Transparent Thin Films via Single-Crystal-to-Single-Crystal Photopolymerization of a Flexible Diene for Femtosecond-Scale Third-Order Optical Nonlinearity

Two-photon absorption (TPA) as a subclass of nonlinear optics has attracted much attention owing to its rich applications in optical limiting, fluorescence imaging, and microfabrication. However, the problem of low transmittance and large light scattering due to aggregation at high concentration into host polymer severely limits their practical applications. In this work, we present a transparent thin film (∼26 nm) synthesized via single-crystal-to-single-crystal (SCSC) [2+2] photopolymerization of a flexible diene coordination polymer (PPMA). The resulting polymer film, (PPMA′)_n, exhibits exceptional third-order nonlinear optical (NLO) properties, including a TPA coefficient (β_eff) of 0.88185 × 10⁶ cm GW^–1 and a TPA cross-section (σ_TPA) of 37.34 × 10⁸ GM at a pulse energy of 1.25 pJ. Additionally, the film demonstrates optical limiting behavior with an impressively low value of 5.52 × 10^–8 J cm^–2. The superior optical performance is attributed to its uniform transparency, extended hydrogen-bonding network, and efficient charge transfer between electron-rich and electron-deficient regions. The saturation in β_eff at high intensities due to the high repetition rate of laser pulses is also explained by a customized model. This work introduces a promising strategy for fabricating high-performance NLO materials, paving the way for advanced photonic and optoelectronic applications.