Khadijeh Bayat

Photonic-Crystal-Based Polarization Converter for Terahertz Integrated Circuit

In this paper, the fabrication and characterization of newly developed photonic crystal (PC) polarization-controlling devices on a silicon-on-insulator wafer for integrated terahertz applications are presented. The polarization converter is composed of periodic asymmetric loaded PC slab waveguide. Square- and circular-hole PC slab waveguides were studied using a 3-D finite-difference time-domain method. For a square-hole PC-based polarization rotator, polarization rotation efficiency higher than 90% was achieved within the normalized frequency band of a/ λ = 0.258-0.267 . In circular-hole PC polarization converter, the polarization conversion efficiency dropped to 70% for the aforementioned frequency band. Low polarization conversion efficiency of the circular-hole PC-based device is attributed to scattering loss at the top loaded layers. Thus, the square-hole PC structure is a better candidate for integrated terahertz polarization-controlling devices. Planar terahertz integrated circuit technology was developed to implement the proposed device. Characterization setup was designed using rigorous numerical methods to use the newly introduced Agilent Millimeter-wave PNA-X network analyzer (up to 500 GHz) as a source. Scattering parameter characterizations provide a good measure of polarization extinction ratio. For the devices designed for the central frequency of f = 200 GHz, it was observed that, within the frequency band of 198-208 GHz (α/λ = 0.26-0.272), the ratio of S21 to S11 was higher than 15 dB. The bandwidth is in good agreement with our preliminary design presented before.

Geometry dependence of field enhancement in 2D metallic photonic crystals

Geometry dependence of surface plasmon resonance of 2D metallic photonic crystals (PCs) was assessed using rigorous 3D finite difference time domain analysis. PCs of noble metallic rectangular and cylindrical nanopillars in square and triangular lattices on thick noble metal film were simulated for maximum field enhancement. It was found that the period, size and thickness of the nanopillars can be tuned to excite of surface plasmons at desired wavelengths in visible and near-infrared ranges. Maximum electric field enhancement near the nanopillars was found to be greater than 10X. The detail analysis of PCs tuned for 750 nm wavelength showed that thickness of nanopillars was the most sensitive parameter for field enhancement, and triangular lattice PCs had the wider enhancement bandwidth than square lattice PCs. Results showed that these PCs are sensitive with incident angle (theta) but not with polarization angle (phi).

Efficient, uniform, and large area microwave magnetic coupling to NV centers in diamond using double split-ring resonators.

We report on the development and utilization of a double split-ring microwave resonator for uniform and efficient coupling of microwave magnetic field into nitrogen-vacancy (NV) centers in a diamond over a mm(2) area. Uniformity and magnitude of delivered microwave field were measured using the Rabi nutation experiment on arrays of diamond nanowires with ensemble NV centers. An average Rabi nutation frequency of 15.65 MHz was measured over an area of 0.95 × 1.2 mm, for an input microwave power of 0.5 W. By mapping the Rabi nutation frequency to the magnetic field, the average value of the magnetic field over the aforementioned area and input microwave power was 5.59 G with a standard division of 0.24 G.

Plasmon resonance modes in two-dimensional arrays of metallic nanopillars

Surface plasmon polaritons (SPPs) and localized plasmon resonance modes in two-dimensional arrays of silver nanopillars on silver surface were analyzed using the three-dimensional finite-difference time domain method. What we believe to be a new type of plasmon resonance modes at oblique incident angle for p-polarized light was observed in two-dimensional arrays of silver nanopillars in a square lattice. This resonance mode is associated with two SPP-like electric field patterns along the metal surface. We found that this resonance mode is localized and excited by the transverse polarization mode of nanopillars. Using Poynting vector plots, it was observed that the plasmon resonances in arrays of nanopillars are always associated with large energy cyclones near the nanopillars leading to light absorption.

Design, fabrication, and characterization of a plasmonic upconversion enhancer and its prospects for photovoltaics

Abstract. The design, fabrication, and characterization of an upconversion-luminescence enhancer based on a two-dimensional plasmonic crystal are described. Full-wave finite-difference time domain analysis was used for optimizing the geometrical parameters of the plasmonic crystal for maximum plasmon activity, as signified by minimum light reflection. The optimum design produced >20× enhancement in the average electromagnetic field intensity within a one-micron-thick dielectric film over the plasmonic crystal. The optimized plasmonic upconverter was fabricated and used to enhance the upconversion efficiency of sodium yttrium fluoride: 3% erbium, 17% ytterbium nanocrystals dispersed in a poly(methylmethcrylate) matrix. A thin film of the upconversion layer, 105 nm in thickness, was spin-coated on the surface of the plasmonic crystal, as well as on the surfaces of planar gold and bare glass, which were used as reference samples. Compared to the sample with a planar gold back reflector, the plasmonic crystal showed an enhancement of 3.3× for upconversion of 980-nm photons to 655-nm photons. The upconversion enhancement was 25.9× compared to the same coating on bare glass. An absorption model was developed to assess the viability of plasmonically enhanced upconversion for photovoltaic applications.

