Ultrafast control of electrons by opitcally controlling the lattice

Recent experiments have demonstrated the potential for modifying the properties of materials using ultrafast optical pulses to selectively and coherently excite particular phonon modes. One such mechanism involves optical excitation of an IR-active phonon Q_IR, which produces a displacement of a Raman-active mode Q_R due to a special anharmonic coupling between the modes, Q²_IRQ_R. This nonlinear phononic effect, and the subtle structural changes it induces, has been invoked to interpret, for example, observations of a five-orders-of-magnitude decrease in resistivity in Pr_0.7Ca_0.3MnO₃ (a material that is insulating at equilibrium at all measured temperatures), and the observation of room temperature superconductivity in YBa₂Cu₃O_6.5. The nonlinear phononics mechanism essentially exploits the strong coupling between an optically excited IR-active mode and some order parameter of the system. Where should we look for materials that might exhibit a strong nonlinear phononics response? In this talk, I will argue (using work from my own group and others) that approaches developed in the search for strongly coupled multiferroics – materials in which an electrical polarization is strongly coupled to magnetism – may aid the search for materials that exhibit large changes in their functional properties when optically excited. In particular, I hope to show that discovering mechanisms through which the lattice, spin and orbital degrees of freedom are strongly coupled may provide a particularly fruitful path forward. Requirements for experimental realization will be discussed, as well as challenges and opportunities unique to the nonlinear phononics field.