M. Joseph Roberts

Effective permittivity near zero in nanolaminates of silver and amorphous polycarbonate

Experiments were conducted to demonstrate a material with epsilon near zero (ENZ). Dimensions estimated by effective medium theory guided the fabrication of nanolaminate composites of silver and amorphous polycarbonate. This approach ensures that the ordinary component (not the extraordinary component) of the relative permittivity of a uniaxial material equals zero. The nanolaminates were characterized for optical properties using spectroscopic ellipsometry, reflectance, and transmittance. Simulations using both, a new scattering retrieval method, and an effective-medium approximation (EMA) were compared to the experimental results. These results indicate that nanolaminates should enable further exploration into the new optical phenomena predicted for ENZ materials.

Computational Fluid Dynamics Models of Molecularly Imprinted Materials in Microfluidic Channels

Abstract : Current research will lead to rapid-prototyping of chemical sensors that utilize microfabricated molecularly imprinted (MI) materials. CFD/CAD software may be used to model flow and chemical binding properties of Ml materials in microfluidic channels. Use of this type of software expedites results when changes in properties are made. The surface concentration of bound analyze on a monolithic molecularly imprinted polymer (MIP) within microfluidic channels can be modeled using its experimental binding kinetics. The time necessary to reach a detection limit is calculated and optimized as a function of flow parameters. In this report, we discuss the unique issues associated with the modeling of chemical sensors that utilize MI materials.

Alternating polyanion/polycation second-order nonlinear optical films by aqueous solution deposition

Alternating polyelectrolyte deposition (APD) in aqueous solutions may be used to process nonlinear optical polymers (NLOPs) into noncentrosymmetric ordered films at ambient temperature. Second-order NLOP films were prepared by alternately dipping a substrate into aqueous solutions of a polycation and a polyanion. Polyepichlorohydrin substituted with stilbazolium side-chain chromophore was used as the cationic NLOP. The inactive polyanion was polystyrene sulfonate. Uniform layer to layer deposition is observed as evidenced by a linear increase of UV-Visible absorbance and quadratic increase of second harmonic generated light intensity as a function of film thickness. Films have been uniformly deposited up to 24 bilayers. Films have been further characterized by contact angle measurements, interferometry, and polarized light microscopy. Work is in progress to deposit thicker films of the same quality and to quantify NLO figures of merit.

Accordion polymers for nonlinear optical applications

Abstract The design and synthesis of mainchain chromophore polymers in a syndioregic (head-to-head) configuration is described. Polymers were designed for both electric-field poling and Langmuir-Blodgett-Kuhn (LBK) deposition. LBK deposition gives intrinsically polar films (no E-field poling) as evidenced by nonlinear optical (χ (2) ) properties.

Fabrication of Multilayer Metal-Dielectric Nanofilms for Coupled Plasmon Resonant Devices

We have developed a process for production of laterally continuous silver layers alternated with glassy polymer films in which the thickness is on the order of 15 nm and 100 nm respectively. Such films can be used to study physical phenomena associated with the coupled plasmon resonance and the resonant transmission in the forbidden bands. Such films may also find applications in photonic bandgap and other nanoplasmonic devices. Since the surface plasmon and evanescent coupling is a means to propagate light inside nanocircuits, the investigation of the coupled surface plasmon in the multilayer structures provides us with fundamental knowledge for the 3D integration. The fabrication technique also allows design flexibility, for example, systems with regions of single M-D-M plane together with multilayer structures facilitating tunable multiple plasmon resonance wavelength response from a single system. Multiple plasmon wavelength resonance absorptions may be obtained from such systems. Utilizing polymer films as the dielectric enables design flexibility and increases the number of applications of the fabricated devices.

Nonlinear optical films by alternating polyelectrolyte deposition

Acentric films of stilbazolium-substituted polyepichlorohydrin (SPECH) and poly(sodium 4- styrenesulfonate) (PSS) were formed using alternating polyelectrolyte deposition (APD) on hydrophobic glass substrates. APD is a layer-by-layer technique for the formation of polymer films by alternately immersing a substrate in aqueous solutions of a polyanion and a polycation. APD provides precise control of the overall film thickness that through automated processing may exceed a thousand layers. The peak maximum UV-visible absorbance in transmission through the SPECH/PSS films was linear as a function of the number of bilayers. Second harmonic generation (SHG) was used as a tool to indicate acentric order to polarizable side-chain chromophores within the APD films. The SHG exhibited the expected quadratic intensity increase with film thickness after 30 bilayers had been deposited. During slow temperature ramping of the SPECH/PSS APD films on hydrophobic glass, in situ SHG measurements revealed that 90 percent of the polar order is retained at temperature well over 120 degrees C. The additional minima in the Maker fringe data, created by interference of the second harmonic waves generated at the two sides of a double-coated substrate, were found to go to zero, which is indicative of high quality films. The observed high thermal stability of the polar order makes the APD films attractive for device applications.

