Abstract

For novel spatio-temporal pulse characterization techniques, adaptive materials processing [1], advanced microscopy, tweezers and other emerging applications, adaptive axicons capable to shape highly robust nondiffracting beams of variable parameters are of increasing interest. Conical phase profiles can be approximated by pixellated liquid crystal based spatial light modulators [2] which reach a high spatial resolution even at ultrashort pulses but suffer from limitations with respect to obtainable phase steps, switching speed, diffraction losses and damage resistance. One alternative approach is to combine the specific advantages of highly reflective analogue micro-electromechanical systems (MEMS) with the optical functionality of continuous conical axicon profiles [3]. This promises to work at minimum dispersion and minimum diffraction and, simultaneously, with extended undistorted propagation zones. Here we report on first experimental results obtained with a new type of tunable mechanical axicon mirror. Its design is based on a piezo-disk with 5 concentric circular electrodes (Ag) and a broadband reflecting aluminium layer on top (Figure 2). The local bending radii of the disk correspond to the voltages at the individual electrodes (≤ 60 V). The envelope phase function was approximated by automated FEM simulations (about 50 iterations). Beam propagation of shaped ultrashort pulses was simulated with VirtualLab (Light Trans). Supporting structures and the substrate for the mirror were fabricated from Si wafers coated with polydimethylsiloxane (PDMS) and aluminium, respectively. The surface quality was characterized interferometrically (Figure 1). A maximum initial bending of < 3 µm over 15 mm diameter was found. Voltage dependent shape variations were detected (a) for the stationary case by phase shift interferometry and with a laser profilometer, (b) by analyzing the far field intensity patterns of a cw source at 532 nm (Figures 2 c-e), and dynamically with a stroboscopic setup. It was shown that the device can be operated up to a frequency of 3 kHz. Variable Bessel-like zones (here indicated by narrow rings in the far field) were created by addressing variable combinations of electrodes (Figures 2 c-e). Femtosecond pulse transfer was studied with a Ti:sapphire oscillator (Venteon, pulse duration < 6 fs, spectral bandwidth > 200 nm, repetition rate 80 MHz) and an LX-SPIDER (APE) for detecting spectral phase changes. Scattering and slightly asymmetric angular profiles indicate small deviations from a perfect geometry which still have to be overcome. To conclude, adaptive MEMS axicon mirrors based on piezo-disks with multiple individually addressable electrodes were studied in proof-of-principle experiments. The capability to flexibly shape Bessel-like beams at ultrashort pulse durations was demonstrated. Improvements of quality and design, a further development of characterization techniques and applications in ultrashort-pulse physics are a subject of continuing experiments.

© 2011 Optical Society of America