Mechanically controllable nonlinear dielectrics.

Strain-sensitive Ba x Sr1-x TiO3 perovskite systems are widely used because of their superior nonlinear dielectric behaviors. In this research, new heterostructures including paraelectric Ba0.5Sr0.5TiO3 (BSTO) and ferroelectric BaTiO3 (BTO) materials were epitaxially fabricated on flexible muscovite substrate. Through simple bending, the application of mechanical force can regulate the dielectric constant of BSTO from -77 to 36% and the channel current of BTO-based ferroelectric field effect transistor by two orders. The detailed mechanism was studied through the exploration of phase transition and determination of band structure. In addition, the phase-field simulations were implemented to provide theoretical support. This research opens a new avenue for mechanically controllable components based on high-quality oxide heteroepitaxy.

Author Correction: Sculpting nanoparticle dynamics for single-bacteria-level screening and direct binding-efficiency measurement.

The original version of this Article omitted the author Kuan Wang, who is from the 'College of Biomedical Engineering, Taipei Medical University, Taipei 11031, Taiwan' and 'Nanyang Environment & Water Research Institute, Nanyang Technological University, Singapore 637141, Singapore.'Also, the author S.H. Lim was incorrectly given as L.S. Hoi and A. Larsson was incorrectly given as A. Larson.The "Author contributions" was amended to reflect the authorship changes. It previously read 'Y.Z.S., C.-W.Q., and A.Q.L. jointly conceived the idea. Y.Z.S., S.X., Y.Z., J.B.Z., W.S., J.H.W., T.N.C., Z.C.Y., Y.L.H., B.L., P.H.Y., D.P.T., and C.-W.Q. performed the numerical simulations and theoretical analysis. Y.Z.S., S.X., and L.K.C. did the fabrication and experiments of particle hopping, biomolecule binding and flow cytometry. A.L. and L.S.H. did the SPR experiments. S.X., Y.Z.S., Y.Z., C.-W.Q., Y.-Y.C., L.K.C., T.H.Z., and A.Q.L. prepared the manuscript. S.X., Y.Z., C.-W.Q., and A.Q.L. supervised and coordinated all the work. All authors commented on the manuscript.' The correct version states 'B.L., K. W., P.H.Y.' instead of 'B.L., P.H.Y.' and 'S.H.L.' in place of 'L.S.H.'This has been corrected in both the PDF and HTML versions of the Article.

Sculpting nanoparticle dynamics for single-bacteria-level screening and direct binding-efficiency measurement.

Particle trapping and binding in optical potential wells provide a versatile platform for various biomedical applications. However, implementation systems to study multi-particle contact interactions in an optical lattice remain rare. By configuring an optofluidic lattice, we demonstrate the precise control of particle interactions and functions such as controlling aggregation and multi-hopping. The mean residence time of a single particle is found considerably reduced from 7 s, as predicted by Kramer's theory, to 0.6 s, owing to the mechanical interactions among aggregated particles. The optofluidic lattice also enables single-bacteria-level screening of biological binding agents such as antibodies through particle-enabled bacteria hopping. The binding efficiency of antibodies could be determined directly, selectively, quantitatively and efficiently. This work enriches the fundamental mechanisms of particle kinetics and offers new possibilities for probing and utilising unprecedented biomolecule interactions at single-bacteria level.

Effects of gain medium on the plasmonic enhancement of Forster resonance energy transfer in the vicinity of a metallic particle or cavity.

We perform theoretical studies on the plasmonic enhancement for the Forster resonance energy transfer (FRET) between a donor and an acceptor molecule in the vicinity of a metallic particle or cavity, with focus on the possible role of the addition of a clad layer of gain material can play in such a process. The results show that while the plasmonic resonances can be shifted with higher order plasmonic enhancements emerged in the presence of such a layer of gain material, optimal enhancement of the FRET rate can be achieved when gain just balances with the loss in the metal. This then leads to the existence of an optimal thickness for the gain material layer, for both particle and cavity enhancement. In addition, it is observed that the FRET efficiency can always be increased with the coating of the gain material even at the dipole plasmonic resonance when nonradiative transfer from the donor to the metal is high, provided that the gain level is not beyond a certain critical value.