Nonlinear Optical Films by Alternating Polyelectrolyte Deposition on Hydrophobic and Hydrophilic Substrates

The formation of acentric films using alternating polyelectrolyte deposition (APD) has been achieved on hydrophobic and hydrophilic glass substrates. APD is a layer-by-layer technique for the formation of polymer films by alternately immersing a substrate in aqueous solutions of a polyanion and a polycation. APD provides precise control of the overall film thickness that through automated processing may exceed a thousand layers. In this study, APD films were made of an NLO-active polycation, stilbazolium-substituted polyepichlorohydrin (SPECH), and NLO-inactive polyanions. The peak maximum UV-Visible absorbance in transmission through the films was linear as a function of the number of bilayers. Second harmonic generation (SHG) was used as a tool to indicate acentric order of polarizable sidechain chromophores within the APD films. The SHG exhibited the expected quadratic intensity increase with film thickness.

Second-order nonlinear optical films for alternating polyelectrolyte deposition

This is a preliminary report on alternating polyelectrolyte deposition (APD) with the goal of making electro-optic films at room temperature. By means of the APD method, films were built up layer-by-layer at room temperature by a dipping process. APD requires two complementary polymers, one being negatively charged and the other being positively charged. The polymer in solution is attracted to the solid substrate by coulombic forces to form the next layer on the substrate. For the first time, second-order nonlinear optical (NLO) polymer films were prepared by APD in which both polymers were NLO-active. The cationic polymer contained a side-chain stilbazole chromophore, and the anionic polymer contained a main-chain cinnamoyl chromophore. The intensity of the second harmonic signal generated in the film increased quadratically with each layer for the first twenty layers.

AgInS2 quantum dots for the detection of trinitrotoluene.

AgInS2 (AIS) quantum dots (QDs) were synthesized via a thermal decomposition reaction with dodecylamine as the ligand to help stabilize the QDs. This reaction procedure is relatively easy to implement, scalable to large batches (up to hundreds of milligrams of QDs are produced), and a convenient method for the synthesis of chalcogenide QDs. Metal powders of AgNO3 and In(NO3)3, were used as the metal precursors while diethyldithiocarbamate was used as the sulfur source. The AIS QDs were characterized via transmission electron microscopy, atomic force microscopy, and energy dispersive x-ray spectroscopy. As an application for these less toxic nanomaterials, we demonstrate the selective detection of Trinitrotoluene (TNT) at concentrations as low as 6 micromolar (μM) and without the functionalization of a ligand that is specifically designed to interact with TNT molecules. We also demonstrate a simple approach to patterning the AIS QDs onto filter paper, for the detection of TNT molecules by eye. Collectively, the ease of the synthesis of the less toxic AIS QDs, and the ability to detect TNT molecules by eye suggest an attractive route to highly sensitive and portable substrates for environmental monitoring, chemical warfare agent detection, and other applications.

Nanofiber waveguides and photophysics of an organic dye

Abstract. We present previously unreported properties of the organic molecular dye compound, 2,3,7,8-tetracyano-5,10-dimethyl-5,10-dihydrodipyrazino[2,3-b:2’,3’-e]pyrazine (DMTC-pyrazine) (CAS Reg. No. 150960-11-5), including photophysical properties with particular emphasis on optical waveguiding. We show that DMTC-pyrazine can self-assemble into monoclinic crystals that are thermally stable over 300°C. Through shape-engineering, optimized growth conditions produced high yields of nanofiber (aspect ratio >6) crystals. No photobleaching of this dye was observed in air even after hours of on-resonant illumination. Excitation of photoluminescence within the DMTC-pyrazine nanofibers propagates in tightly confined guided modes (up to 25  μm). The finesse, end-facet reflectance, and group refractive index of nanofiber DMTC-pyrazine Fabry-Pérot cavities are reported.