James R. Heflin

Analysis of optical response of long period fiber gratings to nm-thick thin-film coatings

We theoretically and experimentally demonstrated that the resonant wavelength of long period fiber gratings (LPG) could be shifted in a large magnitude by coating only nm-thick thin-film whose refractive index is higher than that of the glass cladding. The resonant wavelength shift results from either the variation of the thickness of the film and/or the variation of the refractive index of the film. These results demonstrate the sensitivity of LPG-based sensors could be enhanced by using a sensing thin film with an allowed large thickness and high refractive index. This coating schematic offers an efficient platform for achieving high-performance index-modulating fiber devices and high-performance index/thickness-sensing LPG-based fiber sensors for detecting optical property variations of the sensing thin-film coating.

Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers

An ionically self-assembled monolayer (ISAM) technique for thin-film deposition has been employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second-order, χ(2), nonlinear optical response. As a result of the ionic attraction between successive layers, the ISAM χ(2) films self-assemble into a noncentrosymmetric structure that has exhibited no measurable decay of χ(2) at room temperature over a period of more than one year. The second-harmonic intensity of the films exhibits the expected quadratic dependence on film thickness up to at least 100 bilayers, corresponding to a film thickness of 120 nm. The polarization dependence of the second-harmonic generation yields a value of 35° for the average tilt angle of the nonlinear optical chromophores away from the surface normal.

Creation of a gradient polymer-fullerene interface in photovoltaic devices by thermally controlled interdiffusion

Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface.

Ion transport and storage of ionic liquids in ionic polymer conductor network composites

We investigate ion transport and storage of ionic liquids in ionic polymer conductor network composite electroactive devices. Specifically, we show that by combining the time domain electric and electromechanical responses, one can gain quantitative information on transport behavior of the two mobile ions in ionic liquids (i.e., cation and anion) in these electroactive devices. By employing a two carrier model, the total excess ions stored and strains generated by the cations and anions, and their transport times in the nanocomposites can be determined, which all depend critically on the morphologies of the conductor network nanocomposites.

Imidazolium sulfonate-containing pentablock copolymer–ionic liquid membranes for electroactive actuators

Block copolymer–ionic liquid composite materials emerge as promising candidates for electromechanical transducer applications. Herein, a novel imidazolium sulfonate-containing pentablock copolymer–ionic liquid composite was prepared and fabricated into thermally stable electroactive actuators, which exhibit effective actuation response under a low applied potential of 4 V.

Thickness dependence of curvature, strain, and response time in ionic electroactive polymer actuators fabricated via layer-by-layer assembly

enhanced curvature (0.43 mm � 1 ) and large net intrinsic strain (6.1%). The results demonstrate that curvature and net strain of IEAP actuators due to motion of the anions increase linearly with the thickness of the CNC as a result of the increased volume in which the anions can be stored. In addition, after subtracting the curvature of a bare Nafion actuator without a CNC, it is found that the net intrinsic strain of the CNC layer is independent of thickness for the range of 20‐80 nm, indicating that the entire CNC volume contributes equivalently to the actuator motion. Furthermore, the response time of the actuator due to anion motion is independent of CNC thickness, suggesting that traversal through the Nafion membrane is the limiting factor in the anion motion. V C 2011 American Institute of Physics. [doi:10.1063/1.3590166]

The influence of void space on antireflection coatings of silica nanoparticle self-assembled films

This study investigates the deposition by ionic self-assembly of alternating silica nanoparticle and poly(allyamine hydrochloride) layers with the goal to create a single-material antireflection coating. The condition that the optical thickness of the film be equal to λ∕4 can be satisfied by depositing the requisite number of bilayers to obtain minimum reflectivity at the chosen wavelength. The second condition for antireflection, that the index of refraction of the film be equal to nc=n1n2, where n1 and n2 are the refractive indices of the media on each side of the film, requires that nc=1.22 for a film with air on one surface and glass (assuming n=1.50) on the other. Such a low index of refraction can be created in films consisting of nanoparticles if the proper volume fraction of void space exists in the film. In the wavelength range λ=350–700nm, minimum reflectivities of ⩾2.0%, ⩽0.2%, and ⩽0.2% were obtained with films created on both sides of a glass slide using 15, 45, and 85nm average diameter sili...

Highly sensitive optical response of optical fiber long period gratings to nanometer-thick ionic self-assembled multilayers

Ionic self-assembled multilayers deposited on long period fiber gratings (LPGs) yield dramatic resonant-wavelength shifts, even with nanometer-thick films. Fine control of the refractive index and the thickness of these films was achieved by altering the relative fraction of the anionic and cationic materials combined with layer-by-layer deposition. We demonstrate the feasibility of this highly controllable deposition technique for fine-tuning grating properties. In addition a variety of biological and chemical sensing agents can easily be incorporated into these films, which makes this an attractive platform for realization of high-performance LPG-based sensors.

Layer-by-layer self-assembled conductor network composites in ionic polymer metal composite actuators with high strain response

We investigate the electromechanical response of conductor network composite (CNC) fabricated by the layer-by-layer (LbL) self-assembly method. The process makes it possible for CNCs to be fabricated at submicron thickness with high precision and quality. This CNCs exhibits high strain ∼6.8% under 4 V, whereas the RuO2/Nafion CNCs exhibit strain ∼3.3%. The high strain and submicron thickness of the LbL layers in an ionic polymer metal composite (IPMC) yield large and fast actuation. The response time of a 26 μm thick IPMC with 0.4 μm thick LbL CNCs to step voltage of 4 V is 0.18 s.

Millisecond switching in solid state electrochromic polymer devices fabricated from ionic self-assembled multilayers

The electrochromic switching times of solid state conducting polymer devices fabricated by the ionic self-assembled multilayer method has been investigated. The devices were composed of bilayers of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) and poly(allylamine hydrochloride) on indium tin oxide substrates. Devices fabricated from 40 bilayer thick films have coloration and decolaration switching times of 31 and 6ms, respectively, with low applied voltage (1.4V) for an active area of 0.6cm2. The switching times have been shown to decrease with the active area of the electrochromic device suggesting that even faster electrochromic switching times are possible for devices with smaller areas.