Experimental Techniques

This chapter presents basic experimental techniques and various apparatus for measuring the complex index of refraction and related quantities. Generally, measurements of transmittance, reflectance, and emittance are made using spectrometers or lasers. Other important techniques, which measure directly the real refractive index, n, the absorption coefficient, βabs , and the scattering coefficient, βsca, such as interferometry, ellipsometers, calorimetry, and scatterometers, are also introduced. Ultimately, experimental procedures must be taught in the laboratory. Thus, devoting only one chapter to experimental technique and five to theory is not indicative of the importance of this fundamental topic. By discussing the measurement of basic optical parameters, it is intended that the concepts developed in the first five chapters will be reinforced. All of the theoretical models developed in the previous chapters contain measurable parameters. Basic theory often helps guide the design of a good experiment. Once data is available, it can be used to check the assumptions of the theory. This interplay between experiment and theory is an essential part of definitive work. The chapter has two main parts; the first covers measurements of the real and imaginary parts of the complex index of refraction and the second covers measurements of scattering. As established in Chapter 2, the characterization of bulk absorption mechanisms on optical propagation is accomplished by the complex index of refraction. Considerable effort was expended in Chapters 3, 4, and 5 to obtain models of the complex index. Thus, at this point, we wish to find ways to experimentally measure the complex index of refraction for various media. The broad-band spectral response of a medium is commonly measured by a spectrometer. There are two main types of spectrometers, dispersive and interferometric. Generally, spectrometers make broad-band transmission, emission, and reflection measurements, and therefore indirectly measure, n̄. Interferometric measurements, are the exception. Lasers, which feature narrow-band, high-intensity, highly directional light are often used to complement and calibrate broad-band spectrometer measurements. The highest accuracy measurements of the absorption coefficient are obtainable by laser techniques, which can directly measure the components of the complex index.