We performed the transmission-mode terahertz (THz) pulsed spectroscopy of several THz-wave penetration- enhancing agents (PEAs): glycerol, propylene glycol, ethylene glycol, and polyethylene glycol, featuring the molecular weight of 200, 300 and 400. We vacuumized the THz beam path in order to reduce an impact of water vapor on measured data. We reconstructed optical properties and dielectric constants of the abovementioned PEAs in the spectral range of 0.1 to 2.5 THz. We analyzed measured THz optical properties along with the literature data for coefficients of PEAs' diffusion into tissues in order to objectively uncover strength and weaknesses of their use in the immersion optical clearing of tissues at THz frequencies.
Application of terahertz (THz) spectroscopy for biological tissues is strongly limited by the extremely low penetration depth due to THz absorption by tissue water. One of the possible solution of such problem is the usage of THz wave penetration-enhancing agents (PEA) for optical clearing of tissues. In the present paper, the transmission-mode THz spectroscopy of a set of PEAs (polyethylene glycol with different molecular weight, propylene glycol, ethylene glycol, and dimethyl sulfoxide) was performed in order to reconstruct their dielectric properties and compare them with that of water. The obtained results emphasize the feasibility of using PEG to enhance the depth of THz wave penetration into tissues.
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