Hossein Alisafaee

Compact one-dimensional magnetophotonic crystals with simultaneous large Faraday rotation and high transmittance

We have investigated the case of transmission-type one-dimensional magnetophotonic crystals (MPCs) in order to achieve simultaneous high transmittance and large Faraday rotation utilizing few magnetic layers. In a MPC that includes two Ce:YIG magnetic layers, we have achieved a Faraday rotation as large as 85.82° and a transmittance of 95.66%. In addition, another structure in the form of a triple-cavity MPC with a transmittance of 100% and a Faraday rotation as huge as 87.72° has been achieved through precise selection of layer thicknesses and positions. Both of these high-performance structures are very compact and thin, which makes them excellent candidates for application in integrated MO devices.

Optimization of all-garnet magneto-optical magnetic field sensors with genetic algorithm

In this article, we introduce a simple magnetophotonic crystal structure for magnetic field sensing applications. Design procedure, which is performed using a global optimization tool called genetic algorithm, provides great flexibility for structures with layers having nonquarter-wavelength thickness. Results show that our proposed genetic sensor comparatively exhibits higher simplicity, sensitivity, and spatial resolution, with better photo-response and performance. We also analyze the underlying physical phenomenon responsible for such improvement by inspection of electric field distribution in the interior of the structure.

Spectral properties of Au–ZnTe plasmonic nanorods

Coupled plasmonic nanoparticles of Au and nanorods of ZnTe are modeled and fabricated. Full-wave simulation is performed to obtain an optimum design for enhanced light absorption and to explain scattering properties of the structure. The fabrication method of such arrays is described. Modeling the spectral properties using equivalent circuit theory is also implemented to provide an intuitive approach regarding the design of optical metamaterials with predetermined properties.

Polarization insensitivity in epsilon-near-zero metamaterial from plasmonic aluminum-doped zinc oxide nanoparticles

Abstract. We investigated the optical characteristics and polarization insensitivity of an epsilon-near-zero metamaterial structure comprising aluminum-doped zinc oxide nanoparticles (NPs) hosted by a medium of ligands. By the use of an equivalent circuit model for the pairs of NPs, or dimers, and also of fullwave simulations, we studied the response of this self-assembled metamaterial for near-infrared applications. Considering the coupling of localized surface plasmons, we demonstrated the dominance of a certain dimer configuration and then applied this result to the whole medium as a simplifying approximation for a random structure. The consequent results showed a polarization insensitivity and also a general redshift in the plasmon resonance of the structure.

Low-index metamaterials comprised of plasmonic dimers of aluminum-doped zinc oxide

Transparent conducting oxides (TCO) are an interesting class of plasmonic materials, which are under intensive study for their use in low-loss metamaterials and a range of applications such as sensing, imaging and transformation optics. Here, using both full-wave simulations and an equivalent circuit model for pairs of nanoparticles of aluminum doped zinc oxide (AZO), we study the plasmonic effects for low loss low index metamaterials for infrared applications. The behavior of localized surface plasmon resonances (LSPR) of AZO nanoparticle dimers embedded in a host polymer medium is investigated for different dimer orientations with respect to the indicent electromagnetic wave. In doing this, the role of dressed polarizability to enhance and quench the plasmonic effects is also considered. The effects of the nanoparticles relative size and the spacing between them are studied. Understanding these resonances and their dependence on dimer orientations, provides a means to design metamaterial structures for use in the near infrared (NIR) region with epsilon-near-zero properties leading also to low index metamaterials. In our studies, we demonstrate how nanospheres with radii less than 100 nm that are distributed with an average spacing less than their diameter, can result in an effective medium with refractive index less than one. We utilize a full-wave frequency domain finite element method in conjunction with an equivalent-circuit model for the nanoscale dimers in order to describe the spectral response of the bulk low index properties. We also present a statistical analysis to obtain the effective refractive index for incident light having different polarizations.

