Efficient multiband absorber based on one-dimensional periodic metal-dielectric photonic crystal with a reflective substrate.

We propose an efficient multiband absorber comprised of a truncated, one-dimensional periodic metal-dielectric photonic crystal and a reflective substrate. The reflective substrate is essentially an optically thick metallic film. Such a planar device is easier to fabricate compared to absorbers with complicated shapes. For a four-unit cell device, all four of the absorption peaks can be optimized with efficiencies higher than 95 percent. Moreover, those absorption peaks are insensitive to the polarization and incident angle. The influences of the geometrical parameters and the refractive index of the dielectric on the device performance also are discussed. Furthermore, we found that the number of absorption peaks within each photonic band precisely corresponds to the number of unit cells because the truncated photonic crystal lattices select resonant modes. We also show that the total absorption efficiency gradually increases when there are more periods of the metal-dielectric composite layer placed on top of the metallic substrate. We expect this work to have potential applications in solar energy harvesting and thermal emission tailoring.

Infrared perfect absorber based on nanowire metamaterial cavities.

An infrared perfect absorber based on a gold nanowire metamaterial cavities array on a gold ground plane is designed. The metamaterial made of gold nanowires embedded in an alumina host exhibits an effective permittivity with strong anisotropy, which supports cavity resonant modes of both electric dipole and magnetic dipole. The impedance of the cavity modes matches the incident plane wave in free space, leading to nearly perfect light absorption. The incident optical energy is efficiently converted into heat so that the local temperature of the absorber will increase. Results show that the designed absorber is polarization-insensitive and nearly omnidirectional for the incident angle.

Graphene nano-ribbon waveguides of record-small mode area and ultra-high effective refractive indices for future VLSI.

Electronics circuits keep shrinking in dimensions, as requested by Moore's law. In contrast, photonic waveguides and circuit elements still have lateral dimensions on the order of the wavelength. A key to make photonics have a microelectronics-like development is a drastic reduction of size. To achieve this, we need a low-loss nanoscale waveguide with a drastically reduced mode area and an ultra-high effective refractive index. For this purpose, we propose here several low-loss waveguide structures based on graphene nano-ribbons. An extremely small mode area (~10(-7)λ(0)(2), one order smaller than the smallest mode area of any waveguide that has ever been reported in the literature; here λ(0) is the operating wavelength in vacuum) and an extremely large effective refractive index (several hundreds) are achieved. As a device example, a nano-ring cavity of ultra-small size (with a diameter of ~10(-2)λ(0)) is designed. Our study paves the way for future VLSI (very-large-scale integration) optoelectronics.

Giant transverse optical forces in nanoscale slot waveguides of hyperbolic metamaterials.

Here we demonstrate that giant transverse optical forces can be generated in nanoscale slot waveguides of hyperbolic metamaterials, with more than two orders of magnitude stronger compared to the force created in conventional silicon slot waveguides, due to the nanoscale optical field enhancement and the extreme optical energy compression within the air slot region. Both numerical simulation and analytical treatment are carried out to study the dependence of the optical forces on the waveguide geometries and the metamaterial permittivity tensors, including the attractive optical forces for the symmetric modes and the repulsive optical forces for the anti-symmetric modes. The significantly enhanced transverse optical forces result from the strong optical mode coupling strength between two metamaterial waveguides, which can be explained with an explicit relation derived from the coupled mode theory. Moreover, the calculation on realistic metal-dielectric multilayer structures indicates that the predicted giant optical forces are achievable in experiments, which will open the door for various optomechanical applications in nanoscale, such as optical nanoelectromechanical systems, optical sensors and actuators.

Revealing the truth about 'trapped rainbow' storage of light in metamaterials.

It was claimed that the incident light can be gradually slowed down and finally trapped in a tapered metamaterial waveguide. Here we show that the energy incident from the input port of the tapered metamaterial waveguide will be totally reflected (instead of being trapped) due to the strong intermodal coupling between the forward and backward modes. The underlying physical mechanism for this strong intermodal-coupling is given. The occurrence of energy reflection is unambiguously proved using several independent methods, (1) the semi-analytical mode matching technique, (2) the numerical finite element simulation, (3) the requirement of energy and momentum conservation, and (4) an experimental verification at microwave frequency. The dream of 'trapped rainbow' for storage of light might still be possible if this intermodal coupling could be blocked, and our study provides a useful guidance for such an endeavor.

Optical field enhancement in nanoscale slot waveguides of hyperbolic metamaterials.

Nanoscale slot waveguides of hyperbolic metamaterials are proposed and demonstrated for achieving large optical field enhancement. The dependence of the enhanced electric field within the air slot on waveguide mode coupling and permittivity tensors of hyperbolic metamaterials is analyzed both numerically and analytically. Optical intensity in the metamaterial slot waveguide can be more than 25 times stronger than that in a conventional silicon slot waveguide, due to tight optical mode confinement enabled by the ultrahigh refractive indices supported in hyperbolic metamaterials. The electric field enhancement effects are also verified with the realistic metal-dielectric multilayer waveguide structure.

Sub-µm2 power splitters by using silicon hybrid plasmonic waveguides.

Nano-scale power splitters based on Si hybrid plasmonic waveguides are designed by utilizing the multimode interference (MMI) effect as well as Y-branch structure. A three-dimensional finite-difference time-domain method is used for simulating the light propagation and optimizing the structural parameters. The designed 1 × 2 50:50 MMI power splitter has a nano-scale size of only 650 nm × 530 nm. The designed Y-branch power splitter is also very small, i.e., about 900 nm × 600 nm. The fabrication tolerance is also analyzed and it is shown that the tolerance of the waveguide width is much larger than±50 nm. The power splitter has a very broad band of over 500 nm. In order to achieve a variable power splitting ratio, a 2×2 two-mode interference coupler and an asymmetric Y-branch are used and the corresponding power splitting ratio can be tuned in the range of 97.1%:2.9%-1.7%:98.3% and 84%:16%-16%:84%, respectively. Finally a 1×4 power splitter with a device footprint of 1.9 µm × 2.6 µm is also presented using cascaded Y-branches.