Studies of Microwave Absorption in Liquids by Optical Heterodyne Detection of Thermally Induced Refractive Index Fluctuations.

This work describes the development and implementation of an optical detection method for determining the microwave absorption properties of liquids and liquid suspensions. The method employs a Mach-Zehnder interferometer illuminated with a single-frequency laser to detect fluctuations in the index of refraction of a transparent or semi-transparent substance placed in one arm of the interferometer. The fluctuations are induced by pulses of microwave energy which are introduced into the sample by a specially designed waveguide sample holder. The laser beams from the sample and reference arms of the interferometer recombine on a photodetector resulting in a heterodyne signal. This general method is called Phase Fluctuation Optical Heterodyne (PFLOH) spectroscopy. The method is theoretically capable of detecting refractive index changes of two parts in 10('12) in liquids with greater sensitivity for gases; however, experimental performance is somewhat more limited. Although there is an abundance of microwave spectroscopy literature, virtually all of it is devoted to the absorption properties of gases. There are two reasons for the lack of information on nongaseous substances. First, methods most commonly used on gases, such as absorption ratio methods, are not practical for liquids in this frequency range due to the difficulties of making accurate microwave measurements. PFLOH spectroscopy avoids this difficulty. Second, fine structure (or anomalous dispersion) in the microwave absorption spectra of liquids has been considered nonexistant or at best uninteresting. This level of interest is changing, however, due to possible biological effects of microwave radiation. A number of different types of biological responses have recently been observed and reported in animals exposed to microwave radiation. Some of these effects are clearly responses of the organism to increased local or whole body temperature levels. In some cases the response is not so clearly thermal in nature. This is particularly true in those cases where the effect is responsive to modulation or to peak rather than average power. Such effects suggest direct interaction of the incident field with biologically significant molecules. Such direct interaction and excitation could result in conformational changes which in turn could affect molecular function. The possibility of such an interaction process was the prime motivation for developing microwave spectroscopic methods for liquid solutions. Spectroscopic studies in the X-band frequency range are here reported on organic liquid solutions and on DHA in solution. Direct interaction and absorption of microwave radiation by the DNA molecule was observed. This absorption was found to be substantially greater than that of water. Comparative measurements were made using methods of dielectrometry. The development of an appropriate means for coupling microwave energy into the liquid sample required a theoretical analysis of several probe/antenna configurations. One probe experimentally used and theoretically analyzed was an open ended coaxial transmission line. Although this probe was experimentally convenient, theoretical examination indicated that analysis of the resulting data would be extremely complicated, if not impossible. The theoretical analysis, however, yielded information on absorption of energy in a lossy medium surrounding the coax, which should be helpful to those using such probes in the hyperthermic treatment of tumors. Therefore, these theoretical results are presented in some detail.