Epitaxial thin films for hyperbolic metamaterials

Recent progress in the area of hyperbolic metamaterials (HMMs) has sparked interest in transparent conducting oxides (TCOs) that behave as plasmonic media in the near-IR and at optical frequencies for imaging and sensing applications. It has been shown that by depositing alternating layers of negative-epsilon/positive-epsilon materials, a medium can be created with unusual index values such as near zero. HMMs support high-k waves corresponding to a diverging photonic density of states (PDOS), the quantity determining phenomena such as spontaneous and thermal emission. Also, modeling such structures allows evanescent fields containing sub-wavelength information to be coupled to propagating radiation. We investigate the optical, electronic, and physical properties of radio frequency plasma-assisted molecular beam epitaxial (RF-MBE) growth of alternating layers of ZnO and TCO of uniform thickness for HMM applications. Preliminary work creating HMMs with ZnO and Al-doped ZnO (AZO) has shown a negative real part of the permittivity at near-IR whose modulus is proportional to the number density of Al dopant. However, increasing the Al content of the AZO increases the transmission losses to unacceptable levels for device applications at industry standard wavelengths. A TCO with conductivity and physical structure superior to that of AZO is gallium-doped ZnO (GZO). Uniformly grown GZO has been demonstrated to possess improved crystal quality over AZO due to the higher diffusivity of Al in the ZnO. AZO and GZO HMM structures grown by RF-MBE are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Hall effect, four-point probing, deeplevel transient spectroscopy (DLTS), ellipsometry, visible and ultraviolet spectroscopy (UV-VIS) and in-situ reflection high energy electron diffraction (RHEED).

Nanoantennas for nanowire photovoltaics

We consider the use of plasmonic nanoantenna elements, hemispherical and cylindrical, for application in semiconductor nanowire (NW) vertical arrays. Using Mie theory and a finite element method, scattering and absorption efficiencies are obtained for the desired enhancement of interaction with light in the NWs. We find an optimal mixture of nanoantennae for efficient scattering of solar spectrum in the NW array. Spectral radiation patterns of scattered light are computed, and, for representing the total response of the nanoantenna-equipped NWs to the solar AM1.5G spectrum, the weighted average of scattering patterns for unpolarized normal incidence is obtained showing an advantageous overall directivity toward the NWs.

Selective field localization in random structured media

We have introduced a method to find optimized structured media exhibiting large internal electric field amplitudes. The method is based on a genetic algorithm in which a spatial fitness function according to the computed field distribution in the interior of media is defined and maximized. The main feature of our method is that it enables localization of light at a desired layer (or more) within the structure. The enhancements are demonstrated to be up to about 70-fold in |E|(2) by use of only seven layers. The results are interesting for nonlinear and sensor applications, which due to compact size and few number of structure layers, are also desirable for fabrication purposes.

Genetic algorithm for true negative index in plasmonic metamaterials

We investigate negative index of refraction in plasmonic metamaterials with an emphasis on distinguishing and isolating contributions to negative refraction from spatial dispersion, as a function of metamaterial dimensions on the scale of the wavelength. We explain the design approach using genetic algorithm and provide sample applications including negative refraction.

Iridescence and its applications in thin-film conformal uniformity

Iridescence is structural color, as opposed to pigment, in which the color is a result of constructive or destructive interference of light traveling through periodic structures, where the thickness of each layer plays a critical role in the color observed. Bragg fibers are man-made cylindrical structures with periodic layer coatings, modeled after the atomic lattice structure found in naturally occurring crystals. Bragg layers with even a slight deviation of micrometers in its thickness can result in a non-uniform color, which is the basis of our study. A non-invasive approach was taken to best approximate the dominant wavelength of the reflected hues through the use of microscopic imaging. The RGB values present in the images were extracted, and transformed into new coordinates using matrix operations, from which we were able to estimate the dominant wavelengths present in the Bragg fiber.