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. 2019 Nov 7;9:16266. doi: 10.1038/s41598-019-52706-4

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Optical gain and ASE spectra from silk/dye films. (a–f), Pump-fluence dependences of the ASE emissions and threshold behaviors on the bulk silk/dye film: (a,d), the silk/dye film with the PMMA opal (b,e), and the silk/dye film with air voids after removing the PMMA opal (c,f). The concentration of the sodium fluorescein dye was 10 wt%. As RI contrast (Δn) of the opal structure was increased, the threshold was greatly reduced owing to the increased optical path. (g) The measured threshold pumping power of ASEs from the bulk dye/silk film with various concentrations of the sodium fluorescein dye. The error bar represents the standard deviation calculated using five data points at each concentration. (h) The variable stripe-length experiment to estimate the net modal gain of the bulk silk film mixed with sodium fluorescein. The concentration of sodium fluorescein was 8 wt%, and all spectra were excited at λ = 355 nm with a 25-ps pulse width.