Abstract
Brillouin systems operating in the quantum regime have recently been identified as a valuable tool for quantum information technologies and fundamental science. However, reaching the quantum regime is extraordinarily challenging, owing to the stringent requirements of combining low thermal occupation with low optical and mechanical dissipation, and large coherent phonon-photon interactions. Here, we propose an on-chip liquid based Brillouin system that is predicted to exhibit large phonon-photon coupling with exceptionally low acoustic dissipation. The system is comprised of a silicon-based "slot" waveguide filled with superfluid helium. This type of waveguide supports optical and acoustical traveling waves, strongly confining both fields into a subwavelength-scale mode volume. It serves as the foundation of an on-chip traveling wave Brillouin resonator with an electrostrictive single photon optomechanical coupling rate exceeding 240 kHz. Such devices may enable applications ranging from ultra-sensitive superfluid-based gyroscopes, to non-reciprocal optical circuits. Furthermore, this platform opens up new possibilities to explore quantum fluid dynamics in a strongly interacting condensate.
Highlights
For many decades, the Brillouin interaction was viewed as a weak nonlinear process, only appearing at high optical powers which were large enough to reveal the bulk electrostrictive properties of a material
This perspective was challenged in the early 2000’s after it was observed that co-localising light and sound into the confined geometry of a photonic crystal fiber resulted in strong Brillouin scattering at relatively low optical powers [1]
We propose an on-chip liquid based Brillouin system that is predicted to exhibit ultra-high light-sound coupling with exceptionally low acoustic dissipation and low thermal occupancy
Summary
The Brillouin interaction was viewed as a weak nonlinear process, only appearing at high optical powers which were large enough to reveal the bulk electrostrictive properties of a material. One limitation of superfluid helium lies in its minute refractive index (nHe 1.029), which makes it difficult to use it to confine light and co-localize light and sound in a small interaction volume This can be overcome to some degree by confining the superfluid to a micron-scale Fabry-Perot cavity [17,26], or by using a higher index waveguiding structure (such as a microdisk or microtoroid) to confine the light, with coupling to the superfluid afforded by the evanescent component of the light which extends outside the resonator [24, 29–31]. We propose an on-chip liquid based Brillouin system that is predicted to exhibit ultra-high light-sound coupling with exceptionally low acoustic dissipation and low thermal occupancy It relies on a slot waveguide resonator geometry [32, 33], where the optical field maximum is localized within a narrow channel which is filled with superfluid helium, as illustrated in Fig. 1(c) and discussed in detail below. This is in contrast to intra-modal or inter-modal Brillouin scattering process where the device’s size and mode structure must be carefully engineered to ensure the energy and momentum matching conditions are satisfied [22]
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