ROTATING-COMPENSATOR MULTICHANNEL TRANSMISSION ELLIPSOMETRY OF A THIN-FILM HELICOIDAL BIANISOTROPIC MEDIUM

We have employed a novel multichannel transmission ellipsometer based on the rotating-compensator principle to characterize a MgF2 thin film helicoidal bianisotropic medium (HBM) deposited on a glass substrate. A HBM is a rotationally inhomogeneous anisotropic material obtained i n thin film form by deposition at a glancing angle onto a rotating sub strate to yield helical columns or helices with (i) an alignment perpe ndicular to the substrate surface, (ii) an in-plane scale of the order of nanometres, and (iii) a helix pitch of the order of the wavelength of visible light. The rotating-compensator multichannel ellipsometer provides a complete description of the optical response of this materi al to an incident linearly polarized plane wave. Evidence is introduce d for circularly birefringent and dichroic behavior based on the obser ved rotation and ellipticity imparted to a linearly polarized monochro matic plane wave when it is transmitted at normal incidence through th e ambient/HBM/substrate system. The spectra in both the optical rotati on and ellipticity exhibit strong features in a narrow wavelength zone (430-450 nm) not unlike the Cotton effect in isotropic chiral media. The features in the experimental spectra are found to be in accord wit h theoretical predictions, using values for the pitch of the helices a nd their packing density estimated from scanning electron microscopy a nd gravimetry, respectively. (C) 1998 Elsevier Science S.A.