Strong optical coupling through superfluid Brillouin lasing

Abstract

Brillouin scattering has applications ranging from signal processing [1, 2], sensing [3] and microscopy [4], to quantum information [5] and fundamental science [6, 7]. Most of these applications rely on the electrostrictive interaction between light and phonons [3, 7, 8]. Here we show that in liquids optically-induced surface deformations can provide an alternative and far stronger interaction. This allows the demonstration of ultralow threshold Brillouin lasing and strong phonon-mediated optical coupling for the first time. This form of strong coupling is a key capability for Brillouin-reconfigurable optical switches and circuits [9, 10], for photonic quantum interfaces [11], and to generate synthetic electromagnetic fields [12, 13]. While applicable to liquids quite generally, our demonstration uses superfluid helium. Configured as a Brillouin gyroscope [14] this provides the prospect of measuring superfluid circulation with unprecedented precision, and to explore the rich physics of quantum fluid dynamics, from quantized vorticity to quantum turbulence [15, 16].Brillouin scattering is an optomechanical process that couples two optical waves via their interaction with travelling acoustic phonons. In the electrostrictive interaction usually employed, the optical electric field induces strain in a bulk medium, and the generated phonons scatter light between the two optical waves via refractive index changes caused by the medium's photoelasticity. However, the inherent weakness of this interaction presents a significant challenge [3], necessitating the use of high optical powers and prohibiting some applications. This can be alleviated by resonant enhancement in an optical cavity, which has allowed recent demonstrations of ultralow linewidth lasers [17, 18], Brillouin lasing in liquid droplets [8], non-reciprocal optical transport [19], Brillouin gyroscopes [14] and low-noise microwave oscillators [2]. Alternatively, optically-induced deformations of the boundary of the medium can be leveraged to provide a Brillouin interaction, with scattering induced by the effective refractive index-modulation caused by the deformation. In purpose-engineered solid structures these surface interactions can be made comparable to, or even exceed, the native electrostriction [20][21][22].Here, we transfer the concept of deformation-induced Brillouin scattering to liquid media, specifically a fewnanometer-thick superfluid helium film that coats the surface of a silica microdisk cavity and couples to its whispering gallery modes (see Fig. 1a,b) via perturbation of their evanescent field. Similar to other liquids [23], the superfluid film has an exceedingly weak restoring force, affording a compliant dielectric interface that easily deforms in the presence of optical forces [24, 25], as illustrated in Fig. 1c. This offers the potential for very large surface deformations and consequently extreme interaction strengths. We show that it allows radiation-pressure interactions with acoustic phonons that are over two orders of magnitude ...

Proposal for a quantum traveling Brillouin resonator

“…However, one limitation of superfluid helium lies in its minute refractive index (n He 1.029), which makes it difficult to use it to confine light and thus 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][30][31]. This approach however generally precludes simultaneous optical and mechanical confinement to wavelength-scale interaction volumes, limiting the light-sound coupling rates.…”

Superfluid Optomechanics With Phononic Nanostructures

“…In this work, we presented the characteristic properties of superfluid 4 He as an acoustic medium that are relevant for these applications. Of particular interest, is superfluid [20] 319 MHz 196 THz 3.6 kHz Childress et al [23] 0.02 -1 kHz 300 THz 0.2-10 kHz He et al [24] 1 -10 MHz 193 THz 133 kHz 4 He's extremely low sound attenuation in the low temperature limit, and naturally high acoustic impedance mismatch with most solid materials. We described the different phonon processes responsible for sound attenuation in superfluid 4 He, and how its properties can be tuned with pressure.…”

“…Furthermore, at much smaller scale (i.e. picogram and femtogram), superfluid 4 He resonators have shown great potential for quantum optomechanics experiments [19,20], the study of quantized vorticity in thin films [21,22] and levitating droplets [23], the enhancement of Brillouin interaction [24,25], and the realisation of qubits mechanical systems [26].…”

Optical Forces in Silicon Nanophotonics and Optomechanical Systems: Science and Applications

“…The optical forces in optomechanics, on the other hand, help cool nanostructures towards quantum regime in the room temperature [93] and also benefit various utilities in the signal storage and processing [28], photon and phonon manipulations [83], optical sensing [127], etc. Meanwhile, more demonstrations on the photon and phonon manipulations could also be done using the intriguing physics in optomechanics such as superfluid [128], minimizing the quantum devices for faster computing and information processing. Last but not least, we could also expect more intriguing effect using the optical forces in the silicon chip.…”