Terahertz Sum-Frequency Excitation of a Raman-Active Phonon

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 sumfrequency 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 THz pulse is directly transferred into the phase of the lattice vibration. New prospects in general infrared spectroscopy, action spectroscopy, and lattice trajectory control in the electronic ground state emerge.

Laser‐Driven Strong‐Field Terahertz Sources

“…Driving these modes to large amplitudes can enable new insights into their properties. [164] Here, two frequencies of the incident THz pulse induce a force on the lattice oscillating at the sum frequency, rather than the difference frequency that occurs in standard Raman-type excitations. [162,163] Even if the phonon of interest is infrared-inactive yet Ramanactive does THz radiation provide new means of phonon excitation by a Raman-type sum-frequency process.…”

“…For instance, excitation of infrared-optical phonons with intense THz pulses and tracing the anharmonic response [161] allowed one to reconstruct the atomic-scale potential the ions are moving in. [164] This process can also be considered as 2PA by an optical transition between two adjacent vibrational levels. [164] Here, two frequencies of the incident THz pulse induce a force on the lattice oscillating at the sum frequency, rather than the difference frequency that occurs in standard Raman-type excitations.…”

“…[162,163] Even if the phonon of interest is infrared-inactive yet Ramanactive does THz radiation provide new means of phonon excitation by a Raman-type sum-frequency process. [164] Here, two frequencies of the incident THz pulse induce a force on the lattice oscillating at the sum frequency, rather than the difference frequency that occurs in standard Raman-type excitations. By using intense THz pulses centered at 20 THz, Raman-active zone-center optical phonons of diamond at a frequency of 40 THz were successfully excited and phasecoherently controlled.…”

“…By using intense THz pulses centered at 20 THz, Raman-active zone-center optical phonons of diamond at a frequency of 40 THz were successfully excited and phasecoherently controlled. [164] This process can also be considered as 2PA by an optical transition between two adjacent vibrational levels.…”

Electron–phonon-driven three-dimensional metallicity in an insulating cuprate

“…The underlying mechanism involves the resonant excitation of large-amplitude infraredactive modes, whose oscillation displaces the crystal along the coordinates of coupled Ramanactive modes. Extending these studies to the insulating parent compounds by employing narrow-band THz fields and sum-frequency ionic Raman scattering [55,56] will realize the mode-selective control of the IMT in the electronic ground state, paving the way to the use of correlated insulators in fast room-temperature devices. Finally, our joint experimentaltheoretical effort points to rational-design strategies for novel quantum materials that exhibit a subtle interplay of electron-electron and electron-phonon interactions.…”