D. Jungbauer

Highly efficient and stable nonlinear optical polymers via chemical cross‐linking under electric field

Extending the novel method of Eich et al. [J. Appl. Phys. 66, 3241 (1989)] to prepare highly stable second‐order nonlinear optical (NLO) polymers via chemical cross‐linking under electric field, we have obtained a new polymer exhibiting large and stable second‐order optical nonlinearities after relaxation at 80 °C. This was achieved by forming a network polymer from two NLO‐active monomers, bifunctional N,N‐(diglycidyl)‐4‐nitroaniline and trifunctional N‐(2‐aminophenyl)‐4‐nitroaniline. Here, every NLO moiety is connected to the network by a single covalent bond. After full cure under corona poling at 120 °C, the sample exhibited at ambient conditions d33≂50 pm/V and d31≂16 pm/V at 1064 nm fundamental wavelength, as estimated from the Maker fringe data. Upon heating to 80 °C, the nonlinearities decreased somewhat initially, but leveled off and remained stable at 80 °C. This stable sample gave d33≂42 pm/V and d31≂14 pm/V at ambient conditions. Furthermore, the linear electro‐optical coefficient of this samp...

Structural and dynamic properties of a new type of discotic nematic compounds

Abstract Thermodynamic, structural and dynamical properties of a new type of discotic compounds, a hydrocarbon without any heteroatoms, displaying a nematic discotic phase have been investigated by means of X-ray diffraction, electro-optical relaxation, and calorimetric studies. Of particular interest are the strength of the first order nematic—isotropic phase transition and the nature of the orientational fluctuations in the isotropic phase. The short range positional order was found to be biaxial in both the isotropic and the nematic phase. The isotropic phase displays strong pretransitional effects originating from orientational fluctuations in the neighbourhood of the transition to the nematic phase. The character of these pretransitional effects differs from that found for calamitic systems in that the number of correlated molecules g 2 is extremely large, of the order of 600 at the clearing temperature and the electro-optical relaxation time is very large, caused by the large value of g 2.

Stability of nonlinear optical characteristics and dielectric relaxations of poled amorphous polymers with main-chain chromophores

We have studied second‐order nonlinear optical properties and dielectric relaxation characteristics for two amorphous epoxy polymers based on bisphenol‐A with nitroaniline‐type nonlinear optical moieties covalently attached as part of the main chain. The nonlinearities, rather large immediately after corona poling, undergo a slow decay at ambient temperature (e.g., ∼29% reduction in the nonlinear coefficient d33 in 28 days), even though the nominal glass transition temperature Tg is ∼80 °C. In dielectric relaxation measurements, both the unpoled and poled samples exhibit two relaxation modes: an α relaxation attributed to glass transition and a β relaxation due to local segmental motions of the main chain. Surprisingly, the critical temperature T∞ associated with the glass transition, obtained by a WLF [M. L. Williams, R. F. Landel, and J. P. Ferry, J. Am. Chem. Soc. 77, 3701 (1955)] fit of the α relaxation characteristics, shows a significant decrease (as much as 40 °C) with increasing poling field. More...

Second‐order nonlinear optical properties and relaxation characteristics of poled linear epoxy polymers with tolane chromophores

A new linear epoxy polymer containing 4‐amino‐4’‐nitrotolane chromophores attached to the chain backbone was synthesized in an attempt to enhance both the poling‐induced nonlinear optical susceptibility and its long‐term stability. The polymer films poled by corona discharge showed very large nonlinear optical doubling coefficients of d33 ≂ 89 pm/V and d31≂25 pm/V for incident light of 1.06 μm wavelength, and a linear electro‐optic coefficient r13≂8 pm/V at 633 nm wavelength. Poling‐induced alignment of the nonlinear optical moieties showed no detectable decay in two weeks at ambient temperature, as studied by the birefringence. Even at 100 °C the relaxation time estimated from the birefringence decay was ca. 450 h, consistent with roughly 16% decrease in dij coefficients in 20 h at this temperature and also the extrapolation from the dielectric α relaxation characteristics. The dielectric α relaxation temperatures of the poled polymer samples exhibited higher values than that of the unpoled counterpart. ...

Nonlinear Optical Epoxy Polymers with Polar Tolan Chromophores

Abstract A variety of optically nonlinear tolan chromophores and epoxy based polymers containing them have been prepared and studied. The molecular hyperpolarizabilities of the dye chromophores have been measured by the EFISH technique while the polymers containing them have been examined by calorimetry, X-ray diffraction, dielectric spectroscopy as well as the appropriate linear and nonlinear optical techniques. Novel liquid crystalline behavior has been encountered in some of these monomers and polymers.

Poled Epoxy Polymers for Optoelectronics

New polymeric thin films containing nonlinear optical chromophores aligned by corona poling for χ (2) experiments have been monitored by spectroscopic and optical waveguiding techniques, as well as by second harmonic generation and electrooptic measurements. Stability has been improved considerably in various crosslinked epoxy polymers by attaching the chromophores at several molecular positions to inhibit relaxation. With increased chromophore concentration and improved alignment, films exhibiting nonlinearities exceeding lithium niobate have now been demonstrated. Several electrooptic phase modulators have been constructed with these new materials and some preliminary results are presented.

Organic nonlinear optical materials and their device applications for frequency doubling, modulation, and switching

Increased interest and activity in optoelectronics has resulted from anticipated applications of optics to communication, electronic interconnections, information storage and possibly even logic. Current nonlinear optical (NLO) devices today employ inorganic crystals, such as lithium niobate for electrooptic modulation and switching and KDP or KNbO3 for frequency doubling. Organic nonlinear optical materials, that is, poled organic polymeric films and organic crystals, have the potential to replace some of these inorganic crystals.1 Key advantages include the high intrinsic nonlinearities of some organic molecules, the ability to optimize the molecular structure for specific applications, low dc dielectric constant, and low temperature processing. Current progress has also been made in improving the long term stability of these materials. Here we wish to report on some of our recent results on measurements of the order parameter, new crosslinked polymeric materials which exhibit more stability and some device applications with polymeric materials and organic crystals.

Electro-Optical Studies on Monotropic Nematic-Smectic Phase Transitions

Abstract The formation of nematic phases which are monotropic either with respect to a smectic A or to a smectic B phase was shown to be possible both for binary mixtures of low molar mass liquid crystals as well as for unmixed systems. The existence of these monotropic states was demonstrated experimentally by means of electro-optical Kerr effect studies. The phase diagrams of such systems are characterized by the fact that the transition temperatures vary continuously in mixtures and in homologeous series as a function of the composition or of the chemical structure, respectively. This holds even if the nematic phase becomes monotropic with respect to a smectic phase.