  We report on the first experimental demonstration of terahertz (THz)
whispering-gallery modes (WGMs) with an ultra high quality (Q) factor of $1.5
\times {10}^{4}$ at 0.62THz. The WGMs are observed in a high resistivity float
zone silicon (HRFZ-Si) spherical resonator coupled to a sub-wavelength silica
waveguide. A detailed analysis of the coherent continuous wave (CW) THz
spectroscopy measurements combined with a numerical model based on
Mie-Debye-Aden-Kerker (MDAK) theory allows to unambiguously identify the
observed higher order radial THz WGMs.
