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
(βeff) of 0.88185 × 106 cm GW–1 and a TPA cross-section (σTPA) 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 βeff 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.