Arash Hosseinzadeh

Battling absorptive losses by plasmon–exciton coupling in multimeric nanostructures

The strong inherent optical losses present in plasmonic nanostructures significantly limit their technological applications at optical frequencies. Here, we report on the interplay between plasmons and excitons as a potential approach to selectively reduce ohmic losses. Samples were prepared by functionalizing plasmonic core–shell nanostructures with excitonic molecules embedded in silica shells and interlocked by silica spacers to investigate the role played by the plasmon–exciton elements separation. Results obtained for different silica spacer thicknesses are evaluated by comparing dispersions of plasmonic multimers with respect to the corresponding monomers. We have observed fluorophore emission quenching by means of steady-state fluorescence spectroscopy, as well as a significant shortening of the corresponding fluorescence lifetime using TCSPC data. These results are accompanied by the simultaneous enhancement of Rayleigh scattering and transmittance, revealing more effective absorptive loss mitigation for multimeric systems. Moreover, upon decreasing the thickness of the intermediate silica layer between gold cores and the external gain functionalized silica shell, the efficiency of exciton–plasmon resonant energy transfer (EPRET) was significantly enhanced in both multimeric and monomeric samples. Simulation data along with experimental results confirm that the hybridized plasmon fields of multimers lead to more efficient optical loss compensation with respect to the corresponding monomers.

Coupling of elementary resonances in 3D metamaterials

Simulation analysis of electromagnetic response of infinite 2D arrays of dielectric ceramic resonators and of parallel 2D arrays comprising 3D metamaterial blocks is presented. Modeling of the arrays has been performed by applying periodic boundary conditions to various unit cells. It was revealed that coupling between elementary magnetic resonances in infinite 2D arrays can affect both the resonance frequency and the resonance Q-factor. Defects in array periodicity were found to cause partial splitting of the resonance modes. It was shown that coupling between parallel 2D arrays stacked in the direction of wave propagation produced a drastic splitting of the resonance modes and led to the formation of the resonance band. The observed effects demonstrate that the uniform medium concept can be not applicable to the description of 3D metamaterial structures.

Highly linear dual ring resonator modulator for wide bandwidth microwave photonic links

A highly linear dual ring resonator modulator (DRRM) design is demonstrated to provide high spur-free dynamic range (SFDR) in a wide operational bandwidth. Harmonic and intermodulation distortions are theoretically analyzed in a single ring resonator modulator (RRM) with Lorentzian-shape transfer function and a strategy is proposed to enhance modulator linearity for wide bandwidth applications by utilizing DRRM. Third order intermodulation distortion is suppressed in a frequency independent process with proper splitting ratio of optical and RF power and proper dc biasing of the ring resonators. Operational bandwidth limits of the DRRM are compared to the RRM showing the capability of the DRRM in providing higher SFDR in an unlimited operational bandwidth. DRRM bandwidth limitations are a result of the modulation index from each RRM and their resonance characteristics that limit the gain and noise figure of the microwave photonic link. The impact of the modulator on microwave photonic link figure of merits is analyzed and compared to RRM and Mach-Zehnder Interference (MZI) modulators. Considering ± 5 GHz operational bandwidth around the resonance frequency imposed by the modulation index requirement the DRRM is capable of a ~15 dB SFDR improvement (1 Hz instantaneous bandwidth) versus RRM and MZI.

Photonic properties of two-dimensional high-contrast periodic structures: Numerical calculations

The photon properties of two-dimensional periodic structures formed by infinite homogeneous dielectric cylinders packed in a square lattice have been investigated theoretically. Depending on the dielectric contrast between the cylinders and the surrounding medium, the photonic band structure, transmission spectra of crystals with a finite number of layers, and spectra of Mie scattering by an isolated cylinder have been calculated. The calculations have been performed for the TE polarization. The transformation of photonic stop-bands corresponding to Bragg and Mie resonances has been analyzed using the obtained data. The main effect consists in “castling” energy positions of the Bragg stop-bands and Mie stop-bands. For low-contrast photonic crystals, the low-frequency region of the energy spectrum is determined by Bragg stop-bands, and Mie stop-bands are located higher in energy. With an increase in the dielectric contrast, the energy of Mie stop-bands decreases, and they intersect the region of Bragg stop-bands weakly varying in the TE polarization and form the low-energy region of the spectrum.

