W. M. Saj

Energy transport in plasmon waveguides on chains of metal nanoplates

An interest in energy transport in 3D chains of metal nanoparticles is oriented towards future applications in nanoscale optical devices. We consider plasmonic waveguides composed of silver nanoplates arranged in several geometries to find the one with the lowest attenuation. We investigate light propagation of 500-nm wavelength along different chains of silver nanoplates of subwavelength length and width and wavelength-size height. Energy transmission of the waveguides is analysed in the range of 400–2000 nm. We find that chain of short parallel nanoplates guides energy better than two electromagnetically coupled continuous stripes and all other considered nonparallel structures. In a wavelength range of 500–600 nm, this 2-μm long 3D waveguide transmits 39% of incident energy in a channel of λ × λ/2 cross section area.

Light focusing on a stack of metal-insulator-metal waveguides sharp edge

Near field light focusing by two-dimensional isosceles triangle shaped stack of silver plasmon-polaritons waveguides is being investigated numerically with full-vectorial Finite Difference Time Domain method for H-polarized light and wavelength lambda = 500 nm. For wide angle of tip, results are in good agreement with theoretically predicted propagation constant of light in stack and while discrepancy becomes significant for smaller angle. Physical phenomena of refraction and interference, similar to ones in dielectric axicons lead to conversion of a Gaussian beam incident on the flat side of the stack into a narrow light jet behind the structure sharp edge. The beam is concentrated into long focal region of 0.37 lambda width and enhancement of field amplitude is achieved in spite of significant absorption in the structure. The results are compared with bulk dielectric structure.

Hollow-core photonic crystal fiber with square lattice

Most works on photonic crystal fibers with a photonic bandgap are concerned with structures made of silica glass with a hexagonal lattice. However, there are many other possible choices for the crystal structure of the fiber. In this paper, we study the optical properties of photonic bandgaps in a hollow-core photonic crystal fiber with a square lattice fabricated from multi-component glass. A composition of oxides was chosen to obtain a refractive index contrast higher than in fused silica fibers. The core size of the fiber is 11 microns and the cladding is made of an array of 17 x 17 air capillaries. A full-vector mode solver using the biorthonormal basis method is employed to analyze the modal properties of the fiber. We verify the guiding properties of the fiber by FDTD simulations. The transmission properties for several lengths of the fiber were measured by using broadband light from a nanosecond-pulse supercontinuum source and an optical spectrum analyzer. Preliminary results show that light is guided around 1650 nm. Possible modifications of the structure and potential applications will be discussed.

Optimization of optical transmittance of a layered metamaterial on active pairs of nanowires

Optical metamaterials with a negative value of the refractive index can be fabricated by means of patterning techniques developed for microelectronics. One of those is a layered metamaterial, where the electric and magnetic response comes from coupled parallel subwavelength size wires. We simulate propagation of EM waves through such a metamaterial. Its properties depend on the density of pairs of nanowires oriented in parallel in one layer. There is a tradeoff between high transmittance and large negative refractive index value n. The smaller is the density of nanowires; 1 masculine - the narrower the range of frequencies, where n is negative; 2 masculine - the less negative is n; 3 masculine - the higher is the transmission.

Beam shaping with nanooptical devices

We present a review of recent achievements in nanoscale optical devices based on energy transport with surface plasmon-polaritons and localized surface plasmons. Chains of metal subwavelength-size particles and stripes are used to build straight waveguides, s-bends, y-junctions and beam shaping devices. Strong enhancement of near-field in nanogaps between particles leads to efficient light emission from such nanoantennas. Development of surface plasmon nanoptics stimulates further progress in near-field imaging. To improve resolution of scanning near-field optical microscope (SNOM) it is necessary to improve light throughput in tapered metal-coated SNOM probes. This is achievable due to resonant surface plasmons that propagate in corrugated probes.

Plasmon waveguides on silver nanoelements

Interest in photonic nanodevices motivates search for efficient transport of energy in plasmon waveguides. Chains of silver nanoelements guide light in channels of below-the-diffraction-limit size due to surface plasmon coupling. We calculate attenuation factors in chains with several geometries of nanoplates using the Finite Difference Time Domain (FDTD) method for visible and near infrared range of wavelengths, where the Drude model of dispersion is valid. Nanoplates considered in simulations are 1 micrometer high, 50 nm thick and 380 nm long and are embedded in a medium with refractive index reaching n = 1.5. Advantages of proposed waveguides are connected with their small size and possible tuneability by adjustment of geometrical parameters. However, the waveguides highly attenuate signals due to radiation into the far field and internal damping. For the optimum considered geometry and 595 nm wavelength, the energy transmission of 2 micrometers long chain of parallel nanoplates reaches 39%.

