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Increasing crop yields is the most sustainable approach toward the escalating food demand. Promoting crop production using genetic and ecological engineering such as reducing photorespirations, and accelerating recovery from photoprotections, is emerging yet debatable for its wide public acceptance. Alternatively, managing light quantity (intensity) and quality (spectrum) provides a promising and secure venue for improving crop yield, but comes with costs. For example, increasingly adopted supplementary electric lighting, consumed nearly 6 TWh of electricity in 2017 in the United States alone. Meanwhile, not every spectral component of sunlight contributes equally to photosynthesis. Recently have spectral-shifting materials been introduced to convert impinging sunlight into an optimized spectrum for photosynthesis. However, a fundamental optics challenge remains unaddressed: the majority of the internally generated photons are trapped inside the material, leading to seemingly encouraging but inconsistent results. Exploiting a photonic microstructure that is simple to manufacture, we demonstrate a spectral-shifting and unidirectional light extracting film that converts the least effective photosynthetic components of sunlight (green light) into the most effective (red) light with an internal quantum efficiency of 90% and a total external quantum efficiency of 43.8%. More importantly, this film breaks the propagation symmetry of light and extracts most of the otherwise trapped light unidirectionally into free space, rendering an easily adaptable greenhouse envelope material. Using leafy green lettuce as model crops, the micro-photonic film allows us to harvest > 20% more aboveground biomass of lettuce both indoor and outdoor. As we demonstrate experimentally, the photonic thin film can serve as greenhouse envelopes to provide more effective photosynthetic light than that of direct sunlight, opening the door for “red-colored” greenhouses and other protected environments with substantially augmented crop yields.
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