Raman phonon dynamics and its control for enhanced optical frequency conversion

Abstract

Raman phonons arise from the inelastic scattering of light and represent quantized molecular motions that mediate a wide range of spectroscopic and nonlinear optical phenomena. In this work, we clarify the physical role of Raman phonons within a previously-developed time-domain framework based on the Raman-induced index modulation, and show that phonons correspond to the oscillatory component of the Raman-induced index modulation. The analysis further reveals a linear phonon-mediated interaction embedded within Raman scattering, in which optical fields couple through wave-vector matching with existing phonons. This mechanism underlies what has long been described as coherent Stokes and anti-Stokes scattering, as well as molecular modulation. Building on this insight, we introduce a phonon-controlled approach that enables efficient conversion into a selected Stokes order by tuning the wave-vector-matching relation between the driven phonons and the targeted Raman process. These results provide a clearer physical interpretation of Raman phonons and its corresponding Raman dynamics and offer new strategies for controlling Raman interactions.

Figures (8)

• Figure 1: Raman-induced index modulation driven by a pulse of different durations Chen2024. The example here use 56.8 as the Raman period, which corresponds to the $S(1)$ rotational Raman transition in H2. Impulsive regime: $\tau_0\ll\tau_R,T_2$, transient regime: $\tau_0\ll T_2,\tau_0\gg\tau_R$, steady-state regime: $\tau_0\gg\tau_R,T_2$. $\tau_0$: pulse duration, $\tau_R=1/\nu_R$: Raman period, $\nu_R=\omega_R/(2\pi)$: Raman transition frequency, $T_2=1/\gamma_2$: dephasing time, $\gamma_2$: dephasing rate.
• Figure 2: Raman scattering due to (a) vibrational or (b) rotational molecular motions. Linearly-polarized field is used for illustration here.
• Figure 3: Oscillatory Raman-induced index modulation $\triangle\epsilon_R^{\text{osc}}$ in different temporal Raman regimes. In the long-pulse regimes, the oscillatory part can be much stronger than the pulse-following part.
• Figure 4: Phonon effects to a pulse. (a) Impulsive phonon effects. (b) Transient and steady-state phonon effects. x-axis represents time. In the steady-state regime with a short phonon lifetime compared to the pulse duration, the pump pulse that generates the phonons must temporally overlap with the probe pulse to be investigated (Fig. \ref{['fig:overlapping_suppression_ss']}).
• Figure 5: Linear phonon-mediated effects on the optical field.