Che Ting Chan

Transformation optics and metamaterials

Underpinned by the advent of metamaterials, transformation optics offers great versatility for controlling electromagnetic waves to create materials with specially designed properties. Here we review the potential of transformation optics to create functionalities in which the optical properties can be designed almost at will. This approach can be used to engineer various optical illusion effects, such as the invisibility cloak.

Photonic band gaps in three dimensions: New layer-by-layer periodic structures

Abstract A new three-dimensional (3D) periodic dielectric structure constructed with layers of dielectric rods of circular, elliptical, or rectangular shape is introduced. This new structure possesses a full photonic band gap of appreciable frequency width. At midgap, an attenuation of 21 dB per unit cell is obtained. This gap remains open for refractive indices n ≥ 1.9. Furthermore, this new 3D layer structure potentially has the additional advantage that it can be easily fabricated using conventional microfabrication techniques on the scale of optical wavelengths.

Acoustic cloaking in three dimensions using acoustic metamaterials

A scheme to achieve two dimensional (2D) acoustic cloaking is proposed by Cummer and Schurig [New J. Phys. 9, 45 (2007)] by mapping the acoustic equations to Maxwell’s equations of one polarization in the 2D geometry. We find that the acoustic equation can be mapped to the direct current conductivity equation in three dimensions, which then allows the design of three-dimensional acoustic cloaking using the coordinate transformation scheme. The perfect cloaking effect is confirmed by solving for the scattering problem using the spherical-Bessel function series expansion method.

Illusion Optics: The Optical Transformation of an Object into Another Object

We propose to use transformation optics to generate a general illusion such that an arbitrary object appears to be like some other object of our choice. This is achieved by using a remote device that can transform the scattered light outside a virtual boundary into that of the object chosen for the illusion, irrespective of the profile and direction of the incident light. This type of illusion device also enables people to see through walls. Our work extends the concept of cloaking as a special form of illusion to the wider realm of illusion optics.

Transformation media that rotate electromagnetic fields

The authors suggest a way to manipulate electromagnetic waves by introducing a rotation mapping of coordinates that can be realized by a specific transformation of the permittivity and permeability of a shell surrounding an enclosed domain. Inside the enclosed domain, the information from the outside will appear as if it is coming from a different angle. Numerical simulations were performed to illustrate these properties.

A transferable tight-binding potential for carbon

An interatomic potential for carbon is developed that is based on an empirical tight-binding approach. The model reproduces accurately the energy-versus-volume diagram of carbon polytypes and gives a good description of the phonons and elastic constants for carbon in the diamond and graphite structures. To test the transferability of the model to different environments further, the authors performed molecular-dynamics simulations to study the liquid phase and the properties of small carbon microclusters. The results obtained are in good agreement with those obtained from ab initio calculations.

Dirac cones induced by accidental degeneracy in photonic crystals and zero-refractive-index materials

A zero-refractive-index metamaterial is one in which waves do not experience any spatial phase change, and such a peculiar material has many interesting wave-manipulating properties. These materials can in principle be realized using man-made composites comprising metallic resonators or chiral inclusions, but metallic components have losses that compromise functionality at high frequencies. It would be highly desirable if we could achieve a zero refractive index using dielectrics alone. Here, we show that by employing accidental degeneracy, dielectric photonic crystals can be designed and fabricated that exhibit Dirac cone dispersion at the centre of the Brillouin zone at a finite frequency. In addition to many interesting properties intrinsic to a Dirac cone dispersion, we can use effective medium theory to relate the photonic crystal to a material with effectively zero permittivity and permeability. We then numerically and experimentally demonstrate in the microwave regime that such dielectric photonic crystals with reasonable dielectric constants manipulate waves as if they had near-zero refractive indices at and near the Dirac point frequency.

Directive emissions from subwavelength metamaterial-based cavities

We use experiment and theory to demonstrate a mechanism for directive emissions, which involves a double-plate resonance cavity made with metamaterials. In contrast to other mechanisms employing Fabry-Perot cavities, photonic crystals, or zero index materials, our system is significantly thinner than the working wavelength and requires a smaller lateral size. We show the physics to be governed by subwavelength resonance modes unique to such metamaterial-based cavities.

Experimental realization of photonic topological insulator in a uniaxial metacrystal waveguide

Photonic analogue of topological insulator was recently predicted by arranging ε/μ (permittivity/permeability)-matched bianisotropic metamaterials into two-dimensional superlattices. However, the experimental observation of such photonic topological insulator is challenging as bianisotropic metamaterial is usually highly dispersive, so that the ε/μ-matching condition can only be satisfied in a narrow frequency range. Here we experimentally realize a photonic topological insulator by embedding non-bianisotropic and non-resonant metacrystal into a waveguide. The cross coupling between transverse electric and transverse magnetic modes exists in metacrystal waveguide. Using this approach, the ε/μ-matching condition is satisfied in a broad frequency range which facilitates experimental observation. The topologically non-trivial bandgap is confirmed by experimentally measured transmission spectra and calculated non-zero spin Chern numbers. Gapless spin-filtered edge states are demonstrated experimentally by measuring the magnitude and phase of the fields. The transport robustness of the edge states is also observed when an obstacle was introduced near the edge.

Fabrication of large area two- and three-dimensional polymer photonic crystals using single refracting prism holographic lithography

We demonstrate an approach for easy fabrication of two-dimensional (2D) hexagonal and three-dimensional (3D) face-centered-cubic (fcc)-type photonic crystal (PhC) microstructures in a photosensitive polymer by applying a simple single refracting prism. This prism enables the splitting and recombining of a single incoming laser beam to form multiple-beam interference pattern simultaneously. Thus, antivibration equipment and complicated optical alignment system are not required, leading to a much more simple optical setup than previously reported laser holographic lithography techniques. Large-scale (over 1cm2) 2D hexagonal and 3D fcc-type PhCs have been produced. Reflection/transmission measurements performed on the fabricated 3D fcc-type PhC structures agree well with the corresponding band structure calculation.We demonstrate an approach for easy fabrication of two-dimensional (2D) hexagonal and three-dimensional (3D) face-centered-cubic (fcc)-type photonic crystal (PhC) microstructures in a photosensitive polymer by applying a simple single refracting prism. This prism enables the splitting and recombining of a single incoming laser beam to form multiple-beam interference pattern simultaneously. Thus, antivibration equipment and complicated optical alignment system are not required, leading to a much more simple optical setup than previously reported laser holographic lithography techniques. Large-scale (over 1cm2) 2D hexagonal and 3D fcc-type PhCs have been produced. Reflection/transmission measurements performed on the fabricated 3D fcc-type PhC structures agree well with the corresponding band structure calculation.