Light can exert forces on objects, promising to propel a meter-scale lightsail to near the speed of light. The key to address many challenges in such an ambition hinges on the nanostructuring of lightsails to tailor their optical scattering properties. In this Letter, we present a comprehensive study of photonic design of lightsails by applying large-scale optimization techniques to a generic geometry based on stacked photonic crystal layers. The optimization is performed by rigorous coupled-wave analysis amended with automatic differentiation methods for adjoint-variable gradient evaluations. Employing these methods, the propulsion efficiency of a lightsail that involves a trade-off between high broadband reflectivity and mass reduction is optimized. Surprisingly, regardless of the material choice, the optimal structures turn out to be simply one-dimensional subwavelength gratings, exhibiting nearly 50% improvement in acceleration distance performance compared to prior studies. Our framework can be extended to address other lightsail challenges such as thermal management and propulsion stability and applications in integrated photonics such as compact mirrors.

中文翻译：

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相对论光帆推进轻量宽带反射器的反设计

光线可以对物体施加作用力，有望将米级的光帆推进到接近光速的状态。解决这种雄心壮志的关键在于轻型帆的纳米结构，以调整其光学散射特性。在这封信中，我们通过将大规模优化技术应用于基于堆叠光子晶体层的通用几何形状，对光帆的光子设计进行了全面的研究。优化是通过严格的耦合波分析进行的，并通过自动微分方法进行了修正，以进行伴随变量梯度评估。使用这些方法，可以优化在高宽带反射率和减少质量之间进行权衡的光帆的推进效率。令人惊讶的是，无论选择哪种材料，最佳结构证明只是一维亚波长光栅，与现有技术相比，其加速距离性能提高了近50％。我们的框架可以扩展，以解决其他轻帆挑战，例如热管理和推进稳定性，以及在集成光子学中的应用，例如紧凑型反射镜。