Elmer Rivera

Plasmon-Induced Transparency by Hybridizing Concentric-Twisted Double Split Ring Resonators

As a classical analogue of electromagnetically induced transparency, plasmon induced transparency (PIT) has attracted great attention by mitigating otherwise cumbersome experimental implementation constraints. Here, through theoretical design, simulation and experimental validation, we present a novel approach to achieve and control PIT by hybridizing two double split ring resonators (DSRRs) on flexible polyimide substrates. In the design, the large rings in the DSRRs are stationary and mirror images of each other, while the small SRRs rotate about their center axes. Counter-directional rotation (twisting) of the small SRRs is shown to lead to resonance shifts, while co-directional rotation results in splitting of the lower frequency resonance and emergence of a PIT window. We develop an equivalent circuit model and introduce a mutual inductance parameter M whose sign is shown to characterize the existence or absence of PIT response from the structure. This model attempts to provide a quantitative measure of the physical mechanisms underlying the observed PIT phenomenon. As such, our findings can support the design of several applications such as optical buffers, delay lines, and ultra-sensitive sensors.

Impact of Substrate and Bright Resonances on Group Velocity in Metamaterial without Dark Resonator

Manipulating the speed of light has never been more exciting since electromagnetic induced transparency and its classical analogs led to slow light. Here, we report the manipulation of light group velocity in a terahertz metamaterial without needing a dark resonator, but utilizing instead two concentric split-ring bright resonators (meta-atoms) exhibiting a bright Fano resonance in close vicinity of a bright Lorentzian resonance to create a narrowband transmittance. Unlike earlier reports, the bright Fano resonance does not stem from an asymmetry of meta-atoms or an interaction between them. Additionally, we develop a method to determine the metamaterial “effective thickness”, which quantifies the influence of the substrate on the metamaterial response and has remained challenging to estimate so far. By doing so, very good agreement between simulated and measured group delays and velocities is accomplished. The proposed structure and method will be useful in designing optical buffers, delay lines, and ultra-sensitive sensors.

Low Leakage Current ZnO Nanowire Schottky Photodiodes Built by Dielectrophoretic Contact

This letter presents the characterization results of nanowire (NW) Schottky photodiodes fabricated from the dielectrophoretic contact between a ZnO NW and a Cu electrode. The device counter-electrode is fabricated using Pt focused ion beam deposition. The current-voltage characteristics exhibit rectifying properties with a leakage current as low as 40 pA at -40 V. The fitting of the forward characteristics reveals a barrier height lowering under UV illumination along with a large reduction of the series resistance. At forward bias, responsivities of ~105 A/W are obtained above the bandgap energy. Under reverse bias, the responsivity reduces up to 104 A/W, but a higher ultraviolet/visible contrast and a faster response are observed. In those conditions, the barrier height lowering is fostered by the drift of photogenerated holes toward the interface with the Cu electrode, yielding lower barrier height values under illumination.

Investigation of tunable terahertz metamaterial perfect absorber with anisotropic dielectric liquid crystal

In this letter, we report the unique design, simulation and experimental verification of an electrically tunable THz metamaterial perfect absorber consisting of complementary split ring resonator (CSRR) arrays integrated with liquid crystal as the subwavelength spacer in between. We observe a shift in resonance frequency of about 5.0 GHz at 0.567 THz with a 5 V bias voltage at 1KHz between the CSRR and the metal backplane, while the absorbance and full width at half maximum bandwidth are maintained at 90% and 0.025 THz, respectively. Simulated absorption spectrum by using a uniaxial model of LC matches perfectly the experiment data and demonstrates that the effective refractive index of LC changes between 1.5 and 1.7 by sweeping a 1 kHz bias voltage from 0 V to 5 V. By matching simulation and experiment for different bias voltages, we also estimate the angle of LC molecules versus the bias voltage. Additionally, we study the created THz fields inside the spacer to gain a better insight of the characterist...

