fresnel mirrors can be used to generate propagation invariant and self-healing interference patterns. The mirrors are actuated using piezobimorph actuators, and the consequent change of the tilting angle of the mirrors changes the fringe spacing of the interference pattern generated. The array consists of four Fresnel mirrors each having an area of 2 × 2 mm2 arranged in a 2x2 configuration. The device, optimized using FEM simulations, is able to achieve maximum mirror deflections of 15 mrad, and has a resonance frequency of 28 kHz.