Showing papers by "Xiang Zhang published in 2008"

Plasmon-Induced Transparency in Metamaterials

Abstract: A plasmonic "molecule" consisting of a radiative element coupled with a subradiant (dark) element is theoretically investigated. The plasmonic molecule shows electromagnetic response that closely resembles the electromagnetically induced transparency in an atomic system. Because of its subwavelength dimension, this electromagnetically induced transparency-like molecule can be used as a building block to construct a "slow light" plasmonic metamaterial.

Three-dimensional optical metamaterial with a negative refractive index

Abstract: Metamaterials are artificially engineered structures that have properties, such as a negative refractive index, not attainable with naturally occurring materials. Negative-index metamaterials (NIMs) were first demonstrated for microwave frequencies, but it has been challenging to design NIMs for optical frequencies and they have so far been limited to optically thin samples because of significant fabrication challenges and strong energy dissipation in metals. Such thin structures are analogous to a monolayer of atoms, making it difficult to assign bulk properties such as the index of refraction. Negative refraction of surface plasmons was recently demonstrated but was confined to a two-dimensional waveguide. Three-dimensional (3D) optical metamaterials have come into focus recently, including the realization of negative refraction by using layered semiconductor metamaterials and a 3D magnetic metamaterial in the infrared frequencies; however, neither of these had a negative index of refraction. Here we report a 3D optical metamaterial having negative refractive index with a very high figure of merit of 3.5 (that is, low loss). This metamaterial is made of cascaded 'fishnet' structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. We construct a prism made of this optical NIM to demonstrate negative refractive index at optical frequencies, resulting unambiguously from the negative phase evolution of the wave propagating inside the metamaterial. Bulk optical metamaterials open up prospects for studies of 3D optical effects and applications associated with NIMs and zero-index materials such as reversed Doppler effect, superlenses, optical tunnelling devices, compact resonators and highly directional sources.

A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation

Abstract: The emerging field of nanophotonics1 addresses the critical challenge of manipulating light on scales much smaller than the wavelength. However, very few feasible practical approaches exist at present. Surface plasmon polaritons2,3 are among the most promising candidates for subwavelength optical confinement3,4,5,6,7,8,9,10. However, studies of long-range surface plasmon polaritons have only demonstrated optical confinement comparable to that of conventional dielectric waveguides, because of practical issues including optical losses and stringent fabrication demands3,11,12,13. Here, we propose a new approach that integrates dielectric waveguiding with plasmonics. The hybrid optical waveguide consists of a dielectric nanowire separated from a metal surface by a nanoscale dielectric gap. The coupling between the plasmonic and waveguide modes across the gap enables ‘capacitor-like’ energy storage that allows effective subwavelength transmission in non-metallic regions. In this way, surface plasmon polaritons can travel over large distances (40–150 µm) with strong mode confinement (ranging from λ2/400 to λ2/40). This approach is fully compatible with semiconductor fabrication techniques and could lead to truly nanoscale semiconductor-based plasmonics and photonics. Xiang Zhang and colleagues from the University of California, Berkeley, propose a new approach for confining light on scales much smaller than the wavelength of light. Using hybrid waveguides that incorporate dielectric and plasmonic waveguiding techniques, they are able to confine surface plasmon polaritons very strongly over large distances. The advance could lead to truly nanoscale plasmonics and photonics.

Superlenses to overcome the diffraction limit.

Abstract: The resolution of conventional optical instruments is limited to length scales of roughly the wavelength of the light used. Nanoscale superlenses offer a solution for achieving much higher resolutions that may find appllications in many imaging areas.

