Towards Understanding Long-Lived Metastable Phases in Ultrafast Optical Experiments

A proposal for light-control of the lattice was presented in a series of theoretical papers published over 50 years ago. The basic concept developed in these papers involved optical excitation of an IR-active phonon, from which energy is coherently transferred into an anharmonically coupled Raman-active phonon. In this picture, changes in materials properties are induced by subtle changes in the crystal structure due to optical excitation of phonon modes, rather than electronic excitations or temperature effects. Experimental demonstration of this effect, now known as nonlinear phononics, was generally not possible at the time the theory was first developed. However, the development of bright laser sources capable of emitting light of IR and mid-IR frequencies has enabled direct optical control of crystal structures.



Observations from several recent nonlinear phononics experiments indicate that changes in materials properties and crystal structures due to optical excitation of IR-active phonons can sometimes persist for hundreds or even thousands of picoseconds after the initial light pulse. This is surprising because, in a basic nonlinear phononics picture, the optically excited phase should have a lifetime approximately comparable to a typical optical phonon lifetime (a few picoseconds to a few tens of picoseconds). In this talk, I will discuss our recent work using theory and first-principles calculations to show that strong coupling between strains and Raman-active phonons can lead to a long-lived metastable phase in optically excited LaAlO₃, as observed in recent experiments. Our work suggests that strong coupling between different order parameters can provide a mechanism for long-lived optically created metastable phases and points towards strategies, such as strain engineering, for modifying or increasing the lifetime of light-induced phases in ultrafast optical experiments.