Stefan Skupin

Femtosecond laser pulse train interaction with dielectric materials

The interaction of trains of femtosecond microjoule laser pulses with dielectric materials by means of a multi-scale model is investigated. Theoretical predictions are directly confronted with experimental observations in soda-lime glass. It is shown that due to the low heat conductivity, a significant fraction of the laser energy can be accumulated in the absorption region. Depending on the pulse repetition rate, the material can be heated to high temperatures even though the single pulse energy is too low to induce a significant material modification. Regions heated above the glass transition temperature in the simulations correspond very well to zones of permanent material modifications observed in the experiments. It turns out that pulse-to-pulse variations of the laser absorption are negligible and of minor influence to permanent material modifications.

Spectral dynamics of THz pulses generated by two-color laser filaments in air: The role of Kerr nonlinearities and pump wavelength

We theoretically and numerically study the influence of both instantaneous and Raman-delayed Kerr nonlinearities as well as a long-wavelength pump in the terahertz (THz) emissions produced by two-color femtosecond filaments in air. Although the Raman-delayed nonlinearity induced by air molecules weakens THz generation, four-wave mixing is found to impact the THz spectra accumulated upon propagation via self-, cross-phase modulations and self-steepening. Besides, using the local current theory, we show that the scaling of laser-to-THz conversion efficiency with the fundamental laser wavelength strongly depends on the relative phase between the two colors, the pulse duration and shape, rendering a universal scaling law impossible. Scaling laws in powers of the pump wavelength may only provide a rough estimate of the increase in the THz yield. We confront these results with comprehensive numerical simulations of strongly focused pulses and of filaments propagating over meter-range distances.

Boundary conditions for arbitrarily shaped and tightly focused laser pulses in electromagnetic codes

Investigation of laser matter interaction with electromagnetic codes requires to implement sources for the electromagnetic fields. A way to do so is to prescribe the fields at the numerical box boundaries in order to achieve the desired fields inside the numerical box. Here we show that the often used paraxial approximation can lead to unexpected field profiles with strong impact on the laser matter interaction results. We propose an efficient numerical algorithm to compute the required laser boundary conditions consistent with the Maxwells equations for arbitrarily shaped, tightly focused laser pulses.

Simultaneous spatial and temporal focusing: a route towards confined nonlinear materials processing

Ultrashort pulse lasers enable reliable and versatile high precision ablation and surface processing of various materials such as metals, polymers and semiconductors. However, when modifications deep inside the bulk of transparent media are required, nonlinear pulse material interactions can decrease the precision, since weak focusing and the long propagation of the intense pulses within the nonlinear media may induce Kerr self-focusing, filamentation and white light generation. In order to improve the precision of those modifications, simultaneous spatial and temporal focusing (SSTF) allows to reduce detrimental nonlinear interactions, because the ultrashort pulse duration is only obtained at the focus, while outside of the focal region the continuously increasing pulse duration strongly reduces the pulse intensity. In this paper, we review the fundamental concepts of this technology and provide an overview of its applications for purposes of multiphoton microscopy and laser materials processing. Moreover, numerical simulations on the nonlinear pulse propagation within transparent media illustrate the linear and nonlinear pulse propagation, highlighting the differences between conventional focusing and SSTF. Finally, fs-laser induced modifications in gelatine are presented to compare nonlinear side-effects caused by conventional focusing and SSTF. With conventional focusing the complex interplay of self-focusing and filamentation induces strongly inhomogeneous, elongated disruptions. In contrast, disruptions induced by SSTF are homogeneously located at the focal plane and reduced in length by a factor >2, which is in excellent agreement with the numerical simulations of the nonlinear pulse propagation and might favor SSTF for demanding applications such as intraocular fs-laser surgery.

Towards compact efficient fs-laser-induced THz sources from microplasmas

Terahertz (THz) sources are indispensable for various applications such as THz time-domain spectroscopy. A promising approach to generate broadband THz radiation is to employ laser-induced gas plasmas, which has already been demonstrated for various settings [1, 2]. In order to miniaturize such sources, it has been proposed to exploit single-color laser-induced microplasmas [3]. Then, a conical THz emission is produced by longitudinal low-frequency currents driven by the ponderomotive force. However, the laser-to-THz conversion efficiency for this scenario has been shown to saturate for higher pulse energies to approximately 10−6 [4]. In order to increase the efficiency, we investigate theoretically [4] the ionization current mechanism driven by two-color laser pulses [2] for microplasmas.

Suitability of the unidirectional approach for describing laser-driven terahertz emission

Over the past decade, the increasing potential of terahertz (THz) radiation has stimulated intensive efforts to develop efficient emitters based on the ionization of gases by ultrashort laser pulses [1]. In order to properly calibrate dedicated experiments, nonlinear propagation codes have to be particularly accurate to describe the low-frequency part of the pulse spectrum. Usually, solving full Maxwell models accounting for multiple ionization processes as well as optical and plasma nonlinear effects is computationally expensive when simulating long propagation ranges. Therefore, reduced models such as the Unidirectional Pulse Propagation Equation (uppe) [2] are often preferred, assuming that the forward propagating component conveys the major part of the laser energy. So far, no detailed numerical comparison has been performed to confirm the validity of this uppe approach, which is widely used in the scope of laser-driven THz generation [3]. Here, we examine by means of 1D simulations the differences occurring in the THz pulse spectra and fields when these quantities are described either by a full Maxwell-fluid (maxflu) model, encompassing both backward and forward propagations, or by its uppe approximation, modeling only the forward propagating wave.

Impact of the pump wavelength in THz emissions by two-color femtosecond laser filaments in air

Laser filamentation is actively studied for its rich variety of applications, from supercontinuum generation to lightning control [1]. In air, femtosecond filaments result from the self-focusing of ultrashort light pulses that couple to their own plasma channel and stay self-guided upon extended paths at high intensity levels. Driven by strong nonlinearities, these optical structures are able to promote broadband THz-to-far-infrared radiation when using laser fields composed of two colors, e.g., a fundamental and its second harmonic [2].

Modelling Laser Matter Interaction with Tightly Focused Laser Pulses in Electromagnetic Codes

We propose an algorithm to introduce arbitrarily shaped laser pulses into electromagnetic codes. In contrast to the often used paraxial approximation our approach models accurately tightly focused laser pulses and their interaction with matter.

Femtosecond laser cutting of glass by controlled fracture propagation

We present the use of a compact femtosecond laser with 300-fs pulse duration and pulse energy on the order of 10s of μJ for the cutting of glass by controlled fracture propagation.

Effects of burst mode on transparent materials processing

We investigated the effect of burstmode with nanosecond (ns) time delay between subpulses on sodalime glass volume machining. We observed in tight focusing configuration that the use of burstmode with ns time delay between subpulses does not increase the absorption efficiency and does not bring a significant effect on the heat affected zone diameter with respect to single pulse mode. On the contrary in loose focusing configuration the use of burst mode allows increasing the aspect ratio of the heat affected zone without extra energy absorption. This effect is highly interesting for filamentation glass cutting applications.