No description available.

No description available.

The dichroism induced by a polarized beam of near bandgap light was measured in thin films of amorphous As2Se3. The polarized pump-beam causes a decrease in the absorption coefficient for light polarized in the same direction, and the resulting anisotropy in absorption coefficients is referred to as dichroism. An unpolarized beam of light passing through the sample will be preferentially absorbed in the same direction as the pump-beam polarization direction. That preferential absorption can be measured as a function of time to represent the time dependence of the induced dichroism.<p> The time dependence of the induced dichroism follows a stretched exponential of the form exp((-t/ô)^â). Three parameters from the fitting function are of interest; the saturation level Asat, the time constant ô, and the stretching exponent â. The kinetics of dichroism were extracted from measurements on samples of various thicknesses, samples subjected to various pump-beam intensities, samples doped with small quantities of iodine (0.1%), and for samples in the presence of a DC electric field. It was found that the induced dichroism saturates at a value that depends linearly on sample thickness, but is independent of inducing beam intensity. The time constant (ô) was found to have an inverse relationship with pump-beam intensity and decreases with increasing sample thickness. The stretching exponent â has little or no dependence on either intensity or thickness. The addition of iodine (0.1%) increased the time constant (ô) in some but not all cases; meanwhile the DC electric field had no effect on any of the fitting parameters. <p> Measurement of the kinetics of the induced dichroism in amorphous As2Se3 can be used to constrain models of the phenomenon in Chalcogenide glasses. The results of the intensity dependence study fit well within the framework of the current theoretical models; however, the iodine doping and electric field measurements do not. The model predicts a decrease in the rate for samples doped with iodine, and those subjected to a transverse electric field; neither effects were observed. Providing a framework for photoinduced dichroism is important if device level applications are to be realized using the effect. In order for any material to be used in commercially viable applications it needs to be cheap and easy to produce; and it needs to be completely described from an optical and electrical standpoint. A better understanding of photoinduced dichroism in amorphous As2Se3 may lead to the realization of optical switches, optical memory, polarizers, waveguides, and diffraction gratings.

The dichroism induced by a polarized beam of near bandgap light was measured in thin films of amorphous As2Se3. The polarized pump-beam causes a decrease in the absorption coefficient for light polarized in the same direction, and the resulting anisotropy in absorption coefficients is referred to as dichroism. An unpolarized beam of light passing through the sample will be preferentially absorbed in the same direction as the pump-beam polarization direction. That preferential absorption can be measured as a function of time to represent the time dependence of the induced dichroism.<p> The time dependence of the induced dichroism follows a stretched exponential of the form exp((-t/ô)^â). Three parameters from the fitting function are of interest; the saturation level Asat, the time constant ô, and the stretching exponent â. The kinetics of dichroism were extracted from measurements on samples of various thicknesses, samples subjected to various pump-beam intensities, samples doped with small quantities of iodine (0.1%), and for samples in the presence of a DC electric field. It was found that the induced dichroism saturates at a value that depends linearly on sample thickness, but is independent of inducing beam intensity. The time constant (ô) was found to have an inverse relationship with pump-beam intensity and decreases with increasing sample thickness. The stretching exponent â has little or no dependence on either intensity or thickness. The addition of iodine (0.1%) increased the time constant (ô) in some but not all cases; meanwhile the DC electric field had no effect on any of the fitting parameters. <p> Measurement of the kinetics of the induced dichroism in amorphous As2Se3 can be used to constrain models of the phenomenon in Chalcogenide glasses. The results of the intensity dependence study fit well within the framework of the current theoretical models; however, the iodine doping and electric field measurements do not. The model predicts a decrease in the rate for samples doped with iodine, and those subjected to a transverse electric field; neither effects were observed. Providing a framework for photoinduced dichroism is important if device level applications are to be realized using the effect. In order for any material to be used in commercially viable applications it needs to be cheap and easy to produce; and it needs to be completely described from an optical and electrical standpoint. A better understanding of photoinduced dichroism in amorphous As2Se3 may lead to the realization of optical switches, optical memory, polarizers, waveguides, and diffraction gratings.

No description available.

No description available.

