Diffusion Brillouin | EPFL Graph Search

La 'diffusion Brillouin' est la diffusion inélastique de la lumière par les ondes acoustiques d'un milieu. Dans une expérience de diffusion Brillouin, on illumine un milieu à l'aide d'un faisceau laser et on détecte la lumière diffusée à une fréquence légèrement différente. Les décalages en fréquence observés sont de l'ordre de 1 à 200 GHz environ. La mesure de ce décalage permet de remonter à certaines propriétés du milieu. Cet effet a été prédit en 1914 par Léon Brillouin. Dans ses principes, la diffusion Brillouin est tout à fait semblable à la diffusion Raman. Les deux techniques diffèrent par les ordres de grandeurs des décalages (supérieur à 300 GHz ou 10 cm-1 pour le Raman), ce qui implique des appareillages différents pour analyser les spectres diffusés. Dans le cas de la diffusion Raman, on utilisera un spectromètre à réseaux, et un Interféromètre Fabry-Pérot dans le cas de la diffusion Brillouin. Description Description classique Dans une description

Opto-acoustic coupling and Brillouin phenomena in microstructure optical fibers

Stella Foaleng Mafang, Duc Minh Nguyen, Luc Thévenaz

Like photonic crystals have revolutionized the way of manipulating optical waves at the sub-micron scale, phononic crystals have more recently played similar decisive role for sound waves, or more generally elastic waves. Then, the idea of coupling light and sound in purposely designed microstructures is now emerging. In this respect, the periodic, wavelength-scale (for both optic and high-frequency acoustic waves) transverse air-hole microstructure of photonic crystal fibers (PCFs) provides additional degrees of freedom for light-sound interactions. PCFs can indeed exhibit photonic and phononic bandgap effects, allowing for tight confinement and joint waveguiding of both types of waves [1]. Electrostriction-driven Brillouin phenomena, namely backward Stimulated Brillouin Scattering (SBS) and forward Guided Acoustic Wave Brillouin Scattering (GAWBS), constitute an important category of such opto-acoustic coupling. The geometry of PCFs can dramatically modify the Brillouin spectrum, the gain and the stimulated Brillouin threshold, globally yielding much richer opto-acoustic dynamics and spectral features than in conventional fibers [2-11]. Specific transverse or longitudinal guided acoustics modes in the 100 MHz-10 GHz range can thus be selectively excited, resonantly enhanced and tightly confined within the microstructure, with an intimate dependence on its μm or sub-μm geometry. All these specific features have great potential for developing novel PCF-based distributed Brillouin sensors [6,7,12-14], for high-resolution longitudinal mapping of the intrinsic fluctuations of the fiber microstructure [15], and more generally for developing original tools of optical signal processing [1-4,9,16,17]. This talk will give a comprehensive overview of these original behaviors in a range of PCFs.

IEEE2012

Atomic layer deposition of Ni and Ni80Fe20 for tubular spin-wave nanocavities

Maria Carmen Giordano

Magnetic thin films and magnetic nanostructures have become essential components of modern technological applications. Modern branches of magnetisms focus on spin-charge coupling (spintronics) and the collective excitation of spin waves in magnetically ordered materials (magnonics). In particular, spin waves represent a promising charge-free medium to encode and transmit information in the GHz frequency regime, relevant for microwaves technologies. The ever-increasing demand for compact and portable electronic and telecommunication devices leads to the need for further miniaturization and high integration density of their functional components. These aspects have prompted nanomagnetism to venture the third dimension. This development is also motivated by novel physical phenomena and functionalities predicted to arise from three-dimensional (3D) complex magnetic configurations. In order to advance the research in 3D spintronics and 3D magnonics, it will be essential to have control over the fabrication of 3D nanoarchitectures and to access the new properties of individual nanostructures emerging from new complex 3D spin-textures. These two challenges are addressed in this thesis. Ferromagnetic nanotubes (NTs) represent the ideal 3D system for the study of shape dependent static and dynamic magnetic properties. Their magnetic configurations, as well as the spin wave confinement and propagation inside them, can be engineered by changing their geometrical parameters. First, in order to fabricate 3D magnetic device architectures, we explored atomic layer deposition (ALD). The thickness and quality of the thin films deposited with ALD are irrespective of the geometry of the surface on which they are deposited. This characteristic makes it ideal to fabricate 3D nanomagnets by magnetically coating 3D nanotemplates, e.g. nanowires (NWs) in the case of NTs. In this thesis we propose ALD processes for the conformal coating by means of two ferromagnetic materials conventionally used in planar spintronics and magnonics systems: Ni and permalloy (Py, Ni80Fe20), the second material expected to have a lower spin wave damping. By identifying correlations between ALD process parameters and thin film properties we optimized the materials in terms of conformality, electrical resistivity, anisotropic magnetoresistance (AMR) effect, spin wave damping and, in the case of permalloy, also stoichiometry. The optimized materials were then transferred onto GaAs nanowires templates, obtaining ferromagnetic NTs with hexagonal cross sections. We achieved Ni NTs with the lowest resistivity ever reported in similar ALD-grown Ni systems and a large AMR effect. We observed the lowest spin waves damping for permalloy NTs, exhibithing a value comparable with permalloy thin films achieved with conventional planar deposition techniques. Second, we investigated magnetic states and spin waves confinement in nanotubes experimentally and via micromagnetic simulations. We combined magnetoresistance measurements with Brillouin light scattering spectroscopy (BLS) and micromagnetic simulations to study Ni and Py NTs. BLS measurements, performed while irradiating NTs with microwaves, show that these nanotubes form spin-wave nanocavities which impose discrete wave vectors and confine GHz microwave signals on the nanoscale. Our experimental results show that the confinement can be engineered by changing the NT's geometrical parameters and magnetic states.

EPFL2021

Waveguide and Gas: the Advent of a New Tool for Photonics

Flavien Gyger, Luc Thévenaz, Fan Yang

The realisation of hollow-core fibres and nano-waveguides offers a unique opportunity to optimise the interactions between light and a gaseous medium. Stimulated Brillouin scattering is exploited to achieve unprecedented levels of amplification and to realise efficient all-optical processing.

IEEE2021

Atomic layer deposition of Ni and Ni80Fe20 for tubular spin-wave nanocavities

Maria Carmen Giordano

EPFL2021