Interface Optical Effects

Figure Bl.5.5 Schematic representation of the phenomenological model for second-order nonlinear optical effects at the interface between two centrosynnnetric media. Input waves at frequencies or and m2, witii corresponding wavevectors /Cj(co and k (o 2), are approaching the interface from medium 1. Nonlinear radiation at frequency co is emitted in directions described by the wavevectors /c Cco ) (reflected in medium 1) and /c2(k>3) (transmitted in medium 2). The linear dielectric constants of media 1, 2 and the interface are denoted by E2, and s, respectively. The figure shows the vz-plane (the plane of incidence) withz increasing from top to bottom and z = 0 defining the interface.

Heinz T F 1991 Second-order nonlinear optical effects at surfaces and interfaces Noniinear Surfaoe... 

Reider G A and Heinz T F 1995 Second-order nonlinear optical effects at surfaces and interfaces recent advances Photonio Probes of Surfaoes ed P Halevi (Amsterdam Elsevier) pp 413-78... 

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

We present here a condensed explanation and summary of the effects. A complete discussion can be found in a paper by Hellen and Axelrod(33) which directly calculates the amount of emission light gathered by a finite-aperture objective from a surface-proximal fluorophore under steady illumination. The effects referred to here are not quantum-chemical, that is, effects upon the orbitals or states of the fluorophore in the presence of any static fields associated with the surface. Rather, the effects are "classical-optical," that is, effects upon the electromagnetic field generated by a classical oscillating dipole in the presence of an interface between any media with dissimilar refractive indices. Of course, both types of effects may be present simultaneously in a given system. However, the quantum-chemical effects vary with the detailed chemistry of each system, whereas the classical-optical effects are more universal. Occasionally, a change in the emission properties of a fluorophore at a surface may be attributed to the former when in fact the latter are responsible. 

Bhargava, R., Wang, S. Q. and Koenig, J. L. (1998) FTIR imaging of the interface in multicomponent spatially-separated systems using optical effects induced by differences in refractive indices. Appl. Spectrosc. 52(3), 323-8. 

Fig. 6 Optical effects in dynamers. Color change and fluorescence generation induced by component recombination at the interface between two dynameric films. (Top) Schematic representation of the process yielding two new combinations at the interface of two superimposed films of dynamers. (Bottom) Photographs of an actual experiment, before (left) and after (center and right) recombination induced by heating...

Since the turn of the twentieth century, interesting optical effects from optically small metal interfaces and structures have been the subject of much scrutiny. Molecules adsorbed onto rough metal surfaces, particles, or island films exhibit dramatically different optical properties to those of the free molecules [1]. Perhaps... 

X and x are the non-linear contributions to the susceptibility. Non-linear optical effects become possible with high-powered lasers. SHG and VSFS depend on x the second-order non-linear susceptibility of the medium at an interface. More details about these two methods are given below. 

Kleijn JM, Cohen Stuart MA, De Wit A. Electro-optic effect in the solid phase of the indium tin oxide electrolyte solution interface observed by reflectometry. Colloids Surfaces A 1996 110 213-217. 

Radeva Ts. Frequency behavior of the electro-optical effect from colloid particles in polyelectrolyte solutions with counterion mixtures. J Colloid Interface Sci 1997 187 57-61. 

Let us now turn our attention to the effect of a thin monolayer of soap molecules present on both film surfaces. In the first place it was found (see Section III.B) by Bouchiat and Langevin that density and orientation fluctuations of the soap molecules (in the case of a single interface) make only a negligible contribution to the scattering process, except possibly at a (two-dimensional) critical point of the monolayer, or by addition of a few fluorescent soap molecules. Thus we are left with the optical effect of the monolayers on the interference of the light waves. It turns out (see Section VI) that this interference effect can be taken into account by redefining the (optical) film thickness. 

Figure Bl.5.5 Schematic representation of the phenomenological model for second-order nonlinear optical effects at the interface between two centrosymmetric media. Input waves at frequencies (o j and a 2, with corresponding wavevectors Aj(co j) and are approaching the interface from medium 1. Nonlinear...

Heinz T F 1991 Second-order nonlinear optical effects at surfaces and interfaces Nonlinear Surface Electromagnetic Phenomena ed H-E Ponath and G I Stegeman (Amsterdam North-Holland) pp 353-416... 

Typically, a potential-difference spectrum of the electrode-electrolyte interface presents negative, positive, and bipolar bands and smooth background absorption, which are due to aU species in the path of IR radiation that are affected by the electrode potential O igs. 3.40, 3.43a, 4.47a, 4.50a, 7.45, and 7.47). In addition to the absorption bands that reflect change in the population in the HL of reagents/products for the reaction under study, these may include the bands due to (1) the electrolyte species in the diffuse layer, (2) the electrolyte species in the HL, (3) the reagent/product species whose absorption (parameters of the elementary oscillators) is modulated by potential and coadsorption of electrolyte species, (4) delocalized and localized charge carries, and (5) optical effects of various nature. Consider these effects in more detail. 

---