This paper presents a number of metamaterial (MTM) structures and devices, with emphasis on the so-called transmission line (TL) MTMs, which have lead to most practical applications so far. Both results obtained from 2002 to mid-2005 [1] and recent advances over the past few months are described.

In this paper, we present a method to realize a three dimensional (3D) homogeneous and isotropic negative index materials (3D-NIMs) fabricated using a low cost and massively parallel manufacturable microfabrication and microassembly technique. The construction of self-assembled 3D-NIM array was realized through two dimensional (2-D) planar microfabrication techniques exploiting the as-deposited residual stress imbalance between a bi-layer consisting of e-beam evaporated metal (650nm of chromium) and a structural layer of 500nm of low stress silicon nitride deposited by LPCVD on a silicon substrate.

A periodic continuation of a single rectangular unit cell consisting of split-ring resonators (SRR) and wires were fabricated to generate a 3D assembly by orienting them along all three Cartesian axes. The thin chromium and silicon nitride bi-layer is formed as hinges. The strain mismatch between the two layers curls the structural layer (flap) containing the SRR upwards. The self-assembled out-of-plane angular position depends on the thickness and material composing the bi-layer. This built-in stress-actuated assembly method is suitable for applications requiring a thin dielectric layer for the SRR. The split-ring resonators and other structures are created on the membrane which is then assembled into the 3-D configuration.

It has been experimentally demonstrated that a single layer of silver functions as a “superlens” [Fang et al, Science 308, 534 (2005)], providing image resolution much better than the diffraction limit. Resolution as high as 60 nanometer (λ/6) half-pitch was achieved. In this paper, we explore the possibility of further refining the image resolution using a “multilayer superlens” design. With optimized design of silver-alumina multilayer superlens, our numerical simulations show a feasibility of resolving 15nm features, about 1/26th of the illumination wavelength. We present preliminary experimental results targeted towards achieving the molecular scale imaging resolution. The development of potential low-loss and high resolution superlens opens the door to exciting applications in nanoscale optical metrology and nanomanufacturing.

We have developed a process for production of laterally continuous silver layers alternated with glassy polymer films in which the thickness is on the order of 15 nm and 100 nm respectively. Such films can be used to study physical phenomena associated with the coupled plasmon resonance and the resonant transmission in the forbidden bands. Such films may also find applications in photonic bandgap and other nanoplasmonic devices. Since the surface plasmon and evanescent coupling is a means to propagate light inside nanocircuits, the investigation of the coupled surface plasmon in the multilayer structures provides us with fundamental knowledge for the 3D integration. The fabrication technique also allows design flexibility, for example, systems with regions of single M-D-M plane together with multilayer structures facilitating tunable multiple plasmon resonance wavelength response from a single system. Multiple plasmon wavelength resonance absorptions may be obtained from such systems. Utilizing polymer films as the dielectric enables design flexibility and increases the number of applications of the fabricated devices.

Negative permeability of single-ring split ring resonator (s-SRR) is theoretically investigated in the visible light frequency region [1, 2]. To estimate magnetic responses of conductive elements precisely, we determined internal impedance by considering the delay of the current inside the metal structure. The increase of the surface resistivity, which is the real part of the internal impedance, results in the decrease of the resonator's Q-value. This means the degradation of the tunable range of the permeability. The increase of the internal reactance results in the reduction of the resonant frequency. In our calculation, the surface resistivity saturates at the inherent frequency of each metal as the frequency increases. On the silver case, the saturation value is 0.4 Ω and this value is remarkably smaller than that of gold and copper. On the other hand, the internal reactance is increasing as the frequency increases independently of metal. We concluded that the internal reactance is dominant factor to realize the negative magnetic permeability in the optical frequency region. We also show the frequency dependence of the magnetic permeability of s-SRRs. In the case of s-SRRs made of copper, the minimum value of the magnetic permeability becomes positive value at 550 THz even in the high filling factor condition (11%). In the case of s-SRRs made of gold, only on the filling factor was 11%, the minimum value of the magnetic permeability takes negative value in the entire visible range. On the other hand, the silver SRR exhibits negative magnetic permeability in the visible range even under the low filling factor condition of 3%. Moreover, we concluded that reducing the geometrical capacitance and using silver for SRR are necessary to realize the negative magnetic permeability in the visible light range.

We propose a quasi-periodic dendritic resonant magnetic model that can be used for the realization of the negative permeability at infrared frequencies. The numerical simulation exhibits a magnetic response when the incident light is perpendicular to the plane of the model. The copper dendritic structures are prepared by the electro-deposition method. And the transmission spectrum of this dendritic structure shows a magnetic response at infrared frequencies with a maximum transmission ¨C7.95dB. Further studies demonstrate that the magnetic response relate to the fractal dimension of the dendritic structure to a certain extent.