Molecular fluorescence in the vicinity of a charged metallic nanoparticle.

The modified fluorescence properties of a molecule in the vicinity of a metallic nanoparticle are further studied accounting for the possible existence of extraneous charges on the particle surface. This is achieved via a generalization of the previous theory of Bohren and Hunt for light scattering from a charged sphere, with the results applied to the calculation of the various decay rates and fluorescence yield of the admolecule. Numerical results show that while charge effects will in general blue-shift all the plasmonic resonances of the metal particle, both the quantum yield and the fluorescence yield can be increased at emission frequencies close to that of the surface plasmon resonance of the particle due to the suppression of the nonradiative decay rate. This provides a possibility of further enhancing the particle-induced molecular fluorescence via the addition of surface charge to the metal particle.

Resonant transparency and non-trivial non-radiating excitations in toroidal metamaterials.

Engaging strongly resonant interactions allows dramatic enhancement of functionalities of many electromagnetic devices. However, resonances can be dampened by Joule and radiation losses. While in many cases Joule losses may be minimized by the choice of constituting materials, controlling radiation losses is often a bigger problem. Recent solutions include the use of coupled radiant and sub-radiant modes yielding narrow asymmetric Fano resonances in a wide range of systems, from defect states in photonic crystals and optical waveguides with mesoscopic ring resonators to nanoscale plasmonic and metamaterial systems exhibiting interference effects akin to electromagnetically-induced transparency. Here we demonstrate theoretically and confirm experimentally a new mechanism of resonant electromagnetic transparency, which yields very narrow isolated symmetric Lorentzian transmission lines in toroidal metamaterials. It exploits the long sought non-trivial non-radiating charge-current excitation based on interfering electric and toroidal dipoles that was first proposed by Afanasiev and Stepanovsky in [J. Phys. A Math. Gen. 28, 4565 (1995)].

Effects of extraneous surface charges on the enhanced Raman scattering from metallic nanoparticles.

Motivating by recent experiments on surface enhanced Raman scattering (SERS) from colloidal solutions, we present here a simple model to elucidate the effects of extraneous surface charges on the enhanced Raman signal. The model is based on the well-established Gersten-Nitzan model coupled to the modified Mie scattering theory of Bohren and Hunt in the long wavelength approximation. We further introduce corrections from the modified long wavelength approximation to the Gersten-Nitzan model for the improvement of its accuracy. Our results show that the surface charge will generally lead to a blueshift in the resonance frequency and greater enhancements in the SERS spectrum. Possible correlations with the recent experiments are elaborated.

Microelectromechanical Maltese-cross metamaterial with tunable terahertz anisotropy.

Dichroic polarizers and waveplates exploiting anisotropic materials have vast applications in displays and numerous optical components, such as filters, beamsplitters and isolators. Artificial anisotropic media were recently suggested for the realization of negative refraction, cloaking, hyperlenses, and controlling luminescence. However, extending these applications into the terahertz domain is hampered by a lack of natural anisotropic media, while artificial metamaterials offer a strong engineered anisotropic response. Here we demonstrate a terahertz metamaterial with anisotropy tunable from positive to negative values. It is based on the Maltese-cross pattern, where anisotropy is induced by breaking the four-fold symmetry of the cross by displacing one of its beams. The symmetry breaking permits the excitation of a Fano mode active for one of the polarization eigenstates controlled by actuators using microelectromechanical systems. The metamaterial offers new opportunities for the development of terahertz variable waveplates, tunable filters and polarimetry.

Transformation optofluidics for large-angle light bending and tuning.

Transformation optics is a new art of light bending by designing materials with spatially variable parameters for developing wave-manipulation devices. Here, we introduce a transformation optofluidic Y-branch splitter with large-angle bending and tuning based on the design of a spatially variable index. Differing from traditional splitters, the optofluidic splitter is achieved in an inhomogeneous medium by coordinate transformation. The designed bidirectional gradient index (GRIN) distribution can be achieved practically by the convection-diffusion process of liquid flowing streams. The transformation optofluidic splitter can achieve a much larger split angle with little bend loss than the traditional ones. In the experiments, a large tunable split angle up to 30° is achieved by tuning the flow rates, allowing optical signals to be freely transferred to different channels. Besides the symmetrical branch splitting, asymmetrical Y-branch splitting with approximately equal power splitting is also demonstrated by changing the composition of the liquids. The optofluidic splitter has high potential applications in biological, chemical and biomedical solution measurement and detection.

