Nonlinear Optical Moieties Research Articles

High‐Contrast Photoswitching of Nonlinear Optical Response in Crosslinked Ferroelectric Liquid‐Crystalline Polymers

A conceptually novel materials design, based on crosslinked ferroelectric liquid-crystalline polymers, is demonstrated for efficient switching of a second-order nonlinear optical (NLO) response in the solid state. By controlling the molecular alignment of the NLO moieties through two-photon isomerization of azobenzene molecules, reversible isothermal photocontrol of second-harmonic generation is achieved with contrast of up to 20.

Novel Photocrosslinking Nonlinear Optical Polymer Systems

ABSTRACT We designed and synthesized novel photocrosslinking guest-host nonlinear optical (NLO) polymer systems. In the system, guest NLO molecules bearing two photocrosslinking groups such as cinnamate or acrylate double bonds were homogeneously dispersed in the host photocrosslinking polymer matrix. The NLO molecules are then aligned by using the corona poling technique at an elevated temperature and the system was subsequently crosslinked by irradiating UV light. We expect the photocrosslinking keeps the NLO molecules from relaxing to random orientation. Formation of network structure was confirmed by comparing the absorbances of the NLO moiety in the crosslinked film before and after washing the film with methanol or acetone, exhibiting little leaching out of the NLO moiety from the polymer matrix.

Highly Efficient and Thermally Stable Electro‐Optical Dendrimers for Photonics

After a brief review on electro-optical (EO) polymers, the recent development of EO dendrimers is summarized. Both single- and multiple-dendron-modified nonlinear optical (NLO) chromophores in the guest–host polymer systems showed a very significant enhancement of poling efficiency (up to a three-fold increase) due to the minimization of intermolecular electrostatic interactions among large dipole moment chromophores through the dendritic effect. Moreover, multiple NLO chromophore building blocks can also be placed into a dendrimer to construct a precise molecular architecture with a predetermined chemical composition. The site-isolation effect, through the encapsulation of NLO moieties with dendrons, can greatly enhance the performance of EO materials. A very large EO coefficient (r33 = 60 pm/V at 1.55 μm) and high temporal stability (85 °C for more than 1000 h) were achieved in a NLO dendrimer (see Figure) through the double-end functionalization of a three-dimensional phenyl-tetracyanobutadienyl (Ph-TCBD)-containing NLO chromophore with thermally crosslinkable trifluorovinylether-containing dendrons.

Synthesis of polyurethanes and polyimides for photorefractive applications

Well-defined multifunctional polymers, bearing charge transporting (CT) and nonlinear optical (NLO) functionality covalently attached to each repeat unit, were synthesized and characterized. High glass transition temperature (Tg) polyurethanes, comprised of alternating CT and NLO units, were synthesized from NLO functionalized diols and N,N-bis(4-isocyanatophenyl)aniline. Polyimides, exhibiting high thermal stabilities, were obtained from a fluorinated dianhydride and a multifunctional (CT and NLO) diamine. The functionalized polyimides were obtained in two ways: by formation of a brominated polyimide derivative and subsequent Heck coupling with styrene derivatives; and direct polycondensation of a fully functionalized triarylamine diamine analog with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6-FDA). Phosphonate ester and nitro electron-withdrawing groups were utilized in the NLO moieties.

New second-order nonlinear optical (NLO) epoxy polymer treated by sol-gel processing

We have demonstrated a novel approach to achieve a stable second-order NLO polymer by incorporating NLO moieties into a processable epoxy polymer and hardening with crosslinkable sol-gel technique. The crosslinked polymer showed high nonlinearity (d 33 = 41 pm/V) and better thermal stability compared to the prepolymer, which maintained its nonlinearity up to 100°C without significant loss of NLO activity.

