Quantum Cafe: Premala Chandra

Title: Light-Induced Orderings that Oscillate in Space and in Time

Abstract: The control and design of quantum materials are outstanding goals both to address fundamental questions and to develop applications with quantum advantages. Recent advances in ultrafast optics have led to tremendous opportunities to modify their low-energy physics. To date, work in this area has been focused on materials close to instabilities which are then enhanced when light interacts with the systems. As a result, the nature of the transient order is determined by the soft modes in the equilibrium system, and its characteristic features are generally not tunable.

In this talk I will present a general mechanism for light-induced generation of tunable incommensurate orders in materials not initially close to any phase transition; this alternative pathway results from the interplay of nonlinearity, parametric resonance and spatial dispersion. I will discuss how driving optical phonons above a threshold fluence induces spatiotemporal orders, where material properties oscillate at an incommensurate wavevector qO in space and at half the drive frequency in time. The order is robust against temperature on timescales much larger than the lifetime of the excited modes and can be accompanied by a static 2q0 modulation. Predictions for time-resolved diffraction and estimates for candidate materials. will be presented. These results show the possibility of using THz waves in solids to realize tunable incommensurate orders on the nanoscale.