Pump induced new polaritons in ultrafast experiments

We theoretically investigate the effect of nonlinear terms of infrared-active phonons in pump-probe experiments. We show that after the phonon is intensely excited, the collective mode spectra of the material is qualitatively changed due to the phenomenon of parametric resonance. Specifically, new polariton branches are created near the integer multiples of the frequency ω₀ of the pumped phonon. In non-centrosymmetric materials, the new branches appear primarily near 2ω₀, while in centrosymmetric materials they appear near 3ω₀. We demonstrate this effect in realistic materials such as hBN flakes, and propose to observe this effect by near-field optical spectroscopy. We emphasize that this phenomenon is distinct from simple high harmonic generation.