Optical Negative Refraction in Bulk Metamaterials of Nanowires

Abstract: Negative refraction in metamaterials has generated great excitement in the scientific community. Although negative refraction has been realized in microwave and infrared by using metamaterials and by using two-dimensional waveguide structures, creation of a bulk metamaterial showing negative refraction at visible frequency has not been successful, mainly because of the significant resonance losses and fabrication difficulties. We report bulk metamaterials made of nanowires that show such negative refraction for all incident angles in the visible region. Moreover, the negative refraction occurs far from any resonance, resulting in a low-loss and a broad-band propagation at visible frequencies. These remarkable properties can substantially affect applications such as imaging, three-dimensional light manipulation, and optical communication.

First observation of the Greisen-Zatsepin-Kuzmin suppression.

Abstract: The High Resolution Fly's Eye (HiRes) experiment has observed the Greisen-Zatsepin-Kuzmin suppression (called the GZK cutoff) with a statistical significance of five standard deviations. HiRes' measurement of the flux of ultrahigh energy cosmic rays shows a sharp suppression at an energy of $6\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{eV}$, consistent with the expected cutoff energy. We observe the ankle of the cosmic-ray energy spectrum as well, at an energy of $4\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{eV}$. We describe the experiment, data collection, and analysis and estimate the systematic uncertainties. The results are presented and the calculation of the statistical significance of our observation is described.

Confinement and propagation characteristics of subwavelength plasmonic modes

Abstract: We have studied subwavelength confinement of the surface plasmon polariton modes of various plasmonic waveguides and examined their relative merits using a graphical parametric representation of their confinement and propagation characteristics. While the same plasmonic phenomenon governs mode confinement in all these waveguides, the various architectures can exhibit distinctive behavior in terms of effective mode area and propagation distance. We found that the waveguides based on metal and one dielectric material show a similar trade-off between energy confinement and propagation distance. However, a hybrid plasmon waveguide, incorporating metal, low index and high index dielectric materials, exhibits longer propagation distances for the same degree of confinement. We also point out that plasmonic waveguides with sharp features can provide an extremely strong local field enhancement, which is not necessarily accompanied by strong confinement of the total electromagnetic energy. In these waveguides, a mode may couple strongly to nearby atoms, but suffer relatively low propagation losses due to weak confinement.

Flying plasmonic lens in the near field for high-speed nanolithography

Abstract: The commercialization of nanoscale devices requires the development of high-throughput nanofabrication technologies that allow frequent design changes1,2. Maskless nanolithography3,4,5,6,7,8,9,10,11,12,13, including electron-beam and scanning-probe lithography, offers the desired flexibility but is limited by low throughput. Here, we report a new low-cost, high-throughput approach to maskless nanolithography that uses an array of plasmonic lenses that ‘flies’ above the surface to be patterned, concentrating short-wavelength surface plasmons into sub-100 nm spots. However, these nanoscale spots are only formed in the near field, which makes it very difficult to scan the array above the surface at high speed. To overcome this problem we have designed a self-spacing air bearing that can fly the array just 20 nm above a disk that is spinning at speeds of between 4 and 12 m s−1, and have experimentally demonstrated patterning with a linewidth of 80 nm. This low-cost nanofabrication scheme has the potential to achieve throughputs that are two to five orders of magnitude higher than other maskless techniques. Maskless nanolithography is a flexible nanofabrication technique but it suffers from low throughput. By developing a new approach that involves 'flying' an array of plasmonic lenses just 20 nm above a rotating surface, it is possible to increase throughput by several orders of magnitude.

Cloaking of matter waves.

Abstract: Invariant transformation for quantum mechanical systems is proposed. A cloaking of matter wave can be realized at given energy by designing the potential and effective mass of the matter waves in the cloaking region. The general conditions required for such a cloaking are determined and confirmed by both the wave and particle (classical) approaches. We show that it may be possible to construct such a cloaking system for cold atoms using optical lattices.

