Edwin Arthur Chandross

Absorption and Emission Spectra of Anthracene Dimers

The preparation of two types of anthracene dimer is described and their absorption and fluorescence spectra are reported. The basic spectral features of each dimer, together with a consideration of the method of formation, enable us to assign a geometry to each pair. One dimer, prepared by photolytic dissociation of dianthracene, has a symmetrical sandwich configuration. The geometry of the other dimer, prepared in solution at low temperatures, is deduced from an analysis of the vibronic structure of its first absorption band (1A1g→1B2u transition of a single molecule) using the theory of vibronic coupling in dimers. The two molecules have a configuration related to that of the two molecules in the unit cell of the crystal; one in which the short in‐plane axes make an angle of 55 deg with each other while the long axes are parallel. The resonance splittings of the 1B2u and 1B3u states in each dimer are deduced from the spectra and the results are discussed in relation to the theories of the energy levels ...

Reversible Photodimerization: a New Type of Photochromism

The photodimers formed by polycyclic aromatic hydrocarbons, such as anthracene, and their derivatives can be photodissociated to the original monomers. When the photodimer is held in a rigid matrix, the dimer can be remade efficiently because the two monomers are held in the proper relationship for photodimerization. The absorption spectra of monomer and photodimer differ substantially, and associated with this is a difference in refractive index. These characteristics as well as the stability of both states at room temperature make such a system attractive for reversible phase holographic recording with nondestructive readout. We have investigated several photodimer systems as solutions in rigid transparent polymer matrices, as well as some single crystals of photodimers. We have been able to write and erase diffraction gratings repeatedly. The photosensitivities ( 0.1 J cm(-2)), which are close to the maximum possible for a system with no gain, are high enough to be useful. Grating scattering efficiencies in the range of 0.01-5% have been observed. The physical, chemical, and photochemical characteristics of these systems, as well as some basic general considerations, are discussed.

Photolocking‐A new technique for fabricating optical waveguide circuits

Low‐loss dielectric optical waveguides have been made by a new process in which a photochemical reaction is used to lock or fix a dopant in a polymer film with a lower refractive index, and unreacted dopant is then removed by heating. Narrow guides ([inverted lazy s]4 μm wide, Δn [inverted lazy s] 1%) were formed by laser beam writing at λ=364 nm or by contact printing at λ=313 nm, and had losses of 0.20±0.05 dB/cm at λ=633 nm.

Multicomponent photopolymer systems for volume phase holograms and grating devices

Novel photopolymer systems for the fabrication of high-resolution volume phase holograms and grating devices are reported. Previously reported techniques use a single monomer (or a mixture of similar monomers) and rely solely on density modulation. In contrast, we have found it advantageous to use a mixture of components chosen to have differing reactivities and polarizabilities, which results in a composition modulation. During the image-forming exposure the more reactive monomer is polymerized while the less reactive species is excluded from the irradiated regions. Two-way diffusion, which must be invoked to explain our results, leads to a modulation of the chemical composition and hence of the polarizability of the final material. In some systems the polarizability effect was strong enough to exceed the density effect, even when these factors were in opposite directions. As in previous systems, an over-all exposure is used to fix the images. With appropriate monomer systems, stable images were obtained even when one component was unreactive. The peak-to-peak refractive index differentials achieved in our systems were as high as 1.5%. This is an order of magnitude larger than that reported for initially all liquid systems and 50% larger than that reported for related solid materials. The use of a liquid system enables us to fill small cavities with our materials and then record a high-resolution image. We have made gratings of >3000 lines mm(-1) inside hollow fibers, with 80 microm i.d., filled with a photopolymer material. For white light guided in the core, the expected band-stop filter response was observed.

Thermal and Photochemical Origin of Carbonyl Group Defects in Poly-(P-Phenylenevinylene)

Abstract The thermal stability of poly(p-phenylenevinylene) (PPV), along with its ability to form flexible devices, has sparked a considerable amount of research in the area of polymer light emitting diodes (LEDs). The presence of carbonyl groups as defects in the PPV structure has been reported elsewhere.1 An increase in carbonyl groups correlates well with a dramatic reduction in the photoluminescence and electroluminescence of this polymer. We have discovered that these carbonyl defects can be introduced thermally as well as photochemically. This paper reports the mechanisms for the thermal and photochemical oxidation of PPV. Although PPV is thermally quite stable, its hydroxy containing defects present in the precursor polymer are very susceptible to reaction with traces of oxygen at the elevated temperatures used in thermal conversion, resulting in the formation of ketone based carbonyl impurities. In addition, pristine PPV is very unstable in the presence of light and oxygen. The photooxidation of P...

Evaluation of cycloolefin-maleic anhydride alternating copolymers as single-layer photoresists for 193-nm photolithography

We have developed a fundamentally new class of photoresist matrix resins for use in 193 and 248 nm lithography based on cycloolefin-maleic anhydride alternating copolymers. When used in three-component formulations with cholate-based dissolution inhibitions (DIs) and conventional photoacid generators, these copolymers afford positive-tone resists with potential sub-0.25 micrometer image fidelity. The resists exhibit high contrast (3 - 5.5) and high sensitivity (3 - 5 mJ/cm2 at 248 nm, depending on exact formulation) with low loadings (ca. 1 wt%) of triphenylsulfonium salt photoacid generators. These formulations are sufficiently transparent to be used at 193 nm without further modification.

Embossed Optical Waveguides

Dielectric optical waveguides have been made by embossing grooves in a thermoplastic substrate and filling them with a liquid which is then solidified. Grooves (7 μm wide and 3.5 μm deep) in poly(methyl methacrylate) (n=1.49) were filled with cyclohexyl methacrylate which was photopolymerized to produce a permanent guide (n=1.505). The attenuation was 2–4 dB/cm at λ=0.633 μm.

Submicrometer resolution replication of relief patterns for integrated optics

We report on three techniques for replicating relief patterns in dielectric materials for use as substrates for planar integrated optical circuits: embossing with a metal die; casting from a metal die; and casting from a rubber mold. Techniques for overcoating the substrates with light‐guiding films and filling of grooves are also described. Measured losses in sheet film guides were ≤0.5 dB/cm. Guiding was also observed in narrow light guides fabricated by each technique. Embossing is the simplest process, but for small‐scale production the casting processes are simpler to implement.

Photodimerization of Crystalline Anthracene. The Photolytic Dissociation of Crystalline Dianthracene

The photodimerization of anthracene dispersed in potassium bromide matrix has been reinvestigated. The hydrocarbon is shown to be present as ordinary crystalline anthracene. Dianthracene, prepared in potassium bromide discs, can be photolytically dissociated at low temperatures to give a new sandwich crystal form of anthracene which has a yellow‐green excimer fluorescence. This crystal structure can also be used to explain recent studies of the fluorescence of crystalline anthracene under high pressure.

A novel approach to o‐nitrobenzyl photochemistry for resists

We have developed a two‐component (resin‐solution inhibitor) resist system having good photosensitivity in the 230–300 nm range. The resin is an optically transparent methyl methacrylate‐methacrylic acid copolymer that is soluble in aqueous alkali. The second component is one of a family of o‐nitrobenzyl carboxylates that are initially insoluble in the alkaline developer but are cleaved to soluble components upon irradiation. A number of o‐nitrobenzyl alcohol esters have been prepared and examined for use in this two‐component system. They have proven useful in the development of sensitive high‐resolution short‐wavelength photoresists which are described.