Leonard S. Fifield

From molecules to opto-chips: organic electro-optic materials

Recent advances in polymeric electro-optic materials and device fabrication techniques have significantly increased the potential for incorporation of these materials and devices into modern high bandwidth (fiber and wireless) telecommunication, information processing, and radar systems. Charge transfer π-electron chromophores characterized by molecular first hyperpolarizability (second order optical non-linearity) values approaching 3000×10 –30 esu have been synthesized. Elucidation of the role of intermolecular electrostatic interactions in inhibiting the efficient translation of molecular optical non-linearity to macroscopic electro-optic activity has permitted systematic modification of materials to achieve electro-optic coefficients approaching 100 pm V –1 . Improvements in the optical loss of polymeric materials at wavelengths of 1.3 and 1.55 µm have been effected. Mode matching of passive transmission and active electro-optic waveguides has been addressed, permitting a dramatic reduction in insertion loss. The putative ability of polymeric electro-optic materials to be efficiently integrated with very large scale integration semiconductor electronic circuitry and with passive optical circuitry has been demonstrated. Several devices of varying degrees of complexity have been fabricated and evaluated to operational frequencies as high as 150 GHz. The operational stability of polymeric devices is very competitive with devices fabricated from lithium niobate and gallium arsenide.

Conducting polymer, carbon nanotube, and hybrid actuator materials

The electromechanical actuation performance of carbon nanotube mats, polypyrrole films and hybrid nanotube-polypryrrole materials has been compared. The hybrid materials were formed by coating nanotube mats with polypyrrole using vapour deposition and electropolymerisation techniques. When the coating time was short, the hybrid materials showed the electrochemical responses typical of polypyrrole and retained the porous structure of the nanotube mats. The actuator response of the different materials was determined isotonically at different applied loads. The nanotube mat and hybrid materials gave actuator strains that were largely insensitive to the applied stress up to ~ 10 MPa. The hybrid materials were virtually identical to the uncoated nanotube mats in terms of actuator performance. A simple model showed that the actuator strain depends upon the difference in elastic modulus of the actuator material in the doped and undoped states.

Electrochemically driven actuators from conducting polymers, hydrogels, and carbon nanotubes

The mechanisms of actuation operating in polymeric actuators are reviewed along with a comparison of actuator performance. Polymer hydrogel actuators show very large dimensional changes, but relatively low response times. The mechanism of actuation involves several processes including electro-osmosis and electrochemical effects. Conducting polymer actuators operate by Faradaic reactions causing oxidation and reduction of the polymer backbone. Associated ion movements produce dimensional changes of typically up to 3%. The maximum stress achieved to date from conducting polymers is not more than 10 MPA. Carbon nanotubes have recently been demonstrated as new actuator materials. The nanotubes undergo useful dimensional changes (approximately 1%) but have the capacity to respond very rapidly (kHz) and generate giant stresses (600 MPa). The advantages of nanotube actuators stem from their exceptional mechanical properties and the non-Faradaic actuation mechanism.

Spin-echo in the phosphorescent triplet state of crystalline 2-indanone

Abstract The relative ordering of the three sublevels of the phosphorescent triplet state of 2-indanone was determined by correlating optically detected magnetic resonance (ODMR) spectra taken with the sample in a Zeeman field with calculated spectra. Spin coherence was observed in the phosphorescent triplet state of 2-indanone in neat and doped single crystals by conventional and real time detection. The Hahn electron spin-echo signal was observed in real time by a pulse sequence which terminates with a continuous-wave low power field (LPF). The Fourier transform of the echo decay as a function of τ yielded a peak at 9.0 MHz which was attributed by isotopic substitution studies to nuclear coupling of the triplet electrons via the α-protons in the molecule. Dephasing of the Hahn and stimulated spin echo in 2-indanone due to energy trapping was measured with chemically mixed crystals of 2-indanone prepared with varying concentrations of quinoxaline. Energy transfer to quinoxaline which served as a deep trap contributed to the dephasing of the echo.

Pneumatic Actuator Response from Carbon Nanotube Sheets

Reversible actuation strains in excess of 2% in the sheet direction and over 300% in the thickness direction have been produced by single wall carbon nanotube mats when electrochemically charged to +1.5V (vs. SCE) in aqueous sodium chloride solution. The observed strains represent a ten-fold increase over that previously reported for carbon nanotube actuators, and is considerably larger than that achievable with polymer ferroelectric actuators. The enhanced actuator strains result from a new mechanism of electrochemically induced “pnuematic actuation” where high pressure gas forms within the porous structure of the nanotube mat causing partial delamination and swelling. An erasable “memory” effect was also observed for pneumatic actuation driven by hydrogen gas evolution/storage in the nanotube electrodes.

Optical Fiber Switch Based on Carbon Nanotube Actuation

Carbon nanotubes represent an attractive material option for many applications, including electromechanical actuators. Though single wall carbon nanotubes exhibit advantageous actuator properties, such as large force generation and low operating voltage, functional devices based on carbon nanotube actuation have not yet been reported. Here we describe the fabrication and performance evaluation of a 1×2 electromechanical optical fiber switch based on a carbon nanotube actuator. The side-to-side movement of the input fiber of the device between two output fibers is a result of the actuation of an assembly of carbon nanotubes that have been attached to the fiber. The intensities of optical signals exiting the two outputs are monitored, and switching times down to 30 ms are demonstrated. Initial results indicate that mechanical optical switches using carbon nanotube actuators may be preferable to switches using alternative technologies due to the inexpensive assembly, low operating power, potentially high switching speeds, and potentially low insertion loss of the carbon nanotube based devices.

Investigation of the Capacitance Minimum of Unannealed Single-walled Carbon Nanotube Papers in Aqueous Sodium Chloride

The electrochemical properties of carbon nanotube (NT) assemblies are relevant for many potential nanotube applications including super-capacitors, batteries, fuel cells and actuators. In this work, the double-layer capacitance of a paper of single-walled carbon nanotubes is determined for a series of concentrations of NaCl in water. The dependence of capacitance on potential was also determined in an effort to locate the potential of zero charge (PZC) for each NaCl concentration. The double-layer capacitance of the NT paper is seen to increase with electrolyte concentration, while the PZC (capacitance minimum) is seen to depend more on the sequence of electrolyte concentration tested (sample history) than on the concentration of electrolyte itself.