Susan P. Ermer

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.

Low-voltage electro-optic modulation using amorphous polycarbonate host material

The selection process leading to the development of a guest- host electro-optic material based on an amorphous polycarbonate is described. The optical loss at 1300 nm of this material system is under 2 dB/cm, which is the confidence limit of the slab measurement used. A Mach- Zehnder Modulator fabricated using the push-pull poling technique has a low switching voltage of 1.2V.

Space environment testing of polymer photonic modulators

Polymer photonic modulators and switches offer several attractive features for space systems, including rf bandwidth of 50 GHz or higher, and the potential for high sensitivity and low cost. We have shown that polymer modulators experience little if any degradation (less than 2%) in the half-wave voltage due to total dose exposure of up to 5 MRad from a Co-60 source. An outgassing test at 125 degrees Celsius in vacuum for 24 hr resulted in a mass loss of less than 0.2% of the polymer material.

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.

Progress in optoelectronic polymers and devices

Optoelectronic devices based on organic materials are uniquely suited to applications requiring high rf bandwidth. There have been significant advances in lithium niobate technology, but fundamental frequency-sensitivity tradeoffs are generally required in device design. Using a guest-host electro-optic polymer system, we have demonstrated a Mach- Zehnder modulator with a switching voltage of 3.5 V and interaction length of 2.6 cm. Anisotropic V-groove etching for fiber attachment provides a path to low-cost packaging of these devices. Materials and process optimization are expected to enhance device performance, allowing more compact, sensitive devices. Issues related to electro-optic device development are discussed in this paper, and an update on our development of new chromophores for use in electro-optic polymers is given.

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.