Attenuation contrast between biomolecular and inorganic materials at terahertz frequencies

Wideband photomixing spectroscopy is used in the present work to contrast the transmission spectra of macromolecules commonly found in biomaterials such as potato starch, wheat flour and cornstarch, and proteins (Cytoplex™), and micromolecules such as sucrose, and inorganic materials such as sodium bicarbonate, and calcium sulfate. Powdered samples were measured at $0.1–0.5\mathrm{THz}$ frequencies. A significant difference in attenuation is found between these samples. At $300\mathrm{GHz}$ starch shows an absorption coefficient of $\sim 6{\mathrm{cm}}^{-1}$ whereas Cytoplex shows $1–3{\mathrm{cm}}^{-1}$ , while inorganic micromolecules have $\sim 1{\mathrm{cm}}^{-1}$ . The absorption in starch increases rapidly with frequency tending to follow a power law $\alpha =f^n$ with $n$ typically between 1.5 and 2.0. In contrast, protein materials display a slower dependence on frequency with $n$ between 1.0 and 1.5, and simple molecules show the least $n$ among all three categories. The difference between these ubiquitous macromolecular and micromolecular materials is explained in terms of water content and molecular structure.