William Angerer - Department of Physics and Astronomy - University ...

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SHG is a virtual process, Le. there is no population transfer, even for lossy materials.
Population transfer occurs under a variety of circumstances including two
common mechanisms: one photon absorption and two photon absorption. In one
photon absorption, the absorption of a single photon excites a material. This process
is mediated by the imaginary part of the linear susceptibility, X(l). Two photon absorption
is a process in which a material absorbs two photons simultaneously. This
process is mediated through the imaginary part of the of the third-order nonlinear
susceptibility, X(3). The existence of one and two photon channels in a material does
not alter the form of equation (2.8), but influences the propagation of the fundamental
and SHe fields.
Second, X;~k(w
= 2wo) is nonzero only if the initial state Ii) is occupied. This
restriction has been exploited by J. Qi et at. to distinguish between nonlinear optical
process that originate from a midgap state as opposed to the valence band [10j.
Third, the second-order susceptibility is enhanced when the energy of the incident
or second-harmonic photons, nwo and 2nwo respectively, match an energy difference
between two states in the medium. This phenomena forms the basis for nonlinear
spectroscopy: an increase in measured SHG signal can be correlated with an electronic
or vibrational excitation in the material. Often one uses SHG to probe systems that
are inaccessible when using conventional linear optics or competing spectroscopic techniques.
The measurement of electronic states at the interface between two materials
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