Equivalence between the mechanical model and energy-transfer theory for the classical decay rates of molecules near a spherical particle.

In the classical modeling of decay rates for molecules interacting with a nontrivial environment, it is well known that two alternate approaches exist which include: (1) a mechanical model treating the system as a damped harmonic oscillator driven by the reflected fields from the environment; and (2) a model based on the radiative and nonradiative energy transfers from the excited molecular system to the environment. While the exact equivalence of the two methods is not trivial and has been explicitly demonstrated only for planar geometry, it has been widely taken for granted and applied to other geometries such as in the interaction of the molecule with a spherical particle. Here we provide a rigorous proof of such equivalence for the molecule-sphere problem via a direct calculation of the decay rates adopting each of the two different approaches.

Optofluidic waveguide as a transformation optics device for lightwave bending and manipulation.

Transformation optics represents a new paradigm for designing light-manipulating devices, such as cloaks and field concentrators, through the engineering of electromagnetic space using materials with spatially variable parameters. Here we analyse liquid flowing in an optofluidic waveguide as a new type of controllable transformation optics medium. We show that a laminar liquid flow in an optofluidic channel exhibits spatially variable dielectric properties that support novel wave-focussing and interference phenomena, which are distinctively different from the discrete diffraction observed in solid waveguide arrays. Our work provides new insight into the unique optical properties of optofluidic waveguides and their potential applications.

Toroidal dipolar response in a metamaterial.

Toroidal multipoles are fundamental electromagnetic excitations different from those associated with the familiar charge and magnetic multipoles. They have been held responsible for parity violation in nuclear and particle physics, but direct evidence of their existence in classical electrodynamics has remained elusive. We report on the observation of a resonant electromagnetic response in an artificially engineered medium, or metamaterial, that cannot be attributed to magnetic or charge multipoles and can only be explained by the existence of a toroidal dipole. Our direct experimental evidence of the toroidal response brings attention to the often ignored electromagnetic interactions involving toroidal multipoles, which could be present in naturally occurring systems, especially at the macromolecule level, where toroidal symmetry is ubiquitous.

Pure angular momentum generator using a ring resonator.

This paper reports a pure angular momentum generator using a ring resonator surrounded by a group of nano-rods. The evanescent waves of the circulating light in the ring are scattered by the nano-rods and generate a rotating electromagnetic field, which has only angular momentum but no linear momentum along the axis of rotation. The angular order is determined by the difference between the order of Whispering Gallery mode and the number of the rods, the rotating frequency is equal to the light frequency divided by the angular order. The maximum amplitude of the rotating electromagnetic fields can be 10 times higher than the amplitude of the input field when there are 36 rods (R(rod) = 120 nm, nr = 1.6). The pure angular momentum generator provides a new platform for trapping and rotation of small particles.

Spectral collapse in ensembles of metamolecules.

We report on the first direct experimental demonstration of a collective phenomenon in metamaterials: spectral line collapse with an increasing number of unit cell resonators (metamolecules). This effect, which is crucial for achieving a lasing spaser, a coherent source of optical radiation fuelled by coherent plasmonic oscillations in metamaterials, is linked to the suppression of radiation losses in periodic arrays. We experimentally demonstrate spectral line collapse at microwave, terahertz and optical frequencies.

Dynamic modifications of polarizability for large metallic spheroidal nanoshells.

