Microwave-millimetre wave WGM resonators for evanescent sensing of nanolitre liquid substances

In this paper we describe a novel approach of high sensitivity liquid analysis for volumes in the nanolitre range with challenging perspectives for practical sensor applications in chemistry, biology and medicine. Whispering-gallery modes (WGMs) in cylindrically shaped dielectric disks machined from low-loss single crystalline materials such as sapphire or quartz allow for very high quality factors. The interaction of extremely small volumes of the liquid under test with the evanescent field located in the vicinity of the dielectric disk surface at micro-to-millimetre wave frequencies was employed for the investigation of aqueous solutions with relevance to biological applications. Based on this resonator type, three different liquid sensing approaches were developed and analysed at 10, 35 and 170 GHz with emphasis on the determination of the complex dielectric permittivity of liquids of nanolitre volumes.