Harnessing spontaneous Brillouin scattering for nonequilibrium phonon spectroscopy

Phonon spectroscopy allows access to phonon populations of individual acoustic modes within a material, thereby offering information about the material’s optical and mechanical properties. Selectivemanipulation of phonon populations has been demonstrated using spontaneous backward Brillouin scattering in photonic systems [29, 43, 20]. In this work we demonstrate phonon spectroscopy in liquid core optical fibers using spontaneous backward Brillouin scattering and introduce a novel method for isolating and measuring thermal phonon populations. This inelastic scattering process paired with heterodyne spectroscopy offers an elegant solution for measuring Stokes and anti-Stokes phonon mode populations where one backscattered photon corresponds to one phase-matched phonon. An optical field is injected into a liquid core optical fiber and interacts with the material’s spontaneously generated vibrational modes. In the case of optical scattering from a forward propagating wave, the backscattered Stokes light is red-shifted in frequency where energy is deposited from the optical field into the phase-matched acoustic mode [32, 34, 2]. Similarly, backscattered anti-Stokes light is blue-shifted when the optical field interacts with a counter-propagating traveling acoustic wave. Through heterodyne detection, the backscattered signal combined with a known local oscillator produces a beatnote frequency directly corresponding to the phase-matched phonon population distribution. Additional damping of the anti-Stokes acoustic mode scales with optical power allowing for a form of laser modal cooling for the backward traveling acoustic phonons [29]. We introduce a method including two orthogonally polarized optical fields to isolate thermal phonon contributions intrinsic to the material from those driven by the optical field for the first time. In this way, we determine the thermal phonon occupation for the Stokes and anti-Stokes sidebands and demonstrate attenuation of the anti-Stokes phonon population