  Metamaterial perfect absorbers from microwaves to optical part of the
electromagnetic spectrum has been intensely studied for its ability to absorb
electromagnetic radiation. Perfect absorption of light by metamaterials have
opened up new opportunities for application oriented functionalities such as
efficient sensors and emitters. We present an absorber based sensing scheme at
the terahertz frequencies and discuss optimized designs to achieve high
frequency and amplitude sensitivities. The major advantage of a perfect
metamaterial absorber as a sensor is the sensitive shift in the absorber
resonance frequency along with the sharp change in the amplitude of the
resonance due to strong interaction of the analyte with the electric and the
magnetic fields at resonant perfect absorption frequency. We compare the
sensing performance of the perfect metamaterial absorber with its complementary
structural design and planar metasurface with identical structure. The best FoM
values obtained for the absorber sensor here is 2.67 which we found to be
significantly higher than the identical planar metamaterial resonator design.
We further show that the sensitivity of the sensor depends on the analyte
thickness with the best sensitivity values obtained for thicknesses approaching
{\lambda}/4n, with {\lambda} being the free space resonance wavelength and n
being the refractive index of the analyte. Application of metamaterial
absorbers as sensors in the terahertz spectral domain would be of tremendous
significance due to several materials having unique spectral signature at the
terahertz frequencies.
