Thanks to a process called “high-order harmonic generation” significant progress has been made in generating ultrashort X-ray pulses over the past few years, with a duration of a few attoseconds (equivalent to dividing a second into 1,000,000,000,000,000,000 parts). This extremely short duration is comparable to the time it takes for electrons to transfer between atoms, making these pulses exceptional tools for exploring high-speed physical phenomena.

The required experimental setup and desired characteristics of the light pulses vary depending on their application. While it is possible to simulate this process to understand and predict its behavior under different circumstances, performing these calculations requires an extremely long time, even on the world’s most powerful supercomputers. Therefore, it is common to resort to approximations that provide acceptable but improvable results.

However, this can be addressed with intelligence, specifically with Artificial Intelligence (AI). A recent study conducted by the Research Group in Laser Applications and Photonics (ALF) has shown that it is possible to use artificial neural networks to accelerate these simulations and obtain nearly immediate results with a level of accuracy that had not been achieved until now.

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.