arXiv subjects

Luc Froehly

Publications and source records attributed to Luc Froehly.

8 recordsLinked to original sources

arXiv2024

Research and teaching in physics at the University of Franche-Comt\'e 1845-1970

Recently uncovered archives at the University of Franche-Comt\'e in Besan\c{c}on (France) reveal a rich history of research and teaching in physics since the Faculty of Science was first established in 1845. Here, we describe a selection of notable activities conducted by the named Chairs of Physics during the period 1845-1970. We uncover a long tradition of major contributions to physics education and research, including the production of highly regarded physics textbooks that were widely used in Europe, as well as pioneering contributions to electron diffraction and microscopy, Fourier optics, and holography. These discoveries yield valuable insights into the historical development of physics research in France, and show how even a small provincial university was able to stay up-to-date with international developments across several areas of physics.

physics.hist-ph↗

arXiv2022

High energy density plasma mediated by collisionless resonance absorption inside dielectrics

We demonstrate for the first time to our knowledge the generation of overcritical plasma densities inside transparent solids over long distances using femtosecond laser pulses. This opens new avenues for high energy density physics in confined geometry such as warm dense matter study or the synthesis of new material phases. We show both with experiments and first-principles simulations, that femtosecond conical interference via a Bessel beam creates a dense plasma rod with typically 100 nm diameter in sapphire. The interaction is in ideal conditions to trigger collisionless resonance absorption. This mechanism plays a primary role in the energy deposition process, yielding a plasma with an energy density on the order of MJ/cm3 and a length that can reach several cm using only tabletop femtosecond lasers.

physics.plasm-ph↗

arXiv2021

In-situ diagnostic of femtosecond probes for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations. It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time. Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate, based on wavelength or spatial frequency encoding. However, further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams. Because of the difference in group velocities between pump and probe in the bulk of the material, the determination of the absolute pump-probe delay depends on the sample position. In addition, pulse-front tilt is a widespread issue, unacceptable for ultrafast imaging, but which is conventionally very difficult to evaluate for the low-intensity probe pulses. Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse. It allows solving the two issues. Our approach is valid whatever the transparent medium, whatever the probe pulse polarization and wavelength. Because it is nondestructive and fast to implement, this in-situ probe diagnostic can be repeated to calibrate experimental conditions, particularly in the case where complex wavelength, spatial frequency or polarization encoding is used. We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in all fields of ultrafast science and high field physics at the micro- and nanoscale.

physics.optics↗

arXiv2019

Advancing Fourier: space-time concepts in ultrafast optics, imaging and photonic neural networks

The concepts of Fourier optics were established in France in the 1940s by Pierre-Michel Duffieux, and laid the foundations of an extensive series of activities in the French research community that have touched on nearly every aspect of contemporary optics and photonics. In this paper, we review a selection of results where applications of the Fourier transform and transfer functions in optics have been applied to yield significant advances in unexpected areas of optics, including the spatial shaping of complex laser beams in amplitude and in phase, real-time ultrafast measurements, novel ghost imaging techniques, and the development of parallel processing methodologies for photonic artificial intelligence.

physics.optics↗

arXiv2019

Diffractive coupling for photonic networks: how big can we go?

Photonic networks are considered a promising substrate for high-performance future computing systems. Compared to electronics, photonics has significant advantages for a fully parallel implementation of networks. A promising approach for parallel large-scale photonic networks is realizing the connections using diffraction. Here, we characterize the scalability of such diffractive coupling in great detail. Based on experiments, analytically obtained bounds and numerical simulations considering real-world optical imaging setups, we find that the concept in principle enables networks hosting over a million optical emitters. This would be a breakthrough in multiple areas, illustrating a clear path toward future, large scale photonic networks.

physics.optics↗

arXiv2017

Reinforcement Learning in a large scale photonic Recurrent Neural Network

Photonic Neural Network implementations have been gaining considerable attention as a potentially disruptive future technology. Demonstrating learning in large scale neural networks is essential to establish photonic machine learning substrates as viable information processing systems. Realizing photonic Neural Networks with numerous nonlinear nodes in a fully parallel and efficient learning hardware was lacking so far. We demonstrate a network of up to 2500 diffractively coupled photonic nodes, forming a large scale Recurrent Neural Network. Using a Digital Micro Mirror Device, we realize reinforcement learning. Our scheme is fully parallel, and the passive weights maximize energy efficiency and bandwidth. The computational output efficiently converges and we achieve very good performance.

cs.NE↗

arXiv2015

Caustics and Rogue Waves in an Optical Sea

There are many examples in physics of systems showing rogue wave behaviour, the generation of high amplitude events at low probability. Although initially studied in oceanography, rogue waves have now been seen in many other domains, with particular recent interest in optics. Although most studies in optics have focussed on how nonlinearity can drive rogue wave emergence, purely linear effects have also been shown to induce extreme wave amplitudes. In this paper, we report a detailed experimental study of linear rogue waves in an optical system, using a spatial light modulator to impose random phase structure on a coherent optical field. After free space propagation, different random intensity patterns are generated, including partially-developed speckle, a broadband caustic network, and an intermediate pattern with characteristics of both speckle and caustic structures. Intensity peaks satisfying statistical criteria for rogue waves are seen especially in the case of the caustic network, and are associated with broader spatial spectra. In addition, the electric field statistics of the intermediate pattern shows properties of an optical sea with near-Gaussian statistics in elevation amplitude, and trough-to-crest statistics that are near-Rayleigh distributed but with an extended tail where a number of rogue wave events are observed.

physics.optics↗

arXiv2006

Optical Coherence Spectro-Tomography by all-Optical Depth-Wavelength analysis

Current spectroscopic optical coherence tomography (OCT) methods rely on a posteriori numerical calculation. We present an alternative for accessing optically the spectroscopic information in OCT, i.e. without any post-processing, by using a grating based correlation and a wavelength demultiplexing system. Conventional A-scan and spectrally resolved A-scan are directly recorded on the image sensor. Furthermore, due to the grating based system, no correlation scan is necessary. In the frame of this paper we present the principle of the system as well as first experimental results.

physics.optics↗