The interaction between Terahertz radiation and biological tissue

Terahertz (THz) radiation occupies that region of the electromagnetic (EM) spectrum between approximately 0.3 and 20 THz. Recent advances in methods o f producing THz radiation have stimulated interest in studying the interact ion between radiation and biological molecules and tissue. Given that the p hoton energies associated with this region of the spectrum are 2.0 x 10(-22 ) to 1.3 x 10(-20) J, an analysis of the interactions requires an understan ding of the permittivity and conductivity of the medium (which describe the bulk motions of the molecules) and the possible transitions between the mo lecular energy levels. This paper reviews current understanding of the inte ractions between THz radiation and biological molecules, cells and tissues. At frequencies below approximately 6 THz, the interaction may be understoo d as a classical EM wave interaction (using the parameters of permittivity and conductivity), whereas at higher frequencies, transitions between diffe rent molecular vibrational and rotational energy levels become increasingly important and are more readily understood using a quantum-mechanical frame work. The latter is of particular interest in using THz to probe transition s between different vibrational modes of deoxyribonucleic acid. Much additi onal experimental work is required in order to fully understand the interac tions between THz radiation and biological molecules and tissue.