Research

Introduction: gravitational waves and optomechanical coupling

The optomechanics research community emerged as a product of the great scientific effort to directly observe a gravitational wave propagating through earth and altering the relative length between the arms of a giant Michelson interferometer [1]. This effort commenced in the early 80's when theoreticians like Caves and Braginsky [2,3] posed the question that defined the mission of optomechanics: “can such a measurement be infinitely precise?”. Since then, several research groups started experimental activities to test the fundamental limits associated to this measurement. One of these groups, led by optomechanics pioneer A. Heidmann, developed in the late 90's a table-top optical experiment to simulate in the laboratory the experimental conditions of a gravitational antenna. I received my training as an experimentalist in his group in the prestigious Laboratoire Kastler Brossel (Paris), where I produced my Doctoral thesis [4]. The experiment was based on a high-finesse optical cavity and a pump-probe experimental scheme, combined with a state-of-the-art detection system which until today holds the record of the most accurate displacement measurement [5]. There, I focused my efforts in observing radiation pressure effects onto the cavity's movable mirror, at room temperature, which constitutes one of the fundamental quantum noises that will be limiting the next generation of gravitational antennas. I learned the use of all the experimental tools of quantum optics (optical cavities, homodyne detections etc.) and I obtained a high proficiency in analog feedback protocols used to actively stabilise the experiment.

From macro-resonators to nano-resonators

The technological advances in the fabrication of nano structures offered to the optomechanics community an additional boost. By increasingly reducing the dimensions of the movable object -thus making it lighter- quantum optomechanical effects became more easily measurable [6,7]. This led me to take up the next challenge in my academic career at the Barcelona-based Institut of Photonics Sciences - ICFO Barcelona, where I developed novel experimental techniques to measure the tinniest existing resonator: a suspended carbon nanotube (CNT). To succeed this goal, I developed and used two different experimental platforms. Firstly, I mounted a novel experimental setup based on a tightly focused laser beam and a sensitive avalanche photodetector. A robust nano-engineering protocol enabled the accurate deposition of a metallic nano particle on the edge of the CNT assuring a detectable amount of scattering photons and providing a spectacular signal-to-noise ratio (> 30 dB) at room temperature. The stability of this platform permitted the study of cavity-free dynamical back-action effects, the resonator non-linear effects and the closed-loop manipulation of the resonator (frequency noise cancellation, cold damping, etc.) [8].

Secondly, I have used a focused electron beam provided by a commercial Scanning Electron Microscope (SEM) to perform real time measurements of carbon nanotube resonators. This technique permitted for the first time the extraction of the complete information on the dynamics of the motion of a carbon nanotube (i.e. the amplitude and phase). With careful experimental manipulation the complete motion of the carbon nanotube was reconstructed in real space while with the use of an active feedback scheme was implemented to assure real-time tracking of the frequency thus making it the first demonstration of mass sensing using a carbon nanotube resonator at room temperature [9].

Beyond...

A suspended carbon nanotube in combination with a metallic nano particle is a promising hybridised nano optomechanical system; it may combine the measurement purity of cavity optomechanics with all the advantages by a nano-mechanical system. Its extreme light mass (~ ag) makes it an ideal candidate to be used as a probe for fundamental forces (radiation pressure) but also for sensor-based applications (force/acceleration/mass sensor). In my next career steps, I plan to use this system for both fundamental studies and application-oriented development.