Modeling of reflectometric and ellipsometric spectra from the skin in the terahertz and submillimeter waves region

Abstract

The human skin is modeled as a stack of homogeneous layers in the terahertz and submillimeter waves regions with some anisotropy due to the helical sweat glands and other elongated entities. A dielectric model for the skin is presented, valid for a wider frequency range (up to the terahertz region) taking into account the dispersive nature of the effective conductivity. Polarized reflectivity and generalized ellipsometric parameters are calculated versus angle and wavelength. Recent studies have claimed that the helical sweat ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the sub-terahertz region. We found that water absorption, dispersion and multiple interference effects play the major role in shaping the spectrum without the need for the assumption of the sweat ducts acting as low-Q helical antennae. High sensitivities to the water content are found particularly in the ellipsometric parameters at large incidence angles. Hence a new methodology is proposed to detect skin cancer using variable angle ellipsometry or polarized reflectometry. The parameter found with the highest sensitivity to water content is cos Δ(pp) with Δ(pp) being the phase of the on-diagonal reflection matrix ratio between p-to-p polarization.

Biomedical Applications of Terahertz Spectroscopy and Imaging

“…Again, water absorption has a dominant role in shaping the variable angle ellipsometry or polarized reflectometry THz spectrum in a human skin layer model [73,74]. A recent simulation study found that InN-enhanced THz imaging enables ultrahighly sensitive diagnosis of early-stage skin cancer at a scale comparable to tiny water content changes [75].…”

Morphology of human sweat ducts observed by optical coherence tomography and their frequency of resonance in the terahertz frequency region

“…Moreover, as the sweat duct is controlled by the sympathetic nervous system, it reflects the psychological state of the subject, which allows for remote sensing of mental stress using sub-THz electromagnetic waves30. In contrast, some reports have suggested that multiple interference effects play a major role in shaping the spectrum3132. The duct acts as a helical antenna based on electromagnetic and antenna theory; the dimensions of the duct and the dielectric properties of the surrounding medium are major factors that determine the resonance frequency33.…”

A Real-Time Terahertz Time-Domain Polarization Analyzer with 80-MHz Repetition-Rate Femtosecond Laser Pulses

“…These techniques have become important tools for investigating low-energy dynamical phenomena in various kinds of materials such as molecules [11,12], superconductors [13–17], multiferroics [18], two-dimensional electron gases [19], graphene [20], magnetic materials [21–25] and topological insulators [26,27]. They are also important in many fields of applied science for investigating carrier density and mobility in semiconductors [8,28–30]; local stress and optical axis, probed by terahertz birefringence measurements [31–34] and physiological conditions, probed by reflection polarimetry of human skin [35,36]. In addition, terahertz time-domain polarimetry, which allows us to extract the instantaneous direction of the electric-field (E-field) vectors within a single-cycle of the electromagnetic oscillation, opens new avenues for investigating the surface topography of materials [37] and for separate determination of the optical Faraday and Kerr rotation angles using echo signals [26], a result that cannot be attained with the conventional polarization measurement technique for much higher frequencies.…”