Synthesis and Characterization of New Photorefractive Polymers with High Glass Transition Temperatures

Polyamides based on 2',5'-diamino-4-(dimethylamino)-4-nitro-stilben (DDANS) and aliphatic diacids represent a well-established class of polymers for nonlinear optical (NLO) applications, in which the NLO units are fixed in the polymer backbone with their dipole moments oriented transversly to the polymer main chain. The introduction of a hole transport (HT) agent, [4((diphenylhydrazono)-methyl)phenyl]diethylamin (DEH) or 4-(bis(2-hydroxyethyl)amino)benzaldehyde 1,1-diphenylhydrazone (BBDH), was found to generate or enhance photorefractivity in these materials. Using appropriate chemical pathways, the NLO and HT moieties were connected in various ways, and polymer systems with dramatically different properties were obtained. Thus, (i) polyamides based on DDANS and aliphatic diacids, (ii) polyamide esters based on DDANS, BBDH, and adipic acid, (iii) polymer blends of the DDANS polyamides and a polyester based on BBDH and adipic acid, and (iv) DDANS polyamides doped with DEH have been prepared. All types of polymer systems were processed into transparent thin films by spin coating and could be oriented at elevated temperatures, applying a corona discharge poling technique. Second-order NLO coefficients (d 33 ) of up to 45 pm/V at a fundamental wavelength of 1542 nm have been measured. The poling efficiency was found to depend strongly on the content of the hydrazone moiety and the polymer system, decreasing with increasing content of hydrazone. Preliminary temperature-dependent two-beam coupling experiments demonstrated photorefractivity for DDANS-based systems, even at temperatures below the glass transition.

Correlation between photoconductivity and holographic response time in a photorefractive guest host polymer

The standard model for photorefractivity, which was originally developed for inorganic photorefractive crystals, yields a relation between the pertinent parameters of photoconductivity and the holographic response time. We investigated a photorefractive guest host polymer with respect to these properties by photoconduction and holographic experiments. Our system consists of a photoconducting siloxane polymer, which is sensitized by trinitrofluorenone, and an azo chromophore as nonlinear optical moiety. We compare measurements of photoconduction and response time as a function of the external electric field and the incident light intensity in order to test the applicability of the standard model to photorefractive polymers.

Photodecay mechanisms in side chain nonlinear optical polymethacrylates

The chemical and physical properties of photobleached nonlinear optical (NLO)-polymethacrylates were studied in order to investigate photobleaching mechanisms. We have shown by three key measurement techniques (infrared-spectroscopy, mass spectrometry, and gel permeation chromatography) that the predominant bleaching process is an irreversible decomposition of the NLO moieties accompanied by a broadening of the polymer molecular weight distribution. The quantum yield of the chromophore degradation determined by the evaluation of the bleaching kinetics fit is in agreement with typical data from comparable chromophores. It was found that bleaching causes an increase in surface roughness of NLO-polymer films, causing an increase in waveguide loss. Smoothing of the film surface by an annealing step leads to a lowering of bleaching induced waveguide loss.

Origin of the poling-induced optical loss in a nonlinear optical polymeric waveguide

The excess waveguiding optical loss of nonlinear optical polymers induced by poling was studied both experimentally and theoretically. The loss was found to be caused by the inhomogeneities in the orientation of the nonlinear optical moiety after poling and is strongly dependent on the uniformity of the guiding and cladding films and the magnitude of the poling field.

Langmuir‐Blodgett films of polymethacrylates with side chains containing functionalized aromatic schiff's bases

AbstractLangmuir‐Blodgett (LB) film formation has been investigated for polymers in which non‐linear optical (NLO) moieties are attached to the side chain of a polymethacrylate (PMA) backbone. Polymer monolayers were successfully transferred onto hydrophilic glass substrates using a moving‐wall type LB trough. The LB films were characterized by pressure‐area isotherms, polarized ultraviolet visible spectra, ellipsometry, and second harmonic generation measurements. The characterization shows that NLO moieties in the side chain are inclined at a shallow angle to the plane of the substrate. The second‐order nonlinear optical coefficient and molecular hyperpolarizability were also calculated. © 1993 John Wiley & Sons, Inc.

Langmuir-Blodgett films of polymers containing nonlinear optical molecules in the side chains

Langmuir-Blodgett (LB) film formation has been investigated for two different types of polymers in which nonlinear optical (NLO) moieties are attached in the side chain of polymethacrylate (PMA) and polyvinylalcohol (PVA) backbones. The LB films, where the polymer backbone is polymethacrylate were characterized by pressure-area isotherms, polarized UV-visible spectra and second harmonic generation measurements. The NLO moieties in the side chain are inclined to the plane of the substrate at a shallow angle. The second order nonlinear coefficient and molecular hyperpolarizability were also calculated.

