Optical Characterization of Organic Thin Films and Interfaces with Evanescent Waves
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ETIN/JULY1991
light, called e v a n e s c e n t w a v e s , has proved in récent years to be particularly helpful because their high surface specificity allows sensitive monitoring of the properties of interfaces and thin layers without the interférence of information from the bulk. This article r e p o r t s on the use of evanescent optical modes for investigating polymeric ultrathin films. The examples concern the buildup of wellordered, highly organized multilayer assemblies prepared by the LangmuirB l o d g e t t - K u h n (LBK) t e c h n i q u e 1 : Monomolecular layers, spread and compressed at the water/air interface, are transferred layer by layer onto a solid s u p p o r t by d i p p i n g and w i t h d r a w i n g this s u b s t r a t e t h r o u g h the monolayer-covered wafer surface. The r e s u l t i n g films r a n g e from a single layer ~ 2 nm thick up to multilayers more than 1 /xm thick capable of carrying optical waveguide modes. Other examples concern spin-coated films prepared by the technique well-known from photoresist applications. Finally, spécifie récognition reactions between a biotin-functionalized lipid monolayer and the protein streptavidin are presented as an example of a biopolymer adsorption and binding reaction that can be observed with thèse techniques at the solid-solution interface without the need for a spécial label as, for example, in fluorescence techniques. This article first briefly reviews the physics of evanescent light in gênerai,
and surface plasmons as well as waveguide modes in particular. I demons t r a t e t h a t t h e e x c i t a t i o n of t h è s e modes by laser gives valuable information about the optical architecture of the interface. Next, I describe imaging techniques that allow microscopic characterization of laterally heterogeneous thin film properties or interfacial reactions. Finally, I présent the use of evanescent light for inelastic scattering experiments and demonstrate the high sensitivity that can be achieved in Raman scattering and imaging. Evanescent Waves as Interfacial Light The simplest case for the existence of an evanescent wave is the well-known total internai ref lection of a plane electromagnetic wave at the base of a glass prism (index of refraction «,) in contact with an optically less dense médium (with n2 < «1), e.g., air (« 2 = 1)- This geometry is schematically sketched in Figure la (top). If the reflected light intensity is recorded as a function of the angle of incidence, 6, the reflectivity, R, reaches unity as one approaches the critical angle, 0C, for total reflection (Figure la, bottom). Closer inspection of the E-field distribution in the immédiate vicinity of the interface shows that above 6C the light intensity does not fall abruptly to zéro in air, but there is instead a harmonie wave traveling parallel to the surface with an amplitude decaying exponentially normal to the surface. The depth of pénétration / defined by the 1/e atténuation, is given by
27rV(n, • sinfl) 2 - 1 and is found to be on the order of the wavelen
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