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University of Nebraska - Lincoln

Stephen Ducharme Publications

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Full-Text Articles in Physics

Effect Of Dipolar Molecules On Carrier Mobilities In Photorefractive Polymers, Arosha Goonesekera, Stephen Ducharme May 1999

Effect Of Dipolar Molecules On Carrier Mobilities In Photorefractive Polymers, Arosha Goonesekera, Stephen Ducharme

Stephen Ducharme Publications

The grating formation speed in photorefractive polymers is greatly reduced by highly polar molecules incorporated by necessity in large concentrations to produce large diffraction efficiency and two-beam energy coupling gain. The random electric fields generated by these dipoles interfere with charge transport by increasing the width of the hopping site energy distribution and thus greatly reducing the carrier mobility and the photorefractive speed. We conducted transport studies of several model systems consisting of combinations of two polymer binders, six charge transport agents (four for holes and two for electrons), and varying concentrations of two highly polar electro-optic chromophores. The results …

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Influence Of Dynamical Scattering In Crystalline Poly„Vinylidene, C.N. Borca, Jaewu Choi, Shireen Adenwalla, Stephen Ducharme, Peter A. Dowben, Lee Robertson, V.M. Fridkin, S.P. Palto, N. Petukhova Jan 1999

Influence Of Dynamical Scattering In Crystalline Poly„Vinylidene, C.N. Borca, Jaewu Choi, Shireen Adenwalla, Stephen Ducharme, Peter A. Dowben, Lee Robertson, V.M. Fridkin, S.P. Palto, N. Petukhova

Stephen Ducharme Publications

The effective Debye temperature of poly(vinylidene fluoride-trifluoroethylene) copolymers was measured using photoemission and neutron diffraction techniques. An effective Debye temperature of 53611K is obtained from the photoemission data and 6963.5K from neutron diffraction measurements. This effective Debye temperature is a consequence of the temperature-dependent dynamic motions perpendicular to the surface of these crystalline polymer films.

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Measurement Of The Photorefractive Grating Phase Shift In A Polymer By An Ac Phase-Modulation Technique, Martin M Liphardt, Stephen Ducharme Jul 1998

Measurement Of The Photorefractive Grating Phase Shift In A Polymer By An Ac Phase-Modulation Technique, Martin M Liphardt, Stephen Ducharme

Stephen Ducharme Publications

The complex coupling constant of a photorefractive polymer was measured as a function of an applied electric field by use of a modified ac phase modulation technique. We determined that both a photorefractive index grating and a nonphotorefractive absorption grating were present. The electric field dependencies of the amplitude and the phase of the photorefractive gain coefficient were accurately described by standard photorefractive theory. The accuracy of the photorefractive phase shift as measured by this phase-modulation technique was ±1° near phase shifts of 90° and ±3° near phase shifts of 45°.

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Two-Dimensional Ferroelectric Films, Alexander V. Bune, Vladimir M. Fridkin, Stephen Ducharme, Lev M. Blinov, Serguei P. Palto, Alexander V. Sorokin, S. G. Yudin, A. Zlatkin Feb 1998

Two-Dimensional Ferroelectric Films, Alexander V. Bune, Vladimir M. Fridkin, Stephen Ducharme, Lev M. Blinov, Serguei P. Palto, Alexander V. Sorokin, S. G. Yudin, A. Zlatkin

Stephen Ducharme Publications

Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second- order ferromagnetic phase transitions have been observed in monolayer magnetic films [1,2], where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature [3]. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature [4]. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 Å in diameter [5], in perovskite films …

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Critical Point In Ferroelectric Langmuir-Blodgett Polymer Films, Stephen Ducharme, A.V. Bune, L.M. Blinov, V.M. Fridkin, S.P. Palto, A.V. Sorokin, S.G. Yudin Jan 1998

Critical Point In Ferroelectric Langmuir-Blodgett Polymer Films, Stephen Ducharme, A.V. Bune, L.M. Blinov, V.M. Fridkin, S.P. Palto, A.V. Sorokin, S.G. Yudin

