Negative Effective Index Research Articles

Open cavity formed by a photonic crystal with negative effective index of refraction

An open cavity formed by three 60 degrees wedges of a photonic crystal with negative effective index is designed and studied. The quality factor of the open cavity can be larger than 2000. The influence of the interface termination on the resonant frequency and the quality factor is studied.

Superconducting metamaterials

Evanescent wave amplification has been predicted under the ideal condition that the index of refraction, n=−1+i0 precisely, but is difficult to observe in practice because current metamaterials suffer from high losses. We present experimental results on a metamaterial that employs superconducting Nb metals and low-loss dielectric materials. Results include transmission data on a wire, split-ring resonator, and a combination medium at temperatures between 4.2 and 297K. Evidence of negative effective permittivity, permeability, and a negative effective index passband are seen in the superconducting state between 50MHz and 18GHz. We find a dielectric loss of εeff,2=2.6×10−3 in a superconducting wire array at 10.75GHz.

A completely open cavity realized with photonic crystal wedges

A completely open cavity, which is formed by three 60-degree wedges of a photonic crystal with negative effective index, is introduced. Such a realizable design for a completely open cavity (i.e., without any reflective wall in the radial direction) is the first of its kind ever been reported. Due to the negative effective refraction index of the photonic crystal and the high transmission at the photonic crystal/air interfaces, a closed path with zero optical path is formed for the resonance. The present open cavity is very suitable for use as a biosensor as it has large air wedges into which a liquid measurand can flow easily.

Negative refraction in a photonic crystal with a metallic cross lattice basis

A metamaterial with a negative effective index of refraction is made from a two-dimensional square lattice photonic crystal with a metallic cross lattice basis. A simple procedure is given to design a negative index band within a desired frequency range. The operating frequency is made sufficiently low so as to avoid high orders of diffraction from a slab. An effective index of $\ensuremath{-}1$, which is a requirement for perfect lensing, is designed. In addition, the structure is optimized to exhibit the largest possible bandwidth of negative refraction, while ensuring an isotropic response and efficient coupling from free space. Simulation results show negative refraction of a Gaussian beam through a prism of such a metamaterial. The simplicity and versatility of the structure make it a suitable candidate for frequencies into the infrared region. The dispersive nature of the metallic crosses and planar fabrication are also discussed.

TE and TM guided modes in an air waveguide with negative-index-material cladding

We numerically demonstrate that a planar waveguide in which the inner layer is a gas with refractive index n0 = 1, sandwiched between two identical semi-infinite layers of a negative index material, can support both transverse electric and transverse magnetic guided modes with low losses. Recent developments in the design of metamaterials with an effective negative index suggest that this waveguide could operate in the infrared region of the spectrum.

Effective negative refraction index in periodic metal–ferrite–metal film composite

We analyze the refraction index of periodic metal–ferrite–metal film composite in microwave frequencies by the transfer matrix method. We show that the composite can be equal to a homogeneous material with a negative effective refraction index in certain frequencies. Two negative index states have been found in the composite. One has the negative effective permittivity and negative effective permeability simultaneously, and the other only has negative effective permeability. The transition between them can be controlled by the values of electron damping coefficient in the metal films.

Isotropic three-dimensional left-handed metamaterials

We investigate three-dimensional left-handed and related meta-materials based on a fully symmetric multi-gap single-ring SRR design and crossing continuous wires. We demonstrate isotropic transmission properties of a SRR-only meta-material and the corresponding left-handed material which possesses a negative effective index of refraction due to simultaneously negative effective permeability and permittivity. Minor deviations from complete isotropy are due to the finite thickness of the meta-material.

Multifocal flat lens with left-handed metamaterial

We show experimental results demonstrating multiple focal lengths at microwave frequencies in a flat lens constructed of left-handed metamaterial (LHM). In contrast to conventional lenses, which are constructed of materials with positive index and require a curved surface or inhomogeneous structures to focus light or other electromagnetic radiation, no curvature is needed with a LHM because of a negative effective index of refraction. Such a flat lens has the advantage of being capable of changing the focal length by simply changing the distance between the electromagnetic source and the lens.

Analysis of wave propagation in a two-dimensional photonic crystal with negative index of refraction: plane wave decomposition of the Bloch modes

This work presents a comprehensive analysis of electromagnetic wave propagation inside a two-dimensional photonic crystal in a spectral region in which the crystal behaves as an effective medium to which a negative effective index of refraction can be associated. It is obtained that the main plane wave component of the Bloch mode that propagates inside the photonic crystal has its wave vector k' out of the first Brillouin zone and it is parallel to the Poynting vector ( S' ? k'> 0 ), so light propagation in these composites is different from that reported for left-handed materials despite the fact that negative refraction can take place at the interface between air and both kinds of composites. However, wave coupling at the interfaces is well explained using the reduced wave vector ( k' ) in the first Brillouin zone, which is opposed to the energy flow, and agrees well with previous works dealing with negative refraction in photonic crystals.

NEGATIVE REFRACTION AND SUBWAVELENGTH FOCUSING USING PHOTONIC CRYSTALS

In this article, we present an experimental and numerical study of novel optical properties of two-dimensional dielectric photonic crystals (PCs) which exhibit negative refraction. We investigate two mechanisms which utilize the band structure of the PC to generate a negative effective index of refraction (neff<0) and demonstrate the negative refraction experimentally. To the isotropic extend of neff, different PC slab structures are employed to focus the radiation of a point source. It is shown experimentally that the PC can generate an image of the source with subwavelength resolution in the vicinity of the PC interface. Using a different PC, one can also obtain a far field focusing. In the latter case, we explicitly show the flat lens behavior of the structure. These examples indicate that PC-based lenses can surpass limitations of conventional lenses and lead to novel optics applications.

