Abstract
Melanin, with its high refractive index (RI) and broadband absorption, is an important biomaterial responsible for many of the vibrant structural colors observed in nature and for UV protection. Even though the RI plays an important role in the function of melanin, there is an ambiguity in its reported complex RI and a lack of understanding of whether and how the UV radiation, these materials are likely to experience under normal use, will affect the complex RI. Here, we measured the wavelength-dependent (360-1700 nm) complex RI of synthetic melanin films before and after in situ UV treatment using ellipsometry. We modeled the ellipsometric data using a modified Tauc-Lorentz dispersion model and measured the thickness independently using atomic force microscopy. The UV radiation reduces the film thickness. Interestingly, we find that both the real and imaginary terms of the RI increase upon UV radiation. These experiments provide accurate measurements of the optical properties of melanin and a surprising result that synthetic melanin absorbs more light (∼25% increase in extinction coefficient) below 600 nm after UV exposure.
| Original language | English (US) |
|---|---|
| Article number | 203701 |
| Journal | Applied Physics Letters |
| Volume | 117 |
| Issue number | 20 |
| DOIs | |
| State | Published - Nov 16 2020 |
Funding
The authors thank Cheng Liu and Steven S.C. Chuang for the use of their FTIR equipment and Abraham Joy for his UV spot lamp. A.D., A.P., and W.L. thank the National Polymer Innovation Center (NPIC) and Andrew Knoll for providing access and technical assistance on the ellipsometer. A.P. and W.L. would also like to thank Matthias Duwe and Arash Mirhamed (Accurion GmbH) for their input on modeling ellipsometer data. The authors would also like to thank Austin Garner for his assistance in statistical analysis. We also extend our gratitude to Mario Echeverri and Saranshu Singla for helpful discussion and comments on this manuscript. The authors acknowledge financial support from the Air Force Office of Scientific Research (AFOSR) under the Multi-University Research Initiative (MURI) Grant (No. FA9550–18-1–0142). M.D.S. acknowledges support from an AFOSR Grant (No. FA9550–18-1–0477), an FWO Grant (No. G007117N), and an HFSP Grant (No. RGP 0047).
ASJC Scopus subject areas
- Physics and Astronomy (miscellaneous)