Stephen Ducharme Publications

The ferroelectric critical point has been found in a ferroelectric polymer by exploring the influence of the electric field on the paraelectric-ferroelectric phase transition. Dielectric and pyroelectric measurements on 30-monolayer-thick films of the crystalline copolymer poly(vinylidene fluoride-trifluoroethylene) grown by Langmuir-Blodgett deposition show a single hysteresis loop below the zero-field phase transition temperature Tc0=80±10 °C, double hysteresis loops between Tc0 , and the critical temperature Tcr=145±5 °C, and no hysteresis above Tcr where the critical electric field is Ecr=0.93±0.1X109 V/m.

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Low-Field Hole Mobility In A Photorefractive Polymer, Arosha Goonesekera, Stephen Ducharme, James M. Takacs, Lei Zhang Nov 1997

Low-Field Hole Mobility In A Photorefractive Polymer, Arosha Goonesekera, Stephen Ducharme, James M. Takacs, Lei Zhang

Stephen Ducharme Publications

We present the time-of-flight measurements of hole mobility in a photorefractive polymer composite as a function of temperature and applied electric field. The analysis shows that the temperature dependence of the low-field mobility is in apparent disagreement with the predictions of the Gaussian disorder model and also with polaron models.

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Effect Of Beam Attenuation On Photorefractive Grating Erasure, Martin M Liphardt, Arosha Goonesekera, Stephen Ducharme, James M. Takacs, Lei Zhang Oct 1996

Effect Of Beam Attenuation On Photorefractive Grating Erasure, Martin M Liphardt, Arosha Goonesekera, Stephen Ducharme, James M. Takacs, Lei Zhang

Stephen Ducharme Publications

We investigate the influence of attenuation on the speed of erasure of photorefractive gratings by solving the coupled-wave equations in the undepleted pump approximation and by taking into account the attenuation and Gaussian intensity profile of all the beams. The extrinsic grating decay rate is significantly lower than the intrinsic photorefractive decay rate in samples with overall attenuation as low as 10%. The Gaussian beam profiles of the readout and the erasing beams result in a further reduction of the extrinsic decay rate. The results of these calculations are used to determine the spectrum of intrinsic decay rates in a …

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Novel Switching Phenomena In Ferroelectric Langmuir-Blodgett Films, A. Bune, Stephen Ducharme, V.M. Fridkin, L. Blinov, S. Palto, N. Petukhova, S. Yudin Dec 1995

Novel Switching Phenomena In Ferroelectric Langmuir-Blodgett Films, A. Bune, Stephen Ducharme, V.M. Fridkin, L. Blinov, S. Palto, N. Petukhova, S. Yudin

Stephen Ducharme Publications

We have established ferroelectric switching and observed a novel conductance switching phenomenon in ferroelectric polymer films fabricated by the Langmuir–Blodgett technique. The films consist of 10–30 monolayers (ML) of a copolymer of vinylidene fluoride (70%) and trifluoroethylene (30%), exhibiting a first-order ferroelectric phase transition at 70 °C and nearly rectangular ferroelectric and conductance hysteresis at 24 °C. Ferroelectric switching is accompanied by switching of the conductance by three orders of magnitude. A model describing this switching phenomenon is proposed.

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Photoconductivity And Grating Response Time Of A Photorefractive Polymer, Brian E. Jones, Stephen Ducharme, Martin M Liphardt, Arosha Goonesekera, James M. Takacs, Lei Zhang, Rajan Athalye Jun 1994

Photoconductivity And Grating Response Time Of A Photorefractive Polymer, Brian E. Jones, Stephen Ducharme, Martin M Liphardt, Arosha Goonesekera, James M. Takacs, Lei Zhang, Rajan Athalye

