WAYS TO REDUCE ERRORS IN MEASURING THE DIELECTRIC CONSTANT OF WEAKLY ABSORBING DIELECTRICS IN THE MILLIMETER AND SUBMILLIMETER (TERAHERTZ) WAVELENGTH RANGES BY THE SURFACE PLASMON RESONANCE METHOD

Abstract

Subject and Purpose. The sources of errors are identified that may arise in the course of terahertz-range measurements of the dielectric constant of weakly absorbent dielectrics, if performed within the surface plasmon resonance (SPR) technique. Possible ways are analyzed for reducing or fully eliminating such errors. Methods and Methodology. Specific details of applying the SPR method for dielectric constant measurements have been analyzed, with the aim of identifying the major factors that particularly affect the measurement accuracy. Results. It has been noted that in order to reduce the level of backlight interference (which may lead to blinding the receiver), it is expedient to make surface resonance records via frequency scanning. In that case the impact of the interference signal nonstationarity arising from the partial conversion of the surface wave energy into that of the volume wave, which occurs at the grating edges, is markedly lower than in the case of angular scanning. A mathematical expression has been derived which suggests a relation between scanning step sizes in angle and in frequency (for the angular and frequency scanning, respectively). As has been shown, a better measurement accuracy is achievable if the SPR is recorded as a function of frequency. Indeed, the frequency can be varied, with the use of familiar technologies, in steps of a much smaller size than such adopted for angular scanning. Errors in the above measurements can also arise if the resonance is excited on a grating whose Fourier spectrum contains many high-frequency components which carry a noticeable portion of the diffracted radiation energy. These energy losses can be greatly reduced if the SPR is excited on a grating whose profile involves the lowest number of spatial Fourier harmonics. Conclusions. The method suggested allows a significant reduction in the level of errors of the dielectric constant measurements in weakly absorbing dielectrics if the surface plasmon resonance effects are registered in dependence on the incident frequency, while the SPR is excited at a diffraction grating whose troughs-and-peaks profile is close to harmonical.

Keywords

• dielectric constant measurements
• electromagnetic radiation
• diffraction
• surface plasmon resonance