Paper MN-WeM6
GaAs Disks Optomechanical Resonators in Liquid

Wednesday, November 12, 2014, 9:40 am, Room 301

Vibrating nano or micromechanical structures, such as cantilevers, have been the subject of extensive research for the development of ultrasensitive mass sensors for mass spectrometry, chemical sensing and biomedical analysis. In liquids, the energy losses due to viscous damping, acoustic losses and squeeze film effects are high and the mass sensitivity diminishes dramatically. Additionally, viscous damping in a fluid is often larger when the devices are miniaturized.

To circumvent these problems, novel structures have been proposed, such as microchannels, where the liquid is placed directly inside the resonator. They have indeed shown lower energy losses, but they can hardly be miniaturized. External feedback loops have been applied as well in order to diminish artificially energy losses [4]. Besides this, another technique to reduce mechanical losses in a liquid has been to use higher order modes or contour/extensional modes. On one hand, these modes indeed show lower dissipation, on the other hand the related displacement is extremely small, making it more difficult to detect, especially in a liquid.

Here we study the potential of GaAs disk resonators in this context, in particular focusing on mechanical radial breathing modes. GaAs mechanical disks, with their high mechanical Q even in air (>10³), their low mass (pg) and high mechanical frequency (GHz), have been shown to be potential powerful sensors. Their use in liquids has been never investigated or even suggested, and it is still uncertain what energy losses and sensitivity they will present in such environment. GaAs disks support optical whispery gallery modes (WGMs), with high optical quality factor (several 10⁵). This fact, together with the outmost optomechanical coupling that they possess (up to 4 MHz), provide them with an extremely high displacement sensitivity in the 10^-18 m/√Hz range, allowing measuring the thermomechanical noise of the resonator even in a liquid.

We measure for the first time a GaAs mechanical disk vibrating in a liquid, directly in the Brownian motion regime. Employing finite element simulations and an analytical fluid-structure model, we investigate the mechanical damping mechanisms at play in this situation. We study the fluidic dissipation as a function of disk’s dimensions and of the physical properties of the liquid, such as density and viscosity, performing experiments in a family of different liquids. We finally analyze the sensing capabilities of this object and compare it to other existing approaches.