| Organic electro-optic (OEO) polymers are one of the key enabling elements in thedevelopment of advanced optical interconnect technologies for achieving substantiallyenhanced energy saving and bandwidth in future communication and computation.Due to their large and rapid NLO responses, low dielectric constant, high laserdamage thresholds, low cost, superior chemical flexibility and ease of processing ascompared to conventional inorganic crystalline materials, extensively research hadbeen developed.A lot of molecules with large hyperpolarizabilities and excellentthermal and chemical stability were designed and synthesized. However, it ischallenge to design and synthesize EO materials with large NLO effects, high thermalstability of the dipole moment after poling and low optical loss to fit the requirementsof EO device.In this dissertation, we have some promising attempts to optimize EO materials fordevice requirement, especially to improve the poling efficiency and thermal stabilityof materials. We develop different methods to optimize the EO coefficient (r33). First,we design and synthesize the head-tail linked chromophore (AJL60), which coulddynamically assist poling progress. The guest-host doped film with head-tail linkedchromophore shows better EO coefficient and order parameter, comparing to similar size and hypepolarizability reference chromophore. Secondly, we develop andsynthesize two head-to-tail linked (TCF-28and TCF-DOT) to AJL60and investigateEO properties of the newly designed chromophores and AJL60as monolithicmolecular glass comprised. Poling results shows that r33of AJL60could reach258pm/V. Thirdly, we develop a new photo-assisted poling method that could efficientlypole EO polymer at sub-Tg. Because we found leaking current during poling is quitehigh which limit the higher applied field on the EO film, attenuating poling efficiency.High electric field (up to250V/μm) could be applied to get large EO activity at RTfor the guest-host film. At last, to efficiently translate the high EO activity from singlelayer to EO device, we successfully develop PVDF-HFP derivatives as claddingmaterials to improve EO activity. And the cross-linked film could potentially be usedfor high Tg EO polymer.Recent progress of OEO materials is reviewed in Chapter1. Based on a briefintroduction to EO modulators, the design and synthesis of EO materials, relationshipbetween poling method and EO activity was focused. Then the future developingdirections of EO polymers are discussed. Finally, the strategies and main contents ofthis thesis have also been outlined.In Chapter2, a considerable poling efficiency of guest-host electro-optic materialshad been achieved assisted by dynamically changeable dipolar group on the donor endof chromophore. Chromophore C1consisting of two dipolar structure, the acceptorend of one short dipole flexibly linking to the donor end of another longer push-pullconjugated structure, has been designed and synthesized. According to the polingresults of electro-optic coefficient (r33) and order parameter (), the dipole moment ofthe head-to-tail linked chromophore C1could be enhanced to different extent afterpoling under different applied electric field. And the r33of C1/PMMA could reach upto115pm V-1,~47%higher than the reference chromophore(C2). TSD studiesfurther illustrated higher orientation polarization obtained by C1/PMMA and also exhibited good thermal stability. The dipolar pendant has not only assisted orientationof chromophore but also has an effect on stabilizing orientation.In Chapter3, we present the synthesis of two other head-to-tail linked chromophore(TCF-28and TCF-DOT) and investigate EO properties of the newly designedchromophores and AJL60as monolithic molecular glass. We use TCF as acceptorlinking it to primary push-pull structure, instead of cyanoacetate, which enhance thedipole moment of short dye part. All of the chromophores were characterized by1HNMR and13C NMR. DSC thermograms showed two Tgs of TCF-DOT, while it failedto improve poling efficiency. Apart from TCF-DOT, other two molecular glassexhibits ultra large r33and temporal stability after poling, we introduce dendriticbromide onto donor and bridge end of chromophore. The molecular glass of thebromide chromophore also show very high EO coefficient. Moreover, temporalstability showed record-high in molecular glass material, which retain>90%oforiginal r33after85℃for200h.In Chapter4, an efficient room temperature poling of a guest-host electro-opticpolymer is activated by photo-induced denitrogenation of triazoline moieties. Thehost polymer (P1)-1,2,3-triazoline groups on the backbone issynthesized by1,3-dipolar cycloaddition polymerization between abis-N-phenylmaleimide and an aromatic bis-azide. The guest chromophore (C1) is apush-pull compound with a strong dialkylaminophenyl donor and a CF3–TCFacceptor. By irradiating the thin films of P1with a compact UV lamp at365nm, theangular and asymmetric1,2,3-triazoline structures can be readily converted into amore straight and symmetrical aziridine structure by losing nitrogen. Thisphoto-induced denitrogenation provides the clean conversion of an aziridine-basedpolymer P2. This process with high conversion efficiency is insensitive to thepresence of light-absorbing nonlinear optical chromophores, and the reaction rate canbe controlled by the irradiation power and the thin film UV absorbance. Upon the photo-induced denitrogenation, considerable structural change of the polymerbackbone provides sufficient rotational freedom to the chromophores for the poling ofEO polymers. This has been verified by the successful demonstration of high fieldpoling (up to250V/μm) at RT for thin films containing2025wt%of chromophore1in P1, which can be efficiently activated by photochemical denitrogenation oftriazoline moieties by irradiating with low-power UV light. To improve the photostability for high efficient chromophore, new fluorinated based polymer (6FA-BMI) isdesign and synthesized. High efficient chromophore shows high stability in6FA-BMIduring photo-induced reaction. AJL33/6FA-BMI film showed good EO activity.In Chapter5, we present the synthesis of spindle-type chromophoe with differentside groups on conjugated bridge. Dimethylamine was used as eletron donor,3,4,5-trifluorophenyl, phenyl, trifluoromethylphenyl and4-methyloxyphenyl was usedas side groups for chromophore OHSTC-1, OHSTC-2, OHSTC-3, OHSTC-4. Allchromophore was funtionlized by–OH on the donor end. And we synthesized a seriesof Y-type polyurethane based on the spindle-type chromophores. All of thecopolymers were characterized by H NMR and IR spectrum. The molecular weightand molecular weight distribution were determined by gel permeationchromatography-rafractive index-multi-angle lase light scattering detection techniques.DSC thermograms showed that polyurethanes generally have different Tg. TGA curveshows good thermal stability of PU film. EO propteties of film had been investigated.It showed high order parameter (0.26) of poled film. It also showed good temperalstability, which retain>85%of original r33after85℃for200h.In Chapter6, the potential of ferroelectric materials uncross-linked and cross-linkedPVDF-HFP as cladding materials in optical waveguides has been investigated.Optical quality thin film (1μm) of PVDF could be prepared by spin-coating aftersuitable thermal annealing. The PVDF-HFP and EO Bilayer stack shows25%higherelectro-optic coefficient (r33) than single layer. PVDF-HFP networks exhibit improved morphology, breakdown strength and reliability, while maintaining similarconductivity at poling temperature. As optical cladding, absorbance of PVDFderivatives at RF wavelength is quite low. The commercial available material, simplecross-linking method and properties described above suggest their potential for opticalcladding materials. |
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