Spatial and spectral behavior of the optical near field studied by a terahertz near-field spectrometer

We have developed a technique to obtain spectroscopic information in the optical near field. 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. Small pinholes (λ/100–λ/10) in thin copper foils serve as near-field probes. For an accurate, fast, and contact-less control of the probe–sample distance a confocal microscope is employed. We experimentally test the diffraction theory of electromagnetic waves on subwavelength pinholes. Based on our findings we formulate requirements for near-field spectroscopy at THz frequencies.