Optoacoustic Cooling of Traveling Hypersound Waves

We experimentally demonstrate optoacoustic cooling via stimulated Brillouin-Mandelstam scattering in a 50 cm long tapered photonic crystal fiber. For a 7.38 GHz acoustic mode, a cooling rate of 219 K from room temperature has been achieved. As anti-Stokes and Stokes Brillouin processes naturally break the symmetry of phonon cooling and heating, resolved sideband schemes are not necessary. The experiments pave the way to explore the classical to quantum transition for macroscopic objects and could enable new quantum technologies in terms of storage and repeater schemes.

Figure 1

(a) Diagram of the experimental setup used for the measurement of the backscattered SBS signal as a function of input power via heterodyne detection. CW: continuous wave; EOM (AOM): electro-optical (acousto-optical) modulator; EDFA: erbium-doped fiber amplifier; BPF: band-pass filter; ESA: electrical spectrum analyzer. (b) Scanning electron microscope image of the cross section of the untapered chacogenide glass PCF. The air-filling ratio of this fiber is 0.48 and it has a core diameter of $12\mathrm{\mu }\mathrm{m}$. (c) Diagram of the tapered sample. The taper waist has a length (l) of 50 cm and a core diameter of $3\mathrm{\mu }\mathrm{m}$. The waist is connected to 30 cm sections of untapered fiber, one on each side, via transitions of 2–3 cm in length.

Figure 2

(a) Diagram of the different processes affecting the population of resonant anti-Stokes phonons at ${\mathrm{\Omega }}_B$. (b) Light blue crosses show the experimentally measured decrease of resonant anti-Stokes phonon population (${\mathrm{\Omega }}_B/2\pi =7.38\mathrm{GHz}$) and its respective effective temperature as a function of pump power. From an initial population at room temperature (293 K) of 830 phonons, a final population of 212 is measured, corresponding to 74 K. This results in a temperature decrease of 219 K, or 74.7%. The solid blue line shows the theoretical decrease of phonon population according to Eq. (4). The horizontal black dashed lines are added as visual aids and indicate the initial and final temperature of the phonon mode.

Figure 3

(a) In red, behavior of the Stokes resonance for three different pump powers. (b) In blue, anti-Stokes resonance for the same powers as (a). Note the different $y$-axis scales. (c) Peak height of the Stokes (red triangles) and anti-Stokes (blue crosses) resonances in logarithmic scale as a function of pump power. (d) Peak linewidth (${\mathrm{\Gamma }}_{\mathrm{eff}}$) of the Stokes (red triangles) and anti-Stokes (blue crosses) resonances as a function of pump power. The Stokes resonance narrows, but the anti-Stokes peaks broadens. This indicates an increase of the effective dissipation rate (${\mathrm{\Gamma }}_{\mathrm{eff}}={\mathrm{\Gamma }}_m+{\mathrm{\Gamma }}_{\mathrm{opt}}$) of the addressed phonons, caused by the SBS interaction and a relaxation of the phase-matching condition.