Three-photon reststrahl bands near exciton resonances

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Abstract
Excitation of a KI crystal by a high-power laser in the visible (around 630 nm) leads to ‘‘reflected’’ light at the third harmonic (around 210 nm). There is a pronounced structure in the spectrum between the longitudinal and the transverse exciton, which is interpreted as a nonlinear reststrahl band. Study of third-harmonic generation in transmission also reveals pronounced structure. An analysis according to Maxwell boundary conditions leads to a good fit to the experimental results.