Materials Requirements for Electro-Optic Polymers (for electronic systems applications)

ABSTRACTThe field of electro-optic (EO) polymer materials for integrated optics has been developing rapidly during the past several years. Recent advances include the formulation of poled crosslinked epoxies and guest-host polyimides exhibiting thermal stability at temperatures significantly higher than that previously achieved with thermoplastic acrylate chemistry. These developments are an essential first step toward achieving practical materials exhibiting stability to manufacture, assembly, and end-use in modern electronic systems applications.Despite these developments, there is still much basic research to be performed to achieve a practical EO polymer material system. A polymer material system for integrated optics consists of a set of three compatible materials: cladding polymers, passive core polymers, and active core polymers. The three materials should ideally be derived from the same base polymer so that the layers of the entire waveguide structure have nearly identical thermal, mechanical, chemical, electrical, and optical properties. In an ideal syst', the active core material would consist of passive materials highly loaded with aligned nonlinear optical moieties exhibiting large hyperpolarizabilities and suitable size and shape for thermal stability.This paper provides a detailed review of the material requirements for EO polymer waveguide devices in electronic systems and points to new areas of research required for practical systems.

New polymers with large and stable second-order nonlinear optical effects

Second order nonlinear optical (NLO) polymers containing NLO moieties with large optical nonlinearities and a cross-linkable unit have been synthesized using difunctionalized disperse red dye. The enhanced second order NLO coefficients, X exp (2) of 250 pm/v for the first polymer and 500 pm/v for the second, were obtained at 532 nm using the corona poling method. The introduction of flexible chains into the polymer backbone is found to cause a decrease in glass transition temperature and, hence, a decrease in the stability of the second-order NLO effects.

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. This unusual behavior may be due to a significant contribution of Maier–Saupe thermotropic interactions among the tolane moieties that favor parallel alignment.

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. Moreover, the relaxation times of temporal decay of field-induced alignment at ambient temperature, as determined from the stretched exponential function fit of birefringence changes, are found to be consistent with extrapolations from the the dielectric α relaxation characteristics of poled samples.

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 sample, measured with a Mach–Zehnder interferometer, was r13≂6.5 pm/V at 530.9 nm. From this r13 coefficient one estimates d31≂11 pm/V at 1064 nm, in good agreement with the Maker fringe value.

Novel second-order nonlinear optical polymers via chemical cross-linking-induced vitrification under electric field

We present a novel method of preparing highly efficient and stable second-order nonlinear optical (NLO) polymers via chemical cross-linking induced vitrification under electric field. In this method a soluble prepolymer is first prepared that contains cross-linking sites attached to the NLO-active groups. Upon preparing samples of desired thicknesses, the prepolymer is heated (precured) to enable some chemical cross-linking and thus to increase the glass transition temperature (Tg) to an optimum for poling. The precured polymer is then heated above its Tg and subjected to a high electric field to obtain the desired alignment of NLO moieties. Subsequent chemical cross-linking (curing) under electric field continues to advance the Tg and hence leads to in situ vitrification of the polymer that stabilizes the electric-field-induced orientation of the NLO moieties. Detailed results of thermal, linear optical, poling kinetics, and NLO properties are described for the polymer system prepared from tetrafunctional 4-nitro 1,2-phenylenediamine and bifunctional Bisphenol-A diglycidylether as the starting monomers. The polymer which has been cured finally at 140 °C under a very high corona field exhibits d33≂14 and d31≂3 pm/V, determined from the Maker-fringe experiments. Most significant, however, is the fact that this polymer shows no detectable decay in second harmonic generation for over 500 h under ambient conditions and no tendency of relaxation even at 85 °C.

Electro-optic phase modulation and optical second-harmonic generation in corona-poled polymer films

Electro-optic phase modulation was measured along with optical second-harmonic generation in thin films of a new copolymer containing a dicyanovinyl-terminated azo dye side chain. Orientational order was imparted to these films by poling with a corona discharge. Details of the electro-optic measurement technique, in which the real part of the electro-optic coefficient can be determined directly, are presented. Taking advantage of the increased orientation imparted by corona poling and the hindered motion of the nonlinear optical moiety in the side chain of the polymer leads to substantial improvements in both the magnitude and stability of nonlinear optical susceptibilities compared to guest-host polymer systems ordered by electrode poling.