NSF-BSF:Fluid-fluid interfaces with diminished surface tension and giant thermal and quantum fluctuations as novel materials for ultrasoft photonics

Nontechnical abstract

The behavior of materials under extreme conditions is often where new fundamental discoveries are made and new applications are developed. The authors of this project study the behavior of interfaces between two different liquids in the limit of extreme softness such that by making the material even a little bit softer will result in the destruction of the interface by thermal oscillations.

This project will study light confined in the interior part of the spherical interface by using the difference in refraction indexes on the opposite sides of the interface. This novel confinement enables the researchers to study the details of the thermal fluctuations of the ultrasoft interface with an unprecedent resolution and accuracy using the modes of the confined light.

In addition to providing information about thermal motion of the molecules, optical excitations confined by the interface allow researchers to optically affect its mechanical oscillations, which will, in their turn affect the optical excitations. Such so-called optomechanical interaction is expected to be significantly enhanced in the ultrasoft materials and will allow studying novel optomechanical phenomena.

This international collaborative project will also contribute to educational activities of both participating institutions (Queens College, New York and Tel-Aviv University, Israel) by developing new courses around the themes of the project and involving students, especially those from underrepresented groups, in work on the project. In particular, this project will fund a research internship opportunity for a student at Queens College’s M.S. in Photonics program, designed to help students from underrepresented groups to enter the labor market in photonics related industries.

Technical abstract

This project deals with optomechanical properties of fluid-in-fluid spherical droplets actuated as whispering-gallery-mode resonators. The elasticity of the droplet-forming interface is controlled by surfactants and can be reduced to the limit of ultimate softness such that any additional surfactant would destruct the droplet by Brownian fluctuations.

The mechanical softness of the droplets does not affect their performance as optical resonators, which is determined by the refractive index contrast between the material of the droplet and the surrounding medium. The combination of mechanical softness and the resonance enhanced sensitivity due to the formation of whispering gallery modes enables the optical interrogation of Brownian fluctuations of the surface with resolution exceeding that of existing Rayleigh- limited imaging techniques.

The extreme softness of the droplets results in the giant mechanical response to resonantly enhanced optical forces allowing researchers to achieve a strength of optomechanical coupling far exceeding that in any other available optomechanical system. The regime of the extremely strong optomechanical coupling enables the study of optomechanical phenomena well outside of typically available range of parameters.

For instance, in these systems one can achieve efficient cavity-mediated cooling of the surface oscillations to the smallest phonon population numbers despite the penalty imposed by the lower phonon frequencies. The high-value goal of the project is to cool the capillary oscillations of the softened droplets toward the quantum mechanical ground-state while maintaining the bath at room temperature.

The high risks of such a project will be mitigated by testing various gas-liquid and liquid-liquid interfaces at different temperatures, pressures, and surfactant adsorption-concentration. The experimental efforts will be accompanied by theoretical research, with a novel perturbation approach based on generalized theory of Mie scattering to computing optical spectra of resonators with shapes deviating from spherical will be developed.

A theory will also be developed of sideband cooling to minimize the phonon number at the non-resolved sideband limit.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.