Rapid assessment of nonlinear optical propagation effects in dielectrics

Ultrafast laser processing applications need fast approaches to assess the nonlinear propagation of the laser beam in order to predict the optimal range of processing parameters in a wide variety of cases. We develop here a method based on the simple monitoring of the nonlinear beam shaping against numerical prediction. The numerical code solves the nonlinear Schrödinger equation with nonlinear absorption under simplified conditions by employing a state-of-the art computationally efficient approach. By comparing with experimental results we can rapidly estimate the nonlinear refractive index and nonlinear absorption coefficients of the material. The validity of this approach has been tested in a variety of experiments where nonlinearities play a key role, like spatial soliton shaping or fs-laser waveguide writing. The approach provides excellent results for propagated power densities for which free carrier generation effects can be neglected. Above such a threshold, the peculiarities of the nonlinear propagation of elliptical beams enable acquiring an instantaneous picture of the deposition of energy inside the material realistic enough to estimate the effective nonlinear refractive index and nonlinear absorption coefficients that can be used for predicting the spatial distribution of energy deposition inside the material and controlling the beam in the writing process

Size-dependent stability of ultra-small α-/β-phase tin nanocrystals synthesized by microplasma

Key features of tin, including electronic band structure and opto-electronic properties, are influenced by the crystal structure. Here the authors report a microplasma process for the synthesis of ultra-small tin nanocrystals in which the crystal structure is dependent on crystallite size

Are fluctuations in coal, oil and natural gas consumption permanent or transitory? Evidence from OECD countries

publishedVersio

Glass cutting

The state of the technology of ultrashort pulse laser applications such as glass cutting is dominated by direct ablation of a dielectric material, however the first installations used in-volume filament-like modifications. The variety of intriguing physical phenomena range from numerous nonlinear effects of ionisation to propagation of radiation strongly coupled to electron dynamics and include the formation of filaments. However, the potential as well as the challenge with respect to glass cutting is to tailor the combination of material composition and the laser radiation, which enables the suppression of unwanted damage and stable propagation of an optical and electronic channel; both might be called filaments. Ultrashort laser pulses interacting with the dielectric material generate free electrons dominantly via multiphoton ionisation (MPI) and cascade ionisation (CI). The dense plasma produced results in great changes of the refractive index and the surface reflectivity. When laser-induced plasma density reaches the well-known critical value ρcrit = ω2ε0me/e2 dependent on the laser frequency ω, the material gets highly absorbing. Laser ablation induced by relaxation of electron energy to the atoms takes place after the laser pulse has ceased. This ablation mechanism allows the use of the critical free-electron density ρcrit as the criterion ρablation = ρcrit for modelling ablation. The material near the ablated wall is characterised by a free electron density ρ<ρcrit. Here indeed the material is not ablated but will be modified or damaged due to the energy released by high-density free-electrons. Once more, a threshold value ρdamage for the free electron density can be identified. As result, the shape of the ablation front as well as the morphology of a damaged region is described nearly quantitatively

Effects of dynein on microtubule mechanics and centrosome positioning

When microtubules are severed by laser ablation, newly created minus ends increase in curvature, but they straighten when dynein is inhibited. It is found that cytoplasmic dynein generates tension and friction along microtubule lengths and that these forces govern the dynamics of centrosome centering