Dean P. Brown

Nonlinear chiro-optical amplification by plasmonic nanolens arrays formed via directed assembly of gold nanoparticles.

Metal nanoparticle assemblies are promising materials for nanophotonic applications due to novel linear and nonlinear optical properties arising from their plasmon modes. However, scalable fabrication approaches that provide both precision nano- and macroarchitectures, and performance commensurate with design and model predictions, have been limiting. Herein, we demonstrate controlled and efficient nanofocusing of the fundamental and second harmonic frequencies of incident linearly and circularly polarized light using reduced symmetry gold nanoparticle dimers formed by surface-directed assembly of colloidal nanoparticles. Large ordered arrays (>100) of these C∞v heterodimers (ratio of radii R1/R2 = 150 nm/50 nm = 3; gap distance l = 1 ± 0.5 nm) exhibit second harmonic generation and structure-dependent chiro-optic activity with the circular dichroism ratio of individual heterodimers varying less than 20% across the array, demonstrating precision and uniformity at a large scale. These nonlinear optical properties were mediated by interparticle plasmon coupling. Additionally, the versatility of the fabrication is demonstrated on a variety of substrates including flexible polymers. Numerical simulations guide architecture design as well as validating the experimental results, thus confirming the ability to optimize second harmonic yield and induce chiro-optical responses for compact sensors, optical modulators, and tunable light sources by rational design and fabrication of the nanostructures.

Forward and backward unidirectional scattering from plasmonic coupled wires

We analyze the resonant electromagnetic response of sub-wavelength plasmonic dimers formed by two silver strips separated by a thin dielectric spacer and embedded in a uniform dielectric media. We demonstrate that the off-resonant electric and resonant, geometric shape-leveraged, magnetic polarizabilities of the dimer element can be designed to have close absolute values in a certain spectral range, resulting in a predominantly unidirectional scattering of the incident field due to pronounced magneto-electric interference. Switching between forward and backward directionality can be achieved with a single element by changing the excitation wavelength, with the scattering direction defined by the relative phases of the polarizabilities. We extend the analysis to some periodic configurations, including the specific case of a perforated metal film, and discuss the differences between the observed unidirectional scattering and the extraordinary transmission effect. The unidirectional response can be preserved and enhanced with periodic arrays of dimers and can find applications in nanoantenna devices, integrated optic circuits, sensors with nanoparticles, photovoltaic systems, or perfect absorbers; while the option of switching between forward and backward unidirectional scattering may create interesting possibilities for manipulating optical pressure forces.

Holographic photopolymerization for fabrication of electrically switchable inorganic-organic hybrid photonic structures

Holography offers a versatile, rapid and volume scalable approach for making large area, multi-dimensional, organic PBGs; however, the small refractive index contrast of organics prevents formation of a complete band-gap. The introduction of inorganic nanoparticles to the structure provides a possible solution. In contrast to the multiple steps (exposure, development and infiltration) necessitated by lithographic-based holography (e.g. photoresists), holographic photopolymerization of monomer-nanoparticle suspensions enables one-step fabrication of multidimensional organic-inorganic photonic band gap (PBG) structures with high refractive index contrast. The PBGs are formed by segregation of semiconductor nanocrystals during polymerization of the polymer network. Addition of CdSe/ZnS polymerization of the highly cross-linked polymer network. Addition of CdSe/ZnS quantum dots or ZnO nanocrystals to the H-PDLCs formulation results in phase segregation of the nanoparticles into the liquid crystal rich lamellae, producing photonic structures with high diffraction efficiencies that may be modulated by application of an external electric field.

Influence of morphology on the lasing behavior of pyrromethene 597 in a holographic polymer dispersed liquid crystal reflection grating

Interference lithography of polymer dispersed liquid crystals allows rapid, facile fabrication of complex polymeric photonic structures that have an inherent electro-optic component for agile structures. The polymerization mechanism (step-growth or chain growth) strongly influences the morphology of the LC droplet and distribution within the polymer matrix. Using a multi-functional acrylate monomer that undergoes chain growth polymerization leads to asymmetrical LC droplets of random size and distribution, in contrast to the step-growth mechanism of thiol-ene formation where LC droplets form with a nearly uniform size distribution and spherical shape. Thiol-ene holographic polymer dispersed liquid crystals (H-PDLCs) diffraction structures have narrower bandwidth and less baseline scatter than the acrylatebased H-PDLCs. Furthermore, distributed feedback lasers constructed from thiolene-based H-PDLC lasers show marked improvement in the optical and electro-optical properties as evinced by the factor of two decrease in switching voltage and the reduction of lasing threshold from 0.17 mJ cm-2 to 0.07 mJ cm-2. These differences in optical and electro-optic properties directly correlate with the difference in microscale morphology of the H-PDLCs giving insight to the importance of microscale structure on macroscale phenomenon.

Direct measurement of group delay dispersion in metamagnetics for ultrafast pulse shaping

In this paper, we explore the use of magnetic resonant metamaterials, so called metamagnetics, as dispersive elements for optical pulse shaping. We measure both positive and negative group delay dispersion (GDD) values in a metamagnetic material using the multiphoton interference phase scan (MIIPS) technique and show pulse temporal profiles numerically. The results are compared with finite element models. These GDD properties of metamagnetics, along with previously shown tunability and loss control with gain media, enable their use in ultrashort pulse optical applications.

