Richard R. Barto

Near-infrared optical-absorption behavior in high-beta nonlinear optical chromophore-polymer guest-host materials. II. Dye spacer length effects in an amorphous polycarbonate copolymer host

In the second of a three-part series, spectral absorption behavior of nonlinear optical (NLO) dyes incorporated into amorphous polycarbonate, comprised of a homologous series of dialkyl spacer groups extending from the midsection of the dye molecule, is characterized by UV-Vis and photothermal deflection spectroscopy. The dyes are structural analogs of the NLO dye FTC [2-(3-cyano-4-{2-[5-(2-{4-[ethyl-(2-methoxyethyl)amino]phenyl}vinyl)-3,4-diethylthiophen-2-yl]vinyl}-5,5-dimethyl-5H-furan-2-ylidene)malononitrile]. Previous Monte Carlo calculations [B. H. Robinson and L. R. Dalton, J. Phys. Chem. A 104, 4785 (2000)] predict a strong dependence of the macroscopic nonlinear optical susceptibility on the chromophore waist: length aspect ratio in electric-field-poled films arising from interactions between chromophores. It is expected that these interactions will play a role in the absorption characteristics of unpoled films, as well. The spacer groups range in length from diethyl to dihexyl, and each dye is studied over a wide range of concentrations. Among the four dyes studied, a universal dependence of near-IR loss on inhomogeneous broadening of the dye main absorption peak is found. The inhomogeneous width and its concentration dependence are seen to vary with spacer length in a manner characteristic of the near-IR loss-concentration slope at transmission wavelengths of 1.06 and 1.3 mum, but not at 1.55 mum. The lower wavelength loss behavior is assigned to purely Gaussian broadening, and is described by classical mixing thermodynamic quantities based on the Marcus theory of inhomogeneous broadening [R. A. Marcus, J. Chem. Phys. 43, 1261 (1965)], modeled as a convolution of dye-dye dipole broadening and dye-polymer van der Waals broadening. The Gaussian dipole interactions follow a Loring dipole-broadening description [R. F. Loring, J. Phys. Chem. 94, 513 (1990)] dominated by the excited-state dipole moment, and have a correlated homogeneous broadening contribution. The long-wavelength loss behavior has a non-Gaussian dye-dye dipole contribution which follows Kadors broadening analysis [L. Kador, J. Chem. Phys. 95, 5574 (1991)], with a net broadening described by a convolution of this term with a Gaussian van der Waals interaction given by Obata et al. [M. Obata, S. Machida, and K. Horie, J. Polym. Sci. B 37, 2173 (1999)], with each term governed by the dye spacer length. A minimum in broadening and loss-concentration slope at a spacer length of four carbons per alkyl at all wavelengths has important consequences for practical waveguide devices, and is of higher aspect ratio than the spherical limit shown by Robinson and Dalton to minimize dipole interactions under a poling field.

Nanoscale architectural control of organic functional materials for photonics

Recent progress in developing high-performance organic polymers for electro-optics and photonics is reviewed. A highly fluorinated hyperbranched aromatic polymer with the degree of branching around 0.51 was prepared by a mild one-step polyesterification of an AB2 type monomer. Further post-functionalization with and thermally cross-linking by aromatic trifluorovinyl ethers (TFVE) afforded thermally stable, low loss optical polymer with improved solvent resistance. By more precisely controlling the molecular nano-architecture, we have developed a series of highly fluorinated crosslinkable dendrimers. These materials possess most of the desirable properties needed for the fabrication of optical waveguides, such as high solubility in common organic solvents (up to 50 wt%), very low optical loss, and excellent thermal stability. To overcome the “nonlinearity-stability tradeoff,” a facile and reversibly crosslinkable NLO polymer system is developed that combines both advantages of high poling efficiency and good alignment thermal stability. By smartly controlling the poling and crosslinking processes through the reversible Diels-Alder (DA) reactions, it allows highly polarizable chromophores to be efficiently poled at the stage of low viscosity linear thermoplastic polymer. The resulting nonlinear optical polymer exhibits a combination of a very large r33 value (76 pm/V at 1.3 μm) and good temporal stability at 70°C.

