  Ultrasensitive terahertz sensing of highly absorptive aqueous solutions
remains challenging due to strong absorption of water in the terahertz regime.
Here, we experimentally demonstrate a cost-effective metamaterial-based sensor
integrated with terahertz time-domain spectroscopy for highly absorptive
water-methanol mixture sensing. This metamaterial has simple asymmetric wire
structures that support multiple resonances including a fundamental Fano
resonance and higher order dipolar resonance in the terahertz regime. Both the
resonance modes have strong intensity in the transmission spectra which we
exploit for detection of the highly absorptive water-methanol mixtures. The
experimentally characterized sensitivities of the Fano and dipole resonances
for the water-methanol mixtures are found to be 160 and 305 GHz/RIU,
respectively. This method provides a route for readily available
metamaterial-assisted terahertz spectroscopy for ultrasensitive sensing of
highly absorptive chemical and biochemical materials with multiple resonances
and high accuracy.