Negative refraction of acoustic waves in two-dimensional phononic crystals

Negative refraction of acoustic waves in two-dimensional phononic crystals has been demonstrated through both analysis and exact numerical simulation. The methods to achieve this behavior have been discussed. A microsuperlens for acoustic waves has also been designed. It is shown that refractive devices based on phononic crystals behave in a manner similar to that of optical systems. Therefore, a negative square root of the effective density or negative refraction index for acoustic waves can be introduced to describe this phenomena very well as the case of electromagnetic waves in the photonic crystals.

Left-handed material containing spherical and nonspherical metallic and magnetic particles

We investigate a composite medium consisting of two types of dispersive particles. By invoking the effective medium approximation both the effective permittivity and permeability are derived separately in the limit of the long wavelength, and thus the effective refractive index can be obtained. We realize the left-handed property of the composite characterized by the negative effective index through the suitable control of the volume fraction. Then, we pay attention to the effective refractive index of the composite material consisting of two components with different shape in the low frequency limit. Numerical results show that the shape of granular inclusions has a large influence on the effective refractive index, and the oblate ellipsoid will be more useful for preparing the left-handed material than the spherical one. Moreover, by adjusting the shape of granular inclusions, a transition of the material properties between the left- and right-handed can be realized.

Time-Domain Measurement of Negative Group Delay in Negative-Refractive-Index Transmission-Line Metamaterials

We have simulated and constructed a one-dimensional metamaterial composed of a periodically loaded transmission line that exhibits both negative and positive group velocities in a band of effective negative index of refraction. The negative group velocity or, equivalently, the negative group delay, is demonstrated theoretically and experimentally in the time domain using modulated Gaussian pulses. Due to this negative delay, we can show an output pulse peak emerging from the loaded transmission line prior to the input peak entering the line, i.e., the output pulse precedes the input pulse. The fact that this surprising behavior does not violate the requirements of relativistic causality is illustrated with time-domain simulations, which show that discontinuities in the pulse waveforms are traveling at exactly the speed of light in vacuum. The pulse-reshaping mechanism underlying this behavior is also illustrated using time-domain simulations.

Experimental and theoretical analysis of the self-focusing of light by a photonic crystal lens

We demonstrate the self-focusing of electromagnetic waves in the microwave regime by using a two-dimensional photonic crystal lens with rectangular shape. A detailed study of the band structure shows that the focusing is due to the refractivelike behavior of the photonic crystal with a negative effective index of refraction in the frequency range under study. In contrast to lenses based on homogeneous isotropic materials, it is shown that the symmetry of the mode and the direction of incidence play a fundamental role in the performance of the photonic crystal lens.

Negative refraction without negative index in metallic photonic crystals

It is shown that certain metallic photonic crystals can enable negative refraction and subwavelength imaging without relying on a negative effective index. These metallic structures are very simple in design and appear straightforward for fabrication. Their unusual electromagnetic response should provide an interesting comparison with the metallic left-handed materials.

All-angle negative refraction without negative effective index

We describe an all-angle negative refraction effect that does not employ a negative effective index of refraction and involves photonic crystals. A few simple criteria sufficient to achieve this behavior are presented. To illustrate this phenomenon, a microsuperlens is designed and numerically demonstrated.

Negative refraction in photonic crystals

We demonstrate that light propagation in strongly modulated 2D/3D photonic crystals (PhCs) becomes refraction-like in the vicinity of the photonic bandgap, which is contrary to the fact that light propagation in weakly modulated PhCs is very different from refraction and thus the definition of refraction index becomes meaningless. Such a crystal behaves like a material having an effective refractive index controllable by the band structure. This situation is analogous to the effective-mass approximation in electron-band theory. The propagation states having a negative effective index exhibit unusual properties, such as mirror-like imaging effect, image-transfer effect. These properties are confirmed by finite-difference time-domain simulations.

Buried-ridge striped planar GaAlAs/GaAs lasers with a wide range of effective index steps

A novel self-aligned buried-ridge striped planar (BRSP) GaAlAs/GaAs diode laser structure is presented. The structure was grown by two-step liquid phase epitaxy using preferential growth and melt etch. A Ga1−y Aly As layer with a wide range of Al concentration (y>0.05) is selectively grown outside the ridge and can be an effective melt-etching mask. As a result, the BRSP structure can incorporate virtually all effective index steps, i.e., the positive, negative, and complex index steps. The threshold current of the BRSP structure is between 70 and 100 mA. The BRSP structure may prove to be of importance in the realization of linear array lasers.

Leaky-guided channeled substrate planar (LCSP) laser with reduced substrate radiation and heating

A semiconductor laser design, the leaky-guided channeled substrate planar (LCSP) structure, is presented. The structure with a buffer layer in the region outside the channel provides a built-in negative effective index step. The transverse mode is well damped in the buffer layer, and the substrate radiation in the region outside the channel is virtually eliminated. As a result, the significant heating and facet degradation in the region outside the channel that is related to the substrate radiation in conventional CSP lasers is minimized. The calculated local temperature rise in the region outside the channel of the CSP laser is about 45 degrees C for an output power level of 100 mW and is in reasonably good agreement with previously experimental results. Analysis using the complex domain effective index method shows that the LCSP structure provides better lateral mode discrimination than either gain-guided or positive index step structures. The LCSP structure with reduced heating and strong lateral coupling effect may be useful for high-power linear array lasers. >