Multilayer Dielectric Coatings

The constructive interference found for the reflection of light from plane-parallel interfaces between two regions with different refractive indices can be utiUzed to produce highly reflecting, essentially absorption-free mirrors. The improved technology of such dielectric mirrors has greatly supported the development of visible and ultraviolet laser systems. 

The reflectivity R of a plane interface between two regions with complex refractive indices i = n[ — iK and ni = n — iiC2 can be calculated from Fresnel s formulas [129]. It depends on the angle of incidence a and on the direction of polarization. For the polarization component with the electric field vector E parallel to the plane of incidence (defined by the incident and the reflected beam), the reflectivity is 

The reflectivities and are illustrated in Fig. 4.54 for three different materials for incident light polarized parallel (R ) and perpendicular (R ) to the plane of incidence. 

For vertical incidence a = 0, P = 0), one obtains from Fresnel s formulas for both polarizations 

Since this case represents the most common situation for laser mirrors, we shall restrict the following discussion to vertical incidence. 

To achieve maximum reflectivities, the numerator (n — 2) should be maximized and the denominator minimized. Since n is always larger than one, this implies that ri2 should be as large as possible. Unfortunately, the dispersion relations (3.36), (3.37) imply that a large value of n also causes large absorption. For instance, highly polished metal surfaces have a maximum reflectivity of R = 0.95 in the visible spectral range. The residual 5% of the incident intensity is absorbed and therefore lost. 

The reflectivity R of a plane interface between two regions with complex refractive indices n2 - n - i 2 can be calculated from 

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However, in an attempt to integrate the SFA and spectroscopic techniques, the use of silver for optical interferometry has been seen as a drawback due to the fact that it precluded sufficient excitation source intensity to illuminate the buried interface. In order to circumvent this problem Mukhopadhyay and co-workers in an experimental set-up where the SFA was combined with fluorescence correlation spectroscopy (FCS) used, instead of silver, multilayer dielectric coatings that allowed simultaneous interferometry and fluorescence measurements in different regions of the optical spectrum [75]. Using this set-up they succeeded in measuring diffusion in molecularly thin films with singlemolecule sensitivity. 

It was already pointed out that single or multilayer dielectric coatings can reduce the loss of a leaky capillary waveguide substantially, but that it was difficult to see this improvement in practice due to fabrication imperfections [11]. Advances in fabrication technology have helped improve these imperfections. In addition, a strong need for liquid-core waveguides with small cross sections of a few microns... 

Veiling glare caused by reflection of an automobile instrument panel in the windshield can be reduced by coating the windshield with an antireflective multilayer dielectric coating for oblique incidence [156]. 

Examples of devices in which only two partial beams interfere are the Michelson interferometer and the Mach-Zehnder interferometer. Multiple-beam interference is used, for instance, in the grating spectrometer, the Fabry-Perot interferometer, and in multilayer dielectric coatings of highly reflecting mirrors. 

VR. Costich Multilayer dielectric coatings . In Handbook of Lasers, ed. by R.J. Pressley (CRC, Cleveland, Ohio 1972)... 

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