Barrett G Potter

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• McCloy, J. S., & Potter, B. G. (2013). Photoluminescence in Chemical Vapor Deposited ZnS: Insight into electronic defects. Optical Materials Express, 3(9), 1273-1278.
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  Abstract: Photoluminescence spectra taken from chemical vapor deposited (CVD) ZnS are shown to exhibit sub-band-gap emission bands characteristic of isoelectronic oxygen defects. The emission spectra vary spatially with position and orientation with respect to the major axis of CVD growth. These data suggest that a complex set of defects exist in the band gap of CVD ZnS whose structural nature is highly dependent upon local deposition and growth conditions, contributing to inherent heterogeneity in optical behavior throughout the material. © 2013 Optical Society of America.
• Beal, R. J., Kana, J. K., & Potter Jr., B. G. (2012). Phase assembly and photo-induced current in CdTe-ZnO nanocomposite thin films. Applied Physics Letters, 101(3).
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  Abstract: Sequential radio-frequency sputtering was used to produce CdTe-ZnO nanocomposite thin films with varied semiconductor-phase extended structures. Control of the spatial distribution of CdTe nanoparticles within the ZnO embedding phase was used to influence the semiconductor phase connectivity, contributing to both changes in quantum confinement induced spectral absorption and carrier transport characteristics of the resulting nanocomposite. An increased number density of CdTe particles deposited along the applied field direction produced an enhancement in the photo-induced current observed. These results highlight the opportunity to employ long-range phase assembly as a means to control optoelectronic properties of significant interest for photovoltaic applications. © 2012 American Institute of Physics.
• Potter Jr., B. G., Beal, R. J., & Allen, C. G. (2012). Nanoassembly control and optical absorption in CdTe-ZnO nanocomposite thin films. Journal of Applied Physics, 111(3).
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  Abstract: The spatial distribution of CdTe nanoparticles within a ZnO thin-film matrix was manipulated using a dual-source, sequential radio-frequency (RF)-sputter deposition technique to produce nanocomposite materials with tuned spectral absorption characteristics. The relative substrate exposure time to each sputtering source was used to control the semiconductor phase connectivity, both within the film plane and along the film growth direction, to influence the degree of photocarrier confinement and the resulting optical transition energies exhibited by the CdTe phase. Significant changes (up to δE ≈ 0.3 eV) in the absorption onset energy for the CdTe nanoparticle ensemble were produced through modification in the extended structure of the semiconductor phase. Raman spectroscopy, cross-sectional transmission electron microscopy, and x-ray diffraction were used to confirm the phase identity of the CdTe and ZnO and to characterize the nanostructures produced in these composite films. Isochronal annealing for 5 min at temperatures up to 800 C further indicated the potential to improve film crystallinity as well as to establish the post-deposition thermal processing limits of stability for the semiconductor phase. The study highlights the significance of ensemble behavior as a means to influence quantum-scale semiconductor optical characteristics of import to the use of such materials as the basis for a variety of optoelectronic devices, including photosensitized heterojunction components in thin film photovoltaics. © 2012 American Institute of Physics.
• Shih, G. H., Allen, C. G., & Potter Jr., B. G. (2012). Interfacial effects on the optical behavior of Ge:ITO and Ge:ZnO nanocomposite films. Nanotechnology, 23(7).
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  PMID: 22261039;Abstract: Nanophase semiconductors are of interest for their unique, size-tunable solar spectral absorption characteristics as well as their potential to contribute to the improved energy conversion efficiency of photovoltaics (PV). Embedding these nanoparticles within electrically active transparent conductive oxides (TCO) can also provide an opportunity for enhanced, long-range carrier transport. However, differences in the atomic and electronic structure, dielectric behavior, and chemistry between the matrix and semiconductor phases highlight the influence of interfacial effects on the optical absorption properties of the composite. In this work, nanocomposites of Ge:indium tin oxide (Ge:ITO) and Ge:ZnO were fabricated with sequential RF-magnetron sputtering and annealed at temperatures from 310 to 550°C to investigate the impact of matrix identity on this interface and its contribution to nanostructure-mediated optical absorption. Transmission electron microscopy showed a decrease in Ge nanocrystal size relative to the initial semiconductor domain size in both matrices that was correlated with an increase in absorption onset energy after annealing. The effect was particularly pronounced in Ge:ITO composites in which Raman spectroscopy indicated the presence of germanium oxide at the semiconductorITO interface. These results support the primary contribution of carrier confinement in the Ge nanophase to the shifts in absorption onset energies observed. © 2012 IOP Publishing Ltd.
• Allen, C. G., Shih, G. H., & Potter Jr., B. G. (2010). Inorganic photo-sensitized transparent conductive oxide (TCO) nanocomposite thin films for photovoltaic (PV) energy conversion. Materials Research Society Symposium Proceedings, 1211, 20-25.
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  Abstract: Nanophase semiconductor composites are widely researched for the development of third-generation photovoltaic (PV) devices. Through quantum-size effects and phase assembly manipulation the optical absorption and carrier transport properties of nanocomposite films can be influenced. We investigate the potential for improved PV-relevant material performance by examining the photo-sensitization of indium-tin-oxide (ITO) with nanophase germanium (Ge). Nanocomposite films are produced by a sequential, RF-magnetron sputter deposition technique. Deposition control and post-deposition annealing are used to demonstrate the manipulation of the extended-assembly of the nanocrystalline Ge phase. Optical absorption characteristics were correlated to variations in the composite film structure as confirmed by transmission electron microscopy. In addition to structure-dependent variation in spectral absorption, spectrally resolved photoconductivity measurements demonstrate enhanced photoconductivity of composite films associated with the incorporation of the Ge phase into the ITO host. These results support the further evaluation of such nanocomposite TCO materials in optoelectronic devices, including PV systems. © 2010 Materials Research Society.
• Allen, C. G., Shih, G. H., & Potter Jr., B. G. (2010). Microstructural and optical characterization of germanium:indium tin oxide (Ge:ITO) nanocomposite films. Materials Science and Engineering B: Solid-State Materials for Advanced Technology, 175(2), 150-158.
