Distinguishing bulk and interface modulation of optical third harmonic generation due to coherent optical phonon excitation

Through use of coherence-modulated optical third harmonic generation, we simultaneously observe and distinguish coherent phonon oscillations from the bulk and interface of Bi3Ge4O12, which lacks inversion symmetry. We report the first observation of a coherent second hyper-Raman signal in bulk, along with phonon-induced Fresnel boundary modulation at the interface, both affecting the signal amplitude of the far-field third harmonic.