Single-valued determination of second-order nonlinear susceptibilities by quarter-wave-plate rotation

We present a new technique to determine the complex-valued tensor components of the nonlinear surface susceptibility _s ⁽²⁾ of isotropic adsorbate layers. It is based on a change of the state of the fundamental polarization by rotation of a quarter-wave plate. We verify the theoretical predictions by measurements on a Langmuir-Blodgett film of a highly nonlinear hemicyanine dye in total reflection geometry. The obtained second-harmonic rotation pattern is analyzed in Fourier components, which are related to the absolute value and relative phase of the tensor components of the surface susceptibility. We compare the obtained results with separate measurements in which a half-wave plate is used to rotate the fundamental polarization. Our results are confirmed by an additional interference experiment. The method presented here allows the single-valued determination of _s ⁽²⁾ with measurements at only one angle of incidence, in contrast to the half-wave-plate technique.