Near-field spectroscopy at THz frequencies - a new tool for solid state spectroscopy and biomedical imaging

We developed a novel technique to obtain spectroscopic information in the optical near-field of small pinholes (/spl lambda//100-/spl lambda//10). The spectrometer operates in the frequency range from 30 GHz to 1.5 THz using highly monochromatic and coherent radiation which can be continuously tuned. For an accurate, fast and contact-less control of the probe-sample distance a confocal microscope is employed. We experimentally tested the diffraction theory of electromagnetic waves on sub-wavelength pinholes. Based on our findings we formulate requirements for near-field spectroscopy at THz-frequencies. Besides the possibility to make images at discrete frequencies, a new near-field modulation technique allows us to detect absorption spectra from strongly absorbing materials, which cannot be measured with normal transmission or reflection spectroscopy.