Variable-focus terahertz lens.

We present a variable focus lens for the THz range. The focal length can be changed by pumping a medical white oil in and out of the lens body. Due to the optical transparency of the liquid and a similar refractive index in the visible frequency range, the THz beam path can be aligned using conventional optical light sources. This type of lens might find applications in terahertz based quality control, stand-off detection and wireless communication systems.