  Fingerprint spectral response of several materials with terahertz
electromagnetic radiation indicates that terahertz technology is an effective
tool for sensing applications. However, sensing few nanometer thin-film of
dielectrics with much longer terahertz waves (1 THz = 0.3 mm) is challenging.
Here, we demonstrate a quasi-bound state in the continuum (BIC) resonance for
sensing of nanometer scale thin analyte deposited on a flexible metasurface.
The large sensitivity originates from strong local field confinement of the
quasi-BIC Fano resonance state and extremely low absorption loss of a low-index
cyclic olefin copolymer substrate. A minimum thickness of 7 nm thin-film of
germanium is sensed on the metasurface, which corresponds to a deep
subwavelength length scale of {\lambda}/43000, where {\lambda} is the resonance
wavelength. The low-loss, flexible and large mechanical strength of the
quasi-BIC micro structured metamaterial sensor could be an ideal platform for
developing ultrasensitive wearable terahertz sensors.
