Abstract
Nonlinear phononics is the phenomenon in which a coherent dynamics in a material along a set of phonons is launched after its infrared-active phonons are selectively excited using external light pulses. The microscopic mechanism underlying this phenomenon is the nonlinear coupling of the pumped infrared-active mode to other phonon modes present in a material. Nonlinear phonon couplings can cause finite time-averaged atomic displacements with or without broken crystal symmetries depending on the order, magnitude and sign of the nonlinearities. Such coherent lattice displacements along phonon coordinates can be used to control the physical properties of materials and even induce transient phases with lower symmetries. Light-control of materials via nonlinear phononics has become a practical reality due to the availability of intense mid-infrared lasers that can drive large-amplitude oscillations of the infrared-active phonons of materials. Mid-infrared pump induced insulator–metal transitions and spin and orbital order melting have been observed in pump–probe experiments. First principles based microscopic theory of nonlinear phononics has been developed, and it has been used to better understand how the lattice evolves after a mid-infrared pump excitation of infrared-active phonons. This theory has been used to predict light-induced switching of ferroelectric polarization as well as ferroelectricity in paraelectrics and ferromagnetism in antiferromagnets, which have been partially confirmed in recent experiments. This review summarizes the experimental and theoretical developments within this emerging field.
Highlights
Light is a popular probe that is widely used to investigate the structure and properties of materials
Nonlinear phononics is an emerging field that has the potential to develop as a powerful method for controlling materials by stabilizing novel crystal structures that cannot be accessed in equilibrium
This is made possible by coherent atomic displacements along a set of phonon coordinates after a selective excitation of the IR-active phonons of a material, and it contrasts with the incoherent atomic motions that result from heating
Summary
Light is a popular probe that is widely used to investigate the structure and properties of materials. More convincing evidence of light-induced displacement due to nonlinear phononics has been proposed [19] and observed in ferroelectric LiNbO3, which has a highfrequency IR-active phonon with a large oscillator strength [16] In this material, a strong reduction and sign reversal of the electric dipole moment and a simultaneous oscillation at the frequency of the pumped IR-active mode has been observed in pump–probe SHG experiments. Subedi et al started the use of a theoretical framework to study nonlinear phononics that is based on symmetry principles to identify the symmetry-allowed nonlinear couplings, first principles calculations of the coefficients of these nonlinear terms, and solution of the equations of the motions for the coupled phonon coordinates [38] This framework was used to explain the light-induced phenonmena observed in the pioneering mid-IR pump experiments in the manganites [13, 20]. This review attempts to summarize the experimental and theoretical developments in the field of nonlinear phononics, emphasizing how theoretical calculations have helped the experimentalists drive this field forward
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