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  Abstract: The nanophase assembly and resulting optical and electronic properties of Ge:ITO composite thin films, produced by a sequential RF-sputtering deposition approach, were manipulated via deposition conditions and subsequent isochronal thermal anneals. The study examined the combined influences of thermally induced changes in phase crystallinity, semiconductor-phase morphology, and Ge-ITO interfacial structure on properties of relevance to photovoltaic function. A range of Ge-phase spatial distributions within the ITO embedded phase were produced, including isolated Ge nanocrystals and two-dimensional-extended semiconductor structures, as evaluated using cross-sectional transmission electron microscopy. The magnitude of a quantum-confinement induced blue-shift in the Ge absorption onset increased monotonically with increased isochronal anneal temperature and was concomitant to the decrease in connectivity of the as-deposited Ge-phase assembly. Raman spectroscopy, over the range of nanocomposite structures examined, confirmed the evolution of a germanium oxide interfacial structure anticipated to affect both carrier confinement within the Ge and long-range charge transport in the nanocomposite. Shifts in the near-infrared transmission edge with anneal temperature were further correlated, using Hall-effect measurements, with a thermally equilibrated free carrier population. An increased free carrier density in composite films, over that of similarly treated single-phase ITO, was attributed to the presence of the Ge semiconductor-phase. While a general reduction in carrier mobility accompanied the increased carrier density, resistivities of the composite films were found to be largely insensitive to the nanostructure morphology changes and, moreover, were comparable to that of single-phase ITO films produced under similar preparation conditions. Finally, optical excitation at energies resonant with the Ge absorption onset, but below the band-gap of the ITO, resulted in a photoconductive response attributed to photoelectron transfer from the Ge-phase to the ITO. © 2010 Elsevier B.V. All rights reserved.
• Allen, C. G., Shih, G. H., Beal, R. J., & Potter Jr., B. G. (2010). Nanophase semiconductors embedded within transparent conductive oxides matrices as optical sensitizers for photovoltaic applications. Proceedings of SPIE - The International Society for Optical Engineering, 7772.
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  Abstract: The optical absorption of a transparent conductive oxide (TCO), which is often used as the basis for junction or contact layers in thin film photovoltaics, can be tailored by incorporating a nanophase semiconductor (SC) component. Using a, dual-source, sequential R.F. magnetron sputter deposition technique, we manipulate the optical and electronic properties of SC:TCO composites by varying the local and extended nanophase assembly and composition. The present study explores nanocomposite systems based on Ge:ZnO and Ge:ITO. The impact of host material (ITO vs. ZnO) on the evolution of nanostructure is investigated. Heat treatment of the as-deposited films results in an increased crystallinity of the TCO and SC components, confirmed by X-ray diffraction and Raman spectroscopy studies. The presence of the SC phase is found to influence TCO grain growth and crystallographic orientation, and modification of the SC phase distribution is coincident with the morphological development of the TCO phase in both composite systems. Upon heattreatment, the high-energy optical absorption edge of the nanocomposite is blue-shifted compared to that of the corresponding as-deposited material. This indicates the development of quantum-confinement conditions for photocarriers within the Ge phase which leads to an increased energy gap over that expected for the more bulk-like, asdeposited Ge material. Under the deposition and thermal treatment conditions used in the present study, the spectral absorption response is consistent between the ZnO and ITO-based thin films examined. This suggests that carrier confinement conditions are mediated by the development of similar Ge-phase local spatial extent and Ge:TCO interfacial structures in both systems, regardless of TCO identity. © 2010 SPIE.
• Beal, R. J., Kalva, M., & Potter Jr., B. G. (2010). Elevated-temperature structural transformation and optical transmission behavior of KH₂PO₄ single crystals. Journal of the American Ceramic Society, 93(4), 1089-1095.
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  Abstract: Thermally induced optical opacity in macroscopic potassium dihydrogen phosphate (KDP) single crystals is a significant issue in the application of these materials in optical systems subjected to adverse environments. The present study investigates the relationship between thermally mediated structural transformations in KDP (associated with both a high-temperature phase transition and lattice dehydration effects) and the development of microcracking-induced optical scattering. In this case, the kinetics of these processes are examined under elevated-temperature, isothermal conditions. A significant temperature-dependence in the elapsed time required to observe both the structural transformation (via in situ Raman spectroscopy) and crystal opacification (via single-beam optical transmission) is observed. The phenomena are further correlated spatially within partially transformed specimens examined using optical microscopy and micro-Raman spectroscopy, demonstrating the inherent heterogeneous nature of the structural transformation in these bulk crystals. In this study, a change in the isothermal hold temperature of only 10°C about the phase transition point decreased the elapsed time to transformation by nearly two orders of magnitude, significantly impacting an evaluation of optical performance in these materials under thermally variable conditions. © 2010 The American Ceramic Society.
• Shih, G. H., Allen, C. G., & Potter Jr., B. G. (2010). RF-sputtered Ge-ITO nanocomposite thin films for photovoltaic applications. Solar Energy Materials and Solar Cells, 94(5), 797-802.
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  Abstract: Nanocomposite thin films, composed of a germanium nanocrystalline phase embedded within a tin-doped indium oxide (ITO) matrix, were produced using a multisource, sequential, RF-magnetron sputter deposition technique. The influence of nanocomposite structure on the resulting optical absorption and carrier transport properties was investigated in the context of the use of such materials as functional elements in thin film photovoltaic architectures. Deposition controls and post-deposition thermal anneals were successful in modifying the phase assembly of the nanocomposites, enabling the manipulation of Ge volume fraction, nanocrystallite size and morphology, and spatial distribution within the ITO embedding phase. Modifications in semiconductor nanostructure were correlated with changes in nanocomposite spectral absorption that were consistent with quantum-size-induced variation in Ge absorption onset energy, despite the close agreement in electron affinity between the Ge and ITO components. This suggests the formation of a high band-gap (low electron affinity) interfacial phase between the Ge and ITO components of the nanocomposite. Increased free-carrier (n-type) densities and spectrally resolved photoconductivity were also associated with the presence of the Ge phase. These results emphasize the impact of local and extended length scale structure on properties of importance to photovoltaic performance in semiconductor-based nanocomposites and the utility of the sequential sputter deposition method as a means to manipulate nanocomposite structure. © 2010 Elsevier B.V. All rights reserved.
• Musgraves, D. J., Potter Jr., B. G., & Boyle, T. J. (2009). Nanostructure development in photodeposited, titania-based thin films. Journal of Materials Research, 24(11), 3372-3379.