The motivation for the current work stems from a unique application, i.e. the photopatterning of optical functionality in a photosensitive material immediately prior to use. In this case, optical devices such as diffraction gratings and optical interconnects are produced in thin films using integrated photonic sources under relatively uncontrolled environmental conditions. The compatibility of the material photoexcitation mechanism with wavelength and fluence levels available from compact solid-state optical sources and the need to understand the impact of local atmospheric composition and temperature on the photosensitive material response are therefore of primary concern. The primary goal of the current study was to investigate photoexcitation mechanisms and photoinduced optical and structural changes in promising candidate material systems for this application: polysilane and polygermane-based molecular hybrid polymers. The work pursued the development of a fundamental understanding of the key photophysical and photostructural responses of thin films composed of both pure, linear-chain polysilanes and of a Ge-Si copolymer. The effects of molecular modifications to the polymers, including polymer backbone catenate structure and side-group identity, on the optical and photosensitive behavior observed in these systems are examined. Through such effort, an understanding of how such structural characteristics influence key photosensitive properties, i.e. the excitation wavelength and the resulting photoinduced optical property changes, was attained. A related objective in the present work was to characterize the thermal stability of these hybrid polymers, specifically in terms of the effect of thermal treatment on as-deposited and photomodified materials. In this case, an evaluation of the similarities and differences in structural modification in response to both thermal and optical fields was pursued. The primary mechanism associated with the photoinduced phenomena observed in both polysilane and polygermane involves backbone chain scissioning and the formation of silane-radicals upon absorption of near-UV (λ ≈ 300 to 400 nm) photons, resonant with the lowest energy, σ - σ* (HOMO-LUMO) transition of the Group IVA backbone. The final photoproducts obtained result from a mixture of different competing processes which occur subsequent to this initial photoscissioning. In aerobic atmospheric environments, the radicals formed capture oxygen and form oxide linkages forming the dominant photoproducts. On the other hand, under anaerobic conditions, photooxidation is suppressed while hydride passivation of the radical dominates the response. The oxidized product, resulting from irradiation under the aerobic environment, exhibited higher refractive index changes than irradiation under anaerobic conditions. Photoexcitation using higher energy photons (typically λ ≈ 230 to 300 nm) are resonant with side-group transitions associated with π-conjugated states of the cyclic moieties. Under these conditions, the excitation accesses both these organic side-groups as well as the Group IVA backbone structure. Such excitation conditions resulted in a larger photoinduced structural modification in the irradiated polymer, as observed both in terms of its electronic structure as well as the resulting refractive index change. Thermally induced structural modification to the backbone and side-group moieties were found to be qualitatively similar those produced under optical irradiation. For example, the primary structural changes were again associated with backbone chain scissioning. Photoinduced structural modifications through resonant optical excitation of the material, however, tended to be more focused on the specific structural moieties accessed.

