High power vortex generation with volume phase holograms and non-linear experiments in gases

Abstract

An experimental method for the production of high power optical vortices by using volume phase holographic plates is presented. Experiments in air, N 2 and Ar have been performed as an example of the method's potential, observing non-linear effects and filamentation. Theoretical calculations support the conclusions. Optical vortices (OVs) have been the object of intensive investigation due to their special properties concerning orbital angular momentum and spatial distribution [1]. Most of the works related to OVs have involved continuouswave (cw) and quasi-cw regimes. Recently, the generation of OVs in very short and intense light bursts has been a subject of interest. This will allow us to have access into the high-power regime for studying experimentally the behavior of non-linear vortex structures (i.e. propagation in Kerr media and filamentation).In order to generate the OVs with femtosecond pulses, two main approaches can be found in the literature. Both techniques have been exported from the cw regime, where they had already been consolidated. First, spiral phase plates (SPPs) are media where the optical path increases with the azimuthal angle, yielding an output beam with a helical phase front [2]. It has been shown that it is possible to generate vortex structures at the Ti:sapphire emission wavelength with efficiency of 55% and high damage thresholds [3]. Other materials, such as resists, offer higher efficiencies (around 80% [4]) and still quite high damage thresholds in the case of photoresists [5]. Therefore, SPPs appear as promising candidates to high power OV generation [6]. However, a recent work establishes some limitations of the technique when very short pulses are used [7]. In fact, for spectra presenting a FWHM higher than 40 nm, spatial chirp effects appear. Therefore, this u Fax: +34-923-2945-84, E-mail: ijsola@usal.es technique may have problems for pulses shorter than 30 fs. On the other hand, the manufacture of SPPs is expensive and demanding.The second technique uses the so-called computer-generated holograms (CGHs), gratings presenting some dislocations in the diffraction pattern that creates the vortex. Usually they consist of an amplitude mask [8], but alternative processing of CGHs has been reported [9]. Also, dynamic CGHs have been tested using liquid-crystal media [10]. In general, the CGH technique presents lower efficiencies than those obtained with the SPPs. However, because of its simplicity of production compared to the SPP making up process, it is a quite popular technique, too. In addition, CGHs have been adapted to the femtosecond regime using 2f-2f set-ups, in order to compensate the spatial chirp [11,12]. The main limitations of CGHs to date are the low damage threshold of the materials used, preventing the generation of high-power OVs, and the very low efficiency (as an exception, CGHs consisting of bleached plates [9] and liquid crystals (LCs) [10] present higher efficiency, compared to the more common amplitude CGHs).We propose an alternative way to generate...

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