  We report a new scheme of realizing terahertz fingerprint detection with
ultrahigh sensitivity. Instead of using the direct absorption of terahertz
through a bare sample in the regular transmission scheme, a cavity mode
resonating at the characteristic frequency of the sample is used and due to the
high dependence of the cavity mode transmission on the material loss, an
amplified transmission decaying is observed when the sample is loaded into the
cavity. Furthermore, this scheme retains the feature of substance
identification. A one-dimensional photonic crystal cavity is used as the
example for the detection of {\alpha}-lactose and an efficient detection of 7nm
{\alpha}-lactose can be achieved, which corresponds to 1/80000 of the free
space wavelength at the characteristic frequency of 0.529THz, exhibiting
sensitivity 500 times higher than the regular method.
