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Reflection and absorption of millimeter waves by thin absorbing films
Author(s) -
Alekseev S.I.,
Ziskin M.C.
Publication year - 2000
Publication title -
bioelectromagnetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.435
H-Index - 81
eISSN - 1521-186X
pISSN - 0197-8462
DOI - 10.1002/(sici)1521-186x(200005)21:4<264::aid-bem3>3.0.co;2-3
Subject(s) - optics , refractive index , materials science , penetration depth , absorption (acoustics) , thin film , dielectric , reflection (computer programming) , attenuation coefficient , analytical chemistry (journal) , molar absorptivity , chemistry , physics , optoelectronics , chromatography , computer science , programming language , nanotechnology
Reflection, transmission, and absorption of mm‐waves by thin absorbing films were determined at two therapeutic frequencies: 42.25 and 53.57 GHz. Thin filter strips saturated with distilled water or an alcohol–water solution were used as absorbing samples of different thicknesses. The dependence of the power reflection coefficient R ( d ) on film thickness ( d ) was not monotonic. R ( d ) passed through a pronounced maximum before reaching its steady‐state level [ R (∞)]. Similarly, absorption, A ( d ), passed two maximums with one minimum between them, before reaching its steady‐state level [ A (∞)]. At 42.25 GHz, A ( d ) was compared with absorption in a semi‐infinite water medium at a depth d . When d < 0.3 mm, absorption by the film increased: at d = 0.1 mm the absorption ratio for the thin layer sample and the semi‐infinite medium was 3.2, while at d = 0.05 mm it increased up to 5.8. Calculations based on Fresnel equations for flat thin layers adequately described the dependence of the reflection, transmission, and absorption on d and allowed the determination of the refractive index ( n ), dielectric constant (ε), and penetration depth (δ) of the absorbing medium for various frequencies. For water samples, ε was found to be 12.4−19.3 j , δ = 0.49 mm at 42.25 GHz, and ε = 9.0−19.5 j , δ = 0.36 mm at 53.57 GHz. The calculated power density distribution within the film was strongly dependent on d . The measurements and calculations have shown that the reflection and absorption of mm‐waves by thin absorbing layers can significantly differ from the reflection and absorption in similar semi‐infinite media. The difference in reflection, absorption, and power density distribution in films, as compared to semi‐infinite media, are caused by multiple internal reflections from the film boundaries. That is why, when using thin phantoms and thin biological samples, the specifics of the interaction of mm‐waves with thin films should be taken into account. Bioelectromagnetics 21:264–271, 2000. © 2000 Wiley‐Liss, Inc.