Off resonance second order optical activity of isotropic layers of chiral molecules: Observation of electric and magnetic contributions

We perform nonresonant surface second harmonic reflection from an isotropic layer of a chiral pentamethinium salt. The combination of gated integration and photon counting techniques allows for a very high sensitivity of our experimental setup. We measure a second harmonic optical rotation angle of 5°, as well as linear and circular dichroisms of 5% to 140%. We quantitatively compare these three experimental techniques and obtain a good agreement. The nonresonant configuration of our experiments enables us to distinguish between electric and magnetic contributions to the second order optical activity. The nonvanishing circular dichroism, as well as the nonvanishing $s$ -polarized second harmonic signal with $s$ -polarized fundamental beam, indicates that electric and magnetic effects are of the same order of magnitude.