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  Abstract: Ultraviolet irradiation (λ = 248 nm) was used to photocatalyze a solution of the heteroleptic titanium alkoxide (OPy)2Ti(TAP) 2 [where OPy = pyridine carbinoxide and TAP = 2,4,6 tris(dimethylamino)phenoxide], leading to the deposition of a titania-based thin film only in the exposed region. The effect of water addition to the (OPy) 2Ti(TAP)2 pyridine solution on the properties of the final photodeposited film structure was examined by using vibrational spectroscopy and electron microscopy. Under consistent optical exposure conditions, the amount of water added altered the nanoscale porosity of the final material produced. Films deposited from a solution with a 1:1 H2O/Ti content exhibited surface pores ∼100 nm in diameter, whereas a 4:1 ratio yielded 10-nm pores, and material produced from a 8:1 solution appeared fully condensed. In addition, the effect of postdeposition thermal treatments on the nanostructure and chemistry of the photodeposited films was examined. © 2009 Materials Research Society.
• Musgraves, J. D., Potter Jr., B. G., & Boyle, T. J. (2008). Direct fabrication of physical relief structures via patterned photodeposition of a titanium alkoxide solution. Optics Letters, 33(12), 1306-1308.
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  PMID: 18552940;Abstract: Ultraviolet (λ=248nm) excitation of a photosensitive Ti alkoxide solution was found to generate a metal-oxide-based insoluble film on substrates in contact with the solution during illumination. Patterned deposition of 100 μm wide lines of material was demonstrated using a slit-shaped aluminum shadow mask during exposure. Stylus profilometry confirmed that the average thickness of the photodeposited film monotonically varied with accumulated UV fluence, exhibiting thicknesses of 10 to 310 nm for fluences of 12 and 192 J/cm2, respectively. Moreover, the surface profile of the film surface at fluences greater than 12 J/cm2 was found to reproduce the near-field Fresnel diffraction pattern anticipated from the slit mask used. © 2008 Optical Society of America.
• Musgraves, J. D., Potter Jr., B. G., & Boyle, T. J. (2008). Photo-induced deposition of nanostructured thin films by UV-exposure of heteroleptic Ti-alkoxide solutions. Materials Research Society Symposium Proceedings, 1074, 64-70.
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  Abstract: Photodeposited physical relief structures have been formed via the UV-irradiation of solutions of the heteroleptic titanium alkoxide, (OPy) 2Ti(TAP)2. Raman studies of the films produced confirm the photoinitiation of hydrolysis and condensation reactions in the molecular structure, leading to the development of an insoluble solid phase. Using a shadow mask technique, these films were deposited directly from solution with microscale patterning and their structure and properties were studied as a function of both irradiation conditions (UV-fluence and intensity) as well as precursor solution chemistry (water content). Variations in nanostructure are visible in scanning electron micrographs, with a higher solution water content producing a more condensed, i.e. less porous, film. © 2008 Materials Research Society.
• Potter Jr., B. G., Musgraves, J. D., & Boyle, T. J. (2008). Photo-initiation of intermolecular bonding and oxide deposition in Ti-based alkoxide solutions. Journal of Non-Crystalline Solids, 354(18), 2017-2022.
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  Abstract: Photoexposure of a water-stable, Ti-alkoxide, (OC6H6N)2Ti(OC6H2(CH2N(CH3)2)3-2,4,6)2, dissolved in a water/pyridine solution yielded an insoluble, solid phase, present in the form of suspended particles and a deposited material on the vertical side-wall of the solution container. Raman analysis of these photoproducts confirmed the UV-initiation of hydrolysis and condensation reactions in the alkoxide reagent. The excitation wavelength dependence of the photostructural effects observed provides insight into the mechanisms contributing to these photoinduced phenomena. The results are extended to enable the photopatterned deposition of physical relief structures directly from solution in this system. © 2007 Elsevier B.V. All rights reserved.
• Chandra, H., Potter Jr., B. G., Jamison, G. M., & Thomes Jr., W. J. (2007). Vacuum-ultraviolet spectroscopy measurement of poly(methylphenylsilylene) photosensitivity. Journal of Applied Physics, 102(3).
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  Abstract: Photoinduced optical absorption changes in the vacuum-ultraviolet (VUV) spectral range have been measured in poly(methylphenylsilylene) thin films under varied excitation photon energies and local atmospheric environments. Spectral changes in resonances associated with both the linear chain Si-Si backbone and the side groups of the hybrid structure are consistent with the photodisruption of backbone topology. These effects are more pronounced under a higher energy photon exposure (5.10 eV) resonant with the fundamental π - π* transition of the phenyl moiety. An aerobic environment also favors more dramatic bleaching of VUV absorption in these materials. Finally, the present study enables a Kramers-Kronig analysis of absorption change from the visible to the VUV. These results do not adequately describe the photoinduced refractive index changes measured at 632.8 nm via ellipsometry, indicating the presence of other contributions to the index modifications observed. © 2007 American Institute of Physics.
• Musgraves, J. D., Potter Jr., B. G., Sewell, R. M., & Boyle, T. J. (2007). Photo-induced structural changes in titanium alkoxides for directing molecular assembly. Materials Research Society Symposium Proceedings, 960, 131-136.
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  Abstract: The effects of ultraviolet irradiation on a heteroleptic titanium alkoxide ((OPy)2Ti(4MP)2) have been investigated. The molecule has been studied in solution and in thin film form using FTIR and Raman spectroscopies. Quantum computational modeling was used to associate vibrational modes with structural moieties present in the molecule. In all cases examined, a preferential photoinduced modification in vibrational resonances linked to the 4-mercaptophenol (4MP) ligand was observed. In contrast, little or no change was exhibited in the vibrational structure of the OPy ligands. © 2007 Materials Research Society.
• Musgraves, J. D., Potter, J., Sewell, R. M., & Boyle, T. J. (2007). Preferential photostructural modification of heteroleptic titanium alkoxides for molecular assembly. Journal of Materials Research, 22(6), 1694-1700.
