Christopher R. Moylan

Organic glasses : A new class of photorefractive materials

The performance of amorphous organic photorefractive (PR) materials in applications such as optical data storage is generally limited by the concentration of active molecules (chromophores) that can be incorporated into the host without forming a crystalline material with poor optical quality. In polymeric PR systems described previously, performance has been limited by the necessity of devoting a large fraction of the material to inert polymer and plasticizing components in order to ensure compositional stability. A new class of organic PR materials composed of multifunctional glass-forming organic chromophores is described that have long-term stability and greatly improved PR properties.

LCVD of copper: Deposition rates and deposit shapes

Laser chemical vapor deposition of copper has been performed under a variety of conditions. The results are interpreted using the kinetic model of Ehrlich and Tsao. We find that the kinetics of the process are limited by the rates of surface reactions and not by diffusion of reactant molecules to the surface. Furthermore, we find that deposit shapes are quite sensitive to laser intensity and scan rate.

Design of optimized photorefractive polymers: A novel class of chromophores

It is demonstrated that the microscopic mechanism of the photorefractive (PR) effect in organic composites with low glass transition temperatures involves the formation of refractive index gratings through a space‐charge field‐modulated Kerr effect. A tensorial formulation of the macroscopic aspects of the PR Kerr effect and its microscopic interpretation is presented. The second‐order dipole orientation term containing the anisotropy of the first‐order optical polarizability α(−ω;ω) is shown to yield the dominant contribution to the Kerr susceptibility χ(3)(−ω;ω,0,0). A class of special chromophores having negligible second‐order polarizabilities β(−ω;ω,0) and large dipole moments μ has been identified in order to optimize this term. These chromophores are not subject to the efficiency‐transparency tradeoff typically encountered with second‐order nonlinear optical (NLO) chromophores, providing highly transparent materials with large PR Kerr response. Contrary to previous approaches in this field, the bes...

A comparison of calculated and experimental hyperpolarizabilities for acetonitrile in gas and liquid phases

The nonlinear susceptibility of acetonitrile has been measured in both the gas and liquid phases by dc electric field‐induced second harmonic generation (dc‐SHG or EFISH). The EFISH signal for this molecule is dominated by the first hyperpolarizability β. It is shown that local field factors do not adequately describe the effect of the solvent environment. Thus it is not possible to extract ‘‘gas phase’’ values of hyperpolarizabilities from measurements made in solution. First and second hyperpolarizabilities, β and γ, have been calculated for acetonitrile using ab initio techniques. These calculations are compared to the gas phase experimental measurements. Excellent agreement is obtained when electron correlation effects are included.

A novel sensitized photochromic organic glass for holographic optical storage

A new class of photochromic glasses for use in irreversible holographic optical storage applications is described. The recording mechanism involves a triplet‐sensitized photoreaction of an organic chromophore. The materials are isotropic and red‐sensitive, and exhibit high diffraction efficiencies and long hologram lifetimes. The wavelength sensitivity can be adjusted by the choice of sensitizer; C60, sensitive in the red, is described here. Single digital data page recording and error‐free readout are demonstrated at a density of 0.5 Mb/cm2; multiplexing in a 150‐μm‐thick sample is also demonstrated.

Spectroscopy of reactive intermediates: Electron photodetachment from solvated anions

Electron photodetachment spectra of two complex anions, C6H5CH2OHF− and CH3OHOCH−3, have been recorded, using an ion cyclotron resonance spectrometer to generate, store, and detect the ions. The threshold energies determine the potential well depths of the ion–molecule reactions for which the anions are presumable intermediates. The detachment energies for C6H5CH2OHF− and CH3OHOCH−3 are ≤3.05±0.06 and 2.26±0.08 eV, respectively. The ability to measure absolute photodetachment cross sections is demonstrated in the latter case. The electron affinity of C6H5CH2O is 2.14±0.02 eV (49.3±0.5 kcal/mol). One would predict that complex ions of the form ROHF− where ROH is less acidic than HF would have poor Franck–Condon factors for adiabatic threshold photodetachment. This prediction is confirmed; five such ions display cross sections smaller than about 10−20 cm2 at all wavelengths above 365 nm. A useful correlation between ion structure and observation or nonobservation of visible/near‐UV photodetachment is theref...

Defeating tradeoffs for nonlinear optical chromophores

Materials to be used for optoelectronic, photorefractive, or frequency doubling applications must have high nonlinearities, good thermal stabilities, and low optical loss (high transparency). Organic compounds synthesized for incorporation into poled nonlinear polymers typically exhibit tradeoffs between nonlinearity and each of the other two qualities. By judicious use of arylamino donor groups and cyano-containing acceptor groups, a small set of chromophores that are both highly nonlinear and stable at high temperatures has been prepared. By selecting delocalized bridging moieties that are either tuned for optimum hyperpolarizability or exhibit two charge-transfer excited states, highly transparent chromopohores with excellent nonlinearities can be prepared. The results suggest that thermal stability and nonlinearity are jointly achieved by modifying donor and acceptor groups, while transparency and nonlinearity are jointly achieved by modifying bridging groups.

Multiple decomposition pathways in infrared multiple-photon-induced reactions of ions

Abstract Multiple products have been observed for the first time in collisionless infrared multiphoton decompositions using an unfocused laser. The branching fractions provide evidence for barriers along the potential surfaces for fragmentation; possible origins of such barriers are discussed.

Photorefractivity in new organic polymeric materials

Two experimental studies of new photorefractive polymer materials are described. First, an examination of the fabrication, stability, and photorefractive behavior of a recently-reported high-efficiency polymeric composite 33%PVK:50%DMNPAA:16%ECZ:1%TNF (K. Merrholz et at., Nature 371, 497) shows that this material is metastable, crystallizing on time scales from hours to days depending upon preparation. The grating phase shift also varies considerably from sample to sample. In the second part of this paper, first measurements and analysis of the properties of a new photorefractive polymer composite which contains a liquid nonlinear optical chromophore are presented. This new composite, which is immune to crystallization even at high chromophore loading levels, shows a useful diffraction efficiency of several percent at high fields, but the index of refraction grating is shown to consist of two components; a smaller photorefractive index grating and a grating which is not photorefractive in origin as shown by grating translation measurements.

From electric field-induced second harmonic generation (EFISH) to electro-optic measurements of nonlinear chromophores

Experimental measurements of molecular hyperpolarizabilities can be useful in two ways. First, they allow comparison between different chromophores, so that structure-function relationships can be understood and increasingly nonlinear compounds synthesized. This application requires only good relative measurements. The second and ultimately more important application is prediction of the macroscopic nonlinear optical properties of custom materials. The latter application requires accurate absolute values for the molecular parameters. Examples of both types of comparisons are discussed below. We describe experimental measurements on several types of nonlinear optical chromophores, and the choice of conventions and reference standards that leads to accurate predictions of electro- optic coefficients in poled polymers. The degree to which nonlinearity and thermal stability are correlated is also discussed.