Electro-optic (EO) effects are macroscopic optical properties of nonlinear optical (NLO) materials in response to an applied electric field. The research and development of highly efficient EO materials are crucial as key enabling technology in ultrafast information processing for future 5G/6G technologies. Both inorganic and organic materials have been extensively studied for their EO properties. Organic EO materials offer several advantages, such as significant EO coefficients, low dielectric constants, and compatible integration with other materials. One major challenge in current research of highly active organic EO materials is to optimize the optical transparency, optical power handling, and thermal stability of materials, which are as crucial as EO activity for photonic applications.

By tuning the electron donor and acceptor strength, we could optimize the polymethine chromophores containing tricyanofuran (TCF) acceptors for both the second-order and the third-order nonlinearities. Chapter 2 focused on investigating the molecular structures and the EO coefficient of Michler’s and tetrahydroquinolinyl-based push-pull heptamethine chromophores and their guest-host polymers. The bond length alternations (BLA) and widths of the spectral curve (σ) were studied using the Method of Moments to assist in analyzing the theories. Among these newly designed chromophores is one of the highest hyperpolarizability (β) up to 6,334⨯10^-30 esu in the series of push-pull heptamethine chromophores, corresponding large EO efficient (r₃₃) of 100.3 pm/V at 1304 nm in poled polymer films with modest loading. The structure-property relation study showcased one systematical approach to analyzing the optimal EO performance of chromophores in the series.

Simply changing the electron donor of push-pull polymethines to indoline or benzo[e]indoline derivatives can harness the quadratic electro-optic (QEO) of near-infrared polymethine chromophores over broad telecom wavelength bands, which is a subject of immense potential but remains under-investigated mainly. Chapter 6 reported a series of such push-pull heptamethines for third-order nonlinearities. These dipolar chromophores could attain a highly delocalized “cyanine-like” electronic ground state in solvents spanning various polarities, in some cases even closer to the ideal polymethine state than symmetrical cyanines. We used a transmission-mode electromodulation spectroscopy to study the electric-field-induced changes in optical absorption and refraction of polymer films doped with heptamethine chromophores. We obtained a significant and thermally stable QEO effect with high efficiency-loss figure-of-merits that compared favorably to those from dipolar polyenes in poled or unpoled polymers and III-V semiconductors. This chapter opened a path for developing organic materials based on cyanine-like merocyanines for CMOS-compatible, fast, efficient, and low-loss electro-optic modulation.

The nonlinearity of the EO materials at shorter wavelengths has yet to be investigated in polymethines. However, it has enormous potential to expand the operational range of EO devices, improve fundamental understanding, and enable integration with existing technologies. In Chapter 3, we reported one series of shorter and weaker electron acceptors for a series of new push-pull chromophores. These chromophores could be tuned through molecular design to make a trade-off in optical transparency and considerable nonlinearity at short wavelengths such as 973 nm, 846 nm, and 633 nm. Prism-coupler was utilized to study the electric-field-induced birefringence changes of polymer films doped with this series of chromophores. Relatively large electro-optic coefficients, good thermal stability, and relatively low optical loss were achieved. This chapter delved into a relatively unexplored optical window for short-distance communications.

Optical loss is one of the most significant parameters that should be studied in depth to estimate the overall performance of the EO materials. A full-scale analysis of the absorption edges by modified Tauc-Lorentz (TL) models is essential in determining the optical absorption edge of semiconductors, transparent conductors, ionic compounds, and dielectric materials. This powerful technique has yet to be applied to analyzing organic and polymeric nonlinear optical (NLO) materials. Chapter 4 tackles this problem by developing efficient processing protocols to prepare high-quality films of guest-host NLO polymers with a wide thickness range from sub-micron to 200 microns, which allows accurate measurement of full-spectral absorption coefficients of NLO materials over four orders of magnitude (1-10⁴ cm^-1) by UV-vis-NIR spectroscopy. We study the optical absorption edge of guest-host NLO polymers containing a variety of push-pull chromophores by curve fitting using the Tauc model and a newly developed Monolog-Lorentz (ML) model and analyze the dependence of optical band-gap and Urbach edge on the structure and composition of materials. Our data reveal the critical transition of the Urbach exponential tail of materials to a low energy tail that overlaps with C-H vibrational overtones and combination bands at the telecom wavelength region. Determining the optical absorption edge of organic NLO films in this study provides quantitative insight into the research to harness the resonance-enhanced nonlinear coefficients of materials by operating at the wavelengths near the optical absorption edge without incurring a significant optical loss.

As high glass transition temperature (T_g) polymers are essential to achieve the thermal stability of poled films, the research and development of optical polymers with high T_gs must complement chromophores' rapid progress. Chapter 5 reported a series of high T_g polymers based on poly (N-substituted maleimide-co-(α-methyl styrene)) and compared them with commercial polymers for application in poled polymers. These polymers can attain high poling temperatures, large order parameters, low optical loss, and reasonable thermal stability. A systematic study was carried out into the relationship between resonance forms of highly efficient polyene chromophores and polarities of host polymers. This chapter provided good insights into the intermolecular interactions between the chromophores and high T_g host polymers in achieving optimal EO performance and reduced optical loss.

A systematic investigation evaluated the macroscopic performance of poled polymers containing push-pull chromophores in terms of their EO coefficients, optical loss, and thermal stability. The results revealed significant correlations between molecular design and macroscopic performance. Overall, this thesis provided valuable insights into the relationship between molecular design and materials performance of push-pull chromophores and poled polymer systems for EO modulation. The findings can contribute to developing optimized chromophores and materials with improved EO properties, paving the way for designing and fabricating high-performance EO devices for telecommunications, data communication, and optical signal processing applications.