Millimeter wave measurement of both parts of the complex index of refraction using an untuned cavity resonator

It is demonstrated, both theoretically and experimentally, that simultaneous measurement of the real and imaginary parts of the complex index of refraction of small, strongly absorbing samples can be accomplished by placing the samples inside an oversized untuned resonator cavity. Nearly monochromatic radiation is fed into the cavity, and the change in cavity Q resulting from introduction of a sample is measured. Determination of n and k results from a series of such measurements on lamellar samples of different thickness. An analytic solution of the boundary value problem for a lamellar sample placed between two nonabsorbing windows is used to derive n and k from the measured changes in Q, and as a consequence the procedure is applicable to liquid as well as to solid samples. Although the measurements reported here were made at lambda approximately 4 mm, the technique offers similar advantages at submillimeter wavelengths.