ONE-ELECTRON 2ND-ORDER OPTICAL-ACTIVITY OF A HELIX

The second-order nonlinear-optical response of a chiral molecule is ca lculated. We model the optical response classically using a single ele ctron bound to a helical path. The helical motion of the electron caus es optical activity in the second-order response. The hyperpolarizabil ity tensor of a single helix and the susceptibility tensor for a thin film of helices are given. We examine the process of second-harmonic g eneration from a chiral surface using the calculated susceptibility te nsor. The efficiency of the harmonic generation is different for left- and right-hand circularly polarized fundamental light, which is ascri bed to be a form of nonlinear optical activity. The roles of pitch and radius of the helix are readily seen in the microscopic and macroscop ic second-order optical responses and in the surface second-harmonic g eneration, which may provide some insight for synthesizing new chiral compounds. Our results also allow us to draw conclusions about the rel ative strength and importance to second-order optical activity of elec tric- and magnetic-dipole transitions. For instance, we confirm that o ptical activity can occur in surface second-harmonic generation from e lectric-only response, but we find that magnetic response can make a s imilar contribution and thus should not be ignored. (C) 1996 American Institute of Physics.