Huikan Liu

Subwavelength imaging opportunities with planar uniaxial anisotropic lenses

The imaging properties of a uniaxial anisotropic slab lens, where the dielectric tensor components are of the opposite sign, are studied as a function of the structure parameters. While hypothetical parameters yield various levels of performance, a design principle to achieve good subwavelength resolution is suggested. The anisotropy can be implemented with a metal-insulator stack.

Subwavelength imaging with nonmagnetic anisotropic bilayers

A nonmagnetic compensating bilayer imaging system having two uniaxially anisotropic layers is described. Evanescent fields in vacuum are converted to propagating waves in the bilayer, and one layer compensates the phase progression of the other. In this way, subwavelength imaging performance is possible with substantial flexibility in operating wavelength. Conditions for enhanced resolution and sensitivity to the material parameters are examined with a view to fabrication.

Resonance cones in cylindrically anisotropic metamaterials: a Green’s function analysis

A Greens function analysis for cylindrically anisotropic media is presented that can be used in the design of various metamaterial devices. The resonance cones, which describe the direction of power flow, result from the Greens function singularity when the permittivity tensor elements have opposite signs. Shadow and accessible regions for the source are also identified.

Approximate Green's function for a uniaxially anisotropic metamaterial slab, and its application in analyzing a spectrometer.

An analytic solution for a magnetic line source imaged through a uniaxially anisotropic slab is derived for electric fields, and example results are supported by finite element simulations. The analytical result provides a convenient means to evaluate the effect of material parameters in applications. We introduce a spectrometer that can be realized by illuminating a slab composed of a Ag/ZnO multilayer stack through a small aperture, and we use this example to evaluate the performance of the analytic model.

Imaging performance of an isotropic negative dielectric constant slab

The influence of material and thickness on the subwavelength imaging performance of a negative dielectric constant slab is studied. Resonance in the plane-wave transfer function produces a high spatial frequency ripple that could be useful in fabricating periodic structures. A cost function based on the plane-wave transfer function provides a useful metric to evaluate the planar slab lens performance, and using this, the optimal slab dielectric constant can be determined.

Resonance cone formation in a curved cylindrically anisotropic metamaterial film

Resonance cones, the regions where major power and high-intensity fields are concentrated, form with cylindrically anisotropic media when the permittivity tensor elements have opposite signs. The resonance cones inside a circular layer of cylindrically anisotropic material is shown to experience multiple internal reflections from the layer boundaries. We introduce a spectrometer class by exploiting the dispersive properties of a metal-insulator stack metamaterial. The cones can exhibit negative refraction at the interface of two such circular layers, leading to a far-field bilayer subwavelength imaging system with more flexibility in the material parameter and operating wavelength spaces.

Leaky-wave antennas with anisotropic metamaterials

Leaky wave antennas can offer high directivity and frequency scanning [1]. An isotropic dielectric slab with refractive index n > 1 does not radiate effectively unless spatial variations are introduced to excite fast-wave space harmonics [1]. To realize uniform leaky wave structures, potentially in the optical domain, a metamaterial slab with n ≪ 1 has been proposed to achieve a narrow radiation beam [2, 3]. This approach suffers the limitation of large transverse slab dimensions, because the operating wavelength inside the slab is much greater than the free space wavelength, due to the small value of n [4]. Low refractive index metamaterials that require small material permeabilities face challenges of implementation [3]. In addition, radiation occurs in a narrow angular range between broadside and the critical angle sin−1(n), which limits beam scanning. Finally, we note that while a microstrip transmission line loaded with series capacitors and shunt inductors has been studied to realize metamaterial leaky-wave antennas in the microwave frequency regime [5], identifying the optical counterparts is difficult. To overcome these limitations, we present a forward/backward radiating leaky waveguide at optical wavelengths based on nonmagnetic uniaxially anisotropic metamaterials having permittivity tensors in which one component is different from the others in sign [6]. The resulting hyperbolic dispersion relation enables radiation from backfire (−90°) to endfire (90°), and the slab thickness can be small relative to the free space wavelength.

An anisotropic metamaterial leaky waveguide

We propose a leaky optical waveguide achieved with a uniaxially anisotropic metamaterial that supports both forward and backward leaky waves. The backward leaky nature is exploited in a sub-diffraction imaging system.

Resonance Cones in Cylindrically Anisotropic Metamaterials: A Green's Function Analysis

A Greens function analysis for cylindrically anisotropic media is presented. Resonance cones result from the Greens function singularity when the permittivity tensor elements have opposite sign. This model will facilitate the design of various components.

Optical circuit elements from anisotropic films

Anisotropic films provide a means to achieve optical circuit elements that are relatively independent of incident angle. This observation opens up a variety of circuit synthesis methods for implementing optical devices with important properties. We showed filter and anti-reflection coating examples for TM polarization that are robust to variation in the angle of incidence. Duality would lead to equivalent series capacitors and shunt inductors for TE polarization, if a suitable anisotropic permeability could be achieved (e.g., ferromagnetic materials). Practical realizations, in addition to metals/dielectrics, could have other material options, for example, plasmonic/polaritonic materials and semiconductors, depending on the operating frequency [7].