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  Abstract: The response of a mononuclear, heteroleptic titanium alkoxide [(OPy)2Ti(4MP)2, where OPy = pyridinecarbinol; NC5H4(CH2O) and 4MP = 4-mercaptophenol; OC6H4(SH)] to ultraviolet (UV) irradiation in dilute solution and in solid-state samples has been measured. Vibrational spectroscopy [Fourier transform infrared (FTIR) absorption and Raman scattering] was used to monitor changes in molecular structure upon exposure to 337.1- and 365-nm light. Assignment of spectral features to vibrational modes of the molecule was aided by a normal-mode analysis of the energy-minimized molecular structure within a density-functional theory framework. Photoinduced decreases in peak areas were observed in both FTIR spectra of the precursor solutions and Raman data collected from solution-cast films of the precursor material. These changes were associated with vibrational modes localized at the 4MP ligands. Conversely, no significant modification of vibrational structure associated with the OPy moiety was observed under the excitation conditions examined. In a related study, thin films of the precursor were cast, sampled, and irradiated with UV light in scintillation vials under hydrated air (40% relative humidity) and dry Ar to evaluate the influence of local atmospheric composition on the photoresponse. An increase in the magnitude of photoinduced vibrational changes was observed in the moist-air environment, again associated primarily with the 4MP ligand. The results support an interpretation of these structural changes in terms of a preferential enhancement of hydrolysis at the 4MP site under these conditions. These findings are discussed in the context of an optically driven molecular assembly strategy based on the photoinitiation of intermolecular bonding at selected sites about the metal center. © 2007 Materials Research Society.
• Riley, M. R., DeRosa, D., Blaine, J., Potter Jr., B. G., Lucas, P., Coq, D. L., Juncker, C., Boesewetter, D. E., Collier, J. M., Boussard-Plédel, C., & Bureau, B. (2006). Biologically inspired sensing: Infrared spectroscopic analysis of cell responses to an inhalation health hazard. Biotechnology Progress, 22(1), 24-31.
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  PMID: 16454488;Abstract: This work describes the development of a biologically based sensing technique to quantify chemical agents that pose inhalation health hazards. The approach utilizes cultured epithelial cells (A549 human type II pneumocytes) of the lung, exposed to potential toxins and monitored through the noninvasive means of infrared spectroscopy to quantify changes to cell physiology and function. Cell response to Streptolysin O, a cholesterol-binding cytolysin, is investigated here. Infrared spectra display changes in cell physiology indicative of membrane damage, altered proteins, and some nucleic acid damage. Methods to improve cell adhesion through modification of support surface properties are detailed. This spectroscopic approach not only provides a robust means to detect potential toxins but also provides information on modes of damage and mechanisms of cellular response. © 2006 American Chemical Society and American Institute of Chemical Engineers.
• Potter Jr., B. G., Tikare, V., & Tuttle, B. A. (2000). Monte Carlo simulation of ferroelectric domain structure and applied field response in two dimensions. Journal of Applied Physics, 87(9 I), 4415-4424.
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  Abstract: A two-dimensional, lattice-Monte Carlo approach, based upon the energy minimization of an ensemble of electric dipoles, was developed to simulate ferroelectric domain behavior. The model utilizes a Hamiltonian for the total energy based upon electrostatic terms involving dipole-dipole interactions, local polarization gradients, and the influence of applied electric fields. The impact of boundary conditions on the domain configurations obtained was also examined. In general, the model exhibits domain structure characteristics consistent with those observed in a tetragonally distorted ferroelectric. The model was also extended to enable the simulation of ferroelectric hysteresis behavior. Simulated hysteresis loops were found to be very similar in appearance to those observed experimentally in actual materials. This qualitative agreement between the simulated hysteresis loop characteristics and real ferroelectric behavior was also confirmed in simulations run over a range of simulation temperatures and applied field frequencies. © 2000 American Institute of Physics.
• Barbour, J. C., & Potter, B. G. (1999). Control of optical performance from Er-doped alumina synthesized using an ECR plasma. Materials Research Society Symposium - Proceedings, 504, 387-392.
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  Abstract: Hydrogen in deposited optical ceramics can modify the optical properties, and therefore the role of the hydrogen needs to be understood to control its effects. Erbium-doped amorphous alumina films were deposited using simultaneous electron beam evaporation of aluminum and erbium while bombarding the sample with 30 eV O2(+) ions from an electron cyclotron resonance (ECR) plasma. The hydrogen content was measured, using elastic recoil detection, as a function of isochronal annealing treatments. The data was fit to a simple trap-release model in order to determine an effective activation energy for the thermal release of H from alumina and Er-doped alumina. The intensity of the ion-beam stimulated luminescence from these samples was monitored in the visible and near infrared regions as a function of the thermal treatments. In order to gain a better understanding of the influence of hydrogen, the ionoluminescence (IL) data from samples containing hydrogen were fit with a simple linear equation.
• Boyle, T. J., Tyner, R. P., Alam, T. M., Scott, B. L., Ziller, J. W., & Potter Jr., B. G. (1999). Implications for the thin-film densification of TiO₂ from carboxylic acid-modified titanium alkoxides. Syntheses, characterizations, and X-ray structures of various carboxylic acid modified titanium alkoxides (see abstract). Journal of the American Chemical Society, 121(51), 12104-12112.
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  Abstract: Carboxylic acid (HORc)-modified Ti(OR)4 products were used to study the effect that similarly ligated species with substantially varied structures have on the final densification of the resultant ceramic (in this case TiO2). The 1:1 stoichiometric products isolated from the reactions of [Ti(μ-ONep)(ONep)3]2 (1, ONep = OCH2CMe3) and a variety of sterically hindered carboxylic acids [HORc: HOFc (HO2CH), HOAc (HO2CCH3), HOPc (HO2CCHMe2), HOBc (HO2CCHMe3), or HONc (HO2CCHCMe3)] were identified by single-crystal X-ray diffraction and solid-state 13C MAS NMR spectroscopy as Ti3(μ3-O)(OFc)2(ONep)8 (2), Ti3(μ3O)(OAc)2(ONep)8 (3), Ti6(μ3- O)6(OPc)6(ONep)6 (4), Ti2(μ-OBc)2(ONep)6 (5), and Ti3(μ3- O)(ONc)2(ONep)8 (6). Compounds 2, 3, and 6 adopt a triangular arrangement of Ti atoms linked by a μ3-oxide moiety with ORc and ONep ligands supporting the basic framework. Compound 4 adopts a distorted, hexagon-prism geometry of two offset [Ti-O-]3 rings with each six-coordinated metal possessing a terminal ONep and two monodentate OPc ligands. The unique, nonesterified product 5 is dimeric with two μ-ONep, two unidentate bridging OBc, and two terminal ONep ligands. The solution behaviors of 2-6 were investigated by NMR experiments and were found to retain the solid-state structure in solution with a great deal of ligand rearrangement. Films of TiO2 were made from redissolved crystals of 2-6. The highest density TiO2 thin films were derived from the partially hydrolyzed, trinuclear, low-carbon-containing ONep complexes 2 and 3, as determined from ellipsometric data.