Conventional optical imaging systems cannot resolve the features smaller than approximately half the size of the working wavelength, called the diffraction limit. The superlens theory predicts that a flat lens made of an ideal material with negative permittivity and/or permeability is able to resolve features much smaller than working wavelength through the restoration of evanescent waves. We experimentally demonstrated the superlens concept for the first time using a thin silver slab in a quasi-static regime; a 60nm half-pitch object was imaged with 365nm illumination wavelength, λ/6 resolution, and the imaging of 50nm half-pitch object under the same light source, λ/7, was also reported. Here, we present mainly experimental studies of near-field optical superlens imaging.

We report a true left-handed (LH) behavior and focusing in a composite metamaterial consisting of periodically arranged split ring resonator (SRR) and wire structures. The magnetic resonance of the SRR structure is demonstrated by comparing the transmission spectra of SRRs with that of closed SRRs. We confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined left-handed transmission band with a peak value of -1.2 dB (–0.3 dB/cm) is obtained. We also report the transmission characteristics of a 2D composite metamaterial (CMM) structure in free space. At the frequencies where left-handed transmission takes place, we experimentally confirmed that the CMM structure has effective negative refractive index. Phase shift between consecutive numbers of layers of CMM is measured and phase velocity is shown to be negative at the relevant frequency range. Refractive index values obtained from the refraction experiments and the phase measurements are in good agreement and the experimental results agree extremely well with the theoretical calculations. By measuring the refracted electromagnetic (EM) waves from a LHM slab, we found an effective refractive index of -1.86. A 2D scanning transmission measurement technique was used to measure the intensity distribution of the electromagnetic (EM) waves that radiate from the point source. The flat lens behavior of a 2D CMM slab was demonstrated for two different point source distances of ds = 0.5ë and ë. The full width at half maximum of the focused beams is 0.36ë and 0.4ë, respectively, which are both below the diffraction limit.

In the quest for ever smaller, lighter weight, and conformal components and devices for radar and communication applications, researchers in the RF community have increasingly turned to artificially engineered, composite structures (or “metamaterials”) in order to exploit the extraordinary electromagnetic response these materials offer. One particularly promising class of metamaterials that has recently received a great deal of attention are “left-handed” or negative index materials. Because these metamaterials exhibit the unique ability to bend and focus light in ways no other conventional materials can, they hold great potential for enabling a number of innovative lens and antenna structures for a broad range of commercial and DoD relevant applications. Exploring the possible implementation of negative index materials for such applications will require significant enhancements in the properties of existing Negative Index Materials (NIM) (bandwidth, loss, operational frequency, etc.), as well as improved understanding of the physics of their electromagnetic transport properties. For this reason the Defense Advanced Research Project Agency (DARPA) has initiated a program that seeks to further develop and demonstrate NIM for future DoD missions including, but not limited to, the following: 1) lightweight, compact lenses with improved optics; 2) sub wavelength/high resolution imaging across the electromagnetic spectrum; 3) novel approaches to beam steering for radar, RF, and/or optical communications; and 4) novel approaches for integrating optics with semiconductor electronics. A brief overview of the salient properties of NIM will be presented as well as a general discussion of a few of their potential applications.

Negative index material is expected to exhibit interesting optical properties. Especially, superlens effect, which is predicted by John B. Pendry in 2000, is very attractive to overcome the diffraction limit in optical imaging [1]. Although there is no negative index material in nature, Pendry numerically suggested that several metals, only dielectric constant is negative at optical frequencies, behave like a superlens under the electrostatic limit and for the p-polarized fields. X. Zhang experimentally demonstrated this superlens effect by constructing nanolithography system with silver thin film in 2005 [2].

In this presentation, we newly propose a sub-wavelength imaging system at optical frequency regime in an array of metallic nanorods [3]. The near-field components of dipole sources were plasmonically transferred through the rod array to reproduce the image of the dipoles in the other side.

We calculated the field distribution at the different planes of imaging process using the finite-difference time-domain (FDTD) algorithm and found that the spatial resolution was 40 nm, which was much beyond the diffraction-limit and was limited by the array pitch. The typical configuration is a hexagonal arrangement with 40 nm periodicity of silver rods of 50 nm height and 20 nm diameter. The image formation highly depends on the coherence and the polarization of the dipole sources, array pitch, and the source-array distance. The principle of our near-field imaging is based on the longitudinal resonance of the localized surface plasmon along a metallic nanorod. The spectral responses of the device are also investigated.

In the homogenization of composite metamaterials the role played by the relative positions of the wires and resonators is not well understood, though essential. We present an effective medium approach which can systematically account for these effects. It involves independently homogenizing rows of wires and planes of resonators as slabs with negative permittivity and permeability respectively. The metamaterial is then treated as a 1D single negative anisotropic stack. Using this approach we show that it is in principle possible to satisfy the requirements of Pendry's superlens, [mu]=[epsilon]=-1 , up to losses. We propose a class of structure geometries which seems promising for achieving this holy grail of metamaterial science.