12th of December 2018 (Besançon) – PhD defense of Jérémy Bon

Jérémy BON will defend his thesis on 12th of December 2018 at 10:30 am on the subject “Résonateurs à ondes acoustiques de volume piégées à très basses températures : Application à l’optomécanique”, realized in FEMTO-ST Institute, under the supervision of Serge Galliou.

The defense will take plac eat the Amphitheatre Jules Haag of ENSMM, in front of a jury composed of Ludovic Bellon, Bernard Bonello, Pierre-François Cohadon, Gianpietro Cagnoli, Bernard Dulmet, Serge Galliou et Roger Bourquin.


For a few years, the Time and Frequency department in FEMTO-ST Insitute has been leading research about the behavior of Bulk Acoustic Wave (BAW) trapped in quartz crystal at cryogenic temperatures (near 4K).
The measured quality factor are around a few billions at few tens of MHz for such temperatures. Acoustical quartz cavities are therefore good candidates for ultrastable cryogenic frequency sources.
The work presented here is in the natural continuation of the research cited above. They aim at strenghtening the interest for quartz crystal, but also to consider alternative solutions with non-piezoelectric material with very-low acoustical losses, for which optical excitation is an option.
The following work can be summed up in three main parts:
– The first part is about the determination of a quartz crystal cut for which an turnover point exists in the frequency-temperature curve in the cryogenic region. Indeed, it is not enough to barely control the temperature in an ultrastable frequency source. Such an turnover point needs to be the operation point for thermal regulation. Searching a compensated cut arose the need for a preliminary measurements campaign of thermal coefficients of elastic coefficients of the material, which were unknown at low temperature.
It was then possible, based on these coefficients, to calculate and even realize a cut fulfilling the required condition.
– The second part had the objetive to demonstrate conceptually that using a quartz acoustical cavity as an optical cavity was feasable. In its basic scheme, a BAW quartz resonator is plano-convex (to ensure the trapping of the acoustic wave) and has electrodes (metal-made to ensure electrical excitation) deposited on each face. It has been demonstrated, both theoretically and experimentally, that such a geometry works fine as an optical cavity, with its corresponding advantages and limitations. This scheme is used for the optomechanical coupling discussed in the third part and constitutes the very base for more efficient optomechanical devices.
– The third part is dedicated to the evaluation of how efficient will such devices be while functioning at cryogenic temperature. A theoretical quantitication of the optomechanical coupling that these cavities might reach is also presented.
Producing an optomechanical cavity will allow avoiding the physical constraints imposed by the use of a cryogenerator and will open the way to the study of non-piezoelectric material with very low mechanical losses, similar or even lower than that of quartz.
This kind of experimentation also answers to the needs of other research teams working on quantum optomechanics or hybrid quantum systems (LKB, UWA, …) with which collaborations are currently being held.