A Full-Scale Analysis of Absorption Edges in Dye-Doped Nonlinear Optical Polymers.

A full-scale analysis of the absorption edges by modified Tauc-Lorentz models is essential in determining the optical bandgap and Urbach energy of semiconductors, transparent conductors, ionic compounds, and dielectric materials. This technique has not yet been applied to analyzing organic nonlinear optical (NLO) materials. This problem is tackled by preparing high-quality films of guest-host NLO polymers with a wide thickness range from sub-micron to 200 microns, allowing accurate measurement of full-spectral absorption coefficients of NLO materials over four orders of magnitude by the UV-VIS-NIR spectroscopy. The Tauc model and a new Monolog-Lorentz model are used to study the optical absorption edge of guest-host NLO polymers containing various push-pull chromophores and the dependence of optical bandgap and Urbach edge on the structure and composition of materials is analyzed. The results reveal the critical transition of the Urbach exponential tail to a low energy tail that overlaps with vibrational overtones of materials at the telecom wavelengths. Determining the fundamental absorption region of organic NLO films in this study provides quantitative insight into the research to harness the resonance-enhanced nonlinear coefficients of materials by operating at the wavelengths near the band edge with the control of optical loss.