• Potter Jr., B. G., & Sinclair, M. B. (1998). Photosensitive and rare-earth doped ceramics for optical sensing: A review. Journal of Electroceramics, 2(4), 295-308.
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  Abstract: This work reviews two inorganic materials classes used for optical sensors: photosensitive glasses and rare-earth doped materials. The underlying physical mechanisms contributing to the photosensitive response of germanosilicate glasses are discussed, along with a summary of optical sensor strategies which have been developed based upon photo-imprinted Bragg gratings. Particular attention is paid to the fabrication and characterization of photosensitive germanosilicate thin-films. Insights into the ability to control the photosensitivity of these materials through manipulation of material compositions and structures are also included. The discussion of rare-earth doped optical sensor materials emphasizes the important contribution of the host material to the observed optical behavior of the rare-earth ions. The use of rare-earth doped materials in applications ranging from temperature sensing to fiber-optic gyroscopes is also described.
• Verdozzi, C., Jennison, D. R., Schultz, P. A., Sears, M. P., Barbour, J. C., & Potter, B. G. (1998). Unusual structural relaxation for rare-earth impurities in sapphire: Ab initio study of lanthanum. Physical Review Letters, 80(25), 5615-5618.
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  Abstract: A first-principles density-functional local density approximation study of substitutional La, using 120-atom supercells of α-Al2O3, shows that La assumes a highly displaced position, moving over 0.5 Å from the original Al site. The La-O nearest neighbor distance is much smaller than the sum of standard La+3 and O-2 ionic radii. This is due to La being more ionic than in its own oxide, and to the role of the 5d shell, as the electron density within the displaced La is quite anisotropic. We discuss the implica-tions for a systematic description of rare-earth ions implanted in sapphire.
• Ochoa, O. R., III, E. W., Colajacomo, C., Simmons, J. H., & Potter Jr., B. G. (1997). Raman characterization of CdTe nanocrystallites embedded in a glass matrix. Journal of Materials Science Letters, 16(8), 613-616.
• Potter Jr., B. G., Dimos, D., & Sinclair, M. B. (1997). Waveguide refractometry as a probe of thin film optical uniformity. Journal of Materials Research, 12(2), 546-551.
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  Abstract: Optical inhomogeneities through the thickness of a sol-gel-derived, spin-coated Pb(Zr,Ti)O3 (PZT) thin film have been evaluated using prism-coupled waveguide refractometry. Unusual waveguide coupling angle behavior has been treated using a multilayer model to describe the optical characteristics of the film. Waveguide refractometry measurements, performed after incremental reductions in film thickness, were used to develop a consistent model for optical inhomogeneity through the film thickness. Specifically, a thin film layer model, consisting of alternating layers of high and low refractive index material, was found to accurately predict irregularities in transverse-electric (TE) mode coupling angles exhibited by the film. This layer structure has a spatial periodicity that is consistent with the positions of the upper film surface at intermediate firings during film synthesis. The correlation emphasizes the impact of the multistep thin-film deposition approach on the optical characteristics of the resulting thin film.
• Sidebottom, D. L., Hruschka, M. A., Potter, B. G., & Brow, R. K. (1997). Increased radiative lifetime of rare earth-doped zinc oxyhalide tellurite glasses. Applied Physics Letters, 71(14), 1963-1965.
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  Abstract: We have investigated the structural and optical properties of rare earth-doped zinc tellurite glasses modified by the substitution of ZnF2. Raman and phonon sideband spectroscopies were employed to characterize changes in the glass structure as well as to probe vibrational behavior in the immediate vicinity of the rare earth ion. These measurements are combined with photoluminescence and optical absorption to monitor the effect of halide substitution upon the optical behavior of the rare earth dopant. A substantial increase in the intrinsic radiative lifetime of Nd3+ is observed with increasing halide concentration. © 1997 American Institute of Physics.
• Sidebottom, D. L., Hruschka, M. A., Potter, B. G., & Brow, R. K. (1997). Structure and optical properties of rare earth-doped zinc oxyhalide tellurite glasses. Journal of Non-Crystalline Solids, 222, 282-289.
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  Abstract: Zinc tellurite glasses appear to be excellent candidates for hosting rare earth ions since they provide a low phonon energy environment to minimize non-radiative losses as well as possessing good chemical durability and optical properties. The optical behavior of the rare earth ion can be manipulated by modifying its local environment in the glass host. We report measurements of the emission lifetime, optical absorption, and vibrational density of states of the glass system (ZnO)x(ZnF2)y(TeO2)1-x-y doped (0.1 mol%) with a series of rare earths. Phonon sideband spectroscopy has been successfully employed to probe vibrational structure in the immediate vicinity of the rare earth ion. We observe a significant increase in the emission lifetime (from ∼ 150 μs to 250 μs) of Nd3+ with increasing fluorine substitution. © 1997 Elsevier Science B.V.
• Sidebottom, D. L., Hruschka, M. A., Potter, B. G., Brow, R. K., & Hudgens, J. J. (1997). Optical properties of lanthanide-containing halide-modified zinc tellurite glasses. Materials Research Society Symposium - Proceedings, 453, 253-258.
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  Abstract: As part of an ongoing investigation to characterize the properties and structure of zinc halide-tellurium oxide glasses, we report preliminary measurements of the optical properties of several Nd- and Er-doped tellurites. Measurements include florescence lifetimes and estimates of the theoretical radiative lifetimes (as obtained by traditional Judd-Ofelt analysis of optical absorption spectra) as well as phonon sideband studies sensitive to vibrational characteristics near the rare earth ion. The response of these optical features to the substitution of alternative halides is examined.
• Barbour, J. C., Potter, B. G., Follstaedt, D. M., Knapp, J. A., & Sinclair, M. B. (1996). Er-doped amorphous and crystalline Al₂O₃ and La₂O₃ films grown with low energy ions from an ECR plasma. Materials Research Society Symposium - Proceedings, 438, 465-470.
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  Abstract: The effects of low energy ions from a biased electron cyclotron resonance plasma during growth of Al2O3 and La2O3 are used to modify the density and crystalline quality of these oxide films. The type of phase formed for Al2O3 is varied with the ion-assisted growth from amorphous to crystalline γ-Al2O3. The photoluminescence (PL) properties of different Er-doped Al- and La-oxide phases are examined, and through comparison of the PL spectra, the local environment of Er in these oxide materials is discussed.
