2ND-ORDER NONLINEAR-OPTICAL SIGNATURES OF SURFACE CHIRALITY

Second-harmonic generation can be used to probe chiral properties of s urfaces and thin films with in-plane isotropy. We present a general fo rmalism to analyse such chiral effects and apply it to review known ef fects and to introduce new effects. The formalism is based on expandin g the fundamental and second-harmonic fields in terms of their p-and s -polarized components. Each second-harmonic signal can then be describ ed in terms of only three nonlinear coefficients, which are associated with the quadratic combinations of the fundamental-field components. The coefficients can be classified as achiral (allowed for all isotrop ic surfaces) or chiral (allowed only for chiral surfaces) independent of the details of the nonlinear light-matter interaction. The basic si gnatures of chirality are intensity-difference effects in which the ef ficiencies of second-harmonic generation are different for left-and ri ght-hand circularly-polarized light or two orthogonal linear polarizat ions, for example. These effects depend on proper phase relations betw een the achiral and chiral coefficients. Measurements in which the sta te of polarization of the fundamental beam is continuously varied by a rotating quarter-wave plate are shown to be sensitive to chirality in dependent of any particular phase relation between the coefficients. S uch measurements can also be used to determine uniquely the relative c omplex values of the coefficients:and thus to characterize completely the nonlinear chiral response. The ability of the techniques to distin guish absolutely between the enantiomers of a chiral sample is possibl e only if interference between the electric-dipole and higher-multipol e parts of the coefficients dominates the chiral response.