2-Dimensional mapping of frequency response of a single THz split-ring resonator probed by high speed asynchronous optical sampling

Terahertz spectroscopy is of particular interest for the investigation of low-energy elementary excitations as well as for technical purposes [1]. However, the probing of submicron-scale objects is hindered by the long wavelength of THz waves. Especially for spatially resolved spectroscopy and imaging it is highly desirable to overcome the diffraction limitation at THz frequencies. In order to achieve sub-wavelength resolution, we set up a scattering near-field microscope by combining a THz time-domain reflection spectrometer (TDS) with a scanning tunneling microscope (STM) similar to an approach in [2]. A THz pulse generated by excitation of a large-area photoconductive emitter [3] with a femtosecond laser pulse is focused on the sample underneath the STM tip. The THz electric field of the pulse scattered in the forward direction is measured by electro-optic sampling. In order to obtain the full spatially resolved frequency information for every pixel the electric field of the pulse has to be measured versus time delay. For this time consuming task when performed with a mechanical delay line we employ for the first time high-speed asynchronous optical sampling (ASOPS) where the time delay is realized through two asynchronously locked fs lasers at GHz repetition rate [4]. With this set-up one full temporal trace of 1 ns delay is scanned within 100 µs. Due to the lack of mechanical components in our THz-TDS, this scheme allows for high data acquisition rates that are necessary to scan images in reasonable data acquisition times.