Effects of alkyl spacer group length on Vis-NIR absorption behavior in FTC-like guest-host EO polymers

Spectral absorption behavior of a series of FTC-like dyes of varying shape incorporated into amorphous polycarbonate (APC) is characterized by photothermal deflection spectroscopy. Previous Monte Carlo calculations by Dalton and Robinson predict a strong dependence of the macroscopic nonlinear optical susceptibility on the chromophore waist:length aspect ratio in electric field-poled films. This dependence arises from London interactions between chromophores, which are expected to influence the absorption characteristics of the composite both by changing the local polarity of the medium and through dipole interactions. It is expected that these interactions will play a role in the absorption characteristics of unpoled films as well. Of particular interest are the spectral characteristics of the red edge of the main dye electronic absorption peak, and the fine structure in the near-IR, dominated by overtones of fundamental C-H stretching and bending modes. The spectral structure in these key regions can be influenced by inter- and intramolecular interactions and conformational changes in the dye. The near-IR structure, in turn, will dictate absorption loss in optical devices prepared from these materials at key transmission wavelengths (1.3 and 1.55 um). In this study, a homologous series of spacer lengths, ranging from ethyl to hexyl, attached to an FTC-like NLO chromophore, LMCO-46M, is characterized by a combination of photothermal deflection spectroscopy (PDS) and UV-Vis spectroscopy to examine the effects of the molecular environment on near-IR loss at 1090 nm, 1300 nm and 1550 nm.

Novel perfluorocyclobutane (PFCB)-containing polymers and dendrimers for photonic devices

A wide variety of aromatic trifluorovinyl ether monomers and highly fluorinated crosslinkable dendrimers have been developed via novel synthetic strategies. Through the thermal dimerization of trifluorovinyl ether moieties on the monomers or on the periphery of dendrimers, these monomers or dendrimers can be melt or solution polymerized to form perfluorocyclobutane(PFCB)-containing prepolymers with good processability for optical waveguide fabrication. By further thermal crosslinking, the resulting thermoset materials possess low optical loss (0.3-0.4 dB/cm at 1310 nm with 1% of DR-1 or DCM doping), high thermal stability (Tg: 100-400 °C), good thermo-optic property, high solvent and humid resistance, and excellent mechanical flexibility. The combination of processability and performance in these PFCB-containing thermoset materials make them as ideal candidates for the fabrication of high-performance polymeric planar lightwave circuit components with the applications in the telecom and datacom optical networks.

Polymer molecular environment effects of Disperse Red Vis-NIR absorption behavior

Spectral absorption behavior of Disperse Red-1 and Disperse Red-19 dyes incorporated into a series of polymers by covalent attachment, representing various chemical structures, is characterized by photothermal deflection spectroscopy. Of particular interest are the spectral characteristics of the red edge of the main dye electronic absorption peak, and the fine structure in the near-IR, dominated by overtones of fundamental C-H and O-H stretching modes. The spectral structure in these key regions can be influenced by inter- and intramolecular interactions, or conformational or configurational changes in the dye. The NIR structure, in turn, will dictate absorption loss in optical devices prepared from these materials at key transmission wavelengths (1.3 and 1.55 um) for waveguide devices. A well characterized dye-polymer system, DR1-PMMA, is compared with two other polymer systems. Differences in spectral absorption behavior is assessed in terms of polymer host structure and bonding environment.

Photocrosslinking and SHG stability of PDRMA/PDRFC thin films

A series of functional and photo-crosslinkable maleate or fumarate type polyesters containing azo-type NLO chromophores were studied. The result demonstrated this is a versatile and convenient method of fabricating crosslinked supramolecular ultra-structure polymer thin films for potential nonlinear optical (NLO) and other photonic applications. The unsaturated PDRMA/PDRFC polyester thin films are capable of crosslinking in air via photo polymerization to form a hardened lattice. A ratio of 1:1 of crosslinker double bonds to polyester double bonds was preferred for optimal crosslinking. Thermal stability of Second Harmonic Generation (SHG) signals for a photo crosslinked polymer thin film reached to about 150°C. Films from photo crosslinking of fumarate or maleate vs. vinyl crosslinkers have the advantages of avoiding NH/OH groups and their vibrational overtones that absorb at telecommunication wavelength of 1550 nm.

Dendritic NLO chromophore with fluorinated dendrons for improving poling efficiency in electro-optic devices

Encapsulated by highly-fluorinated dendrons, a nonlinear optical chromophore core, which is based on the phenyl-tetracyanobutadienyl (Ph-TCBD) thiophene-stilbene-based NLO chromophore, exhibits a large ~30-40 nm blue shift of the charge-transfer absorption maximum, 20 oC higher decomposition temperature, and most impressively, three times higher E-O coefficient. The combination of these appreciable improvements makes the molecular design of dendritic modification as a very promising molecular-engineering for next generation of E-O materials.