Ultrafast laser-induced plasma anisotropy in pristine and surface pre-structured zinc telluride, probed by terahertz pulses.

We use THz probe pulses to detect and analyze the dynamics of charge transport anisotropies generated by ultrafast laser two-photon absorption in Zinc Telluride (ZnTe) semi-insulating crystal showing smooth and laser structured surfaces. The detected anisotropy consists in a modulation of the THz transmission as a function of the orientation of the <001 > axis of ZnTe. The change in THz transmission after pump excitation is attributed to free carrier absorption of the THz field in the laser-induced electron-hole plasma. Pre-structuring the surface sample with laser-induced periodic surface structures (ripples) has strong influence on free carrier THz transmission and its associated anisotropic oscillation. Within the relaxation dynamics of the laser-induced free carriers, two relaxation times have to be considered in order to correctly describe the dynamics, a fast relaxation, of about 50 picoseconds in pristine sample (90 picoseconds in sample pre-structured with ripples), and a slow one, of about 1.5 nanoseconds. A theoretical model based on classical Drude theory and on the dependence of the two-photon absorption coefficient with the crystal orientation and with the laser polarization is used to fit the experimental results.

Two-photon excitation and stimulated emission depletion by a single wavelength.

Super-resolved optical microscopy using stimulated emission depletion (STED) is now a mature method for imaging fluorescent samples at scales beyond the diffraction limit. Nevertheless the practical implementation of STED microscopy is complex and costly, especially since it requires laser beams with different wavelengths for excitation and depletion. In this paper, we propose using a single wavelength to induce both processes. We studied stimulated emission depletion of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) dye with a laser delivering a single wavelength in the near infrared. Fluorescence was excited by two photon absorption with a femtosecond pulse, then depleted by one photon stimulated emission with a stretched pulse. Time-resolved fluorescence decay measurements were performed to determine the depletion efficiency and to prove that fluorescence quenching is not affected by side effects. Numerical simulations show that this method can be applied to super-resolved microscopy.

Multiscale electronic and thermomechanical dynamics in ultrafast nanoscale laser structuring of bulk fused silica.

We describe the evolution of ultrafast-laser-excited bulk fused silica over the entire relaxation range in one-dimensional geometries fixed by non-diffractive beams. Irradiation drives local embedded modifications of the refractive index in the form of index increase in densified glass or in the form of nanoscale voids. A dual spectroscopic and imaging investigation procedure is proposed, coupling electronic excitation and thermodynamic relaxation. Specific sub-ps and ns plasma decay times are respectively correlated to these index-related electronic and thermomechanical transformations. For the void formation stages, based on time-resolved spectral imaging, we first observe a dense transient plasma phase that departs from the case of a rarefied gas, and we indicate achievable temperatures in the excited matter in the 4,000-5,500 K range, extending for tens of ns. High-resolution speckle-free microscopy is then used to image optical signatures associated to structural transformations until the evolution stops. Multiscale imaging indicates characteristic timescales for plasma decay, heat diffusion, and void cavitation, pointing out key mechanisms of material transformation on the nanoscale in a range of processing conditions. If glass densification is driven by sub-ps electronic decay, for nanoscale structuring we advocate the passage through a long-living dense ionized phase that decomposes on tens of ns, triggering cavitation.

Tuning and focusing THz pulses by shaping the pump laser beam profile in a nonlinear crystal.

Spatially shaped femtosecond laser pulses are used to generate and to focus tunable terahertz (THz) pulses by Optical Rectification in a Zinc Telluride (ZnTe) crystal. It is shown analytically and experimentally that the focusing position and spectrum of the emitted THz pulse can be changed, in the intermediate field zone, by controlling the spatial shape of the near-infrared (NIR) femtosecond (fs) laser pump. In particular, if the pump consists of concentric circles, the emitted THz radiation is confined around the propagation axis, producing a THz pulse train, and focusing position and spectrum can be controlled by changing the number of circles and their diameter.

Revival of femtosecond laser plasma filaments in air by a nanosecond laser.

Short lived plasma channels generated through filamentation of femtosecond laser pulses in air can be revived after several milliseconds by a delayed nanosecond pulse. Electrons initially ionized from oxygen molecules and subsequently captured by neutral oxygen molecules provide the long-lived reservoir of low affinity allowing this process. A Bessel-like nanosecond-duration laser beam can easily detach these weakly bound electrons and multiply them in an avalanche process. We have experimentally demonstrated such revivals over a channel length of 50 cm by focusing the nanosecond laser with an axicon.

A simple method for determination of nonlinear propagation regimes in gases.

A simple method to evaluate the nonlinear propagation regimes in gases is demonstrated. The principle is to focus ultrashort laser pulses into a gas cell, vary the input pulse power and measure the transmission through a pinhole placed at the output. The resulting transmission curve yields an intuitive signature of various nonlinear propagation regimes. Going from low powers to higher, one first observes a brief decrease in the transmission due to nonlinear moving focus. Then, a sharp rise occurs, indicating the starting of the filamentation process. When the power increases further, the transmission saturates and eventually decreases due to the beginning of multi-filamentation. As a result, this method gives a quick and sensitive measurement of pulse energies required to have single and multiple filaments.