Martin Bock

Efficient second harmonic generation in ZnO nanorod arrays with broadband ultrashort pulses

Broadband frequency-doubling properties of c-axis oriented zinc oxide (ZnO) nanorod arrays grown by low-temperature chemical bath method on glass substrate were studied. The maximum effective nonlinearity was found to be about 7.5 times higher than that of a type-I beta-barium borate crystal for a pump intensity of 5.5×1010 W/cm2. The angular dependence of second harmonic generation (SHG) was determined experimentally. The measured spectral profile of SHG was found to be in good agreement with theoretical simulations.

Femtosecond-laser-induced quasiperiodic nanostructures on TiO2 surfaces

High-spatial frequency, quasiperiodic structures (HSFL, Nanoripples) of 170 nm feature size were induced in rutile-type titanium dioxide surfaces by focused 150 fs Ti:sapphire laser pulses at wavelengths around 800 nm. The ripple formation is distinctly visible for numbers of pulses of N=100–1000. At lower number of pulses (N=10), a significant surface roughening appears instead of ripples which is characterized by randomly meandering nanostructures. These observations confirm an essential contribution of early stage irregular material modifications to the dynamics of quasiperiodic ripple formation. The threshold fluence for ripple generation is estimated on the basis of the conventional theory of laser-induced surface structuring. The decrease in the threshold fluence from 0.34 to 0.24 J/cm2, as it was found for an increase in the number of pulses from N=100 to N=1000, is attributed to a damage accumulation effect. Nanostructuring of spatially extended regions was enabled by utilizing a controlled sample...

Highly efficient THG in TiO 2 nanolayers for third-order pulse characterization

Third harmonic generation (THG) of femtosecond laser pulses in sputtered nanocrystalline TiO2 thin films is investigated. Using layers of graded thickness, the dependence of THG on the film parameters is studied. The maximum THG signal is observed at a thickness of 180 nm. The corresponding conversion efficiency is 26 times larger compared to THG at the air-glass interface. For a demonstration of the capabilities of such a highly nonlinear material for pulse characterization, third-order autocorrelation and interferometric frequency-resolved optical gating (IFROG) traces are recorded with unamplified nanojoule pulses directly from a broadband femtosecond laser oscillator.

Reconfigurable wavefront sensor for ultrashort pulses

A highly flexible Shack-Hartmann wavefront sensor for ultrashort pulse diagnostics is presented. The temporal system performance is studied in detail. Reflective operation is enabled by programming tilt-tolerant microaxicons into a liquid-crystal-on-silicon spatial light modulator. Nearly undistorted pulse transfer is obtained by generating nondiffracting needle beams as subbeams. Reproducible wavefront analysis and spatially resolved second-order autocorrelation are demonstrated at incident angles up to 50° and pulse durations down to 6 fs.

Spectral and temporal response of liquid-crystal-on-silicon spatial light modulators

Spectral and temporal phase response of selected types of liquid-crystal-on-silicon spatial light modulators were studied using femtosecond pulses, determining specific transfer functions of the devices. The phase response resulting from programed gray level distributions was detected by analyzing the diffraction characteristics and by spectral phase interferometry for direct electric-field reconstruction. The results indicate the appearance of distinct parameter ranges that enable minimum spatiotemporal distortion. Weak oscillations in the spectral phase are explained by Gires–Tournois resonances [F. Gires and P. Tournois, Acad. Sci. Paris, C. R. 258, 6112 (1964)].

Ultrashort-pulsed truncated polychromatic Bessel-Gauss beams

Spatio-spectral and spatio-temporal transfer and intensity propagation of truncated ultrashort-pulsed Bessel-Gauss beams were investigated. Extended needle-shaped focal zones were generated using a compact setup with a reflective small-angle axicon and self-apodized truncation by an adapted aperture. Spectral maps of Bessel-Gauss beams were analyzed on the basis of higher order statistical moments. Compared to focused pulsed Gaussian beams with their spectrally dependent propagation, an ultrabroadband spatio-spectral transfer function was detected over Rayleigh ranges exceeding 10 cm. These results indicate favorable pseudo-nondiffracting characteristics not only from the point of view of spatial propagation but also with respect to the spectral and temporal stability.

Few-cycle high-contrast vortex pulses

Few-cycle high-contrast vortex beams with pulse durations around 8 fs were generated from a Ti:sapphire laser oscillator with a single diffractive-refractive component. Angular and temporal pulse properties were characterized with an advanced time-wavefront sensor. The temporal transfer indicates a fairly complete self-compensation.

Multiphoton-absorption induced ultraviolet luminescence of ZnO nanorods using low-energy femtosecond pulses

Multiphoton-absorption (MPA) induced ultraviolet (UV) luminescence of ZnO nanorods grown by vapor phase transport was demonstrated using ultrafast excitation at pulse energies in the few nanojoules range, directly generated by a Ti:sapphire laser oscillator at wavelengths around 800 nm. The dependence of the UV luminescence on the excitation density reveals a two-photon absorption process as the responsible excitation mechanism. The broad spectral bandwidth of the excitation pulses obviously promotes the feasibility of the observed two-photon channel. Theoretical estimates concerning the contribution of nonlinear absorbance strongly support the experimental findings. The essential conditions for proper utilization of this process are discussed.

Angular tolerance of Shack-Hartmann wavefront sensors with microaxicons

Wavefront sensing under nonparaxial conditions was studied with Shack-Hartmann setups based on arrays of microaxicons. The robustness of the generated pseudonondiffracting subbeams against tilt and axial displacement was demonstrated for ultraflat Gaussian- and inverse-Gaussian-shaped elements in transmission and reflection. To characterize slight aberrations and to identify optimum parameter fields, spatial moments of intensity profiles were analyzed with high sensitivity. Reflective design enables for wavefront sensing at oblique incidence as necessary for low-feedback detection and phase diagnostics of ultrashort pulses.

Ultrashort highly localized wavepackets.

The recently introduced concept of radially non-oscillating, temporally stable ultrashort-pulsed Bessel-like beams we referred to as needle beams is generalized to a particular class of highly localized wavepackets (HLWs). Spatio-temporally quasi-nondiffracting pulses propagating along extended zones are shaped from Ti:sapphire oscillator radiation with a spatial light modulator and characterized with spatially resolved second order autocorrelation. Few-cycle wavepackets tailored to resemble circular disks, rings and bars of light represent the closest approximation of linear-optical light bullets known so far. By combining multiple HLWs, complex pulsed nondiffracting patterns are obtained.