Wenjun Fan

Experimental Demonstration of Near-Infrared Negative-Index Metamaterials

Metal-based negative refractive-index materials have been extensively studied in the microwave region. However, negative-index metamaterials have not been realized at near-IR or visible frequencies due to difficulties of fabrication and to the generally poor optical properties of metals at these wavelengths. In this Letter, we report the first fabrication and experimental verification of a transversely structured metal-dielectric-metal multilayer exhibiting a negative refractive index around 2 microm. Both the amplitude and the phase of the transmission and reflection were measured experimentally, and are in good agreement with a rigorous coupled wave analysis.

Near-infrared double negative metamaterials

We numerically demonstrate a metamaterial with both negative epsilon and negative mu over an overlapping near-infrared wavelength range resulting in a low loss negative-index material. Parametric studies optimizing this negative index are presented. This structure can be easily fabricated with standard semiconductor processing techniques.

Optical negative-index bulk metamaterials consisting of 2D perforated metal-dielectric stacks

Numerical simulations of a near-infrared negative-index metamaterial (NIM) slab consisting of multiple layers of perforated metal-dielectric stacks exhibiting a small imaginary part of the index over the wavelength range for negative refraction are presented. A consistent effective index is obtained using both scattering matrix and modal analysis approaches. Backward phase propagation is verified by calculation of fields inside the metamaterial. The NIM figure of merit, [ -Re(n)/Im(n) ], for these structures is improved by ~ 10x compared with previous reports, establishing a new approach to thick, low-loss metamaterials at infrared and optical frequencies.

Second harmonic generation from patterned GaAs inside a subwavelength metallic hole array

By extending GaAs dielectric posts with a large second-order nonlinear susceptibility through the holes of a subwavelength metallic hole array coupled to the metal surface-plasma wave, strong second harmonic (SH) signal is observed. The SH signal is strengthened as a result of the enhanced electromagnetic fields inside the hole apertures.

Enhanced mid-infrared transmission through nanoscale metallic coaxial-aperture arrays

Using a single interferometric lithography patterning step along with self-aligned pattern-definition techniques, uniform, large-area metallic coaxial arrays with ~ 100-nm toroidal gaps are fabricated. Enhanced (5x) mid-infrared (4 mum) transmission through these sub-wavelength coaxial arrays is observed as compared with that through the same fractional opening area hole arrays as a result of the complex coaxial unit cell. Varying the coaxial dimensions shifts the resonance wavelength and impacts the maximum transmission; design rules are derived. The ability to control the transmission wavelength combined with dramatically enhanced transmission represent a promising path toward nanophotonic applications.

Metallic inductive and capacitive grids: theory and experiment.

We present theoretical modeling and experimental validation of both capacitive (dot) and inductive (hole) metallic crossed gratings in the mid-infrared (2-5 microm). The gratings are fabricated by use of interferometric lithography and modeled by use of rigorous coupled-wave analysis. Our experimental and numerical investigations of the transmittance spectra of these gratings suggest that, as in inductive grids, the behavior of capacitive grids is described by the coupling of the incident light into surface plasma waves.

Large-area, infrared nanophotonic materials fabricated using interferometric lithography

In this paper, we discuss the characterization and fabrication of two nanophotonic materials for the infrared region by using IL—realizing a negative refractive index material and enhancing transmission through annular metallic coaxial aperture arrays.

Quantitative determination of tensile stress creation during island coalescence using selective-area growth

Island coalescence during Volmer–Weber thin film growth is generally accepted to be a source of tensile stress. However, the stochastic nature of unpatterned film nucleation and growth prevents meaningful comparison of stress measurements taken during growth to that predicted by theoretical models. We have overcome this by systematically controlling island geometry using selective growth of Ni films on patterned substrates via electrodeposition. It was determined that the measured power-law dependence of mean stress on island size agreed well with theory. However, our data clearly demonstrates that the majority of the tensile stress associated with coalescence actually occurred after the initial contact of neighboring islands as the film planarizes with additional deposition.

Fabrication of 1D and 2D vertical nanomagnetic resonators

We have successfully fabricated the first resonant magnetic nanostructures exhibiting a negative permeability in the midinfrared. These metal-dielectric structures exhibit local resonances based on metallic inductive-capacitive effects that are controlled by the dimensions of the individual nanostructures and are independent of the periodicity, which is much smaller than the resonance wavelengths. Compared to other commonly used structures for obtaining negative permeability in the microwave and THz regions which are based on a planar fabrication paradigm, this structure is oriented vertically so that the smallest features are controlled by deposition rather than by lithography.

Nano-patterned isotropic nonlinear material for second harmonic generation without phase matching

Second-harmonic generation without the need for phase matching from a nanopatterned isotropic nonlinear material located inside the subwavelength gaps of a metallic coaxial array is reported. Numerical simulations are in good agreement with experimental data.