• Ochoa, O., Colajacomo, C., J., E., Simmons, J. H., & Potter Jr., B. G. (1996). Quantum confinement effects on the photoluminescence spectra of CdTe nanocrystallites. Solid State Communications, 98(8), 717-721.
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  Abstract: Quantum size and semiconductor volume fraction effects on the intensity, line width and position of photoluminescence (PL) peaks from CdTe nanocrystallites have been measured. CdTe-glass composite thin films were grown by using a sequential rf-magnetron sputtering process. Postdeposition heat treatments were used to produce average crystallite sizes between 1.2 and 16 nm. The PL spectra consist of single, broad peaks that undergo larger blue shifts with decreasing crystallite sizes. The PL signal degrades with increasing nanocrystallite size and is quenched at the higher semiconductor volume fractions. The spectral characteristics of the clusters are discussed in terms of the electron-hole pair and the effect of surface states.
• Yanxia, L. u., Hoelzer, D. T., Schulze, W. A., Tuttle, B., & Potter, B. G. (1996). Grain-oriented ferroelectric bismuth titanate thin film prepared from acetate precursor. Materials Science and Engineering B, 39(1), 41-47.
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  Abstract: Bismuth titanate (Bi4Ti3O12) thin films were fabricated by a spin coating deposition and rapid thermal processing (RTP) technique. The acetate-derived solution for deposition was synthesized by blending dissolved bismuth acetate in aqueous acetic acid and then adding titanium acetate. A series of electrically insulating, semiconducting and conducting substrates were evaluated for Bi4Ti3O12 films deposition. X-ray diffraction indicated that the initial crystallization temperature of the Bi4Ti3O12 films was 500°C or less; a 700°C crystallization treatment was used to obtain single phase films. The Bi4Ti3O12 film crystallographic orientation is shown to depend on three factors: substrate type; number of coating layers; and thermal processing. While preferred c-direction orientation was observed for films deposited on silver foil substrates, preferred a-direction orientation was obtained for films deposited on both Si and Pt coated Si wafers. The films were dense, smooth, crack-free and had grain sizes ranging from 20 to 400 nm. Film thickness and refractive index were determined using a combination of ellipsometry, waveguide refractometry and TEM techniques. The refractive index is close to the value of single crystal BIT.
• Potter, B., Philipps, A. F., Bierny, J. P., & Crowe Jr., C. P. (1995). Neonatal radiology. Acquired diaphragmatic hernia with group B streptococcal pneumonia.. Journal of perinatology : official journal of the California Perinatal Association, 15(2), 160-162.
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  PMID: 7595779;
• Sinclair, M. B., Dimos, D., Potter Jr., B. G., & Schwartz, R. W. (1995). Angularly and spectrally resolved light scattering from lead zirconate titanate thin films. Journal of the American Ceramic Society, 78(8), 2027-2032.
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  Abstract: The results of light-scattering measurements of a series of Pb(Zr,Ti)O3 thin films prepared by a sol-gel method are presented and analyzed. The films differed due to the addition of different concentrations of acetylacetone to the precursor solution immediately prior to film fabrication. Visual inspection of the films indicated improvements in optical quality with the addition of acetylacetone. To quantify these improvements, two types of light-scattering measurements were performed: angularly resolved light scattering and spectrally resolved light scattering. Surprisingly, only slight differences between the films were observed using angularly resolved light scattering at 633 nm. In contrast, the spectrally resolved scattering revealed large differences between the films, with the films prepared using the largest concentrations of acetylacetone exhibiting the lowest scattering. The apparent contradiction between these findings is resolved using a theoretical model for light scattering due to fluctuations in the dielectric constant occurring within the volume of the thin film and by noting that slight thickness differences exist between the films in the series. Analysis of the light scattering from the best sample yields estimates for the amplitude (ξ0 = 0.08) and the characteristic size (τ0 = 110 nm) of the dielectric constant fluctuations. These estimates are consistent with the variations of the dielectric constant expected due to the birefringent, polycrystalline nature of these films.
• Chen, D., Potter, B. G., & Simmons, J. H. (1994). GeO₂SiO₂ thin films for planar waveguide applications. Journal of Non-Crystalline Solids, 178(C), 135-147.
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  Abstract: Various thin films of germanium silicate glass with high germania content have been fabricated by a sol-gel method. After densification, these films have a high optical quality and can be used as planar waveguides. The densification behavior of these films was studied by measurements of the index of refraction, as a function of heat treatment temperatures, using ellipsometry. The relationship of refractive index versus GeO2 content follows the additive Lorentz-Lorenz model. Waveguiding Raman spectroscopy suggested the formation of SiOGe linkages in these films without phase segregation. Various waveguide propagation characteristics, at 632.8 nm, were studied using a prism coupling technique. The propagation loss rate of a 50GeO250SiO2 glass waveguide was measured as 3.31 dB/cm. © 1994.
• Dimos, D., Potter, B. G., Sinclair, M. B., Tuttle, B. A., & Warren, W. L. (1994). Photo-induced and electrooptic properties of (Pb,La)(Zr,Ti)O₃ films for optical memories. Integrated Ferroelectrics, 5(1 pt 3), 47-58.
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  Abstract: Photo-induced hysteresis changes and electrooptic effects in sol-gel Pb(Zr,Ti)O3 (PZT) and (Pb,La)(Zr,Ti)O3 (PLZT) films have been studied in an effort to evaluate these materials for optical memory applications. The films exhibit two distinct, but related, types of photo-induced changes in their hysteresis behavior: 1) a photo-induced change in the coercive voltage and 2) a photo-induced suppression of the switchable polarization. Both types of photo-induced hysteresis changes are due to trapping of photo-generated charge carriers at sites which minimize internal depolarizing fields. The photo-induced changes are reproducible and stable and are, thus, suitable for optical memory applications. In addition, polarization-dependent changes in the refractive indices can be the basis of a nondestructive optical readout technique. To characterize these electrooptic effects, a waveguide refractometry technique has been used to independently determine field-induced changes in the ordinary and extraordinary indices. For an applied field sufficient to saturate the ferroelectric polarization (E = 125 kV/cm), the ratio of the extraordinary to ordinary index change (Δne/Δno) of a Pb(Zr0.53Ti0.47)O3 film was found to be -4/1, leading to a net birefringence change [Δ(ne - no)] of -0.21.
