Modélisation de l'influence de la géométrie sur des capteurs plasmoniques à détection de phase

L’interface entre un métal et un milieu diélectrique peut supporter des ondes guidées appelées « plasmons de surfaces » liées aux oscillations des électrons à la surface du milieu métallique. Différents types de coupleurs plasmoniques permettent l’excitation de ces ondes de surfaces. Parmi les dispositifs les plus employés, on distingue les coupleurs plans reposant sur le phénomène de réflexion totale atténuée et les nanostructures périodiques qui exploitent la diffraction évanescente de la lumière. L’excitation des plasmons de surface est principalement exploitée au sein de capteurs d’indice de réfraction de haute précision. Un faisceau laser a alors la possibilité de se coupler au système (on parle alors de résonance), la détection se fait donc à partir du faisceau émergeant du système. Le suivi d’indice peut se faire en étudiant la variation de l’intensité ou du déphasage liée au coupleur en fonction d’un paramètre ajustable (longueur d’onde, angle de couplage). Les capteurs à détection d’intensité sont aujourd’hui bien connus et représentent la grande majorité des dispositifs de détection existants. Moins étudiée, la détection de phase représente pourtant une voie intéressante d’amélioration des performances de détection selon la géométrie du coupleur employé. Ce mémoire a pour principal objectif de mieux comprendre le lien entre la géométrie des coupleurs plasmoniques et la réponse de phase qui leur est associée afin de proposer des voies d’amélioration pour ce type de détection. Pour ce faire, les deux principales méthodes employées en détection plasmonique, à savoir le couplage par prisme et le couplage par réseau de diffraction, sont modélisées dans ce travail. Dans le cadre d’un couplage par prisme, le formalisme de Fresnel permet de modéliser la réponse de phase du système. Nous avons étudié une configuration dite de Kretschmann-Raether impliquant le dépôt sur le prisme d’une couche mince métallique. Pour une épaisseur particulière de cette couche, le couplage plasmonique conduit à une annulation de l’intensité réfléchie par le système. Dans un premier temps, on montre l’importance de cette épaisseur dans l’étude de la réponse de phase du même système. Une caractérisation des performances de détection est par la suite proposée. Elle permet de mesurer le gain en sensibilité et les inconvénients en termes de résolution et de plage de détection qu’il peut y avoir à travailler à cette épaisseur dite----------Abstract "Surface plasmons" are electromagnetic waves originating from surface electrons oscillation which can exist at the metal/dielectric interface. These guided waves can be excited by using various types of plasmonic couplers which are usually based on attenuated total reflection phenomenon or diffraction grating (which uses diffraction of evanescent waves). These surface waves can be excited by resonant coupling with an incident light beam. This phenomenon is very sensitive to the surrounding refractive index and can be used in order to build sensors with high performances. Usually, the monitoring of the sensing events is made possible through the analysis of the beam emerging from this plasmonic coupler. The study of either the intensity or the phase variation of this beam allows to follow the refractive index change versus a tunable parameter (wavelength, coupling angle). While nowadays sensors based on intensity detection are commonly used, sensors using phase variation are slowly emerging. However, phase detection is still less known and studied but could lead to performances improvement for some particular couplers. The main purpose of this master’s thesis is to suggest better ways to detect the phase. In order to do so, we have modelized the phase signal obtained from two methods mainly used to detect plasmons: prism coupling and grating coupling. Phase signal with a sensor based on prism coupler can be modeled using the Fresnel formalism. We have used a Kretschmann-Raether configuration which implies a thin metal layer on the prism. For a specific thickness this configuration can lead to null reflectivity during the plasmonic coupling. In this study, we show the importance of such a thickness in phase detection. Then, we analyse the relationship between detection performances and the geometry of the system in order to highlight the advantages (sensitivity) and the drawbacks (resolution and dynamic range) of this "optimal" thickness. We are then able to demonstrate a specific property of phase response which is independent from the metal thickness in certain conditions. This leads us to suggest a new design for the sensor that benefits from this phenomemon and improves the resolution in an angular plasmonic coupling detection. As for the study of phase detection in grating coupler, a numerical method based on coupled waves analysis has been developed and tested. Using this method we have simulated the phas

Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque

Spin-orbit effects [1-4] have the potential of radically changing the field of spintronics by allowing transfer of spin angular momentum to a whole new class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $\mu$m thick Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer spin angular momentum through the interface between these two different materials. The outstanding feature was the detection of auto-oscillation of the YIG when enough dc current was passed in the Pt. This finding has created a great excitement in the community for two reasons: first, one could control electronically the damping of insulators, which can offer improved properties compared to metals, and here YIG has the lowest damping known in nature; second, the damping compensation could be achieved on very large objects, a particularly relevant point for the field of magnonics [6,7] whose aim is to use spin-waves as carriers of information. However, the degree of coherence of the observed auto-oscillations has not been addressed in ref. [5]. In this work, we emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current. This requires to reduce both the thickness and lateral size in order to reach full damping compensation [8] , and we show clear evidence of coherent spin-orbit torque induced auto-oscillation in micron-sized YIG discs of thickness 20 nm

Étude d'une électronique refroidie pour un oscillateur hyperfréquence à résonateur supraconducteur

National audienceLe choix de composants actifs à faible bruit de phase et leur modélisation est indispensable à la réalisation de sources hyperfréquences à haute pureté spectrale. Différents transistors ont donc été testés vis-à-vis de leurs caractéristiques en gain et en bruit de phase à basse température. Un modèle électrique du composant sélectionné a été extrait et les premières simulations de l'oscillateur cryogénique se révèlent particulièrement prometteuses en termes de pureté spectrale (-155 dBc/Hz à 1 kHz de la porteuse à 1 GHz)

Development of a mobile HTS cryogenic oscillator

International audienceHigh-temperature superconductors (HTS) have shown their potential for analog signal processing at microwave frequencies; they provide high Q resonators, the most critical component of an efficient low phase noise oscillator. Thus, a strategy to outperform state of art quartz oscillators could consist in integrating, in a monolithic block, a planar HTS resonator and a cryo-compatible transistor to build a 1 GHz oscillator. However, mobile systems may require reducing hardware size, weight and power consumption while meeting high performances, controlled costs and reliability needs. We discuss these questions and present our approach to validate the integration of HTS based oscillators into embedded systems

Low phase noise cryogenic amplifiers and oscillators based on superconducting resonators

International audienceA cryogenic low phase noise amplifier and an high Q superconductor resonator at 1 GHz have been designed and realized. A good agreement between the measured and simulated data at 80 K for these two devices is observed. An all cryogenic oscillator has also been designed with the same devices on an alumina substrate. This oscillator is still under test

A low phase noise all cryogenic microwave oscillator based on a superconductor resonator

International audienceA 1 GHz full cryogenic oscillator is presented. The oscillator is based on a planar superconductor resonator featuring a loaded Q factor of 200 000 at low microwave input power (unloaded Q of 400 000) and on amplifying parts realized with SiGe bipolar transistors. The circuit is designed with an harmonic balance software and realized on alumina substrate. A nonlinear model is extracted at low temperature both for the transistor and the resonator. This double nonlinearity increases the difficulty of the oscillator design and implies a strategy to limit the power inside the resonator. The vibrations of the cryogenerator are also a serious issue to get high performance. Finally, the oscillator features a phase noise of-112 dBc/Hz at 100 Hz offset frequency and a phase noise floor of -170 dBc/Hz (100 kHz offset) at a temperature of 65 K