  The development of innovative terahertz (THz) imaging systems has recently
moved in the focus of scientific efforts due to the ability to screen
substances through textiles or plastics. The invention of THz imaging systems
with high spatial resolution is of increasing interest for applications in the
realms of quality control, spectroscopy in dusty environment and security
inspections. One of the main restrictions of current THz imaging systems is the
low spatial resolution which is limited by a lack of THz lenses with strong
focusing capabilities. Here we present the design, fabrication and the
measurement of the optical properties of spectrally broadband
metamaterial-based gradient index (GRIN) lenses that allow one to focus THz
radiation to a spot diameter smaller than the wavelength. Due to the
subwavelength thickness and the high focusing strength the presented GRIN
lenses are an important step towards compact THz imaging systems with strongly
improved spatial resolution.