Enhancement of electromagnetic field intensity by metallic photonic crystal for efficient upconversion

2D arrays of cylindrical and rectangular gold nanopillars in square and triangular lattice were investigated for maximum field intensity enhancement above the metal dielectric interface. 3D-FDTD method was used for investigation and design of metallic photonic crystal. Photonic crystal was designed for maximum electromagnetic field intensity enhancement in infrared radiation centered at 980 nm wavelength. It was found that gold nanopillars of thickness 70nm, diameter 310nm and periodic in 620nm enhances the electromagnetic intensity by 100 times above the metallic nanopillars by exciting surface plasmon resonance. Enhancement of near-field intensity by two orders of magnitude is the promising way to increase the upconversion of infrared radiation by rare earth doped nanoparticles, such as NaYF4:Er,Yb.

Unified electronic charge transport model for organic solar cells

This paper provides a comprehensive modeling approach for simulation of electronic charge transport in excitonic solar cells with organic and organic/inorganic structures. Interaction of energy carrying particles (electrons, holes, singlet excitons, and triplet excitons) with each other and their transformation in the bulk of the donor and acceptor media as well as the donor/acceptor interfaces are incorporated in form of coupling matrices into the continuity equations and interface boundary conditions. As a case study, the model is applied to simulate an organic bilayer photovoltaic (PV) device to quantify the effects of photo generation, recombination coefficient, carrier mobility, and electrode work function on its PV characteristics. The study proves that electron-hole recombination at the donor/acceptor interface is the dominant mechanism that limits open circuit voltage of the device.

FDTD simulation of metallic gratings for enhancement of electromagnetic field by surface plasmon resonance

Enhancement of electromagnetic field by two dimensional arrays of rectangular and cylindrical nanopillars of both gold and silver metals arranged in either square or triangular lattices was investigated. We simulated these gratings by 3D Finite Difference Time Domain (3D-FDTD) method in visible and near infrared (NIR) wavelengths regime and investigated field enhancement by exciting surface plasmon polaritons (SPPs) as a function of geometrical parameters of grating. It was found that the geometrical grating parameters such as period, shape, thickness and size can be tuned for excitation of SPPs at particular frequency of interest. The tuned grating would lead to an electric field intensity enhancement by greater than 100× near the grating surface due to excitation of SPPs. Cylindrical gratings tuned for 750 nm at zero degree incident angle showed that the thickness of grating is the most sensitive geometrical parameter of resonance. Furthermore, triangular lattice gratings have wider bandwidth of resonance than square lattice gratings. Meanwhile, wavelength versus incident angle diagram showed that the enhancement was highly sensitive with angle of incidence.

Highly tunable self-assembled plasmonic lattices through nanosphere lithography

This Letter reports a method to produce two-dimensional self-assembled plasmonic nanopillar (NP) arrays with independent control of the diameter (d), spacing (s), and height (h) of the NPs. A plasmonic lattice was designed and optimized for maximum plasmonic activity at 980 nm using three-dimensional finite-difference time-domain simulations. The optimized lattice with d=365 nm, s=410 nm, and h=70 nm was fabricated utilizing a self-assembled nanosphere lithography approach. Outstanding agreement between the observed and predicted results confirms the validity of the design process and the controllability and repeatability of the fabrication process. The excellent short-range order in the lattice structure suggests that this method can replace the electron-beam lithography approach in a scalable and cost-effective manner.

Kinetic Monte Carlo modeling of dark and illuminated current-voltage characteristics of bulk heterojunction solar cells

Comprehensive Monte Carlo simulation of dark and illuminated IV characteristics of polymer bulk heterojunction solar cells has long been blocked by lack of incorporation of physical processes involved in formation of dark current and charge injection models at the electrodes. This paper introduces quasi-neutral electron and hole transport layers for decoupling contact properties from hopping charge transport in the bulk of donor/acceptor blend. This approach led to comprehensive independent simulation of dark and illuminated IV characteristics which better agree with experimental results. Recombination at the donor/acceptor interface was proved to be the origin of dark IV and the limiting factor of open circuit voltage.