Evanescent-field Terahertz time-domain microscopy

Several modalities have previously been presented for the sub-wavelength imaging of objects at terahertz (THz) frequencies. An important limitation of many such techniques is that they require intense near-infrared radiation to be focused onto the sample being studied, making them unsuitable for measuring light sensitive specimens such as biological substrates and charge carriers contained within semiconductor microstructures. We present an alternative approach based on evanescent-field THz time-domain spectroscopy using on-chip THz circuits. In this case, a pulsed evanescent THz electric field, extending above an on-chip THz waveguide is used to probe specimens. A lithographically-defined microstrip waveguide restricts the THz electric field laterally, while the exponential decay of the field above the waveguide into free-space provides controllable sub-wavelength penetration into specimens. We present an overview of recent results showing the interaction of pulsed THz evanescent electric fields with semiconductor samples, and discuss how such on-chip THz waveguides present a useful route towards THz microscopy.