Nat Mater. 2011 Oct 30;11(1):34-8. doi: 10.1038/nmat3148.
The law of refraction first derived by Snellius and later introduced as the Huygens-Fermat principle, states that the incidence and refracted angles of a light wave at the interface of two different materials are related to the ratio of the refractive indices in each medium. Whereas all natural materials have a positive refractive index and therefore exhibit refraction in the positive direction, artificially engineered negative index metamaterials have been shown capable of bending light waves negatively. Such a negative refractive index is the key to achieving a perfect lens that is capable of imaging well below the diffraction limit. However, negative index metamaterials are typically lossy, narrow band, and require complicated fabrication processes. Recently, an alternative approach to obtain negative refraction from a very thin nonlinear film has been proposed and experimentally demonstrated in the microwave region. However, such approaches use phase conjugation, which makes optical implementations difficult. Here, we report a simple but different scheme to demonstrate experimentally nonlinear negative refraction at optical frequencies using four-wave mixing in nanostructured metal films. The refractive index can be designed at will by simply tuning the wavelengths of the interacting waves, which could have potential impact on many important applications, such as superlens imaging.
折射定律最初由斯涅耳推导得出，后来被引入惠更斯-菲涅耳原理，该定律指出，光在两种不同材料界面上的入射角和折射角与每个介质中的折射率之比有关。虽然所有天然材料都具有正折射率，因此在正方向上表现出折射，但人工设计的负折射率超材料已被证明能够负向弯曲光波。这种负折射率是实现完美透镜的关键，完美透镜能够在远低于衍射极限的情况下进行成像。然而，负折射率超材料通常是有损耗的、窄带的，并且需要复杂的制造工艺。最近，已经提出了一种从非常薄的非线性膜中获得负折射的替代方法，并在微波区域进行了实验验证。然而，这些方法使用相位共轭，这使得光学实现变得困难。在这里，我们报告了一种简单但不同的方案，通过在纳米结构金属膜中进行四波混频，在光学频率下实验演示非线性负折射。折射率可以通过简单地调谐相互作用波的波长来任意设计，这可能对许多重要应用具有潜在影响，例如超透镜成像。
