Sensing with THz metamaterial absorbers
  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. 
Longqing Cong and Ranjan Singh
physics.optics