  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.
