Yu-i Hu

Structured Metal Film as a Perfect Absorber

A new type of absorber, a four-tined fish-spear-like resonator (FFR), constructed by the two-photon polymerization process, is reported. An absorbance of more than 90% is experimentally realized and the resonance occurs in the space between the tines. Since a continuous layer of metallic thin film covers the structure, it is perfectly thermo- and electroconductive, which is the mostly desired feature for many applications.

Metallic stereostructured layer: An approach for broadband polarization state manipulation

In this letter, we report a full-metallic broadband wave plate assembled by standing metallic L-shaped stereostructures (LSSs). We show that with an array of LSSs, high polarization conversion ratio is achieved within a broad frequency band. Moreover, by rotating the orientation of the array of LSSs, the electric components of the reflection beam in two orthogonal directions and their phase difference can be independently tuned. In this way, all the polarization states on the Poincare sphere can be realized. As examples, the functionalities of a quarter wave plate and a half wave plate are experimentally demonstrated with both reflection spectra and focal-plane-array imaging. Our designing provides a unique approach in realizing the broadband wave plate to manipulate the polarization state of light.

Assembling optically active and nonactive metamaterials with chiral units

Metamaterials constructed with chiral units can be either optically active or nonactive depending on the spatial configuration of the building blocks. For a class of chiral units, their effective induced electric and magnetic dipoles, which originate from the induced surface electric current upon illumination of incident light, can be collinear at the resonant frequency. This feature provides significant advantage in designing metamaterials. In this paper we concentrate on several examples. In one scenario, chiral units with opposite chiralities are used to construct the optically nonactive metamaterial structure. It turns out that with linearly polarized incident light, the pure electric or magnetic resonance (and accordingly negative permittivity or negative permeability) can be selectively realized by tuning the polarization of incident light for 90°. Alternatively, units with the same chirality can be assembled as a chiral metamaterial by taking the advantage of the collinear induced electric and magn...

Fabrication of stereo metallic resonant structures with polymer droplets as template

Using polymer droplets formed in dewetting of polymer film on silicon surface as template, the silicon columns capped with circular polymer plates are fabricated via plasma etching. By blanket deposition of a gold layer on the structure, an array of metallic microcavities featured by a metal circular plate separated by a silicon column with the bottom metallic film is achieved. The geometrical parameters of the stereo structures can be tuned. We show that the electric field at the edge of the cap is greatly enhanced, which can be used as the hot spot for trace amount of chemical detection.

Localized excitation of polarized light emission by cathodoluminescence spectroscopy

Surface plasmons (SPs), the resonance of free electrons on the metal-air interface, may strongly interact with light and generate some extraordinary optical effects. Instead of using conventional planar light excitation, here we excite SPs with a focused electron beam on metallic nanostructures with different geometrical symmetries. With the help of a polarizer and filter in the detection system, we obtain cathodoluminescence (CL) images with different polarizations at certain wavelengths. The maxima in the CL images show that the focused electron beam may efficiently excite luminescence with different polarizations at different spots. Comparing the data collected on the structures with specific geometrical symmetry, we demonstrate that the polarization of the emitted light depends on both the structural symmetry and the excitation location. We suggest that this Letter is enlightening to understand the relationship between the SP resonance on the structure and the emission of CL with different polarizations.

Selective propagation and beam splitting of surface plasmons on metallic nanodisk chains

Manipulating the propagation of surface plasmons (SPs) on a nanoscale is a fundamental issue of nanophotonics. By using focused electron beam, SPs can be excited with high spatial accuracy. Here we report on the propagation of SPs on a chain of gold nanodisks with cathodoluminescence (CL) spectroscopy. Experimental evidence for the propagation of SPs excited by the focused electron beam is demonstrated. The wavelength of the transmitted SPs depends on the geometrical parameters of the nanodisk chain. Furthermore, we design and fabricate a beam splitter, which selectively transmits SPs of certain wavelengths to a specific direction. By scanning the sample surface point by point and collecting the CL spectra, we obtain the spectral mapping and identify that the chain of the smaller nanodisks can efficiently transport SPs at shorter wavelengths. This Letter provides a unique approach to manipulate in-plane propagation of SPs.

Controlling the polarization state of light with metasurfaces via the excitation of plane-wave and focused electron beam

We demonstrate the general mechanism to construct the dispersion-free metastructure. By designing the parameters of a metallic metamaterial and a dielectric interlayer, the thickness-dependent dispersion of the dielectric spacing layer cancels out the intrinsic dispersion of the metallic structure.