Session E2 - Optical Properties of Surfaces and Interfaces.
INVITED session, Tuesday morning, March 21
102AB, MCC
Nonlinear optical phenomena relying on a second order susceptibility, such as the coefficient \chi _2(-2ømega ;ømega ,ømega ) describing second harmonic generation, exist only if there is no centre of inversion symmetry. As such they serve as surface specific probes for materials that have centre of inversion symmetry in the bulk. That symmetry is clearly broken at the surface, where ``up'' is different than ``down,'' and thus any allowed second harmonic signal, for example, can be associated with the surface region. For some years now surface second harmonic generation, and the related processes of sum and difference frequency mixing, have proved to be powerful optical probes of surfaces.
Other second order phenomena should also provide surface specific probes. Recently it has been realized that the second order response coefficient \chi _2\left( 0;ømega ,-ømega \right) , usually associated with optical rectification, describes as well two effects that have been known for some time, but not in the context of nonlinear optics. They both appear as divergences in \chi _2\left( 0;ømega ,-ømega \right) that arise if \hbar ømega is above the energy gap of a semiconductor, or is associated with an interband transition in a metal, and can be observed in unbiased
samples. One effect, \textitcurrent injection, can be understood as a
quantum interference between absorption associated with two different polarizations of light; it leads to an injection of current in a direction that can be controlled by adjusting the phase relation of those two polarizations. A second effect, \textitshift current, is associated with the motion of the centre of charge in a unit cell as interband absorption occurs.
We review the susceptibility description of these phenomena, identify their dependence on crystal symmetry, discuss recent experimental work and numerical calculations, and consider the relevance of these phenomena as
probes of surfaces and possibly as tools for controlling electronic motion at surfaces in the ultrafast regime.