Sub-10 Femtowatt Sensitivity in Room-Temperature Brillouin Spectroscopy Using a Detuned Pump-Probe Design

Brillouin spectroscopy is a powerful tool for studying acoustic phonons in materials, but traditional techniques are limited by phase-matching requirements and sensitivity constraints at small scales. We present a novel coherently stimulated Brillouin spectrometer utilizing a detuned pump-probe design that overcomes these limitations by relaxing phase-matching requirements at micrometer lengths. This enables room-temperature traveling-wave phonon spectroscopy at the micrometer scale with sub-10 femtowatt sensitivity. Our instrument's sensitivity was validated using 1 cm of UHNA3 fiber and 100 micrometers of bulk carbon disulfide liquid, demonstrating its capability to measure Brillouin scattering in low-gain materials or those with small effective lengths. This advancement opens new possibilities for nanometer-scale Brillouin spectroscopy, with potential applications in nanoacousto-optic devices and materials science.