Realization of High-Q Fano Resonances in Ceramic Dielectric Metamaterials for Sensing Applications

Ceramic all-dielectric metamaterials are found to support very high Q resonances of the Fano-type, which until recently were largely attributed to the atomic physics phenomena. It is shown that proper arrangement of ceramic resonators in the metamaterial array allowsd for obtaining Q factors up to 15000. Thus high Q factors could be employed for new applications, in particular, for advanced sensing. An opportunity to design compact arrays that could be incorporated in a microwave sensor fed by a microstrip line is demonstrated. Numerical experiments have confirmed that Fano resonances in such arrays conserve high sensitivity to the dielectric permittivity of the controlled media.

Design and optimization of polymer ring resonator modulators for analog microwave photonic applications

Efficient modulation of electrical signals onto an optical carrier remains the main challenge in full implementation of microwave photonic links (MPLs) for applications such as antenna remoting and wireless access networks. Current MPLs utilize Mach-Zehnder Interferometers (MZI) with sinusoidal transfer function as electro-optic modulators causing nonlinear distortions in the link. Recently ring resonator modulators (RRM) consisting of a ring resonator coupled to a base waveguide attracted interest to enhance linearity, reduce the size and power consumption in MPLs. Fabrication of a RRM is more challenging than the MZI not only in fabrication process but also in designing and optimization steps. Although RRM can be analyzed theoretically for MPLs, physical structures need to be designed and optimized utilizing simulation techniques in both optical and microwave regimes with consideration of specific material properties. Designing and optimization steps are conducted utilizing full-wave simulation software package and RRM function analyzed in both passive and active forms and confirmed through theoretical analysis. It is shown that RRM can be completely designed and analyzed utilizing full-wave simulation techniques and as a result linearity effect of the modulator on MPLs can be studied and optimized. The material nonlinearity response can be determined computationally and included in modulator design and readily adaptable for analyzing other materials such as silicon or structures where theoretical analysis is not easily achieved.

Optical waveguides using PDMS-metal oxide hybrid nanocomposites

Development of passive and active polymer based optical materials for high data rate waveguide routing and interconnects has gained increased attention because of their excellent properties such as low absorption, cost savings, and ease in fabrication. However, optical polymers are typically limited in the range of their refraction indices. Combining polymeric and inorganic optical materials provides advantages for as development of nano-composites with higher refractive indices with the possibility of being used as an active optical component. In this paper a new composite material is proposed based on polymer-metal oxide nano-composites for use as optical wave guiding structures and components. PDMS (Polydimethylsiloxane) is utilized for the polymer portion while the inorganic material is titanium dioxide. Refraction indices as high as 1.74 have been reported using these composites. For PDMS-TiO2 hybrids, the higher the ratio of titanium dioxide to PDMS, the higher the resulting refractive index. The index of refraction as a function of the PDMS:TiO2 ratio is reported with an emphasis on use as optical waveguide devices. Absorption spectrum of the nano-composites is measured showing low absorption at 850 nm and high absorption in the UV regime for direct UV laser/light curing. Prototype multimode waveguides are fabricated using soft imprint embossing that is compatible with the low viscosity nano-composite material. Cross dimensional shape and profile show the potential for full scale development utilizing the material set.

Effects of magnetic resonance on the band structure of 3D dielectric metamaterial arrays

The magnetic resonance related effects on energy band diagrams of 3D metamaterials composed of cylindrical dielectric resonators are investigated for a wide range of dielectric permittivity of the resonator material. It is shown that at permittivity values exceeding some threshold, the effective negative permeability of the medium provided by magnetic resonances in DR arrays defines the 1st band-gap formation. Below threshold, in contrast, the resonance phenomena are responsible for the formation of the second transmission band providing for the negative refractive index of the medium.

Forward and backward-wave propagation in “below cut-off ” waveguides loaded with dielectric resonators

Dispersion properties of a rectangular metallic waveguide periodically loaded with dielectric resonators (DRs) are investigated both theoretically and by full-wave simulations. Forward and backward wave propagation has been observed in the waveguide at below cut-off frequencies for the fundamental TE10 mode, when the DRs exhibited electric and magnetic resonances, respectively. It is demonstrated that the “below cutoff” forward wave propagation is related to shifting of the cut-off frequency of the waveguide filled with DRs with respect to that of an empty waveguide to lower frequencies in dependence on the effective permittivity component along the electric field direction. This effective permittivity increases substantially at the frequency just below that of the electric-type resonance in DRs.