Zhu Jinlong, Goddard Lynford L
Opt Express. 2016 Dec 26;24(26):30444-30464. doi: 10.1364/OE.24.030444.
For a long time, light focusing from microspheres has been intensively researched. The microsphere has been shown to be capable of generating a high intensity beam with sub-wavelength width, known as a photonic nanojet (PNJ). In this article, we present a detailed report on the properties of a new asymmetrical microstructure, consisting of a supporting stage and a spherical cap, and demonstrate precise engineering of the PNJ characteristics by simply selecting its geometrical dimensions. More importantly, we find that a single asymmetrical microstructure can generate an ultra-elongated PNJ on the shadow side and the cascade of two asymmetrical elements can generate a PNJ with a full width at half maximum (FWHM) waist down to 0.27λ. In addition, because of the presence of energy convergence regions within the second element, an ultra-narrow PNJ can be generated even when the length of the second element in the cascade is many orders of magnitude greater than the wavelength or deviates somewhat from the optimal dimensions. This offers design flexibility and manufacturing tolerance, which has not been demonstrated in the conventional microsphere design or its derivatives. We believe that these remarkable performance features make the asymmetrical structure and its cascade attractive in numerous applications.
长期以来，微球的光聚焦一直是深入研究的课题。微球已被证明能够产生具有亚波长宽度的高强度光束，即所谓的光子纳米喷流（PNJ）。在本文中，我们详细报道了一种由支撑平台和球形帽组成的新型非对称微结构的特性，并通过简单选择其几何尺寸来展示对PNJ特性的精确调控。更重要的是，我们发现单个非对称微结构能够在阴影侧产生超细长的PNJ，并且两个非对称元件的级联能够产生半高宽（FWHM）腰宽低至0.27λ的PNJ。此外，由于第二个元件内存在能量汇聚区域，即使级联中第二个元件的长度比波长大多个数量级或稍微偏离最佳尺寸，也能够产生超窄的PNJ。这提供了设计灵活性和制造容差，这在传统微球设计或其衍生物中尚未得到证明。我们相信，这些卓越的性能特征使得非对称结构及其级联在众多应用中具有吸引力。