All-angle negative refraction and imaging in a bulk medium made of metallic nanowires in the visible region

Abstract: We theoretically demonstrated that all-angle negative refraction and imaging can be implemented by metallic nanowires embedded in a dielectric matrix. When the separation between the nanowires is much smaller than the incident wavelength, these structures can be characterized as indefinite media, whose effective permittivities perpendicular and parallel to the wires are opposite in signs. Under this condition, the dispersion diagram is hyperbolic for transverse magnetic waves propagating in the nanowire system, thereby exhibiting all-angle negative refraction. Such indefinite media can operate over a broad frequency range (visible to near-infrared) far from any resonances, thus they offer an effective way to manipulate light propagation in bulk media with low losses, allowing potential applications in photonic devices.

Observation of Stimulated Emission of Surface Plasmon Polaritons

Abstract: We report a direct experimental evidence of stimulated emission of surface plasmon polaritons (SPPs) at telecom wavelengths (1532 nm) with erbium doped glass as a gain medium. We observe an increase in the propagation length of signal surface plasmons when erbium ions are excited optically using pump SPP. The design, fabrication, and characterization of SPP waveguides, thin gold metal strips, embedded in erbium (Er) doped phosphate glass is presented. Such systems can be suitable as integrated devices coupling electronic and photonic data transmissions as well as SPP amplifiers and SPP lasers.

Contribution of electric quadrupole resonance in optical metamaterials

Abstract: Contribution of electric quadrupole resonance is studied in optical metamaterials through numerical simulation. For nanostructures, its radiation is often comparable to that from magnetic dipole. Their individual contributions can be determined by angle-resolved scattering spectroscopy.

Plasmonic nearfield scanning probe with high transmission

Abstract: Nearfield scanning optical microscopy (NSOM) offers a practical means of optical imaging, optical sensing, and nanolithography at a resolution below the diffraction limit of the light. However, its applications are limited due to the strong attenuation of the light transmitted through the subwavelength aperture. To solve this problem, we report the development of plasmonic nearfield scanning optical microscope with an efficient nearfield focusing. By exciting surface plasmons, plasmonic NSOM probes are capable of confining light into a 100 nm spot. We show by nearfield lithography experiments that the intensity at the near field is at least one order stronger than the intensity obtained from the conventional NSOM probes under the same illumination condition. Such a high efficiency can enable plasmonic NSOM as a practical tool for nearfield lithography, data storage, cellular visualization, and many other applications requiring efficient transmission with high resolution.

Manipulating optical rotation in extraordinary transmission by hybrid plasmonic excitations

Abstract: Polarized optical transmission properties through the L-shaped holes array in silver film was investigated at near infrared wavelength. Besides the enhanced transmission due to the combined plasmonic excitations, strong optical rotation was definitely observed at specific polarized incidences. After elaborate analyses, two eigenmodes were clearly characterized as the results of the hybrid localized plasmon resonances. Any polarization states from the incidences will degenerate into these two eigenstates after transmissions, suggesting a practical method to manipulate the polarization of light. Our result demonstrates the giant rotation rate achieved by the nanothin sample, indicating potential applications in the micro-optical devices.

Projecting deep-subwavelength patterns from diffraction-limited masks using metal-dielectric multilayers

Abstract: We utilize a metal-dielectric multilayer structure to generate deep-subwavelength one-dimensional and two-dimensional periodic patterns with diffraction-limited masks. The working wavelength and the pattern are set by the flexible design of the multilayer structure. This scheme is suitable to be applied to deep-subwavelength photolithography. As an example, we numerically demonstrate pattern periods down to 50nm under 405nm light illumination.

Magnetic resonance hybridization and optical activity of microwaves in a chiral metamaterial

Abstract: The propagation of microwaves through a chiral metamaterial based on a magnetic dimer is experimentally studied. As proposed by our previous theoretical model, two resonance peaks are obtained in the transmission spectrum; these originate from the hybridization effect of magnetic resonance modes in this system. Optical activity is also observed in the transmission wave. The polarization state dramatically changes around the resonance frequency: the transmitted wave becomes elliptically polarized with its major polarization axis approximately perpendicular to that of the linear incident wave. This coupled magnetic dimer system provides a practical method to optically design tunable active medium and device.

