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Ximin He1 Xiaoshi Qian1 Yusen Zhao1 Yousif Alsaid1 Mo Sun1

1, University of California, Los Angeles, Los Angeles, California, United States

Phototropism is well developed in nature as a self-adapting functionality for many living organisms that rely on solar energy to survive. Plant such as the sunflower can turn its head following the sun by producing auxin that promotes stem elongation on the non-illuminated side of the stem. Thus the elongated shaded side of the stem pushes the stem to bend toward the sun. It takes millions of years for the plants to develop such a smart and self-adaptive functionality to harness the solar energy more efficiently. Today, mankind is facing unprecedented challenges including energy shortage, global warming, air pollutions, etc., due to the over burning of the fossil fuel to meet the ever increasing energy demand of the economic growth. Research societies are seeking solutions from the nature. However, there has no such a functionality being developed in synthetic materials to detect and track the light fully autonomously. Photomechano-responsive material systems have been attracting more and more attention throughout recent years, owing to their unique potentials to achieve smart and self-regulating photonic devices. Among various mechanisms that provide the functionality, photo-thermo-mechanical energy conversion has been considered as the one of the most promising routes to address the challenge. In this work, we report a soft material system that can self-adaptively track the incident energy source from arbitrary directions in the three-dimensional space around the material. Uniquely this soft material is physically symmetric in geometry and composition throughout the entire thermal-responsive hydrogel body with evenly distributed Au Nanoparticles as the photonic absorbers for specific frequency of incident light. Such soft photo-tracker can achieve fast and omni-directional real-time tracking in all direction covering 360 degrees of azimuthal plane, within 10s of seconds at a remarkably high tracking accuracy > 99.8%. The dynamic photo-thermo-mechanical process of the spontaneous tracking has been systematically characterized experimentally and elucidated by hydrogel mechanics modeling. With the micron-scale photo-tracker array, we further present an omni-directional solar vapor generator exhibiting extraordinary water vapor generation efficiency at all different incident angles constantly as high as at the 90 degrees normal incidence, indicating a significant compensation of energy loss caused by the angular incidence by the tracking functionality. The presented material system has broad applications that require maximizing the energy input, for enhanced solar harvesting, energetic emissive signal detecting and tracking, energy-efficient smart windows, and other self-regulative optic devices.

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