Refractometry in the Far Infra-red using a Two-beam Interferometer

Abstract

A MICHELSON two-beam interferometer can be used for refractive-index determination by measuring the shift of the achromatic fringe when a specimen of known thickness is placed in one arm of the interferometer. Such a measurement using a broad spectrum and ‘white-light’ fringes gives only a single index value and that cannot be related to a definite wave-length. Monochromatic radiation cannot be used because of the impossibility of identifying a given fringe after displacement. It appeared plausible, however, that a spectrum of refractive index could be obtained by making observations of intensity over a range of path-difference and taking the Fourier transform of the two-beam interferogram, the method being an extension of that used for obtaining infra-red power-spectra¹. This has been realized in the far infra-red region, where the method has been used to determine the refractive-index spectrum of crystalline quartz over the wave-length range 500–180µ (20–56 cm⁻¹). No values have been previously reported over much of this range.