Ray Optics at a Deep-Subwavelength Scale: A Transformation Optics Approach

Abstract: We present a transformation optics approach for molding the light flow at the deep-subwavelength scale, using metamaterials with uniquely designed dispersion. By conformal transformation of the electromagnetic space, we develop a methodology for realizing subwavelength ray optics with curved ray trajectories. This enables deep-subwavelength-scale beams to flow through two- or three-dimensional spaces.

Positive selection acting on splicing motifs reflects compensatory evolution

Abstract: We have used comparative genomics to characterize the evolutionary behavior of predicted splicing regulatory motifs. Using base substitution rates in intronic regions as a calibrator for neutral change, we found a strong avoidance of synonymous substitutions that disrupt predicted exonic splicing enhancers or create predicted exonic splicing silencers. These results attest to the functionality of the hexameric motif set used and suggest that they are subject to purifying selection. We also found that synonymous substitutions in constitutive exons tend to create exonic splicing enhancers and to disrupt exonic splicing silencers, implying positive selection for these splicing promoting events. We present evidence that this positive selection is the result of splicing-positive events compensating for splicing-negative events as well as for mutations that weaken splice-site sequences. Such compensatory events include nonsynonymous mutations, synonymous mutations, and mutations at splice sites. Compensation was also seen from the fact that orthologous exons tend to maintain the same number of predicted splicing motifs. Our data fit a splicing compensation model of exon evolution, in which selection for splicing-positive mutations takes place to counter the effect of an ongoing splicing-negative mutational process, with the exon as a whole being conserved as a unit of splicing. In the course of this analysis, we observed that synonymous positions in general are conserved relative to intronic sequences, suggesting that messenger RNA molecules are rich in sequence information for functions beyond protein coding and splicing.

Fastanova: an efficient algorithm for genome-wide association study

Abstract: Studying the association between quantitative phenotype (such as height or weight) and single nucleotide polymorphisms (SNPs) is an important problem in biology. To understand underlying mechanisms of complex phenotypes, it is often necessary to consider joint genetic effects across multiple SNPs. ANOVA (analysis of variance) test is routinely used in association study. Important findings from studying gene-gene (SNP-pair) interactions are appearing in the literature. However, the number of SNPs can be up to millions. Evaluating joint effects of SNPs is a challenging task even for SNP-pairs. Moreover, with large number of SNPs correlated, permutation procedure is preferred over simple Bonferroni correction for properly controlling family-wise error rate and retaining mapping power, which dramatically increases the computational cost of association study.In this paper, we study the problem of finding SNP-pairs that have significant associations with a given quantitative phenotype. We propose an efficient algorithm, FastANOVA, for performing ANOVA tests on SNP-pairs in a batch mode, which also supports large permutation test. We derive an upper bound of SNP-pair ANOVA test, which can be expressed as the sum of two terms. The first term is based on single-SNP ANOVA test. The second term is based on the SNPs and independent of any phenotype permutation. Furthermore, SNP-pairs can be organized into groups, each of which shares a common upper bound. This allows for maximum reuse of intermediate computation, efficient upper bound estimation, and effective SNP-pair pruning. Consequently, FastANOVA only needs to perform the ANOVA test on a small number of candidate SNP-pairs without the risk of missing any significant ones. Extensive experiments demonstrate that FastANOVA is orders of magnitude faster than the brute-force implementation of ANOVA tests on all SNP pairs.

A hybrid plasmonic waveguide for sub-wavelength confinement and long range propagation

Abstract: We describe the coupling between surface plasmons and semiconductor fiber modes across a nano-scale gap leading to “capacitor-like” energy storage for sub-wavelength transmission and long range propagation. The approach could enable nano-scale semiconductor-based plasmonics.