We present an approach alternative to the hybridization model for the treatment of the coupled interfacial plasmon modes in spheroidal metallic nanoshells. Rather than formulating the problem from the Lagrangian dynamics of the free electronic fluid, we adopt an effective medium approach together with the uniqueness of the solutions to electromagnetic boundary value problem, from which the polarizability of the shells can then be systematically and efficiently derived; and the resonance frequencies for the coupled modes can be obtained from the poles in the polarizability. This approach can treat confocal nanoshells with different geometries for the spheroidal cavity and external surface and allow for a natural extension to incorporate corrections from the finiteness of the optical wavelength which are important for nanoparticles of larger sizes. This thus surpasses the hybridization model which is limited to incorporate only the electrostatic Coulomb interaction between the uncoupled plasmons. Numerical results will be provided for different nanoshell systems, and for the illustration of the various geometric and dynamic effects from our model.

Light well: a tunable free-electron light source on a chip.

The passage of a free-electron beam through a nanohole in a periodically layered metal-dielectric structure creates a new type of tunable, nanoscale radiation source--a "light well". In the reported demonstration, tunable light is generated at an intensity of approximately 200 W/cm(2) as electrons with energies in the 20-40 keV range are injected into gold-silica well structures with a lateral size of just a few hundred nanometers.

Towards the lasing spaser: controlling metamaterial optical response with semiconductor quantum dots.

We report the first experimental demonstration of compensating Joule losses in metallic photonic metamaterial using optically pumped PbS semiconductor quantum dots.

Metamaterials: optical activity without chirality.

We report that the classical phenomenon of optical activity, which is traditionally associated with chirality (helicity) of organic molecules, proteins, and inorganic structures, can be observed in artificial planar media which exhibit neither 3D nor 2D chirality. We observe the effect in the microwave and optical parts of the spectrum at oblique incidence to regular arrays of nonchiral subwavelength metamolecules in the form of strong circular dichroism and birefringence indistinguishable from those of chiral three-dimensional media.

Study of the optical response of phase-change recording layer with zinc oxide nanostructured thin film.

Recently, use of nanostructured materials as a near-field optical active layer has attracted a lot of interest. The non-linear optical properties and strong enhancements of metallic oxide nanostructured thin films are key functions in applications of promising nanophotonics. For the importance of ultra-high density optical data storage, we continue investigating the ultra-high density recording property of near-field optical disk consisting of zinc oxide (ZnO(x)) nanostructured thin film. A carrier-to-noise ratio above 38 dB at a recording mark size of 100 nm can be obtained in the ZnO(x) near-field optical disk by a DVD driver tester directly. In this article, we use an optical pump-probe system (static media tester) to measure the optical response of a phase-change recording layer (Ge(2)Sb(2)Te(5)) and demonstrate the high contrast of optical recording with a ZnO(x) nanostructured thin film in short pulse durations. Also, we investigate the dependence of writing power and the optical response in conventional re-writable recording layers and the phase-change material with ZnO(x) nanostructured thin film.

Evanescent field enhancement due to plasmonic resonances of a metamaterial slab.

The characteristics of plasmonic resonance in a dielectric-sandwiched metamaterial film at visible wavelengths of 650 and 568 nm have been investigated (for both p- and s-polarized light). Our calculated results demonstrate that each mode of plasmonic resonance has maximum resonance strength at a particular film thickness of the metamaterial. We also demonstrated that the effect of evanescent field enhancement is due to plasmonic resonances of the sandwiched metamaterial system. And the stronger the plasmonic resonance strength the larger the evanescent field is enhanced at the interfaces of the metamaterial film. Also we see that the plasmonic resonances in a sandwiched metamaterial are influenced not only by the materials that constitute the interfaces but also by the thickness of surrounding dielectrics or distance between evanescent light source and metamaterial film. Finally, our results show that there might be an effective light propagation length that will let the coupling efficiency between evanescent light source and SPs resonance become a maximum. These properties of plasmonic resonances to structure parameters of metamaterial film and its surrounding dielectrics provide a useful way to control the optical responses of an optoelectronic device when the wavelength of light source is fixed. That is, by suitably choosing light polarizations, thickness of the metamaterial thin film or the surrounding dielectrics and the position of evanescent light source, it is possible to modulate the plasmonic resonance wavenumber or resonance strength of the system. Therefore, the optical responses of the system can be modulated. Our results will be helpful for the structure design to control the behaviours of coupled plasmonic resonances and consequently the optical properties of the dielectric-sandwiched metamaterial film.