介质填充型二次柱面等离激元透镜的亚波长聚焦

Subwavelength light focusing using quadric cylinder surface plasmonic lens with gold film slits filled with dielectric

本文提出了一种亚波长聚焦的表面等离激元透镜,该透镜由二氧化硅填充金膜纳米狭缝阵列组成,金膜的出射表面为二次柱面.表面等离激元在狭缝入口处激发并沿狭缝传输,在狭缝出口转变为带有一定相位延迟的自由空间传播的光波.通过对透镜结构参数的控制,可以调节来自各狭缝的光波间的相对相位,使它们在设定的焦点处进行相长干涉,从而实现聚焦效果.本文用时域有限差分法数值计算了二次柱面等离激元透镜的聚焦特性.数值模拟结果表明,所设计的孔径为2μm的透镜,能够实现微米级焦距和焦深、且焦斑半高宽低至0.4倍波长的亚波长聚焦.该表面等离激元透镜结构简单紧凑、尺寸小,有利于光子器件的集成,在集成光学、光学微操纵、超分辩率成像、光存储、生化传感等相关领域有潜在的应用价值.

A novel plasmonic lens（PL） with simple nano-structure is proposed to realize the subwavelength focusing. The proposed PL is composed of the gold film with only five dielectric-filled nanoslits. The exit surface of the gold film is processed into quadric shape that can be parabolic, elliptical or hyperbolic cylinders. The film is fabricated to form five uniformly spaced nanoslits with different widths and depths. All five slits are symmetrically arranged with respect to the center of lens and filled with a dielectric medium（i.e., Si O2）. Under the illumination of TM polarized beams, the surface plasmon polaritons are excited at the entrance surface of the PL, then pass through the Si O2-filled slits while acquiring specific phase retardations, and are finally coupled to the light waves in the free space. Each light wave originating from the slit can be regarded as an individual point source, and the constructive interference of light waves from slits gives rise to the beam focusing at the focal plane of the PL. We investigate the phase modulation mechanism of the PL and find that the focusing performance relies on the shape of exit surface, filling medium and geometric parameters of the slits. A suitable phase modulation can be achieved by adjusting the structure parameters of the PL with a specific exit surface shape. Three kinds of quadratic cylindrical PLs, i.e., parabolic, elliptical and hyperbolic cylindrical ones with continuous or stepped exit surface are designed to realize the focusing of TM polarized subwavelength beams in visible spectrum. The finite difference time domain method is employed to compute the light field and to investigate the focusing characteristics of the proposed PL. The performance measurements include the focal length, depth of focus and full-width half-maximum（FWHM）. The simulation results confirm that the proposed PL with a 2-μm-diameter aperture can achieve the subwavelength focusing at a focal length of micron scale. The attainable smallest FWHM of the focal spot is 0.405λ0（λ0denoting the wavelength of the incident light） which is well beyond the diffraction limit. It is also worth mentioning that the step-like cylindrical PL can yield a sharper focal spot than the continuous cylindrical PL. For example, the FWHM of focal spot produced by the stepped elliptical cylindrical PL is about 92% of that produced by the continuous elliptical cylindrical PL. The proposed PL has the advantages of simple and compact structure with much smaller lateral dimension and easy integration with other photonic devices. Our study helps design the easy-to-fabricate PLs and facilitates applications of plasmonic devices in the fields such as optical micro manipulation, super-resolution imaging, optical storage and biochemical sensing.