Sub-wavelength terahertz imaging system using a vector network analyzer

Summary form only given. We develop a new terahertz (THz) imaging approach with 90 μm image resolution at 750 GHz using an inverse-microscope quasi-optical setup with an extended hemispherical high-resistivity silicon lens and a vector network analyzer frequency extender with horn-antenna output port. Owing to the large refractive index of silicon lens (n~3.5), the achieved resolution is less than λ₀/3, where λ₀ is the free-space wavelength. The test sample is placed on the focal plane of the silicon lens, making direct contact with the flat focal surface of the back side of the extended hemispherical lens and a raster-scan image is obtained using computer-controlled x-y translation stages. Reflected THz signal is digitized and recorded on a vector network analyzer, providing wideband reflection spectra of the sample as a function of position. Using the proposed imaging system, 2-dimensional THz images of planarized samples, including integrated circuits, chemical substances and biological tissue samples can be readily collected. This cost-effective quasi-optical approach can also easily be adapted to other signal acquisition systems, such as THz time-domain spectroscopy (THz-TDS) and/or continuous wave (THz-CW) systems to improve image resolution and expand their imaging capabilities. However, unlike existing THz-TDS and THz-CW systems where the source and receiver ports are disparate, the proposed VNA-based system uses the same port for the reflection (S₁₁) measurements. As such, it allows a direct measurement of normal reflection from the sample, resulting in more straightforward analysis of the sample properties. We will present the details of the setup and its performance using several samples that exhibit distinct morphological and spectroscopic behavior in the THz band.