Andrey K. Sarychev

Plasmon modes and negative refraction in metal nanowire composites.

Optical properties of metal nanowires and nanowire composite materials are studied. An incident electromagnetic wave can effectively couple to the propagating surface plasmon polariton (SPP) modes in metal nanowires resulting in very large local fields. The excited SPP modes depend on the structure of nanowires and their orientation with respect to incident radiation. A nanowire percolation composite is shown to have a broadband spectrum of localized plasmon modes. We also show that a composite of nanowires arranged into parallel pairs can act as a left-handed material with the effective magnetic permeability and dielectric permittivity both negative in the visible and near-infrared spectral ranges.

Electromagnetic field fluctuations and optical nonlinearities in metal-dielectric composites

Abstract A scaling theory of local field fluctuations and optical nonlinearities is developed for random metal-dielectric composites near a percolation threshold. The theory predicts that in the optical and infrared spectral ranges the local fields are very inhomogeneous and consist of sharp peaks representing localized surface plasmons (s.p.). The localization maps the Anderson localization problem described by the random Hamiltonian with both on- and off-diagonal disorder. The local fields exceed the applied field by several orders of magnitudes resulting in giant enhancements of various optical phenomena. A new numerical method based on the developed theory is suggested. This method is employed to calculate the giant field fluctuations and enhancement of various optical processes in 2D metal-dielectric composites – semicontinuous metal films. The local field fluctuations appear to be highly correlated in space. These fluctuations result in dramatically enhanced Rayleigh and Raman light scattering. The scaling analysis is performed to describe the giant light scattering in a vicinity of the percolation threshold. The developed theory describes quantitatively enhancement of various nonlinear optical processes in percolation composites. It is shown that enhancement depends strongly on whether nonlinear multiphoton scattering includes an act of photon subtraction (annihilation). The magnitudes and spectral dependencies of enhancements in optical processes with photon subtraction, such as Raman and hyper-Raman scattering, Kerr refraction and four-wave mixing, are dramatically different from those processes without photon subtraction, such as sum-frequency and high-harmonic generation. Electromagnetic properties of metal-dielectric crystals and composites beyond the quasistatic approximation are also studied. Equations of macroscopic electromagnetism are presented for these systems. Both linear and nonlinear optical responses are considered in the case of a strong skin effect in metal grains. It is shown that the magnetic field undergoes giant spatial fluctuations. Scaling properties of the local magnetic field and its high-order moments are analyzed.

PLASMON MODES IN METAL NANOWIRES AND LEFT-HANDED MATERIALS

The electromagnetic eld distribution for thin metal nanowires is found, by using the discrete dipole approximation. The plasmon polariton modes in wires are numerically simulated. These modes are found to be dependent on the incident light wavelength and direction of propagation. The existence of localized plasmon modes and strong local eld enhancement in percolation nanowire composites is demonstrated. Novel left-handed materials in the near-infrared and visible are proposed based on nanowire composites.

Electrodynamics of metamaterials

Light is in a sense “one-handed” when interacting with atoms of conventional materials. This is because out of the two field components of light, electric and magnetic, only the electric “hand” efficiently probes the atoms of a material, whereas the magnetic component remains relatively unused because the interaction of atoms with the magnetic field component of light is normally weak. Metamaterials, i.e. artificial materials with rationally designed properties, can enable the coupling of both of the field components of light to meta-atoms, enabling entirely new optical properties and exciting applications with such “two-handed” light. Among the fascinating properties is a negative refractive index. The refractive index is one of the most fundamental characteristics of light propagation in materials. Metamaterials with negative refraction may lead to the development of a superlens capable of imaging objects and their fine structures that are much smaller than the wavelength of light. Other exciting applications of metamaterials include novel antennae with superior properties, optical nano-lithography and nano-circuits, and “meta-coatings” that can make objects invisible. The word “meta” means “beyond” in Greek, and in this sense the name “metamaterials” refers to “beyond conventional materials.” Metamaterials are typically man-made and have properties not available in nature. What is so magical about this simple merging of “meta” and “materials” that has attracted so much attention from researchers and has resulted in exponential growth in the number of publications in this area? The answer you can find in this book.

Negative refractive index in optics of metal-dielectric composites

Specially designed metal-dielectric composites can have a negative refractive index in the optical range. Specifically, it is shown that arrays of single and paired nanorods can provide such negative refraction. For pairs of metal rods, a negative refractive index has been observed at 1.5 µm. The inverted structure of paired voids in metal films can also exhibit a negative refractive index. A similar effect can be accomplished with metal strips in which the refractive index can reach −2. The refractive index retrieval procedure and the critical role of light phases in determining the refractive index are discussed.

Propogation of Surface Plasmons in Ordered and Disordered Chains of Metal Nanospheres

We describe two types of surface plasmons in ordered and disordered chains. The second kind is mediated by far-field interaction and is affected by Ohmic and radiative losses much less than the first kind.

Resonance transmittance through a metal film with subwavelength holes

An analytical theory for extraordinary light transmittance through an optically thick metal film with subwavelength holes is developed. It is shown that the film transmittance has sharp peaks that are due to the Maxwell-Garnet resonances in the holes. There are localized electric and magnetic resonances resulting in, respectively, dramatically enhanced electric and magnetic fields in the holes. A simple analytical expression for the resonance transmittance is derived that holds for arbitrary hole distribution. It is also shown that there are other types of transmittance resonances, when the holes are arranged into a regular lattice. These resonances occur because of the excitation of surface plasmon polaritons propagating over the film surface. A combination of the two kinds of resonances results in a rich spectral behavior in the extraordinary optical transmittance.

Surface-plasmon-enhanced radiation effects in confined photonic systems

Combined action of a resonant cavity and metal nanocomposites is shown to result in dramatic enhancement of spontaneous emisson rate in the optical spectral range. The cavity leads to modification of the optical density of states (Parcell effect), whereas the metal nanoparticles lead to giant local-field effects in spontaneous emission occurring because of surface-plasmon-enhanced vacuum field fluctuations. It is also shown that the surface-plasmon resonances may result in a full photonic gap in a metal-dielectric composite, even for a completely random structure of the composite. In our experiments, lasing at extremely low pump intensities, below 1 mW was observed for a novel class of optical materials, microcavities doped with fractal aggregates of metal nanoparticles.

METAL-DIELECTRIC COMPOSITE FILTERS WITH CONTROLLED SPECTRAL WINDOWS OF TRANSPARENCY

In this report we show the possibility of broadband low-pass filters with windows of transparency in pre-set spectral ranges. Those filters are based on the unique optical properties of metal-dielectric composites near the percolation threshold. In such composites, metal clusters of different sizes and shapes have different plasmon resonances and resonate nearly independently at different wavelengths. All together, the resonant plasmon modes cover a very broad spectral range. Applying the effect of spectrally selective photo-modification, we develop a procedure that creates mid-infrared windows of transparency within the broadband filters. We also investigate the possibilities of making low-pass filters composed of spheroids and conductive sticks with certain distributions of aspect ratios.

Loss and gain in metamaterials

Results of studies in the influence of losses on the superresolution achievability are presented. The studies involve modeling and realization of superresolution devices. It is found that rigorous electrodynamic models that are not based on homogenization of composites can be effectively used in modeling devices with metamaterials. The possibilities to compensate losses in near-optic metamaterials by means of active inclusions or active medium are discussed and the active material design is presented. Alternatively, what we believe to be new designs are suggested for applications where losses are desirable.