MEMS axicons for nondiffracting line shaping of ultrashort pulses

Abstract

For a growing number of applications in nonlinear spectroscopy, micro-and nano-machining, optical data processing, metrology or medicine, an adaptive shaping of ultrashort pulsed, ultrabroadband laser beams into propagation-invariant linear focal zones is required. One example is the femtosecond laser high-speed large area nanostructuring with moving substrates and cylindrical optics we reported about recently. Classical microoptical systems, however, distort the temporal pulse structure of few cycle pulses by diffraction and dispersion. The temporal pulse transfer can be improved with innovative types of reflective MEMS axicons based on two integrated rectangular mirrors tilted by piezo-actuators. In contrast to pixelated liquid-crystal-on-silicon (LCoS) based devices, cutoff frequencies in multi-kilohertz range, a purely reflective setup and continuous profiles with larger phase shift are realized which enable for shaping extended propagation-invariant zones at a faster and more robust operation. Additionally, a fixed phase offset can be part of the structure. Here, the performance of a prototype of linear mechanically tunable MEMS axicon is demonstrated by generating a pseudo-nondiffracting line focus of variable diameter and depth extension from a femtosecond laser pulse. The temporal transfer of 6-fs pulses of a Ti:sapphire laser oscillator is characterized with spectral phase interferometry for direct electric-field reconstruction (SPIDER) and spatially resolved nonlinear autocorrelation. Spatial and temporal self-reconstruction properties were studied. The application of the flexible focus to the excitation of plasmon-polaritons and the self-organized formation of coherently linked deep sub-wavelength laser-induced periodic surface structures (LIPSS) in semiconductors and dielectrics is reported.

Adaptive micro axicons for laser applications

“…The obtaineded FWHM time duration of the autocorrelation function in (a,c) is unrealistically short because of a spatial cut due to incomplete spatial superposition. Incontrast, the spatial superposition in (b,d) is nearly complete and delivers a pulse duration close to the value expected by reference measurements with spectral phase interferometry by direct electric-field reconstruction (SPIDER) [9]. The corresponding pulse duration for the used configuration was found to be about 8 fs.…”

“…By the superposition of both partial beams, this leads to the formation of "light blades" -extended (quasi-nondiffracting) linear focal zones [9]. Their number, thickness and length can be tuned by adjusting the tilt angle of the mirrors.…”

“…Application experiments were performed to generate variable excitation zones for laser-induced periodic surface structures (LIPSS) [10] and to study the adaptive autocorrelation of few-cycle pulses [11]. In this case, the self-referential interferometric setup (known from inducing holographic gratings) in combination with a thin nonlinear crystal (beta barium borate, BBO) detects the second order autocorrelation function, thus acting as a non-collinear autocorrelator.…”

Micro Fresnel mirror array with individual mirror control

“…Fresnel mirrors can, for example, be used to fabricate linear optical gratings directly into the photoresist without the use of a photomask [3]. It was shown that an adaptive Fresnel mirror can be used in adaptive pulse autocorrelation [4], and for direct nano-machining of subwavelength structures on silicon or dielectric substrates by laser-induced periodic surface structures (LIPSS) [5,6]. LIPSS are formed by the interference of incident and scattered light pulses on the surface of the material [7].…”

A tunable piezoelectric Fresnel mirror for high-speed lineshaping

“…Recently, we proposed the use of an adaptive Fresnel mirror for direct nanomachining of sub-wavelength structures into silicon or dielectric substrates by laser-induced periodic surface structures (LIPSS), generated by femtosecond laser pulses [9][10][11]. In this application, the Fresnel mirror is intended to deliver a sharp line pattern, while introducing only a minimal amount of optical dispersion into the system.…”