Ultrathin, Wavelength‐Multiplexed and Integrated Holograms and Optical Neural Networks Based on 2D Perovskite Nanofilms

Holography, as a technique for coherent wavefront reconstruction, is extensively used in numerous optical applications such as optical imaging, 3D display, photolithography, and optical artificial intelligence. In order to achieve highly compact and functional integration with optoelectronic devices, the hologram needs to possess ultrathin thickness as well as multicolor functionality. However, its thickness is typically limited to optical wavelength ranges due to the requirement for pronounced amplitude or phase modulation, and it generally operates at a single wavelength band without wavelength‐multiplexed channels. Here, the hologram is decreased thickness to sub‐ten nanometers by exploiting the large refractive index and strong exciton absorption of 2D perovskites. Ultrathin perovskite holograms and holographic neural networks with high stability are successfully developed by using femtosecond laser direct writing, and their operation wavelength can be rationally tuned from 400–515 nm through halide anion engineering. Consequently, the wavelength can be multiplexed with low cross‐talks by stacked perovskite nanolayers for applications of holographic display and neural networks without the usages of complex optical structures or filters. This work provides a feasible and promising technical route for integrating ultrathin, high pixel density, and multiplexed holographic structures with flat optoelectronic devices for next‐generation integrated optical systems.