Diffractive control of 3D multifilamentation in fused silica with micrometric resolution

Spatio-temporal controlled filamentation using higher order Bessel-Gaussian beams integrated in time

We demonstrate a new method for a systematic, dynamic, high-speed, spatio-temporal control of femtosecond light filamentation in BK7 as a particular example of nonlinear medium. This method is based on using coherent conjugate asymmetric Bessel-Gaussian beams to control the far-field intensity distribution and in turn control the filamentation location. Such spatio-temporal control allows every femtosecond pulse to have a unique intensity distribution that results in the generation of structured filamentation patterns on demand. The switching speed of this technique is dependent on the rise time of the acousto-optic deflector, which can operate in the MHz range while having the ability to handle high peak power pulses that are needed for nonlinear interactions. The proposed and demonstrated spatio-temporal control of structured filaments can enable generation of large filament arrays, opto-mechanical manipulations of water droplets for fog clearing, as well as engineered radiofrequency plasma antennas.

DNA Base Modifications Mediated by Femtosecond Laser-Induced Cold Low-Density Plasma in Aqueous Solutions

Applications based on near-infrared femtosecond laser-induced plasma in biological materials involve numerous ionization events that inevitably mediate physicochemical effects. Here, the physical chemistry underlying the action of such plasma is characterized in a system of biological interest. We have implemented wavefront shaping techniques to control the generation of laser-induced low electron density plasma channels in DNA aqueous solutions, which minimize the unwanted thermo-mechanical effects associated with plasma of higher density. The number of DNA base modifications per unit of absolute energy deposited by such cold plasma is compared to those induced by either ultraviolet or standard ionizing radiation (γ-rays). Analyses of various photoinduced, oxidative, and reductive DNA base products show that the effects of laser-induced cold plasma are mainly mediated by reactive radical species produced upon the ionization of water, rather than by the direct interaction of the strong laser field with DNA. In the plasma environment, reactions among densely produced primary radicals result in a dramatic decrease in the yields of DNA damages relative to sparse ionizing radiation. This intense radical production also drives the local depletion of oxygen.

Tuning the size of gold nanoparticles produced by multiple filamentation of femtosecond laser pulses in aqueous solutions

In the present study, we consider the self-regulated generation of spatially homogeneous low density plasma (LDP) micro-channels as a high intensity ionization source arising from the multi-filamentation of powerful femtosecond (fs) laser pulses in aqueous solutions. We investigate the modulation of the femtosecond laser multiple filamentation for tuning the size of gold nanoparticles (AuNPs) synthesized in an irradiated gold chloride solution. Previous studies on the radiation-induced synthesis of colloidal gold by more conventional ionization sources, such as high energy γ-rays and electron beams, highlighted the dependence of the size distribution of AuNPs on the density of energy deposited per unit of time, i.e. the dose rate. The present method of laser-induced production of AuNPs rests on a similar radiation-assisted process, i.e. the reduction of the solvated trivalent gold ions by the hydrated electrons produced upon ionization of water. We find that trivial optical manipulation varies the rate of deposited energy by laser irradiation, which can be considered equivalent to a variation of the dose rate. We investigate the influence of varying the density of energy deposited on the laser-induced gold cluster size distribution and made a comparison with the high energy radiation-induced synthesis of AuNPs. Here, our results highlight that the present method of laser irradiation, in the regime of LDP generation, mimics the radiolysis of water at an adjustable high dose rate. More generally, these spatially and temporally resolved plasmas could be developed as a tool for the unprecedented control of chemistry under ionizing radiation.

Generation of multifocal irradiance patterns by using complex Fresnel holograms

We experimentally demonstrate Fresnel holograms able to produce multifocal irradiance patterns with micrometric spatial resolution. These holograms are assessed from the coherent sum of multiple Fresnel lenses. The utilized encoded technique guarantees full control over the reconstructed irradiance patterns due to an optimal codification of the amplitude and phase information of the resulting complex field. From a practical point of view, a phase-only spatial light modulator is used in a couple of experiments addressed to obtain two- and three-dimensional distributions of focal points to excite both linear and non-linear optical phenomena.