Superfluid Brillouin Laser

Enabled by superfluid helium's ultra low viscosity, we propose and experimentally demonstrate a superfluid Brillouin laser with ultra-high gain. Our experimental setup consists of an evanescently coupled... [ view full abstract ]

Enabled by superfluid helium's ultra low viscosity, we propose and experimentally demonstrate a superfluid Brillouin laser with ultra-high gain. Our experimental setup consists of an evanescently coupled silica microresonator (see Fig. (a)) covered by a nanometer-thick superfluid 4He film [1,2]. Regions of high light intensity inside the resonator pull in more superfluid due to the optical radiation pressure force (see Fig. (b)) , creating a spatially modulated refractive index grating, in turn scattering more light, leading to spontaneous Brillouin lasing (see Fig. (c)), where a pump photon is scattered into a lower energy Stokes photon and a phonon of frequency f_BS (the brillouin shift) (see Fig. (d)).

In contrast to the commonly studied Brillouin scattering in solids, whereby the light intensity strains the material through electrostrictive forces, this superfluid based approach provides orders of magnitude larger gain. This dramatic enhancement can be intuited by comparing the difficulty of straining a high Young's modulus material such as silica, compared to the ease with which light can continuously deform a (super) fluid interface. This work brings the optical sensitivity of optomechanical systems to the fluid/soft-matter systems, and has applications in terms of on-chip ultra-narrow linewidth lasers and optical stirring of quantum fluids.

[1]  Harris, G. I. et al.. "Laser Cooling and Control of Excitations in Superfluid Helium." Nature Physics 12, 8, 2016.

[2] Baker, Christopher G. et al. "Theoretical Framework for Thin Film Superfluid Optomechanics:Towards the Quantum Regime." New Journal of Physics 18, 12, 2016.