• Potter Jr., B. G., Simmons, J. H., Kumar, P., & Stanton, C. J. (1994). Quantum-size effects on the band edge of CdTe clusters in glass. Journal of Applied Physics, 75(12), 8039-8045.
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  Abstract: The measurement of quantum-size behavior in semiconductor crystals has been examined through an analysis of the size dependence of the semiconductor's absorption edge. In past studies, there appeared to be little agreement between theory and experiment for very small crystals. In this paper, the effects of crystal-size distribution and tunneling of the carrier wave functions into the quantum well barrier are considered. An analysis of the microstructure size and absorption edge of CdTe clusters in a glass matrix is conducted with samples ranging from 0.16 to 0.8 times the exciton Bohr diameter at the Γ point and from 1 to 5 times the exciton Bohr diameter at the L point. Results show fully coupled exciton behavior at the L point and a more complex process at the Γ point. In the latter, the band-gap energy increase with decreasing cluster size is significantly smaller than that calculated using a model in which the photoexcited carriers are assumed to be confined to a monosize set of clusters bounded by an infinite potential well. Analysis presented here shows that this discrepancy can be explained in part by inhomogeneous broadening of the absorptive transition and by carrier penetration into the insulator. The quantum-size behavior of CdTe crystals at the Γ point is subsequently found to follow fully decoupled carrier behavior modeled by Efros and Efros [Sov. Phys. Semicond. 16, 772 (1982)] when the inhomogeneous broadening and tunneling effects are taken into account.
• Potter Jr., B. G., Sinclair, M. B., Dimos, D., Tuttle, B. A., & Schwartz, R. W. (1994). Electro-optical and optical evaluation of Pb(Zr, Ti)O₃ thin films using waveguide refractometry. Journal of Non-Crystalline Solids, 178(C), 69-76.
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  Abstract: Prism-coupled, waveguide refractometry was utilized to independently monitor electric-field-induced changes in the extraordinary and ordinary refractive indices of a Pb(Zr0.53Ti0.47)O3 (PZT 53/47) thin film. Under an electric field, applied normal to the film plane and corresponding to saturation of the electric polarization, the ratio of the extraordinary to ordinary refractive index change, Δne/Δno, is found to be -4/1, contributing to a net birefringence change, Δ(ne - no), of -0.021. The technique thus accesses both diagonal and off-diagonal elements of the electro-optic response tensor describing the macroscopic behavior of the polycrystalline film. In addition, the widths of the waveguide mode reflectivity minima were sensitive to variation in the microstructure of several PZT (40/60) films indicating that the refractometry technique can provide information helpful in evaluating the optical quality in these films. © 1994.
• Simmons, J. H., Pagano, S. J., Downie, L. K., Potter, B. G., Simmons, C. J., Boulos, E. N., & Best, M. F. (1994). Graded-index AR coatings for large angles of inclination. Journal of Non-Crystalline Solids, 178(C), 166-175.
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  Abstract: Antireflective coatings consisting of a single layer of glass with a graded variation in refractive index offer advantages over multi-layer coatings. These include a weaker dependence on angle of incidence at large angles and a flatter response to different wavelenghts over the visible. Consequently, graded-index films are desirable in automotive applications where colorless coatings are required. A sol-gel process is presented for the formation of a graded-index coating of MgOSiO2 with in-diffused fluorine. Refractive index variations over a range 1.46-1.40 are achieved over a few hundred micrometers. A theoretical analysis of the angular and wavelength dependence of the reflectivity of uncoated, single-layer coated, triple-layer coated and graded-index-layer coated samples was performed and compared with experimentally derived films. Good agreement between model and experiment was observed. © 1994.
• Simmons, K. D., Potter Jr., B. G., & Stegeman, G. I. (1994). Red photoluminescence and optical absorption in hydrogen-treated GeO₂-SiO₂ sol-gel-derived planar waveguides. Applied Physics Letters, 64(19), 2537-2539.
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  Abstract: Presented here is a report on laser-excited photoluminescence data obtained in a waveguide geometry from sol-gel-derived germania-silica films whose photosensitive response was previously examined. Through a hydrogen-containing atmosphere, several samples were heat treated for different time periods to induce varied degrees of oxygen-deficiency and hydrogen-center concentrations in the films.
• Simmons, K. D., Stegeman, G. I., Potter Jr., B. G., & Simmons, J. H. (1994). Photosensitivity in germano-silicate sol-gel thin films. Journal of Non-Crystalline Solids, 179(C), 254-259.
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  Abstract: Binary germano-silicate thin-film waveguides were produced by the sol-gel method with 10, 25 and 45 mol% GeO2. Post-depositin, reducing-atmosphere heat treatments were used to induce a variety of photosensitive defects in the films. The waveguides were subsequently exposed to guided counter-propagating beams at 488 nm to seed and grow permanent optical gratings. The mechanisms of defect formation and bleaching following exposure at 488 and at 248 nm are analyzed, and the role of these mechanisms in developing photosensitivity is elucidated. New photo-induced absorptive processes are observed and related to the optical behavior. Models for photosensitivity are reviewed in light of these observations. © 1994.
• Tuttle, B., Voigt, J. A., Headley, T. J., Potter, B. G., Dimos, D., Schwartz, R. W., Dugger, M. T., Michael, J., Nasby, R. D., Garino, T. J., & Goodnow, D. C. (1994). Ferroelectric thin film microstructure development and related property enhancement. Ferroelectrics, 151(1 -4 pt 1), 11-20.
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  Abstract: Factors that control phase evolution, microstructural development and ferroelectric domain assemblage are evaluated for chemically prepared lead zirconate titanate (PZT) thin films. Zirconium to titanium stoichiometry is shown to strongly influence microstructure. As Ti content increases, there is an apparent enhancement of the perovskite phase nucleation rate, grain size becomes smaller, and the amount of pyrochlore phase, if present, decreases. While the pyrochlore matrix microstructure for near morphotropic phase boundary composition thin films consists of two interpenetrating nanophases (pyrochlore and an amorphous phase); the pyrochlore microstructure for PZT 20/80 films deposited on MgO substrates is single phase and consists of 10 nm grains. Zirconium to titanium stoichiometry also has a substantial influence on process integration. Near morphotropic phase boundary films exhibit extensive reaction with underlying TiO2 diffusion barriers; conversely, there is no chemical reaction for identically processed PZT 20/80 thin films. We have attempted to directly correlate the optical quality of PZT thin films to the following microstructural features: 1) presence of a second phase, 2) domain orientation, and 3) nanometer surface morphology.
