Igor M. Burakov

Spatial distribution of refractive index variations induced in bulk fused silica by single ultrashort and short laser pulses

We correlate phase-contrast microscopy of modification tracks induced by tightly focused single ultrashort and short laser pulses inside fused silica with numerical simulations of nonlinear laser excitation footprints. Different pulse durations on the femtosecond and picosecond range are compared in order to validate the experimental and theoretical observations on the subsequent refractive index variations in a regime where linear and nonlinear contributions play a comparable role. The nature of the laser-induced structural changes depends essentially on the characteristics of pulse propagation in different regions of the irradiated zone. Numerical simulations of laser pulse propagation in the excited region show that accumulation of excess energy and swift nonlinear absorption contribute to the formation of either positive or negative phase-shift regions within the same single-pulse-induced damage trace. The decrease in the refractive index can be unambiguously correlated with the regions of maximum energy deposition during prolonged exposure times.

Theoretical Models and Qualitative Interpretations of Fs Laser Material Processing

In this paper a number of numerical models are presented which have been developed to describe the processes taking place at different time and length scales in different classes of materials under the irradiation by ultrashort laser pulses. A unified drift-diffusion approach for modeling charge- carrier transport in metals, semiconductors, and dielectrics allows to elucidate the dynamics of the electric field generated in the target due to photo-emission and to get insight into the origin of the Coulomb explosion process. The widely known two-temperature model is used to follow heating dynamics of irradiated matter and to analyze its phase transformations on the basis of thermodynamic concepts. Being modified for semiconductors, this model has allowed to establish the nature of high-energeti c ion emission using laser pulse tailoring and to undertake a simplified modeling of consequences of ultrafast melting of silicon. A two-dimensiona l model of dielectric breakdown has made possible to uncover the mechanisms which enable the spatial modulation of the structures induced by temporally modulated laser pulses in wide-band-gap dielectric materials. A combined thermal/elasto-plastic model has provided a deep insight into the mechanisms and dynamics of the microbump and nanojet formation on nanosize gold films under femtosecond laser irradiation.

Nanosize structural modifications with polarization functions in ultrafast laser irradiated bulk fused silica

Laser-induced self-organization of regular nanoscale layered patterns in fused silica is investigated using spectroscopy and microscopy methods, revealing a high presence of stable broken oxygen bonds. Longitudinal traces are then generated by replicating static irradiation structures where the nanoscale modulation can cover partially or completely the photoinscribed traces. The resulting birefringence, the observed anisotropic light scattering properties, and the capacity to write and erase modulated patterns can be used in designing bulk polarization sensitive devices. Various laser-induced structures with optical properties combining guiding, scattering, and polarization sensitivity are reported. The attached polarization functions were evaluated as a function of the fill factor of the nanostructured domains. The polarization sensitivity allows particular light propagation and confinement properties in three dimensional structures.

Designing laser-induced refractive index changes in "thermal" glasses

Ultrafast lasers emerged as promising tools to process refractive index changes in band-gap materials, resulting in waveguiding functions. Positive refractive index changes were often reported in fused silica matrices. However, in glasses characterized by slow electronic relaxation and high thermal expansion, the refractive index change is usually negative, detrimental for waveguide writing. This relates to the formation of hot regions, where, due to thermal expansion, material is quenched in low-density phases. We discuss control mechanisms related to spatio-temporal heat-source design which may be tuned by temporally shaped laser radiation. Programmable temporal tailoring of pulse envelopes triggers transitions from thermal expansion to directional inelastic flow. Consequently, material compaction leads to a positive refractive index change and guiding structures may thus be created. From an application perspective, the structuring quality degrades with the focusing depth due to wavefront distortions generated at the air-dielectric interface inducing spatial energy dispersion. Spatial beam tailoring corrects beam propagation distortion, improving the structuring accuracy. The corrective process is becoming important when laser energy has to be transported without losses at arbitrary depths, with the purpose of triggering mechanisms of positive index change.

Tailored excitation sequences for optimized laser-induced modifications in bulk transparent materials exposed to sub-ps irradiation

Tight focusing of ultrashort near infrared laser pulses in the bulk of various transparent materials induces significant modifications of the optical properties by locally changing the material refractive index. Such laser-induced phase objects are of major technological interest, notably for direct writing of embedded optical functions. While extensive studies have been reported on ultrashort pulsed laser induced modifications in several materials, especially with regard to focusing conditions, incubation effects, or the influence of the energy content of the pulse, we emphasize here the role of the temporal design of the excitation sequence. We present phase-contrast microscopy investigations of the resultant morphology and discuss the refractive index topological map induced by different temporal pulse intensity envelopes in various transparent materials. The consequences of temporal profiles generated by a pulse shaping apparatus on the morphology of the interaction zone are illustrated, emphasizing the benefits of the synchronization between the excitation temporal profile and the material response.

Adaptive optimization in ultrafast laser material processing (Plenary Paper)

Ultrafast lasers promise to become attractive and reliable tools for material processing on micro- and nanoscale. The additional possibility to temporally tailor ultrashort laser pulses by Fourier synthesis of spectral components enables extended opportunities for optimal processing of materials. An experimental demonstration of the technique showing the possibility to design particular excitation sequences tailored with respect to the individual material response will be described, laying the groundwork for adaptive optimization in materials structuring. We report recent results related to the implementation of self-learning, adaptive loops based on temporal shaping of the ultrafast laser pulses to control laser-induced phenomena for practical applications. Besides the fundamental interest, it is shown that under particular excitation conditions involving modulated excitation, the energy flow can be controlled and the material response can be guided to improve processing results. Examples are given illuminating the possibility to control and manipulate the kinetic properties of ions emitted from laser irradiated semiconductor samples using excitation sequences synchronized with the phase transformation characteristic times.