S. Mohsen Raeis-Zadeh

Colorimetric sensors using nano-patch surface plasmon resonators

A two-dimensional array of gold nano-patches on a highly reflective mirror is proposed for refractive index sensing based on changes in the reflected colors. The grating on the mirror creates localized surface plasmon resonances resulting in a minimum in the visible reflectance spectra. The wavelength of the resonance can be tuned by changing the width of the nano-patches and is also dependent on the refractive index of the surrounding medium. The color variation due to change in the refractive index is measured and used to realize a simple low-cost sensor with a refractive index resolution better than 10⁻⁵ just using image processing. The efficacy of the proposed sensor is also demonstrated for surface sensing by depositing thin layers of silicon dioxide. The color difference due to the addition of a 3 nm thick layer of silicon dioxide is detectable by the naked eye and deposition thickness of 2 Å can be resolved using image processing.

Analysis of Electromagnetic Wave Scattering by Graphene Flakes Using the Generalized Multipole Technique

The generalized multipole method is applied to the analysis of the electromagnetic wave scattered by graphene flakes. In the proposed method, the graphene flakes are modeled as surface current sheets causing a discontinuity in the magnetic field. This method is used to analyze the scattering from isolated islands of the graphene layer. In addition, the field enhancement between two nearby graphene flakes as well as the field distribution of a two dimensional periodic configuration of these flakes are investigated with the proposed method. To validate the developed method, its numerical results are compared with those of the finite element method (FEM). As compared to the model used in the FEM, the surface current model used in the proposed method significantly reduces the computational complexity.

Graphene-Integrated Plasmonic Structure for Optical Third Harmonic Generation

In this paper, a general recipe is proposed to design an efficient graphene-integrated plasmonic structure for third harmonic generation. Specifically, the design procedure for an integrated graphene-based ultraviolet light generator is presented. In order to enhance the field intensity at the graphene layer, two distinct mechanisms are utilized. A multilayer Bragg structure is used as a perfect magnetic conductor to make a constructive interference between incident and reflected field at the graphene layer. A periodic array of shaped resonant gold nanoparticles is placed on top of the graphene sheet to enhance the field intensity due to the plasmonic resonance at the fundamental frequency. A hybrid and fast numerical method based on the scattering matrix of Floquet modes and the Generalized Multipole Technique is also proposed to analyze the periodic structure. This numerical method is used to optimize the dimensions of the multilayer structure and boost the nonlinear conversion efficiency by more than 106 times.

Application of graphene nanoholes as a terahertz polarizer

A periodic structure of nanoholes in a continuous sheet of graphene over a silicon substrate is analyzed and its application as a terahertz (THZ) polarizer is demonstrated. The effect of the hole geometry on the extinction ratio and the working frequency of the polarizer is investigated. This periodic structure is then used in a double layer structure to realize a substrate-based polarizer with a higher extinction ratio and bandwidth as compared to the single layer configuration. The small thickness of substrate-based structures (2μm) ensures that the Fabry-Perot multi-reflections will not happen in the proposed polarizer.

Space division multiplexing using disordered optical antennas

Using disordered highly coupled nanoantennas, previously we proposed a novel scheme for optical information multiplexing [1]. It was shown that for a given random configuration of nanoantennas, orthogonal transmission channels can be identified and realized. It is desired to increase the number of channels for a given configuration. Here we investigate this in more details.

Radiation properties of THz photomixing antennas loaded with nanoplasmonic periodic structures in the gap

The existence of a nanoplasmonic periodic structure in the gap of a THz photomixing antenna produces the sub-wavelength variation in the induced THz current. We are proposing a model based on the infinitesimal electric dipoles for the inclusion of these sub-wavelength variations in the antenna radiation. We demonstrate that the antenna highly enhances the radiated power of electric dipoles by changing their near field. We also show that the scattering model typically used to characterize the antenna is not accurate and sufficient for this certain application. These results are specifically important in the modeling of the interaction between the THz wave with the small structures such as molecules in the gap of an antenna.

A wide axial ratio beamwidth circularly polarized antenna for Ka-band satellite on the move (SOTM) phased array applications

A wide axial ratio beamwidth circularly polarized (CP) antenna based on aperture-coupled elliptical microstrip patch antenna fed by a GCPW line is presented as a building element for a large-scale Ka-band SOTM phased array. The proposed antenna element is developed with a low-cost PCB planar technology. Full wave solver from ANSYS is used to optimize and evaluate the antenna performance. The simulated return loss bandwidth 31% centered at 31 GHz. Moreover, the proposed antenna element shows a 3 dB axial ratio (AR) bandwidth 3.4% centered 29.6 GHz in all φ angles and ±80° from boresight.

Quantum-Enhanced Second-Order Nonlinearity in Graphene: The Role of Wave Momentum and DC Biasing

A comprehensive and rigorous analysis is presented for the study of terahertz photomixing process in a biased graphene layer, when two obliquely incident waves are used as primary excitation. The second-order nonlinearity tensor of graphene associated with the difference frequency generation (DFG) is calculated to evaluate the amount of induced terahertz current density. In this analysis, we are calculating the significant contribution of the photon drag effect to the DFG and consequently to the terahertz wave generation. We also examine the effect of DC current biasing on the DFG as the Fermi energy level of graphene changes. Our results show that the DFG in graphene can be enhanced by at least two orders of magnitude when the Fermi energy level of graphene becomes equal to the energy of incident photon. This paper provides more insight into the contributing factors in the DFG process and allows the realization of more optimal graphene-based photomixing devices.

Graphene-based controllable antenna for Terahertz photomixer sources

A graphene-based antenna for the THz photomixing application is proposed. The graphene layer was used to control the antennas resonance frequency. A frequency change as high as 270 GHz at the center frequency of 1.4 THz was obtained for the proposed antenna structure.

Colorimetric Sensors using Plasmonics Grating on a Metallic Mirror

We experimentally demonstrate a low-cost colorimetric sensor in which the change in surrounding refractive index is measured using simple image processing. This sensor consists of two-dimensional gold nanopatch grating on a highly reflective mirror.