Record-high near-band-edge optical nonlinearities and two-level model correction of poled polymers by spectroscopic electromodulation and ellipsometry

The determination of nonlinearities near the band edge of organic and polymeric electro-optic (EO) materials is important from the viewpoint of molecular nonlinear optics (NLO) and photonic device applications. Based on transmission-mode Stark effect electromodulation (EM) spectroscopy, we study the electric-field-induced changes in optical absorption and refraction of newly developed EO polymers from the visible to near-infrared (NIR) wavelengths and report record-high near-band-edge complex EO effects from poled thin films. Values of Δn and Δk up to 10⁻³ and 10⁻² are found at an applied electric field of 2.0×10⁵−3.0×10⁵ V/cm. The study of linear optical properties of poled films by spectroscopic ellipsometry shows large poling-induced birefringence and a nearly two-fold increase in the extinction coefficients at the extraordinary polarization. Through the Kramers-Kronig analysis, we obtained the real and imaginary second-order nonlinear coefficients up to ∼3,500 and ∼5,600 pm/V, respectively, which are believed to be the highest NLO coefficients of poled polymers through the resonance enhancement. Our approach goes beyond the previous works, applicable only to several discrete wavelengths, to a full-spectral analysis with independent verification of slab waveguide measurements. By considering both the electroabsorption and electrorefraction effects, our study overcomes the limitation of the classic qualitative two-level model and provides a quantitative understanding of near-resonance optical nonlinearities of organic EO materials. It can inspire the exploration of high-speed, absorptive, or phase-shifting light-modulators using EO polymers for on-chip applications.