Frequency-dependent skin penetration depth of terahertz radiation determined by water sorption–desorption

A multilayered water–skin model is used to experimentally verify a new sensing method for determining the skin penetration depth of radiation with 0.1–0.9 terahertz (THz) frequencies. A water overlayer is dripped on a skin sample to form a multilayered structure for dynamically measuring the reflected THz-wave amplitude during water desorption. Skin penetration depths can be successfully derived by using the multilayered water–skin model and by considering the measured reflectivity, water dielectric constants, and effective thicknesses of the water overlayer on the skin sample. The maximum penetration depth is approximately 0.3 mm and is obtained with wave frequencies of 0.4–0.6 THz. This penetration depth encompasses the stratum corneum (SC) and part of the epidermis. The high penetration depth of 0.4–0.6 THz waves is also confirmed in the dried and damaged SC.