# Terahertz sum-frequency excitation of a Raman-active phonon

## TL;DR

This paper demonstrates the first experimental and theoretical observation of terahertz sum-frequency excitation of a Raman-active phonon, enabling coherent control of lattice vibrations via intense terahertz pulses.

## Contribution

It introduces and explores the neglected up-conversion process of THz sum-frequency excitation of phonons, expanding the understanding of phonon control mechanisms.

## Key findings

- Coherent control of diamond lattice vibration achieved.
- Carrier-envelope phase imprinted on lattice vibration.
- Potential applications in infrared spectroscopy and light storage.

## Abstract

In stimulated Raman scattering, two incident optical waves induce a force oscillating at the difference of the two light frequencies. This process has enabled important applications such as the excitation and coherent control of phonons and magnons by femtosecond laser pulses. Here, we experimentally and theoretically demonstrate the so far neglected up-conversion counterpart of this process: THz sum-frequency excitation of a Raman-active phonon mode, which is tantamount to two-photon absorption by an optical transition between two adjacent vibrational levels. Coherent control of an optical lattice vibration of diamond is achieved by an intense terahertz pulse whose spectrum is centered at half the phonon frequency of 40 THz. Remarkably, the carrier-envelope phase of the driving pulse is directly imprinted on the lattice vibration. New prospects in infrared spectroscopy, light storage schemes and lattice trajectory control in the electronic ground state emerge.