M. K. Bhuyan

Ultrafast laser nanostructuring in bulk silica, a “slow” microexplosion

Ultrafast laser microexplosions in bulk material create extreme conditions at mesoscopic scales and are essential to the synthesis of extraordinary matter structural phases and to light structuring beyond the diffraction limit. Observing the transformation cycle can elucidate their evolution. We discuss multiscale relaxation dynamics in the formation of nanoscale structures in laser-irradiated fused silica. Tightly focused ultrafast nondiffractive Bessel beams are used to generate microexplosions that lead to uniform voids. These trigger thermodynamic nonequilibrium conditions in one-dimensional geometries with record excitation confinement down to 100xa0nm and electronic pressures in the gigapascal range. Time-resolved phase-contrast microscopy on nanosecond to microsecond scales indicates that void formation is a slow process developing from low-viscosity phases after persistent plasma fluid stages signaled via nanosecond-long luminescence. The void evolution is not necessarily driven by rarefaction following initial pressure relaxation, but involves molecular kinetics and stress mechanisms that interfere with the evolution of the liquid phase and induce cavitation. Heat transport is also visualized. Higher energy leads to hydrodynamic instabilities and void fragmentation. The dynamic view helps us understand material transformation under confinement.

Role of free carriers excited by ultrafast Bessel beams for submicron structuring applications

Abstract. Ultrafast Bessel beams are ideal sources for high aspect ratio submicron structuring applications because of their nondiffracting nature and higher stability in nonlinear propagation. We report here on the interaction of ultrafast Bessel beams at various laser energies and pulse durations with transparent materials (fused silica) and define their impact on photoinscription regimes, i.e., formation of positive and negative refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the relevant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses, leading to soft positive index alterations via presumably nonthermally induced structural transitions involving defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to hydrodynamic expansion, resulting in high aspect ratio submicron-size wide voids. Delayed ionization, carrier defocusing, and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis.

Ultrafast laser-induced refractive index changesin Ge15As15S70 chalcogenide glass

Understanding and controlling laser-induced refractive index nmodifications in bulk chalcogenide glasses is important for a range of nphotonics applications targeting the mid-infrared spectral region. We focus nhere on material engineering aspects and pulse spatio-temporal design ncharacteristics that are able to induce and maintain positive refractive index nchanges in laser-irradiated Sulfur-based chalcogenide glass, mandatory for n3D photonic design. Specifically we study the photoinscription process of a nGe-doped Sulfur-based chalcogenide glass, Ge15As15S70, irradiated by nfocused ultrafast near-infrared laser pulses where Ge doping plays a ndeterminant role in generating high-contrast positive index changes. By nmeans of aposteriori and real-time in situ observations we show that npositive refractive index changes (type I) are the result of a restructuring of nthe glass matrix and a photo-induced contraction process initiated by twophoton nelectronic excitation leading to bond softening, molecular mobility, nstructural changes and rearrangements. Oppositely, negative refractive nindex changes (type II) could be associated with two different processes: nphoto-expansion at higher intensities and hydrodynamic evolution initiated nby plasma generation and laser heating, with thermomechanical relaxation nand stress unload. Alongside the role of Ge in setting various degrees of the nmatrix connectivity, the structural arrangement developed under different nthermal history schemes for glass preparation is equally important as it ndefines to which extent further structural flexibility is possible. Thus we nindicate the role of glass matrix metastability in generated high-contrast nrefractive index changes and we show that a higher degree of relaxation is nan impediment for contrasted positive index changes, while these are ndeveloping in unrelaxed glasses, where several degrees of structural nflexibility exist. Alongside dynamic time-resolved imaging experiments nprobing the development and relaxation of excitation, we also show, via nstatic Raman analysis of the modified regions, that significant structural nchanges are induced by laser irradiation and we discuss the potential nprocesses involved.

High fidelity visualization of multiscale dynamics of laser-induced bubbles in liquids containing gold nanoparticles

Cavitation in pure liquids and in liquids containing nanoparticles enables applications in mechanics, bio-medicine, and energy. Its evolution carries a significant interest. We describe the multiscale dynamic evolution of ultrafast-laser-induced cavitation in pure and gold-nanoparticles-doped liquids in one-dimensional geometries induced by non-diffractive ultrashort Bessel-Gauss laser beams. Covering the complete electronic and thermomechanical cycle, from the early plasma phase to bubble cavitation and collapse on ms timescales, we reconstitute, using time-resolved imaging with amplitude and phase sensitivity, the hydrodynamic phenomena concurring to bubble evolution. We indicate geometry-specific instabilities accompanying the collapse. The insertion of gold nanoparticles of 200u2009nm size has subtle effects in the process energetics. Albeit a moderate field enhancement minimizing the contribution to breakdown, the nanoparticles play a role in the overall relaxation dynamics of bubbles. The evolving bubble border in nanoparticles-containing liquids create a snow-plough effect that sweeps the nanoparticles at the gas liquid interface. This indicates that during the macroscopic cavity development, the nanoparticles were removed from the interaction region and dragged by the hydrodynamic movement. We thus shed light on the evolution of cavitation bubbles not triggered but perturbed by the presence of nanoparticles.

Efficient point-by-point Bragg gratings fabricated in embedded laser-written silica waveguides using ultrafast Bessel beams

We demonstrate highly efficient Bragg gratings written point-by-point by sequential single-pulse ultrashort Bessel laser beams in laser photoinscribed single-mode waveguides in bulk fused silica. The use of chirped non-diffractive Bessel beams determines a strong Bragg resonance in a weak-to-strong transitional regime, augmenting to a record value of 40u2009u2009dB/cm at 1550xa0nm in the third order. The Bessel-induced refractive index modulation is negative and localized to sub-micrometer (200xa0nm) transverse scales. The strong light confinement in Bessel beams ensuring uniform one-dimensional void conditions thus allows for enhanced precision in the Bragg grating waveguide design. We demonstrate flexible fabrication of multiplexed waveguide gratings for multiple and tunable spectral resonances.

Optical beam engineering for nanoscale structuring of transparent materials

This Beam engineering solutions are provided to enhance the light-matter interaction processes for extreme scale material structuring applications. We present experimental results demonstrating the potential use of spatially shaped laser pulses (Bessel pulses in particular) for high aspect ratio material structuring applications. We also provide simulation results on nonlinear beam propagation in glass, which show that in addition to the spatial shaping, temporal shaping of a laser pulse can significantly increase the effectiveness of the light-matter interaction processes.

Control of Ultrafast Laser Nanostructuring of Glasses using Temporal and Spatial Pulse Design

Laser transformation of optical glasses depends on the rate of energy feedthrough, affecting the resulting function. We discuss the role of spatio-temporal design of laser pulses in 3D nanostructuring either via self-arrangements or direct writing.