Jianlong Liu

Generation of hollow Gaussian beams by spatial filtering

We demonstrate that hollow Gaussian beams can be obtained from Fourier transform of the differentials of a Gaussian beam, and thus they can be generated by spatial filtering in the Fourier domain with spatial filters that consist of binomial combinations of even-order Hermite polynomials. A typical 4f optical system and a Michelson interferometer type system are proposed to implement the proposed scheme. Numerical results have proved the validity and effectiveness of this method. Furthermore, other polynomial Gaussian beams can also be generated by using this scheme. This approach is simple and may find significant applications in generating the dark hollow beams for nanophotonic technology.

Surface plasmon reflector based on serial stub structure

Plasmonic reflectors based on serial stub structure are studied in this paper. A general theory of periodic stub structure using transmission line model is developed. The transmission characteristics, e.g., periodicity and symmetry of the spectra, are closely related to the ratio of structure period to stub length. Investigation reveals that the transmission valleys of the spectra could be divided into two categories, which is quite different from conventional Bragg reflectors. Finite-Difference Time-Domain (FDTD) method is used in numerical analysis in this paper.

Plasmon flow control at gap waveguide junctions using square ring resonators

A compact add-drop coupler with square ring resonator (SRR) in gap plasmon waveguide is studied. It is shown that there exist both travelling-wave and standing-wave resonant modes in the resonator due to the influence of the four corners in the square ring. Based on SRR, a new type of two-dimensional plasmonic L-shaped bend is proposed. The compact geometry and high add-drop efficiency of the proposed structure enable the realization of flexible flow control at waveguide junctions, e.g. T-splitter and waveguide cross intersections, in nano scale. The optical field propagation patterns in the gap waveguide junctions equipped with SRRs are investigated by the finite-difference time-domain method.

Ultra-thin circular polarization analyzer based on the metal rectangular split-ring resonators

We propose an ultra-thin metasurface of the metal rectangular split-ring resonators (MRSRR) array which can modulate and analyze the wavefront of circularly polarized light efficiently. An incident circularly polarized light could be converted into the corresponding cross-polarized light which would be bent to ± 23° at a wavelength of 808 nm for the normal incidence. And a linearly polarized light would be decomposed into two lights of left and right-handed circular polarizations in the directions of ∓23° respectively. These phenomena have also been observed at 1200 nm with different geometric parameters. And these results depend on controlling the optical-axis profile of the resonators in a subwavelength scale by precisely modulating two degrees of freedom in our nanostructures.

Design of microlenses with long focal depth based on the general focal length function

A general focal length function is proposed to design microlenses with long extended focal depth and high lateral resolution. The focal performance of the designed microlenses, including the actual focal depth, the focal spot size, and the diffraction efficiency, is calculated by rigorous electromagnetic theory and the boundary-element method for several f-numbers. In contrast to conventional microlenses, the numerical results indicate that the designed microlenses can exhibit long extended focal depth and good focal performance. It is expected that the long focal length function will be widely used to design microlenses with long focal depth characteristics.

Analysis of focal-shift effect in planar metallic nanoslit lenses.

A theoretical analysis based on scalar diffraction theory about the recently reported focal-shift phenomena in planar metallic nanoslit lenses is presented. Under Fresnel approximation, an axial intensity formula is obtained, which is used to analyze the focal performance in the far field zone of lens. The relative focal shift is totally dependent on the Fresnel number only. The influences of the lens size, preset focal length and incident wavelength can be attributed to the change of Fresnel number. The total phase difference of the lens is approximately equal to the Fresnel number multiplied by π. Numerical simulations performed using finite-difference time-domain (FDTD) and near-far field transformation method are in agreement with the theoretical analysis. Using the theoretical formula assisted by simple numerical method, we provide predictions on the focal shift for the previous literatures.

Focusing surface plasmon and constructing central symmetry of focal field with linearly polarized light

When linearly polarized light illuminates a plasmonic lens, it is challenging to realize centrally symmetric focal spot in the near field since there is mismatch between the symmetries of the plasmonic structures and the polarization spatial distribution of the incident light. In this paper, we propose a plasmonic lens that can focus surface plasmon to a centrally symmetric field when illuminated by light with linear polarization. Remarkably, the intensity distribution of the focal field is insensitive to the polarization direction, which makes it more reliable in optical applications. Numerical simulations are used to check the performance of the proposed structure. Comparisons with two conventional plasmonic lenses are made to demonstrate its unique superiority in polarization-insensitive focusing.

A side-illuminated plasmonic planar lens

Planar lens based on nanoscale slits has been demonstrated theoretically and experimentally. In this paper, we propose a 2D model of a similar planar lens but with side-illumination. The lens consists of a main bus waveguide to transport plasmonic wave and grooves functioning as antennas. The shapes and filling materials of the waveguide and grooves are assumed to be invariant in the third direction. The phase retardation needed for wavefront shaping comes from the transverse propagation of the plasmonic wave in the waveguide and the well-designed groove positions. The concept is applied to the design of planar lenses and axicons. The simulation results demonstrate that such structures can work as good diffractive elements. The side-illumination property of such structure enables the potential integration of lens on chip.

Circular Polarization Analyzer Based on the Combined Coaxial Archimedes’ Spiral Structure

We have designed a novel plasmonic lens for the circular polarization analyzer based on the combined coaxial Archimedes’ spiral structure which is composed of a Archimedes’ spiral slit (ASS) surrounded by coaxial Archimedes’ spiral gratings (ASG) on the both sides of the metal film. The designed structure is sensitive to polarization of the incident circularly polarized light, and it can realize the focusing effect under circular polarization illumination with specific handedness and defocusing effect under the other handed circular polarization illumination. This phenomenon of spin-dependent intensity distribution can be attributed to the presence of geometric phase effect of ASS. The simulated results show that the combination of the ASS and ASG has a higher efficiency and better performance than the traditional pure ASS.

Chromatic aberration of light focusing in hyperbolic anisotropic metamaterial made of metallic slit array.

The dispersion of a hyperbolic anisotropic metamaterial (HAM) and the chromatic aberration of light focusing in this kind of HAM are studied. The HAM is formed by alternately stacking metal and dielectric layers. The rules of materials and filling factors affecting the optical property of HAM are given. The chromatic aberration of light focusing is demonstrated both theoretically and numerically. By comparing the theory with the simulation results, the factors influencing the focal length, including the heat loss of material and low spatial frequency modes, are discussed. The investigation emphasizes the anomalous properties, such as chromatic aberration and low spatial frequency modes influencing focus position, of HAM compared with that in conventional lens. Based on the analysis, the possibility of using HAM to focus light with two different wavelengths at the same point is studied.