(a) Illustration of the single-band THz all-silicon metamaterial absorber and the inset plot shows the unit cell geometry of the design. (b) Measured absorption spectra of the single-band THz all-silicon metamaterial absorber for 0.05– 4 ppm of 2, 4-D. (c) Regression curves established for the single-band THz all-silicon metamaterial absorber based on spectral intensity variations [21].