ELECTRONIC ABSORPTION ENHANCEMENT FOR TCNQ FILMS ON SILVER BY EXCITATION OF SURFACE-PLASMON POLARITONS

A polarization modulation method has been employed to characterize the optical excitation of surface plasmon polaritons (SPP's) of Ag conduction electrons in the Kretschmann ATR geometry. Absorption measurements in the visible region by either an angular scan or a wavelength scan were performed on tetracyanoquinodimethane (TCNQ) films of 5 and 10 nm in mass thickness deposited by evaporation onto a Ag film of 50 nm thickness. It is demonstrated that the absorption intensity of the TCNQ films is enhanced by a factor of 20 or more upon excitation of the SPP resonance at the Ag substrate surface. This enhancement was accompanied by a sizable decrease in intensity of the SPP resonance band. We also find that the experimental SPP dispersion curve is deformed by the electronic absorption of the TCNQ molecules. Within the limits of our experiments no positive evidence was found for the charge transfer between the Ag surface and the TCNQ molecule.