Simulation of resonant behavior and negative refraction of metal nanowire composites

A metal-in-dielectric metamaterial structure different from that composed of split-ring-resonators and wire units was proposed. The metamaterial layer is composed of randomly distributed parallel pairs of nanowires of subwavelength size that form electromagnetically active units. It was predicted that the metamaterial should exhibit macroscopic negative refraction. In a recent paper fabrication of the metamaterial in the form of periodic array of parallel golden nanorods with trapezoidal cross section was reported and a negative refractive index of n = -0.3 was observed at a wavelength 1.5 μm (200 THz). In this paper we simulate response of a single pair of nanowires to near-infrared illumination and observe surface plasmon resonances using FDTD method. We simulate light propagation through the metamaterial slab made of one, two and three layers. In each layer the nanowires cover 10% of the surface. In simulations made for a single layer medium, negative refraction is observed for wavelengths from 1.55 to 2.1 μm, with Δλ/λ ≈ 0.3. When the number of layers increases, the range of negatively refracted wavelengths becomes narrower. For a narrow range of wavelengths that are close to the resonant frequency the intensity transmission of three layers reaches −7dB for the angle of incidence of 10°. Then layers with two orientations of nanowires are considered. In the first stack of layers all nanowires are oriented in parallel. This configuration assures plasmon resonances for both the electric and magnetic components of electromagnetic wave in all layers. In the second stack, nanowires in two subsequent layers are oriented perpendicularly. In the second layer, the plasmon resonance for the electric component of light is due to the oblique incidence of light. For a small angle of incidence of a near infrared narrow Gaussian beam we calculate two characteristics: the attenuation vs. wavelength and the lateral shift of the beam on the plane-parallel slab vs. wavelength. For a narrow range of wavelengths simulations show negative refraction of a beam incident the plane of the nanowires and a corresponding shift in the far field.

Plasmon Waveguides on Silver Nanoplates

Current interest in photonic nanodevices motivates search for efficient transport of energy in plasmon waveguides. Chains of nanoelements made of noble metals guide light in channels of below-the-diffraction-limit size due to surface plasmon coupling. We calculate attenuation factors in a chain of parallel pairs of nanoplates for visible and near infrared range of wavelengths. Nanoplates are embedded in a medium with refractive index reaching n = 1.5. Advantages of the proposed waveguide are its small size and tunability by adjustment of geometrical parameters. However, the waveguide highly attenuates signals due to radiation into the far field and internal damping. For 595 nm wavelength, the energy transmission of 2 micrometers long chain of parallel nanoplates reaches 39%.We use the finite difference time domain (FDTD) method to investigate properties of a waveguide illuminated with polarized light. The permittivity of silver is described by Drude model. The simulation area is 780times1400times2225 nm discretized with Deltar = 5 nm step surrounded by 10 layers of UPML absorbing conditions at all boundaries. Simulation time step is Deltat = 8.34middot10-18 s. The source has a Gaussian field distribution with half width 372 nm. The source is introduced with total/scattered field split of simulation volume. Theoretically, for the wavelength of 514.5 nm the intensity of surface plasmons propagating on an infinite smooth surface decreases to 37% of its initial value after 22 mum. Our result confirms the prediction that transmission in a waveguide made of discrete plates is considerably smaller. A sample result where z-axis component of the Poynting vector calculated at a cross-section plane of the waveguide at half height is depicted. The output beam has a distinct central maximum with small side lobes in the x-axis direction

Optical Properties of Layered Metamaterial Composed of Pairs of Nanowires

A metamaterial composed of electromagnetically active cells with two parallel wires of sub-wavelength size can be fabricated by means of patterning techniques used in microelectronics. Intensity transmittance of layered metamaterial is assessed as a function of fill factor, that is a ratio of wire to layer surfaces, that in simulations changes from 6 to 20%. Negative refractive index reaching as low as -0.5 is predicted in the range of 1.13-1.29 mum for fill factor 12%. Then in FDTD simulations we analyze propagation of light through single and multiple layers of the metamaterial. A single layer exhibits a negative index of refraction at wavelengths for which absorption results from a high imaginary value of n. FDTD simulations show that the negative n should appear for the range of 1.92-2.19 mum. Theoretical underassessment of the range of wavelengths where n is negative comes from simplicity of the considered dipole model. We conclude that the smaller is the fill factor, the narrower the range of frequencies, where n is negative. Then, the smaller is the fill factor, the less negative is n. Finally, transmission considerably decreases with increasing fill factor

Numerical investigation of energy transport along chains of silver nanorods and nanoplates

We examine the propagation of energy along chains of silver nanoelements oriented perpendicularly to the flow of light and ordered in several ways. The first chain is composed ofvertical silver nanorods arranged in a hexagonal lattice. The second one consists of vertical elongated nanoplates that form a herring-bone pattern. In the third, distribution of vertically oriented nanoplates recalls footsteps. The chains are embedded in a medium with refractive index n = 1 and 1.5. Incident polarized Gaussian beams propagate along chains of nanoelements and have electric field components oriented transversally with respect to the vertical nanoelements. Transport of energy is investigated with the Finite Difference Time Domain (FDTD) method for visible and infrared range ofwavelengths, where the Drude model is valid. Propagation constants and attenuation factors are calculated. Losses are due to absorption in metal and light scattering on structure elements. In the analyzed structures, energy is transported due to localized surface plasmons-polaritons, where the amplitude of optical fields is locally enhanced by orders of magnitude. This property might be useful in the construction of nanoscale photonic devices. The smaller the metallic elements are, the stronger is the concentration of energy. Waveguides of that form may be used for creating a medium with novel effective electromagnetic properties.