Visible light initiated thiol-ene based reflection H-PDLCs

Multifunctional acrylate formulations containing nematic liquid crystals have been shown to form holographic polymer dispersed liquid crystal gratings (H-PDLCs) easily using ultra-violet AND/OR visible photoinitiators. Laser wavelengths of 364, 476, 488, 514, 532 and 647 nm have been used for the fabrication of the gratings. Recently, the use of a thiol-ene based monomer system has been shown to overcome some of the adverse effects like post polymerization, voltage creep, and non-uniform shrinkage incurred when using highly functional acrylate monomers. However, Bragg reflection gratings have only been demonstrated utilizing ultra-violet (UV) (363.8 nm Argon ion) photopolymerization. Using UV irradiation and single prism geometry limits the upper end of the reflection notch wavelength. In this work, we report on new visible photoinitiator systems developed for the formation of reflective H-PDLCs using thiol-ene monomers. Using these new photoinitiator systems, reflection notches have been routinely written from the visible to the near infrared (IR) regions. The visible photoinitiator systems included the photoinitiator and radical generator titanocene organo-metallic complex (commercially known as Irgacure 784 (Ciba-Geigy), Rhodamine 6G, Pyrromethene, and a radical generating organic peroxide as coinitiator. Reflection gratings were written using laser wavelengths 442, 488, and 532 nm with diffraction efficiencies (DEs) above 70%. Angle tuning allowed for gratings with reflection notches in the near IR (900-1500 nm) to be written using these initiator systems. Rhodamine 6G was found to be more efficient than the other two initiators. We discuss here this new chemistry, the morphology, and electro-optical properties of the reflection gratings.

RF Performance of Single Sideband Modulation Versus Dual Sideband Modulation in a Photonic Link

Single sideband optical modulation can be used in multiple RF photonic link applications. However, single sideband optical modulation is often thought to provide lower RF output power than traditional dual sideband modulation techniques. While true in one specific case, it is not true in all cases. We theoretically and experimentally analyze the RF performance of a photonic link utilizing a Z-cut dual-electrode Mach-Zehnder intensity modulator with a 90° RF hybrid that produces optical single sideband modulation. The single sideband performance is compared to double sideband modulation using the same Mach-Zehnder modulator in either a push-pull configuration using a 180° RF hybrid or a single-arm drive configuration. The optical sideband powers, RF output power and the output intercept power and spur-free dynamic range of the third-order intermodulation nonlinearity are compared both theoretically and experimentally for each of the three cases. The performance of a double sideband modulated link using an X-cut inherently push-pull Mach-Zehnder modulator is also compared theoretically, assuming the same Vπ and insertion loss as the dual-electrode Mach-Zehnder modulator.

Cooperative bi-exponential decay of dye emission coupled via plasmons

Bi-exponential decay of dye fluorescence near the surface of plasmonic metamaterials and core-shell nanoparticles is shown to be an intrinsic property of the coupled system. Indeed, the Dicke, cooperative states involve two groups of transitions: super-radiant, from the most excited to the ground states and sub-radiant, which cannot reach the ground state. The relaxation in the sub-radiant system occurs mainly due to the interaction with the plasmon modes. Our theory shows that the relaxation leads to the population of the sub-radiant states by dephasing the super-radiant Dicke states giving rise to the bi-exponential decay in agreement with the experiments. We use a set of metamaterial samples consisting of gratings of paired silver nanostrips coated with Rh800 dye molecules, having resonances in the same spectral range. The bi-exponential decay is demonstrated for AuSiO2ATTO655 core-shell nanoparticles as well, which persists even when averaging over a broad range of the coupling parameter.

Optical characterizations on surface-polished polycrystalline YAG fibers

The superior thermal and optical properties of transparent polycrystalline ceramics make them attractive alternatives to glass-based materials for laser gain media. Fibers have other advantages of compactness, vibration-resistance, and reduced cooling requirements. Recently it was found that surface roughness caused by grain boundary grooving dominated optical scattering even though there were other scattering sources in the fiber. Therefore, a lot of effort went to fabrication of fibers with smooth surfaces. A mechanical polishing method for polycrystalline YAG fibers was developed. The fiber surface roughness was reduced, while maintaining a circular cross-section. Surface-polished 1.5% Ho-doped polycrystalline YAG fiber, 62 mm long with 31 μm diameter, was fabricated, and lasing was demonstrated from this fiber. Effects of surface-polishing on the surface roughness and scattering coefficient are presented, and lasing characteristics are discussed.

Investigation of plasmonic enhancement in a quantum dot-in-a-well structure

Metallic metamaterial structures are used in nanophotonics applications in order to localize and enhance an incident electromagnetic field. We have theoretically and experimentally studied resonant coupling between plasmonic modes of an SRR array and a quantum dot-in-a-well (DWELL) heterostructure. The near-field distribution from the SRRs on the GaAs substrate was first modeled by electromagnetic simulations and optimized SRR dimensions for maximum nearfield coupling at the peak absorption were extracted. The DWELL sample with a ground state emission peak at 1240 nm was grown by molecular beam epitaxy on a semi-insulating GaAs substrate. The sample was uniformly covered with an array of SRRs, and patterned by standard electron-beam-lithography. In order to study the near field coupling of the plasmonic structure into the DWELL, optical characterization was performed on the SRR-DWELL heterostructure, including room temperature photoluminescence, and transmission measurement.