Thesis advisor: Torsten Fiebig / This dissertation introduces a new field of DNA photonics centering on the electronic properties of DNA, which emerges after the initial controversies regarding the long-range conductivity and wire-type behavior of DNA have been widely settled. DNA photonics study is not solely focused on charge transfer phenomena but encompasses all possible photophysical processes and their potentially complex interplays. For instance, ultrafast electronic energy migration, dissipation, and (de)localization on the femtosecond time scale are shown to be crucial for the description of light-induced dynamics in DNA and have been thoroughly investigated in this dissertation. In addition to measurements on natural single and double-stranded DNA, this dissertation also presents experimental data on a series of functionalized DNA systems (derivatized by stilbene, ethidium, 2-aminopurine, etc.), obtained by state-of-the-art femtosecond broadband pump–probe spectroscopy. The results illustrate the distance dependence of charge transfer, emphasize the role of the initial electronic excitation on energy transfer dynamics, and highlight the influence of structural factors on both processes in DNA. Finally, as one initial step towards DNA electronics application, a DNA mimicking system of tertiary arylureas were employed to demonstrate molecular wire behavior, implying its potential use in molecular electronics. Thus, both the experimental and theoretical research accumulated for DNA π–π coupling can be translated into designing and testing various molecular systems with similar π-stacked structures. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The dichroism in amorphous As2Se3 induced by a polarized beam of near band-gap light (λ = 632.8 nm) was measured in films that ranged in thickness from 0.25 µm to 1.93 µm. Most noncrystalline materials are initially isotropic and homogeneous. When amorphous As2Se3 (a chalcogenide glass) absorbs an intense pump-beam of polarized light, the absorption coefficient for light polarized in the same direction as the pump-beam is less than for the perpendicular polarization, i.e. the sample becomes anisotropic (dichroic). The induced dichroism is reversible by rotating the polarization of the pump light by 90°. Induced dichroism is potentially useful in various photonic devices including optical switches, optical memory, and photowritable polarizers.<p>Several aspects of photoinduced anisotropy in a-As2Se3 have been analyzed:<p>(i) rate of photoinduced anisotropy buildup as a function of inducing beam intensity and sample thickness, <p>(ii) the kinetics of the photoinduced anisotropy buildup in terms of a stretched exponential curve, <p>(iii) the stretched exponent, β, as a function of inducing beam intensity and sample thickness, <p>(iv) reversibility of the photoinduced anisotropy, (v) saturation level of photoinduced anisotropy as a function of inducing beam intensity and sample thickness.<p>The anisotropy buildup kinetics has been found to follow a stretched exponential behavior and that there exists an inverse relationship between the pump intensity and the time constant, τ. The τ(I) vs intensity (I) relationship more closely follows a logistic dose response curve than a simple straight line or power law relationship. There exists a direct relationship between the time contact  and the sample thickness, where a longer anisotropy buildup time is required as the sample thickness increases. The stretched exponent, β, was found to be approximately 0.6 and has a slight dependence on the inducing light intensity. The correlation of the stretching exponent, β, to sample thickness, L, was found to have a weak inverse relationship, that is β tends to decrease as the sample thickness increases.<p>The findings in this work demonstrate that the anisotropy orientation could be changed indefinitely since it was found that even after 100 orientation changes the anisotropy saturation had no measurable fatiguing. The anisotropy saturation level was found to be independent of the inducing beam intensity and linearly proportional to the sample thickness.

The dichroism in amorphous As2Se3 induced by a polarized beam of near band-gap light (λ = 632.8 nm) was measured in films that ranged in thickness from 0.25 µm to 1.93 µm. Most noncrystalline materials are initially isotropic and homogeneous. When amorphous As2Se3 (a chalcogenide glass) absorbs an intense pump-beam of polarized light, the absorption coefficient for light polarized in the same direction as the pump-beam is less than for the perpendicular polarization, i.e. the sample becomes anisotropic (dichroic). The induced dichroism is reversible by rotating the polarization of the pump light by 90°. Induced dichroism is potentially useful in various photonic devices including optical switches, optical memory, and photowritable polarizers.<p>Several aspects of photoinduced anisotropy in a-As2Se3 have been analyzed:<p>(i) rate of photoinduced anisotropy buildup as a function of inducing beam intensity and sample thickness, <p>(ii) the kinetics of the photoinduced anisotropy buildup in terms of a stretched exponential curve, <p>(iii) the stretched exponent, β, as a function of inducing beam intensity and sample thickness, <p>(iv) reversibility of the photoinduced anisotropy, (v) saturation level of photoinduced anisotropy as a function of inducing beam intensity and sample thickness.<p>The anisotropy buildup kinetics has been found to follow a stretched exponential behavior and that there exists an inverse relationship between the pump intensity and the time constant, τ. The τ(I) vs intensity (I) relationship more closely follows a logistic dose response curve than a simple straight line or power law relationship. There exists a direct relationship between the time contact  and the sample thickness, where a longer anisotropy buildup time is required as the sample thickness increases. The stretched exponent, β, was found to be approximately 0.6 and has a slight dependence on the inducing light intensity. The correlation of the stretching exponent, β, to sample thickness, L, was found to have a weak inverse relationship, that is β tends to decrease as the sample thickness increases.<p>The findings in this work demonstrate that the anisotropy orientation could be changed indefinitely since it was found that even after 100 orientation changes the anisotropy saturation had no measurable fatiguing. The anisotropy saturation level was found to be independent of the inducing beam intensity and linearly proportional to the sample thickness.