Babak Dastmalchi

Single and multilayer metamaterials fabricated by nanoimprint lithography

We demonstrate for the first time a fast and easy nanoimprint lithography (NIL) based stacking process of negative index structures like fishnet and Swiss-cross metamaterials. The process takes a few seconds, is cheap and produces three-dimensional (3D) negative index materials (NIMs) on a large area which is suitable for mass production. It can be performed on all common substrates even on flexible plastic foils. This work is therefore an important step toward novel and breakthrough applications of NIMs such as cloaking devices, perfect lenses and magnification of objects using NIM prisms. The optical properties of the fabricated samples were measured by means of transmission and reflection spectroscopy. From the measured data we retrieved the effective refractive index which is shown to be negative for a wavelength around 1.8 µm for the fishnet metamaterial while the Swiss-cross metamaterial samples show a distinct resonance at wavelength around 1.4 µm.

Optical activity in sub-wavelength metallic grids and fishnet metamaterials in the conical mount

We report on measurements of optical activity in reflection in the conical mount from two plasmonically resonant nanostructures; a sub-wavelength silver meshed grid and a fishnet metamaterial. The square-centimeter size of the materials, formed by nano-imprint lithography, allows reliable investigation of such materials by plane-wave techniques with minimal focusing. For both materials we observe strong polarization conversion (s- to p-polarization, and vice versa) in generalized ellipsometry measurements. We compared the spectra to analytical predictions using surface plasmon polariton (SPP) theory and find good agreement for the meshed grid. The spectra for the meshed grid are also well modeled using the rigorous coupled wave analysis (RCWA) technique. Simulated results for the more complicated fishnet layer showing qualitative agreement are also presented. We then probe the validity of describing the observations using homogenous parameters such as dichroism and birefringence, by examining the calculated reflection of nominally polarized incident light using simulated and measured Mueller matrices. The results show that the cross-polarization that we observe is primarily related to linear birefringence and dichroism, although circular effects are indeed present.

Oblique incidence ellipsometric characterization and the substrate dependence of visible frequency fishnet metamaterials

We use spectroscopic ellipsometry to investigate the angular-dependent optical modes of fishnet metamaterials fabricated by nanoimprint lithography. Spectroscopic ellipsometry is demonstrated as a fast and efficient method for metamaterial characterization and the measured polarization ratios significantly simplify the calibration procedures compared to reflectance and transmittance measurements. We show that the modes can be well identified by a combination of comparing different substrates and considering the angular dependence of the Woods anomalies. The lack of angular dispersion of the anti-symmetric gap-modes does not agree with the model and requires further theoretical investigation.

Method of calculating local dispersion in arbitrary photonic crystal waveguides.

We introduce a novel method to calculate the local dispersion relation in photonic crystal waveguides, based on the finite-difference time-domain simulation and filter diagonalization method (FDM). In comparison with the spatial Fourier transform method (SFT), the highly local dispersion calculations based on FDM are considerably superior and pronounced. For the first time to our knowledge, the presented numerical technique allows comparing the dispersion in straight and bent waveguides.

Local Dispersion of Guiding Modes in Photonic Crystal Waveguide Interfaces and Hetero-Structures

Recently, we have introduced a numerical method for calculating local dispersion of arbitrary shaped optical waveguides, which is based on the Finite-Difierence Time-domain and fllter diagonalization technique. In this paper we present a study of

Spectroscopic Ellipsometry Study of a Swiss Cross Metamaterial

We present spectroscopic ellipsometry measurements of a metamaterial, taken under different incidence-angles and compared with calculations based on the RCWA method. We find that resonances for (Psi, Delta) do not disappear changing the incidence angle.

Design and simulation of multi-functional layer double negative fishnet metamaterial at optical communication wavelength

In this paper we present multi-functional fishnet metamaterial structures whose dimensions has been engineered and optimized to provide negative permeability, permittivity and consequently negative index of refraction at optical communication wavelength. They are based on silver as metallic layer rather than gold and SiO2 as dielectric layer which are perforated and deposited on glass substrate. Parameter retrieval procedure is done to determine the optical properties of fishnet metamaterial from calculated data using Rigorous Coupled Wave Analysis (RCWA) method.

Study of local dispersion in photonic crystal waveguide interfaces and hetero-structures

We have recently introduced a novel method to calculate local dispersion relation based on the Finite-Difference Time-domain and filter diagonalization method, which is suitable for local study of dispersion in optical waveguide, especially for the cases of non-periodic, curvilinear, and finite waveguides. In this paper, this approach is applied to study the photonic crystal waveguides at interfaces and double hetero-structure waveguides. We also studied the stretching effect, which is increasing the lateral distance between neighboring rods along guiding direction on band gap. Hybrid modes at interface are results of superposition of existing modes in adjacent waveguides. The results present a clear picture of localization mechanism of cavity modes and the transmission in the double-hetero-structures.

Local Dispersion in 2D Photonic Crystals Using Filter Diagonalization Method

Local dispersion in Photonic Crystals (PC) is calculated using the advanced filter diagonalization method and compared to the traditional spatial Fourier transform of the field distribution.