Mining non-redundant high order correlations in binary data

Abstract: Many approaches have been proposed to find correlations in binary data. Usually, these methods focus on pair-wise correlations. In biology applications, it is important to find correlations that involve more than just two features. Moreover, a set of strongly correlated features should be non-redundant in the sense that the correlation is strong only when all the interacting features are considered together. Removing any feature will greatly reduce the correlation.In this paper, we explore the problem of finding non-redundant high order correlations in binary data. The high order correlations are formalized using multi-information, a generalization of pairwise mutual information. To reduce the redundancy, we require any subset of a strongly correlated feature subset to be weakly correlated. Such feature subsets are referred to as Non-redundant Interacting Feature Subsets (NIFS). Finding all NIFSs is computationally challenging, because in addition to enumerating feature combinations, we also need to check all their subsets for redundancy. We study several properties of NIFSs and show that these properties are useful in developing efficient algorithms. We further develop two sets of upper and lower bounds on the correlations, which can be incorporated in the algorithm to prune the search space. A simple and effective pruning strategy based on pair-wise mutual information is also developed to further prune the search space. The efficiency and effectiveness of our approach are demonstrated through extensive experiments on synthetic and real-life datasets.

An upper limit on the electron-neutrino flux from the HiRes detector

Abstract: Air-fluorescence detectors such as the High Resolution Fly's Eye (HiRes) detector are very sensitive to upward-going, Earth-skimming ultrahigh energy electron-neutrino-induced showers. This is due to the relatively large interaction cross sections of these high-energy neutrinos and to the Landau-Pomeranchuk-Migdal (LPM) effect. The LPM effect causes a significant decrease in the cross sections for bremsstrahlung and pair production, allowing charged-current electron-neutrino-induced showers occurring deep in the Earth's crust to be detectable as they exit the Earth into the atmosphere. A search for upward-going neutrino-induced showers in the HiRes-II monocular dataset has yielded a null result. From an LPM calculation of the energy spectrum of charged particles as a function of primary energy and depth for electron-induced showers in rock, we calculate the shape of the resulting profile of these showers in air. We describe a full detector Monte Carlo simulation to determine the detector response to upward-going electron-neutrino-induced cascades and present an upper limit on the flux of electron-neutrinos.

Magnetic plasmon modes in periodic chains of nanosandwiches

Abstract: The magnetic plasmon (MP) modes in periodic chains of metallic trilayer nanostructures (nanosandwich) have been investigated numerically in optical frequency region. By employing the Fourier Transformation (FT) method, the MP modes excited in these chains can be observed directly. We have also used different exciting sources to excite the MP modes in the chain so that we can get clearer physics picture and richer information of the nanosandwich chain. For their long propagating lengths, the nanosandwich chains can well work as subwavelength waveguides to transport electromagnetic field. And one can easily tune the working frequencies and band width of the MP modes by changing the parameters of these chains.

Design and Modeling of Selective Reinforcements for Integral Aircraft Structures

Abstract: A numerical simulation is presented in this paper on the performance of crack retarders bonded to integral metallic structures. The work is described in two main parts. First, a novel modeling approach employing the finite element method has been developed for simulating the various failure mechanisms of a bonded structure and for predicting fatigue crack growth life. Crack growth in the substrate and the substrate/strap interface disbond failure aremodeledintheframeworkoflinearelasticfracturemechanics.Acomputercodeinterfacingwiththecommercial package MSC NASTRAN has been developed and validated by experimental tests. Second, the effectiveness of differentstrapconfigurationsoncrackgrowthretardationhasbeenmodeled;theseincludedifferentstrapmaterials, strap dimensions, and their locations on the substrate. The research has included two substrate materials and four strap materials, and at this stage the specimens were cured at room temperature. Strap stiffness and adhesive toughness are found to be the most influential parameters in designing crack retarders. A design tool has been developedbasedonthe numericalsimulationto achieveoptimalcrackretarderdesigninterms ofprescribedfatigue life target and minimum structural weight added by the bonded reinforcement.