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==Optical properties==
[[File:Envelope Method.tif|thumb|left|The as-measured normal-incidence transmission spectra of ''a''-, ''a''-, and ''a''- samples (open symbols are measured data) and substrate (black solid curve). The inset shows constructed maxima and minima envelope curves for the ''a''-Se1000 sample transmittance]]
There are several approaches to analyze transmittance data of selenium films, from which one can extract their optical constants and other related topics.<ref>{{Cite journal|last=Jafar|first=Mousa M. Abdul-Gader|last2=Saleh|first2=Mahmoud H.|last3=Ahmad|first3=Mais Jamil A.|last4=Bulos|first4=Basim N.|last5=Al-Daraghmeh|first5=Tariq M.|date=2016-04-01|title=Retrieval of optical constants of undoped amorphous selenium films from an analysis of their normal-incidence transmittance spectra using numeric PUMA method|url=https://link.springer.com/article/10.1007/s10854-015-4156-z|journal=Journal of Materials Science: Materials in Electronics|language=en|volume=27|issue=4|pages=3281–3291|doi=10.1007/s10854-015-4156-z|issn=0957-4522}}</ref> However, the algebraic method of Swanepoel envelope method (EM) seems to be trustful and need no dispersion relations in prior, but is limited to transmittance spectra that display a number of peaks (maxima and minima).<ref>{{Cite journal|last=Saleh|first=Mahmoud H.|last2=Ershaidat|first2=Nidal M.|last3=Ahmad|first3=Mais Jamil A.|last4=Bulos|first4=Basim N.|last5=Jafar|first5=Mousa M. Abdul-Gader|date=2017-06-01|title=Evaluation of spectral dispersion of optical constants of a-Se films from their normal-incidence transmittance spectra using Swanepoel algebraic envelope approach|url=https://link.springer.com/article/10.1007/s10043-017-0311-5|journal=Optical Review|language=en|volume=24|issue=3|pages=260–277|doi=10.1007/s10043-017-0311-5|issn=1340-6000}}</ref> However, EM method has a further limitation that is only of good validity in a wide spectral region in the quasi-transparency <ref>{{Cite web|url=https://www.researchgate.net/publication/272292433_Comprehensive_formulations_forthe_total_normal-incidence_optical_reflectance_and_transmittance_of_thin_films_laid_on_thick_substrates|title=Comprehensive formulations forthe total normal-incidence optical reflectance and transmittance of thin films laid on thick substrates|website=ResearchGate|language=en|access-date=2017-11-06}}</ref><ref>{{Cite journal|last=Minkov|first=D.A.|last2=Gavrilov|first2=G.M.|last3=Angelov|first3=G.V.|last4=Moreno|first4=J.M.D.|last5=Vazquez|first5=C.G.|last6=Ruano|first6=S.M.F.|last7=Marquez|first7=E.|title=Optimisation of the envelope method for characterisation of optical thin film on substrate specimens from their normal incidence transmittance spectrum|url=https://doi.org/10.1016/j.tsf.2017.11.003|journal=Thin Solid Films|doi=10.1016/j.tsf.2017.11.003}}</ref> Conventional curve-fitting of the transmittance data, an approach that depends on the availability of proper dispersion formulas to be inserted in the program, usually yield multi-solutions of the problem (that is, local solutions, from which different, sometimes unrealistic, fitting parameters and care should be taken to choose the correct results, and only a global solution of the problem gives physically-meaningful results.<ref>{{Cite journal|last=Saleh|first=Mahmoud H.|last2=Jafar|first2=Mousa M. Abdul-Gader|last3=Bulos|first3=Basim N.|last4=Al-Daraghmeh|first4=Tariq M. F.|date=2014-10-21|title=Determination of Optical Properties of Undoped Amorphous Selenium (a-Se) Films by Dielectric Modeling of Their Normal-Incidence Transmittance Spectra|url=http://www.ccsenet.org/journal/index.php/apr/article/view/40452|journal=Applied Physics Research|language=en|volume=6|issue=6|pages=10|doi=10.5539/apr.v6n6p10|issn=1916-9647}}</ref>
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