Cavity Cooling of Dielectric Nanoparticles

CAVICOOL is a research project targeted at the development of advanced optical cooling techniques for size- and shape selected dielectric nanoparticles in ultra-high vacuum. It is explored as an enabling technology for future quantum interferometry experiments in the regime of ultrahigh masses, i.e. 107-1010 amu. CAVICOOL is based on the recent first successful demonstration of cavity cooling of dielectric nanoparticles by two independent research groups at the University of Vienna [1, 2]. CAVICOOL complements ongoing international efforts on nanoparticle cooling and shall pioneer the next big leaps in the field: CAVICOOL will study new laser-induced volatilization methods for size- and shape selected slow nanoparticles in ultrahigh vacuum. In particular laser-induced acoustic desorption (LIAD) and laser induced thermo-mechanical stress (LITHMOS) shall enable the launch of size-selected spherical particles with diameters between 30-300 nm. CAVICOOL will implement bimodal cooling in a free-standing cavity as a technique to reach temperatures in the millikelvin range for masses around 1010 amu. CAVICOOL will develop microcavities etched into pristine silicon wafers. The expected mirror quality and finesse shall enable focal beam waists as small as 5 m with mirror separations of approximately 500 m. This serves the goal of cooling with particles in the diameter range or 30-50 nm. CAVICOOL will implement fast optical feedback radial to the cavity axis to foster further slowing by increasing the residence time of the particles in the cooling field. This shall allow us to reach millikelvin temperatures also for particles in the 107 amu mass range. All this is an important prerequisite for future near-field quantum diffraction experiments with particles in the 107 amu mass range.

The aim of the FWF individual project CAVICOOL was to develop new methods for cooling dielectric nanoparticles with a view onto future experiments with massive quantum systems as well as highly sensitive acceleration and torque sensors. The starting point of this study was the experimental successes of our research group, in which we were able to show that nanoparticles with 10 billion proton masses can be cooled by laser light in high-quality optical resonators: Even a transparent particle changes the optical properties of a resonator through its refractive index and the light field in this cavity, in turn, exerts a force on the particle. Thus, the movement of a silicon nanoparticle in the resonator modulates the light field such that it always runs against a higher "light mountain" than it can roll down it again. As a result, it loses kinetic energy. Cavity-cooling of nanoparticles reminds us of the mythical hero Sisyphus who had to run uphill perpetually. For novel quantum interference experiments, one now wants to cool particles that are 1000x smaller than those for which this cavity effect has already been proven, before. A particular challenge here is the fact that cooling becomes drastically more challenging as the mass of the particle decreases. To ensure the required light-matter coupling, the CAVICOOL team has therefore developed and tested integrated microfabricated silicon resonators with a tiny mode volume. These micro-resonators have exceptional quality, mechanical stability and open access to allow feeding with nanoparticles. The finesse of the resonators is now sufficient to cool particles with microscopic masses of around 107 atomic mass units, corresponding to 10-20 kg, in a next step. In addition, new sources were developed that allow free-form nanoparticles to be catapulted into high vacuum, using laser light. This method enabled first experiments to observe cavity light to act on the rotational motion of the particles. This paved the way for new experiments in rotational-quantum optomechanics, too.