The wavelength-dependent reflectivity of linearly-polarised Terahertz (THz) radiation has been recorded from a metal grating spaced above a metal mirror by a homogeneously aligned liquid crystal layer. Clearly defined minima were observed in the spectra measured in the range 0.1-3 THz. On application of a voltage across the liquid crystal layer these resonances shift in position by 141) to 30 GHz due to the resulting change in the sensed refractive index. The minima are attributed to resonant absorption through the excitation of standing waves generated both across and along the liquid crystal. The mechanism responsible for this phenomenon and methods for optimising the device are discussed.