Zhu Yechuan, Yuan Weizheng, Sun Hao, Yu Yiting
Key Laboratory of Micro/Nano Systems for Aerospace, Ministry of Education, Xi'an 710072, China.
Key Laboratory of Micro- and Nano-Electro-Mechanical Systems of Shaanxi Province, Northwestern Polytechnical University, Xi'an 710072, China.
Nanomaterials (Basel). 2017 Aug 12;7(8):221. doi: 10.3390/nano7080221.
The development of techniques for efficiently confining energy in the visible and infrared spectral regions to the deep subwavelength spatial scale with dimensions as small as a few nanometers would have great significance for scientific research and engineering practices. Such an ability to manipulate light is impossible for conventional dielectric lenses due to the diffraction limit. Here, we propose a metallic graded-index (MGRIN) lens formed by an array of coupled metallic waveguides with identical nanoscale widths embedded by index-varying dielectrics to enable the optical nanofocusing. The focusing mechanism of the MGRIN lens is theoretically investigated based on Hamiltonian optics, which are verified by the finite-difference time-domain (FDTD) method. Numerical results reveal that an ultra-deep subwavelength focus of 8 nm (/500) with a long focal depth (1.93) and enhanced field intensity can be achieved. Moreover, the nanofocusing capability of the MGRIN lens without redesigning the structure can be well kept when the incident wavelength changes over a broad range from visible to infrared. Our design of optical nanofocusing shows great potential for use in nano-optics and nanotechnology.
开发能够在可见光和红外光谱区域将能量有效地限制在深亚波长空间尺度（尺寸小至几纳米）的技术，对科学研究和工程实践具有重大意义。由于衍射极限，传统介电透镜无法实现这种操纵光的能力。在此，我们提出一种金属渐变折射率（MGRIN）透镜，它由一系列具有相同纳米级宽度的耦合金属波导阵列构成，并嵌入折射率变化的电介质以实现光学纳米聚焦。基于哈密顿光学理论研究了MGRIN透镜的聚焦机制，并通过时域有限差分（FDTD）方法进行了验证。数值结果表明，可以实现8纳米（/500）的超深亚波长聚焦，焦深较长（1.93）且场强增强。此外，当入射波长在从可见光到红外的宽范围内变化时，无需重新设计结构，MGRIN透镜的纳米聚焦能力就能很好地保持。我们的光学纳米聚焦设计在纳米光学和纳米技术中显示出巨大的应用潜力。
