Highly sensitive determination of coating thickness by using the high filling factor in an adiabatically coupled terahertz waveguide

Terahertz time-domain spectroscopy can be used to detect the thickness of thin dielectric layers by measuring the relative pulse delay of the propagating terahertz pulse. The detection limit was considerably increased by using the high filling factor of a 2-cylinder waveguide sensor. The single-pass delay was increased by a factor of more than 150 for dielectric layers down to 2.5 microns thickness. Using the adiabatic coupling and guiding of the metal cylinders, no further optics were necessary.