InP/ZnS core-shell quantum dots sensitized ZnO nanowires for photovoltaic devices

With the increasing worldwide need for energy, fossil fuels will not be able to keep up with demand in the coming decades. Solar energy is one of the best solutions to this problem with the advantages of being clean and sustainable. Among the various designs of solar cells, dye-sensitized solar cells provide relatively high efficiency with large scale for a low cost [1]. Conventional dye-sensitized solar cells operate with light harvesting organic dye molecules adsorbed at the interface between TiO2 nanoparticles and a hole-conducting liquid electrolyte [2]. However, new combinations of materials potentially present an opportunity to further improve the performance and lower the cost of solar cells. One such promising approach is to use quantum dots (QDs) instead of the organic materials as the main photosensitive constituent. Several types of semiconductor QDs, including CdSe [3], CdS [4] and InP [5], have been investigated to realize quantum dot sensitized solar cells by taking advantage of the tunable absorption spectrum of the QDs through changes in their size. Complementarily to this approach, it is desirable to use a semiconductor material with a high structural uniformity and surface area as the framework on which to attach these QDs and achieve efficient electron transport, such as well-aligned ZnO nanowires [6].

Terahertz metamaterials: design, implementation, modeling and applications

Sub-wavelength metamaterial structures are of great fundamental and practical interest because of their ability to manipulate the propagation of electromagnetic waves. We review here our recent work on the design, simulation, implementation and equivalent circuit modeling of metamaterial devices operating at Terahertz frequencies. THz metamaterials exhibiting plasmon-induced transparency are realized through the hybridization of double split ring resonators on either silicon or flexible polymer substrates and exhibiting slow light properties. THz metamaterials perfect absorbers and stereometamaterials are realized with multifunctional specifications such as broadband absorbing, switching, and incident light polarization selectivity.

Liquid crystal frequency tunable terahertz metamaterial absorber

Here, we theoretically and experimentally realize a terahertz (THz) perfect metamaterial absorber with an actively tunable frequency response. The structure is made of a complementary frequency selective surface (CFSS) and a metal backplane where the subwavelength space between them is filled with liquid crystal. By applying a voltage between CFSS and metal backplane a resonance frequency shifts about 4.5 GHz at 0.567 THz is obtained while the absorbance maintains at %90.

Photovoltaic devices based on quantum dot functionalized nanowire arrays embedded in an organic matrix

Quantum dot (QD) functionalized nanowire arrays are attractive structures for low cost high efficiency solar cells. QDs have the potential for higher quantum efficiency, increased stability and lifetime compared to traditional dyes, as well as the potential for multiple electron generation per photon. Nanowire array scaffolds constitute efficient, low resistance electron transport pathways which minimize the hopping mechanism in the charge transport process of quantum dot solar cells. However, the use of liquid electrolytes as a hole transport medium within such scaffold device structures have led to significant degradation of the QDs. In this work, we first present the synthesis uniform single crystalline ZnO nanowire arrays and their functionalization with InP/ZnS core-shell quantum dots. The structures are characterized using electron microscopy, optical absorption, photoluminescence and Raman spectroscopy. Complementing photoluminescence, transmission electron microanalysis is used to reveal the successful QD attachment process and the atomistic interface between the ZnO and the QD. Energy dispersive spectroscopy reveals the co-localized presence of indium, phosphorus, and sulphur, suggestive of the core-shell nature of the QDs. The functionalized nanowire arrays are subsequently embedded in a poly-3(hexylthiophene) hole transport matrix with a high degree of polymer infiltration to complete the device structure prior to measurement.

Hybrid nanostructures based on quantum dots and nanowires

We report the realization of hybrid nanostructures that combine inorganic InP/ZnS core-shell quantum dots and wide bandgap ZnO nanowires with an organic polymer matrix compound, which can potentially enable optoelectronic devices novel functionalities, including photovoltaics. The ZnO nanowires were synthesized by thermal chemical vapor deposition prior to functionalization with the quantum dots and embedding into a poly-3(hexylthiophene) hole transport matrix. The nanostructures were precisely characterized using photoluminescence, Raman spectroscopy, x-ray diffraction, scanning and high resolution transmission electron microscopy. Unique atomic scale resolution analysis of the nanostructures was performed using energy dispersive x-ray spectroscopy mapping and atom probe tomography for the first time.

Quantum dot functionalized ZnO nanowire/P3HT hybrid photovoltaic devices

We report hybrid inorganic/organic photovoltaic heterojunction structures based on quantum dot functionalized nanowires. Single crystalline ZnO nanowires were first functionalized with InP/ZnS core-shell quantum dots and then embedded in a conductive polymer hole matrix to achieve a low cost, high efficiency device structure. The device structures were characterized by scanning electron microscopy, transmission electron microscopy, confocal Raman and photoluminescence spectroscopy. The obtained results show excellent interface properties between the QDs and the ZnO nanowires, a high degree of polymer infiltration into the nanowire array and improved planarization of the polymer matrix.