Stephen Ducharme Publications

We report the photoconductive properties and photorefractive grating response time of a polymer mixture composed of 40-wt. % dissolved diethylamino-benzaldehyde diphenyl hydrazone (DEH) and the non-cross-linking epoxy polymer Bisphenol A 4,4'-nitroaminostilbene. The films have improved photoconductive sensitivities as high as 2.1 × 10–10 cm/(W Ω) at a wavelength of 650 nm with a corresponding reduction of the grating response time constant to 0.11 ± 0.02 s at an intensity of 1 W/cm2. The nitro-aminostilbene chromophore is deduced to be the source of photogenerated charge carriers on the basis of a comparison of the wavelength dependence of the …

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Electric-Field Stabilization And Competition Of Gratings In A Photorefractive Polymer, Stephen Ducharme, Brian Jones, James M. Takacs, Lei Zhang Jan 1993

Electric-Field Stabilization And Competition Of Gratings In A Photorefractive Polymer, Stephen Ducharme, Brian Jones, James M. Takacs, Lei Zhang

Stephen Ducharme Publications

We report observations of electric-field stabilization of gratings and grating competition in a new photorefractive polymer mixture composed of 40 wt. % diethylamino-benzaldehyde-diphenyl hydrazone dissolved in Bisphenol A 4,4'-nitroaminotolane. Gratings formed with a 650-nm optical wavelength in 145-μm-thick films at 138-kV/cm bias field have a diffraction efficiency as high as 0.11%, require an energy density of –1 J/cm2 for formation, and show cancellation, revelation, and electric-field-stabilized persistence. We propose a model in which fast recording is associated with one set of photosensitive traps, whereas a slower cancellation and subsequent revelation are associated with a competing grating formed …

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Pyro-Electro-Optic Phase Gratings, Stephen Ducharme Nov 1991

Pyro-Electro-Optic Phase Gratings, Stephen Ducharme

Stephen Ducharme Publications

A new physical mechanism is proposed for generating transient phase gratings in transparent pyroelectric materials. The mechanism combines the pyroelectric and electro-optic effects to convert spatial intensity variations into transient phase gratings. The grating diffraction efficiency increases with a figure of merit that is proportional to the pyroelectric, electro-optic, and absorption coefficients and is inversely proportional to the specific heat. The grating response time is proportional to the specific heat and inversely proportional to the thermal conductivity. Diffraction efficiencies that approach unity are predicted in several readily available electro-optic materials with modest pulse energies from Q-switched Nd:YAG lasers. Observation …

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Observation Of The Photorefractive Effect In A Polymer, Stephen Ducharme, J.C. Scott, R.J. Twieg, W.E. Moerner Apr 1991

Observation Of The Photorefractive Effect In A Polymer, Stephen Ducharme, J.C. Scott, R.J. Twieg, W.E. Moerner

Stephen Ducharme Publications

We report the first observation of the photorefractive effect in a polymeric material, the electro-optic polymer bisphenol-A-diglycidylether 4-nitro-1,2-phenylenediamine made photoconductive by doping with the hole-transport agent diethylamino-benzaldehyde diphenylhydrazone. The gratings formed exhibit dynamic writing and erasure, strong electric-field dependence, polarization anisotropy, and estimated space-charge fields up to 26 kV/cm at an applied field of 126 kV/cm. Application of similar concepts should provide a broad new class of easily fabricated photorefractive materials.

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Intracavity Frequency Doubling Of A Nd:Yag Laser With An Organic Nonlinear Optical Crystal, Stephen Ducharme, W.P. Risk, W.E. Moerner, Victor Y. Lee, R.J. Twieg, G.C. Bjorklund Aug 1990

Intracavity Frequency Doubling Of A Nd:Yag Laser With An Organic Nonlinear Optical Crystal, Stephen Ducharme, W.P. Risk, W.E. Moerner, Victor Y. Lee, R.J. Twieg, G.C. Bjorklund

