We show measurements and calculations of the terahertz (THz) near field of a metal tip with a specially formed, semicircular apex that allows us to identify the separate contributions of the tip apex and shaft to the measured signal. We find that when the tip–crystal distance is not modulated the measured near-field signal is overwhelmed by contributions from the tip shaft, resulting in a relatively large THz spot size. When the tip–crystal distance is modulated, with subsequent lock-in detection at the modulation frequency, only the near-field distribution of the semicircular apex is observed, resulting in a much smaller THz spot size and thus improved spatial resolution.
© 2004 Optical Society of America
P. C. M. Planken and N. C. J. van der Valk, "Spot-size reduction in terahertz apertureless near-field imaging," Opt. Lett. 29, 2306-2308 (2004)