Ultrafast Nonlinear Chiral Modulation With Hydrogel Helices

ABSTRACT Chirality plays a critical role in light‐matter interactions and is of great interest for applications in photonics, biosensing, and spintronics. However, realizing strong and ultrafast chiral optical responses remains challenging due to the weak intrinsic chirality of natural materials and limited interaction with circularly polarized light. Here, we demonstrate a helical‐structured polyacrylamide (PAAm) hydrogel deposited with gold nanoparticles (Au NPs) to realize giant ultrafast nonlinear chirality. Through femtosecond laser scanning, large‐area PAAm with nano‐periodic structured helices are fabricated, enabling enhanced optical fields and programmable chiral patterns. Polarization‐resolved Z‐scan measurements reveal intensity‐dependent nonlinear circular dichroism (CD) and preferential saturation under one circular polarization. Electromagnetic simulations present vortex‐enhanced field distributions in asymmetrical helical structures. Under chiral‐pump transient absorption, the Au helix reaches a transient CD of 0.7 within 1 ps and relaxes with a 2.19 ps lifetime, consistent with plasmon‐generated hot‐carrier dynamics and structure‐dependent electron coupling. This work uncovers the origin of ultrafast chirality and provides a scalable strategy for programmable high‐chirality architectures in ultrafast chiral photonics, polarized devices, and time‐resolved chiroptical technologies.