Stephen Ducharme Publications

Your access to Appl. Phys. Lett. is provided through the subscription of Univ Nebraska-Lincoln Lib. What is this? Scitation Citing Articles | CrossRef Citing Articles | All Citing Articles [ Previous / Next Abstract | Issue Table of Contents | Bottom of Page ] Applied Physics Letters -- August 6, 1990 -- Volume 57, Issue 6, pp. 537-539 Full Text: [ PDF (639 kB) ] Order Rightslink Permissions for Reuse About Rightslink view MyArticles What is this? Select up to 20 articles at a time. Intracavity frequency doubling of a Nd:YAG laser with an organic nonlinear optical crystal Stephen Ducharme, …

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Photodarkening Profiles And Kinetics In Chalcogenide Glasses, Stephen Ducharme, J. Hautala, P.C. Taylor Jun 1990

Photodarkening Profiles And Kinetics In Chalcogenide Glasses, Stephen Ducharme, J. Hautala, P.C. Taylor

Stephen Ducharme Publications

The spatial and temporal evolution of photodarkening in chalcogenide glasses is simulated by a simple kinetic model and tested against experiments. In the model, photodarkening, a gradual decrease of the energy of the absorption edge caused by above-band-gap illumination, is the additional absorption at defects created by the absorbed light. The defect-creation rate is proportional to the rate at which photons are absorbed either directly at the defect site or, alternatively, at a neighboring site. The depletion of the incident light within the sample is included and the resulting equations cast into a universal form that is applicable to realistic …

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Absorption At Radio Frequencies In Superconducting Y1Ba2Cu3OY, Stephen Ducharme, R. Durny, J. Hautala, D.J. Zheng, P.C. Taylor, O.G. Symko, S. Kulkarni Aug 1989

Absorption At Radio Frequencies In Superconducting Y1Ba2Cu3OY, Stephen Ducharme, R. Durny, J. Hautala, D.J. Zheng, P.C. Taylor, O.G. Symko, S. Kulkarni

Stephen Ducharme Publications

The large magnetic-field-dependent ac absorption in superconducting Y1Ba2Cu3Oy ceramics and powders decreases slowly with increasing frequency in the range 2–16 MHz. The magnetic-field-dependent ac absorption is observed below Tc in superconducting Y1Ba2Cu3Oy ceramics, powders and twinned crystals and in (La,Sr)2 CuO2 and BaPb0.75 Bi0.25 CuO3 powders and appears to contain distinct contributions from trapped flux and/or intragrain tunnel junctions and from ordinary superconductor surface impedance. The contribution from trapped flux and/or intragrain tunnel junctions is important in dc magnetic fields …

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Electrooptic And Piezoelectric Measurements In Photorefractive Barium Titanate And Strontium Barium Niobate, Stephen Ducharme, Jack Feinberg, R.R. Neurgaonkar Dec 1987

Electrooptic And Piezoelectric Measurements In Photorefractive Barium Titanate And Strontium Barium Niobate, Stephen Ducharme, Jack Feinberg, R.R. Neurgaonkar

Stephen Ducharme Publications

We have measured the low-frequency ("unclamped") electrooptic and piezoelectric coefficients in undoped BaTiO3and SrxBa1-xNb2O6 (x = 0.61) crystals using interferometric techniques. The contribution of the piezoelectric effect to the Pockels measurements is discussed. For an applied ac electric field in the range 0.1-200 V/cm, the electrooptic and piezoelectric effects are linear in the magnitude of the applied field and independent of its frequency in the range 10 Hz-100 kHz. The unclamped electrooptic coefficients of poled BaTiO3 single crystals are r13= 19.5 pm ±1 pm/V and r33 = …

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Microwave Absorption In The Superconducting And Normal Phases Of Y-Ba-Cu-O, R. Durný, J. Hautala, Stephen Ducharme, B. Lee, O.G. Symko, P.C. Taylor, D.J. Zheng, J.A. Xu Aug 1987

Microwave Absorption In The Superconducting And Normal Phases Of Y-Ba-Cu-O, R. Durný, J. Hautala, Stephen Ducharme, B. Lee, O.G. Symko, P.C. Taylor, D.J. Zheng, J.A. Xu

Stephen Ducharme Publications

Microwave absorption in a dc magnetic field up to 12 kG, attributed to nonequilibrium contributions to the ac susceptibility, appears at Tc as the sample is cooled. ESR measurements of Y-Ba-Cu-O show that Cu2+ exists only in the fraction of the sample which is not superconducting in a distorted octahedral surrounding.

