Nonlinear Optical Signatures of Collective Vibronic Correlations in NV– Centers

We demonstrate that second harmonic generation (SHG) reveals signatures of a collective vibronic response associated with electron–phonon coupling in NV– centers in diamond. Broadband SHG spectroscopy resolves resonances associated with the zero-phonon line and phonon sideband. Complementary photoluminescence and ultrafast pump–probe measurements reveal a broad vibronic sideband and a coherent 40-THz optical phonon, demonstrating the participation of high-frequency phonons in the nonlinear optical response. Polarization-resolved SHG further reveals a crossover from an electronically dominated response to a collective vibronic response and, at higher defect densities, increasing disorder-induced decoherence. Our results reveal collective vibronic correlations associated with electron–phonon coupling and establish SHG as a phase-sensitive probe of vibronic dynamics in solid-state quantum systems.