We consider and experimentally study two reflection refractometry methods involving reflection of light from the optical surface of a material for nearly normal incidence and for incidence at the Brewster angle. We analyze the sources of error in the methods, and show that it is possible to use laser radiation to measure the refractive index of materials to four decimal places. We note the advantages of the methods in preparing samples for measurements in the case of solid, liquid, film, and weakly absorbing materials.Keywords: reflection refractometry, Fresnel theory, error in measurement methods, Brewster angle in the presence of absorption, thin film.
Introduction.It is commonly known that exact measurements of the refractive index of optical materials are important for scientific, medical, and military instrumentation and materials science. These measurements play a paramount role in chemical analysis and synthesis of new materials. Recently considerable attention has been focused on study of metamaterials with negative refractive index [1,2]. "Profile refractometry" is actively under development for measuring local values of the refractive index of liquids in time and space, associated with application of the Soret effect: mass diffusion in a thermal gradient. Bifocal coherent optical refractometry concerns measurement of the refractive index of scattering media [3].Conventional procedures and instruments for refractometry are quite comprehensively represented in monograph [4]. The most widely used methods used in refractometric instruments can be briefly summarized as involving measurement of the angle of total internal reflection (TIR) or the angle of refraction (goniometric methods). Preference is given to angular methods for measurement of the refractive index of materials because it minimizes sources of systematic measurement error. Development of the indicated techniques using tunable lasers also makes possible further improvement in the accuracy of the measurements of the dispersion of the refraction not only at the characteristic points of the spectrum but also over a broad range.We may suggest a number of reasons why angular refractometry methods have not included application of methods based on measurement of the parameters of reflected light, in particular measurements of the refractive index of materials using the Brewster angle. Certainly the indicated method, requiring measurement of a single angular parameter, is characterized by only systematic error. This method uses a beam reflected from the surface and does not depend on the physical state of the material, its shape, or in part the absorption and scattering coefficients. The indicated advantages of this approach potentially expand the measurement options compared with developed goniometric methods and TIR methods. An insurmountable limitation in this case is still the state of the reflecting surface, which should not be a scattering (rough) surface. The argument that the method of measuring the refractive index of materials using the Brews...
