Analysis and optimization of a focusing metal-dielectric probe for near-field terahertz imaging

Illumination techniques play an important role in millimeter and Terahertz wave imaging. In aperture near-field imaging, the resolution depends on the size of the aperture; in aperture-less scattering near-field imaging, the resolution depends on the tip-sample distance and the size of the scatter. Normally, sharp metal tips acts as an antenna and illuminated by a focusing beam, it is impossible to focus only on the tip of the metal needle, the propagation along the shaft of the needle impacts analyzing the received signal. A combination of an aperture and aperture-less near-field illumination technique is presented in this paper. We use the focusing beam coming out from the aperture probe as a near field illumination source which just covers the sharp end of the aperture-less metal tip to decrease the stray wave. A focusing metal-dielectric tapered probe is analyzed, the material of the dielectric part is discussed and lower permittivity materials are used instead of highly resistive silicon or sapphire, the geometric dimension optimizations are simulated in Microwave studio of CST.