• Yanxia, L. u., Hoelzer, D. T., Schulze, W. A., Tuttle, B., & Potter, B. G. (1994). Ferroelectric thin film bismuth titanate prepared from acetate precursor. IEEE International Symposium on Applications of Ferroelectrics, 348-351.
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  Abstract: Bismuth Titanate (Bi4Ti3O12) thin films were fabricated by spin coat deposition - rapid thermal processing (RTP) technique. Acetate derived solution for deposition was synthesized by blending dissolved bismuth acetate in aqueous acetic acid, and then adding with titanium acetate. A series of electrically insulating, semiconducting and conducting substrates were evaluated for Bi4Ti3O12 films deposition. While X-ray diffraction and TEM analyses indicated that the initial perovskite crystallization temperature was 500°C or less for these Bi4Ti3O12 films, a 700°C crystallization treatment was used to obtain single phase perovskite films. Bi4Ti3O12 film crystallographic orientation was shown to depend on three factors: substrate, the number of coating layers and thermal processing. While preferred c-direction orientation was observed for films deposited on silver foil substrates, preferred a-direction orientation was obtained for films deposited on both Si and Pt coated Si wafers. The films were dense, smooth, crack-free and had grain sizes ranging from 20 nm to 400 nm. Film thickness and refractive index were determined using a combination of ellipsometry, waveguide refractometry and TEM measurements. Both low field dielectric and ferroelectric properties were measured for an 800 nm thick film deposited on a Pt coated MgO substrate. A remanent polarization of 38 μC/cm2 and a coercive field of 98 kv/cm was measured for this film that was crystallized at 700°C.
• Potter Jr., B. G., Sinclair, M. B., & Dimos, D. (1993). Electro-optical characterization of Pb(Zr,Ti)O₃ thin films by waveguide refractometry. Applied Physics Letters, 63(16), 2180-2182.
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  Abstract: Electric field-induced changes in the extraordinary and ordinary refractive indices of a Pb(Zr0.53Ti0.47)O3 thin film were independently determined using waveguide refractometry. Under an electric field, applied normal to the film plane and corresponding to saturation of the electric polarization, the ratio of the extraordinary to ordinary refractive index change (Δne/Δno) is found to be -4/1, contributing to a net birefringence change [Δ(ne-n o)] of -0.021. Using this technique, both diagonal and off-diagonal elements of the electro-optic response tensor describing the macroscopic behavior of the polycrystalline film were accessed, illustrating the importance of this approach in evaluating orientation-specific electro-optic characteristics in these films.
• Simmons, K. D., Stegeman, G. I., Potter Jr., B. G., & Simmons, J. H. (1993). Photosensitivity of solgel-derived germanosilicate planar waveguides. Optics Letters, 18(1), 25-27.
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  Abstract: Photosensitivity of the experiments performed in germanosilicate sol-get planar waveguides containing high concentrations of germanium is studied in which sample composition, index and thickness were closely controlled. The results are discussed based on a model for photosensitive effects in germania-silicate system, where change of photoinduced index depends on local bleaching of 242-nm band.
• Potter Jr., B. G., & Simmons, J. H. (1991). Quantum confinement effects at the L point in CdTe. Physical Review B, 43(3), 2234-2238.
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  Abstract: We report quantum-size-related shifts in optical transitions originating away from the center of the Brillouin zone of a II-VI semiconductor. CdTe-glass composite thin films, containing isolated semiconductor crystallites with average sizes ranging from 46 to 158, exhibit a size-dependent blue shift of the E1 and E1+1 transitions occurring at the L point of the zinc-blende Brillouin zone. Analysis of the effect, in the context of current zone-center confinement theory, supports the existence of bound electron-hole pairs whose internal motion is restricted to two dimensions. The effective mass governing the energy shift of each transition falls within at least 5% of the corresponding total transverse mass of the electron and hole calculated using kp theory for the bulk material. It thus appears that, within the crystal size regime of the present study, optical absorption into these energy states results in the formation of a fully correlated two-particle state whose behavior is analogous to that of a single quasiparticle confined within the crystallite. © 1991 The American Physical Society.
• Potter Jr., B. G., & Simmons, J. H. (1990). Quantum-confinement effects in CdTe-glass composite thin films produced using rf magnetron sputtering. Journal of Applied Physics, 68(3), 1218-1224.
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  Abstract: Quantum-confinement-induced shifts in the fundamental absorption edge of isolated CdTe crystallites are reported in CdTe-glass composite thin films produced using a sequential rf magnetron sputtering process employing two separate sputtering sources. Films ranging in thickness from 0.5 to 4.5 μm and containing as much as 30 vol % CdTe have been produced, illustrating the versatility of this technique over a more conventional melting approach. Post-deposition heat treatments were used to produce average crystallite sizes in the range 46-158 Å. An improved fit to theory at larger crystal sizes is found if a cylindrical crystal morphology is assumed. The effective mass of the confined specie, which governs the shift of the absorption edge with crystal size, is found to be 0.20m0 (spherical morphology) and 0.12m 0 (cylindrical morphology), both of which are greater than the exciton-reduced mass in bulk CdTe. The data suggests, therefore, that a non-negligible Coulomb interaction may still exist in crystals even as small as 0.31 times the size of the bulk Wannier exciton diameter. Planar waveguiding has also been demonstrated in these samples as a preliminary step to the production of waveguide-based nonlinear device structures.
• Potter Jr., B. G., & Simmons, J. H. (1988). Quantum size effects in optical properties of CdS-glass composites. Physical Review B, 37(18), 10838-10845.
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  Abstract: The optical properties of extremely small isolated semiconductors provide a sensitive probe of the developing electronic structure in the materials. Cadmium sulfide and CdSxSe1-x crystallites are precipitated in an insulating glass matrix during a secondary heat-treatment procedure. Variation in the heat treatment significantly alters the final crystallite size, enabling the examination of a size-dependent change in the observed optical-absorption edge and exciton-related photoluminescence peak energies. A diffusion-limited coarsening behavior is exhibited by the CdS crystallites precipitated from a Zn-free base glass, indicating a high degree of purity in these crystals. Growth behavior of mixed crystallites, however, indicates some stoichiometric variation with heat-treatment time. Quantum size effects measured using the CdS precipitates in the size range from 40400 A reflect confinement primarily in the translational motion of the Wannier exciton with some slight modification of its internal electron and hole orbits. © 1988 The American Physical Society.