Terahertz-Driven Phonon Upconversion in SrTiO3

TL;DR

This paper demonstrates that intense terahertz pulses can drive low-frequency phonons in SrTiO3 into anharmonic regimes, enabling energy transfer to higher frequency phonons via nonlinear coupling, visualized through femtosecond x-ray imaging.

Contribution

It introduces a novel method of phonon upconversion in SrTiO3 using terahertz fields, expanding control over lattice dynamics beyond traditional infrared excitation.

Findings

Successful excitation of anharmonic phonons in SrTiO3

Observation of energy transfer to higher frequency phonons

Visualization of atomic displacements with femtosecond x-ray pulses

Abstract

Direct manipulation of the atomic lattice using intense long-wavelength laser pulses has become a viable approach to create new states of matter in complex materials. Conventionally, a high frequency vibrational mode is driven resonantly by a mid-infrared laser pulse and the lattice structure is modified through indirect coupling of this infrared-active phonon to other, lower frequency lattice modulations. Here, we drive the lowest frequency optical phonon in the prototypical transition metal oxide SrTiO3 well into the anharmonic regime with an intense terahertz field. We show that it is possible to transfer energy to higher frequency phonon modes through nonlinear coupling. Our observations are carried out by directly mapping the lattice response to the coherent drive field with femtosecond x-ray pulses, enabling direct visualization of the atomic displacements.