Nonequilibrium optical properties of transition metals upon ultrafast electron heating.

Femtosecond laser excitation of metals triggers swift modifications of the electronic distribution within the band structure. This has direct consequences on optical transitions transiently modifying the optical properties of materials. Influencing in real time the action of the pulse, these changes lead to substantial variations of the amount and the distribution in the energy deposited during the laser irradiation. The effect of the laser pulse can be described considering electrons heated to a range of electronic temperatures. In order to evaluate the dielectric response of ultrafast heated electrons, we performed ab initio molecular dynamic simulations coupled to the Kubo-Greenwood formalism and determined electronic temperature dependent optical properties. A series of representative transition metals was investigated: Cu, Ni, Cr, W, Ti, and Fe. The evolution of the optical properties is optically-pumped based on electronic redistribution within the density of electronic states. The proposed interpretation rely on modifications of the energy range of occupied states undergoing optical electronic transitions. It is found that the degree of filling and the shape of the d-block drive the dynamics of optical processes. Nonequilibrium optical indices, reflectivities and skin depths are reported for electron thermal excitation relevant to near-threshold laser ablation regimes. The effect of the electron temperature on optical properties allows to reconstruct and model ultrafast excitation dynamics in time-resolved diagnostics with relevance in laser micro- and nano-processing.

Control of ultrafast laser-induced bulk nanogratings in fused silica via pulse time envelopes.

Employing a method of in-situ control we propose an approach for the optimization of self-arranged nanogratings in bulk fused silica under the action of ultrashort laser pulses with programmable time envelopes. A parametric study of the influence of the pulse duration and temporal form asymmetries is given. Using the diffraction properties of the laser-triggered subwavelength patterns we monitor and regulate the period and the quality of the periodic nanoscale arrangement via the effective nonlinear excitation dose. Periodicity tuning on tens of nanometers can be achieved by pulse temporal variations, with a minimum around 0.7 ps at the chosen powers. Equally, strong sensitivity to pulse asymmetries is observed. The driving factor is related to increasing carrier densities due to nonlinear confinement and the development of extended nanoroughness domains upon multiple exposure, creating a pulse-dependent effective accumulation dose via a morpho-dimensional effect. The result may impact the associated optical functions.

Evidence of surface plasmon resonance in ultrafast laser-induced ripples.

The sensitivity of grating-coupled Surface Plasmon Polaritons (SPPs) on metallic surface has been exploited to investigate the correlation between ripples formation under ultrashort laser exposure and SPPs generation conditions. Systematic examination of coupling of single ultrashort laser pulse on gratings with appropriate periods ranging from 440 nm to 800 nm has been performed. Our approach reveals that a surface plasmon is excited only for an appropriate grating period, the nickel sample exhibits fine ripples pattern, evidencing the plasmonic nature of ripples generation. We propose a systematic investigation supported by a comprehensive study on the obtained modulation of such a coupling efficiency by means of a phenomenological Drude-Lorentz model which captures possible optical properties modification under femtosecond irradiation.

Tuning spectral properties of ultrafast laser ablation plasmas from brass using adaptive temporal pulse shaping.

Using automated laser pulse temporal shaping we report on enhancing spectral emission characteristics of ablation plasmas produced by laser irradiation of brass on ultrafast time scales. For different input irradiance levels, control of both atomic and ionic species becomes possible concerning the yield and the excitation state. The improved energy coupling determined by tailored pulses induces material ejection with lower mechanical load that translates into hot gas-phase regions with higher excitation degrees and reduced particulates.

Controlled nanostructrures formation by ultra fast laser pulses for color marking.

Precise nanostructuration of surface and the subsequent upgrades in material properties is a strong outcome of ultra fast laser irradiations. Material characteristics can be designed on mesoscopic scales, carrying new optical properties. We demonstrate in this work, the possibility of achieving material modifications using ultra short pulses, via polarization dependent structures generation, that can generate specific color patterns. These oriented nanostructures created on the metal surface, called ripples, are typically smaller than the laser wavelength and in the range of visible spectrum. In this way, a complex colorization process of the material, involving imprinting, calibration and reading, has been performed to associate a priori defined colors. This new method based on the control of the laser-driven nanostructure orientation allows cumulating high quantity of information in a minimal surface, proposing new applications for laser marking and new types of identifying codes.

Transient optical response of ultrafast nonequilibrium excited metals: effects of electron-electron contribution to collisional absorption.

Approaching energy coupling in laser-irradiated metals, we point out the role of electron-electron collision as an efficient control factor for ultrafast optical absorption. The high degree of laser-induced electron-ion nonequilibrium drives a complex absorption pattern with consequences on the transient optical properties. Consequently, high electronic temperatures determine largely the collision frequency and establish a transition between absorptive regimes in solid and plasma phases. In particular, taking into account umklapp electron-electron collisions, we performed hydrodynamic simulations of the laser-matter interaction to calculate laser energy deposition during the electron-ion nonequilibrium stage and subsequent matter transformation phases. We observe strong correlations between optical and thermodynamic properties according to the experimental situations. A suitable connection between solid and plasma regimes is chosen in accordance with models that describe the behavior in extreme, asymptotic regimes. The proposed approach describes as well situations encountered in pump-probe types of experiments, where the state of matter is probed after initial excitation. Comparison with experimental measurements shows simulation results which are sufficiently accurate to interpret the observed material behavior. A numerical probe is proposed to analyze the transient optical properties of matter exposed to ultrashort pulsed laser irradiation at moderate and high intensities. Various thermodynamic states are assigned to the observed optical variation. Qualitative indications of the amount of energy coupled in the irradiated targets are obtained.