The generation of ultra-short light pulses with a good spatial structure is the philosopher’s stone of ultrafast pulse physics. These pulses make it possible to study and modify the properties of matter at time scales unreachable by other procedures.

In recent decades, great strides have been made in the generation of high-quality ultrashort pulses among which post-compression techniques stands out. Post-compression techniques consist of widening the spectrum of a pulse during its propagation thanks to nonlinear effects and then correcting its phase to achieve the shortest possible temporal pulse. The most widely used post-compression technique today is based on the nonlinear propagation of a pulse through a hollow core fiber filled with gas. However, in the last decade, with the rise of new lasers, such as the Yb laser, other post-compression methods that do not have to deal with the restrictions presented by hollow core fibers have gained relevance. One of these new post-compression techniques is the nonlinear propagation in multipass cells.

These multipass cells are cavities formed by two spherical mirrors in which the laser beam is introduced in the cavity off-axis, in such a way that the beam is reflected multiple times forming a hyperboloid before leaving the cell. One of the advantages of these cavities is that we can introduce in them a nonlinear medium through which the beam propagates in nonlinearly during the successive round trips.

Building upon this research, we have theoretically explored a post-compression region in multipass cells that allows the generation of wide spectra with smooth profiles that prevent the pulse from presenting too much structure (pre-pulses or post-pulses) once compressed. In order to accomplish this, we have relied on a particular regime explored already in the 80s known as the enhanced frequency chirp regime, and we have adapted it to multipass cells. In this regime, nonlinear effects and dispersion go hand in hand to widen the spectrum while maintaining a smooth structure that supports a very clean temporal profile. We have optimized the parameters of this region for the case of a multipass cavity filled with argon obtaining pulses whose Fourier limit is compressed more than 10 times with respect to the duration of the initial pulse, but above all maintaining an extremely clean structure, which makes it very useful for a variety of applications.

Recently, researchers belonging to ALF, have been working in the development of a miniaturized spectrometer in collaboration with the European Space Agency, the Department of Physics and Swiss Nanoscience Institute (University of Basel), the Department of Chemistry and Applied Biosciences (ETH Zurich), the Swiss Federal Laboratories for Materials Science and Technology (Empa) and the Optics and Photonics Technology Laboratory (Ecole Polytechnique Fédérale de Lausanne, EPFL). The device belongs to the family of ultracompact Fourier Transform spectrometers, and it consist of a LiNbO3 chip in which a monomodal waveguide was fabricated with an optimized design to produce a light flux in the vertical direction. In the upper part of the chip a nano-detector (gold nanowire) was placed perpendicularly to the waveguide, together with a quantum dot HgTe nanolayer. The gold nanowire acts as scattering element, sensing the light confined in the waveguide. The nanolayer creates a photocurrent that can be measured. An external mirror placed at the output of the waveguide enables the creation of a standing wave that is monitorized by the nano-detector. The controlled motion of the mirror produces a spatial swept of the standing wave, thus obtaining the measurement of the confined intensity, from which the spectrum is extracted by Fourier transform.

The use of fracture mechanics for rationalizing the fracture behavior of cemented carbides is valid, as far as sharp cracks, free of residual stresses and subjected to a well-defined stress state are used for assessing fracture toughness. However, machining a very sharp notch on the surface of hardmetals for fracture toughness testing has been a critical issue during many years. Within this context, introduction of surface “through-thickness” _micronotches (SEμVNB) by means of ultrashort pulsed laser ablation (UPLA) is here proposed, implemented and analyzed as an innovative precracking-like route within flexural testing procedures for appropiated evaluation of fracture toughness of cemented carbides. UPLA parameters used for introducing the micronotch are optimized in terms of induced damage ahead of the notch tip. For comparison purposes, fracture toughness is also determined by means of flexural testing of previously cracked single-edge notch beams (SENB-Cracked) as well as specimens with V-notch tips sharpened through diamond polishing using a razor blade, and Palmqvist indentation microfracture method. The satisfactory agreement found between values measured using UPLA-micronotched and SENB-Cracked (reference) specimens allows to conclude that flexural testing of SEμVNB samples is a valid methodology for reliable determination of fracture toughness of hardmetals. This is feasible because of the extremely short time of laser-matter interaction. It yields small and somehow controlled damage in front of the notch tip as a result of shock wave propagation during ablation, which translates into effective precracking of SEμVNB specimens