Plasmonic Taiji

The Taiji symbol is a very old schematic representation of two opposing but complementary patterns in oriental civilization. Using electron beam lithography, we fabricated an array of 70 × 70 gold Taiji marks with 30nm thickness and a total area of 50 × 50 µm2 on a fused silica substrate. The diameter of each Taiji mark is 500nm, while the period of the array is 700nm. Here we present experimental as well as numerical simulation results pertaining to plasmonic resonances of several Taiji nano-structures under normal illumination. We have identified a Taiji structure with a particularly interesting vortex-like Poynting vector profile, which could be attributed to the special shape and dimensions of the Taiji symbol.

Electromagnetic energy vortex associated with sub-wavelength plasmonic Taiji marks

W. T. Chen, P. C. Wu, C. J. Chen, H. Y. Chung, Y. F. Chau, C. H. Kuan, D. P. Tsai

Optics Express 18(19), 19665 (2010). [Link]

Multidimensional Data Storage

We demonstrate a concept of optical data storage through plasmonic resonances of metallic nanostructures. Metallic nanostructures exhibit strong variations in their reflectance and/or transmittance spectra due to surface plasmon polariton resonances. We study the variations in spectra through 50×50 arrays of repeated unit cells covering a total area of ∼ 50×50 μm2. Each cell contains ten different nanofeatures, such as an ellipse, a ring, a circle, a triangle, a square, etc. The size of each unit-cell is 500×500 nm2, and the periodicity is 1.0 μm. The variations in spectra are obvious enough to be distinguished and then retrieved.

Manipulation of multidimensional plasmonic spectra for information storage

W. T. Chen, P. C. Wu, C. J. Chen, C. J. Weng, H. C. Lee, T. J. Yen, C. H. Kuan, M. Mansuripur, D. P. Tsai

 Applied Physics Letters 98(17), 171106 (2011) [Link]

Gradient Meta-Surfaces

We combine theory and experiment to demonstrate that a carefully designed gradient meta-surface supports high-efficiency anomalous reflections for near-infrared light following the generalized Snell’s law, and the reflected wave becomes a bounded surface wave as the incident angle exceeds a critical value. Compared to previously fabricated gradient meta-surfaces in infrared regime, our samples work in a shorter wavelength regime with a broad bandwidth (750–900 nm), exhibit a much higher conversion efficiency (80%) to the anomalous reflection mode at normal incidence, and keep light polarization unchanged after the anomalous reflection. Finite-difference-time-domain (FDTD) simulations are in excellent agreement with experiments. Our findings may lead to many interesting applications, such as antireflection coating, polarization and spectral beam splitters, high-efficiency light absorbers, and surface plasmon couplers.

High-efficiency broadband anomalous reflection by gradient meta-surfaces

S. Sun, K.-Y. Yang, C.-M. Wang, T.-K. Juan, W. T. Chen, C. Y. Liao, Q. He, S. Xiao, W.-T. Kung, G.-Y. Guo, L. Zhou, D. P. Tsai

Nano Letters 12(12), 6223-6229 (2012) [Link]

Double exposure e-beam lithographic process

Using electron beam lithography with a double exposure process, we fabricated 375 × 375 erected U-shape three-dimensional (3D) gold resonance rings on fused silica substrate, covering a total area of ~75 µm × 75 µm. For the precision alignment of e-beam double exposure process, two golden cross alignment marks are fabricated and used. For the first exposure process, the bottoms of resonance rings are defined in positive resist after first e-beam exposure and lift-off process. Subsequently, the two prongs of resonance ring are made by second e-beam exposure and lift-off process. The Espacer/poly methyl methacrylate bi-layered resist are spin-coated on a cleaned substrate. An e-beam lithography system at the high acceleration voltage and low current is used.

3D Metamaterial of Upright Plasmonic Meta-Molecules

We report the first three-dimensional photonic metamaterial, an array of erected U-shape plasmonic gold meta-molecules, that exhibits a profound response to the magnetic field of light incident normal to the array. The metamaterial was fabricated using a double exposure e-beam lithographic process. It was investigated by optical measurements and finite-element simulations, and showed that the magnetic field solely depends on the plasmonic resonance mode showing either enhanced in the centre of the erected U-shape meta-molecule (16 times enhancement) or enhanced around two prongs of erected U-shape meta-molecule (4 times enhancement).

Optical magnetic response in three-dimensional metamaterial of upright plasmonic meta-molecules

W. T. Chen, C. J. Chen, P. C. Wu, S. Sun, L. Zhou, G.-Y. Guo, C. T. Hsiao, K.-Y. Yang, N. I. Zheludev, D. P. Tsai

Optics Express 19(13), 12837 (2011).[Link] [PDF]

An array of fabricated U-shaped split-ring resonators shows a profound response to the magnetic field of light incident normal to the sample.

Optical magnetic response of upright plasmonic molecules in 3D metamaterial

W. T. Chen, P. C Wu, C. J. Chen, S. Sun, L. Zhou, G.-Y Guo, C. T. Hsiao, K.-Y. Yang, N. I. Zheludev, and D. P. Tsai

20 December 2011, SPIE Newsroom. DOI: 10.1117/2.1201112.003839 [Link]

Magnetic plasmon induced transparency

In a laser-driven atomic quantum system, a continuous state couples to a discrete state resulting in quantum interference that provides a transmission peak within a broad absorption profile the so-called electromagnetically induced transparency (EIT). In the field of plasmonic metamaterials, the sub-wavelength metallic structures play a role similar to atoms in nature. The interference of their near-field coupling at plasmonic resonance leads to a plasmon induced transparency (PIT) that is analogous to the EIT of atomic systems. A sensitive control of the PIT is crucial to a range of potential applications such as slowing light and biosensor. So far, the PIT phenomena often arise from the electric resonance, such as an electric dipole state coupled to an electric quadrupole state. Here we report the first three-dimensional photonic meta­material consisting of an array of erected U-shape plasmonic gold nanostructures that exhibits PIT phenomenon with magnetic dipolar interaction between magnetic meta­molecules. We further demonstrate using a numerical simulation that the coupling between the different excited pathways at an intermediate resonant wavelength allows for a π phase shift resulting in a destructive interference. A classical RLC circuit was also proposed to explain the coupling effects between the bright and dark modes of EIT-like electromagnetic spectra. This work paves a promising approach to achieve magnetic plasmon devices.

Magnetic plasmon induced transparency in three-dimensional metamolecules

P. C. Wu, W. T. Chen, K.-Y. Yang, C. T. Hsiao, G. Sun, A. Q. Liu, N. I. Zheludev, D. P. Tsai

Nanophotonics 1(2), 131-138 (2012) [Link]