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Photorefraction In Batio3, Stephen Ducharme Dec 1986

Photorefraction In Batio3, Stephen Ducharme

Stephen Ducharme Publications

This thesis summarizes the results of experimental investigations of the photorefractive properties of melt-grown BaTiO3 single crystals. Three basic photorefractive material properties are studied: (1) the effective density of photorefractive charges, (2) the photoconductivity of the photorefractive charges, and (3) the relative contributions of electron and hole photoconduction. Volume holographic measurement techniques are used: two-beam energy coupling, four-wave mixing, and erasure of volume holograms. The photorefractive properties were altered experimentally by chemically reducing or oxidizing a BaTiO3 crystal. I have extended the “hopping conduction” model of photorefractive transport to include simultaneous electron and hole photoconduction. The extension of …

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Altering The Photorefractive Properties Of Batio3 By Reduction And Oxidation At 650 °C, Stephen Ducharme, Jack Feinberg Feb 1986

Altering The Photorefractive Properties Of Batio3 By Reduction And Oxidation At 650 °C, Stephen Ducharme, Jack Feinberg

Stephen Ducharme Publications

The photorefractive Properties of a nominally pure single crystal of BaTiO3 were altered by treating the crystal at 650 °C in oxygen at different partial pressures. Treatment altered the effective density of photorefractive charge carriers in the crystal and couls convert an inactive crystal into an active one. Treatment at low oxygen pressure (reduction) decreased the temperature of the tetragonal-to-cubic phase transition of the crystal and also decreased the measured optical band gap, implying that oxygen vacancies had been introduced into the bulk crystal. These oxygen vacancies are associated with negative photorefractive charge sonors. Either hole transport or electron transport …

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Speed Of The Photorefractive Effect In A Batio3 Single Crystal, Stephen Ducharme, Jack Feinberg Aug 1984

Speed Of The Photorefractive Effect In A Batio3 Single Crystal, Stephen Ducharme, Jack Feinberg

Stephen Ducharme Publications

We present data on the speed of light-induced refractive index changes in a BaTiO3 single crystal. The light-induced erasure rate of a refractive index grating is shown to depend on optical intensity as I x where x<1. The exponent x depends weakly on temperature and increases from 0.62±0.02 to 0.71±0.02 when the temperature is varied between 12 and 40 °C. The sublinear dependence of rate on intensity implies that higher optical intensity is required to achieve high-speed operation of BaTiO3 devices than previously thought. The dark erasure rate has an anomolously strong temperature dependence; it increases by a factor of …

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Thermal Propagation And Stability In Superconducting Films, Kenneth E. Gray, Robert T. Kampwirth, John F. Zasadzinski, Stephen P. Ducharme Jan 1983

Thermal Propagation And Stability In Superconducting Films, Kenneth E. Gray, Robert T. Kampwirth, John F. Zasadzinski, Stephen P. Ducharme

Stephen Ducharme Publications

Thermal propagation and stable hot spots (normal domains) are studied in various high Tc superconducting films (Nb3Sn, Nb, NbN and Nb3Ge). The prediction of the thermal propagation velocity of the long-standing model of Broom and Rhoderick (1960) is verified quantitatively in the regime of its validity. A new energy balance model is shown to give reasonable quantitative agreement of the dependence of the propagation velocity on the length of short normal domains. The steady state (zero velocity) measurements indicate the existence of two distinct situations for films on high thermal conductivity (sapphire) substrates. For low power per unit area the …

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