Adaptive spiral phase elements for the generation of few-cycle vortex pulses

摘要

Vortex beams with an optical angular momentum (OAM) are of increasing interest for optical tweezers [1], materials processing [2] or quantum information [3]. For specific applications, ultrashort wavepackets with OAM are required. Recently, 2.3-cycle vortex pulses were generated with an optical parametric amplification system [4]. From a more compact setup with a self-compensated diffractive spiral phase axicon, 8-fs Ti:sapphire laser pulses with OAM were extracted [5]. The combination of intense few-cycle vortex pulses with the flexibility of adaptive elements opens further prospects for leading-edge technologies. Here we present recent results on two alternative approaches: (a) liquid-crystal-on-silicon spatial light modulators (LCoS-SMs), and (b) micro-electro-mechanical systems (MEMS). MEMS axicons were originally developed as variable shapers of Bessel beams [6,7]. The concept was extended to include an OAM as additional degree of freedom. A spiral MEMS was built via the following steps. An Si mould was structured by 3D layer-by-layer UV-laser ablation. Liquid PDMS monomer is poured into the mould and the chip is subjected to vacuum. A second blank Si chip carrying the reflective layer is pressed on top of the PDMS. The polymer is thermally cured. Upon heating, the liquid monomer expands prior to crosslinking and excess polymer is pushed out. The mirror layer is revealed by etching through the top Si substrate. Due to the stress induced by thermal shrinkage of the polymer, the structure of the Si mould is transferred to the mirror layer. The Si roughness is smoothed by the polymer down to 6 nm. The fabrication process is explained more in detail in ref. [7]. The mirror deflection depends linearly on the temperature (Fig. 1a). The topologies of mirror and Si substructure are shown in Figs. 1b and c. The propagation of vortex pulses was studied in spatial and temporal domain with shearography and spectral phase interferometry for direct electric field reconstruc- ion (FC-SPIDER). The OAM was detected with a wavefront sensor [5]. It was found that SLMs enable for a larger spatial variability but are limited concerning the phase, whereas the MEMS work at lower dispersion but higher phase steps.