Galina Todorova

The molecular and supramolecular engineering of polymeric electro-optic materials

Abstract A new class of electro-optic chromophores, of which 2-dicyanomethylen-3-cyano-4-{2-[ E -(4- N , N -di(2-acetoxyethyl)-amino)-phenylene-(3,4-dibutyl)thien-5]- E -vinyl}-5,5-dimethyl-2,5-dihydrofuran (denoted FTC) is the prototype, has been prepared, characterized, and used to fabricate electro-optic devices. The molecular hyperpolarizability and thermal stability of these chromophore molecules are exceptional. Strong intermolecular electrostatic interactions inhibit the efficient poling of these molecules. A statistical mechanical theoretical treatment is used to quantitatively predict the competition of poling, intermolecular electrostatic interactions, and thermal effects in defining achievable acentric order and hence macroscopic optical nonlinearity. Theory is used to predict the optimum chromophore structure and material composition (chromophore loading in a polymer matrix) for maximum electro-optic activity and minimum optical loss. Problems associated with lattice hardening to lock-in poling-induced order are discussed briefly.

Polymer electro-optic devices for integrated optics

Abstract Recent advances in polymer electro-optic polymers and in fabrication techniques have made possible advances in polymer optical guided wave devices which bring them much closer to system ready. The processing of a new thermal set FTC polymer and its incorporation into a high-frequency, low-Vπ optical amplitude modulator are reviewed. The design and fabrication of 100 GHz modulators and their integration with rectangular metal waveguides using an anti-podal finline transition with a flexible Mylar substrate is discussed. High-speed polymer modulators with balanced outputs and the in situ trimming of the output coupler is described. More complex guided wave devices using polymers are demonstrated by the photonic rf phase shifter. Techniques for integrating both passive and active polymers into the same optical circuit without the need for mode matching is presented and demonstrated. To reduce the Vπ of a polymer amplitude modulator to 1 V or under, a technique of constant-bias voltage is demonstrated. Finally, a technique to directly laser write electro-optic polymer devices is reviewed.

Material development and processing for electro-optic device systems

New crosslinked clad polymers were developed for electro-optic polymer modulators with special attention paid to properties such as refractive index tunability, optical loss, and conductivity. These cured polymers showed relatively low optical loss at 1550 nm and desirable conductivity. The clads were used to fabricate electro-optic devices having mode profiles closely matched to that of optical fibers in order to reduce insertion loss. A new hardmasking technique was developed to define Mach-Zehnder rib waveguides by photolithography and dry etching with high reliability and surface smoothness. The hardmasking technique demonstrated flexibility in defining waveguides made with electro-optic polymers having different reactivity towards etchant gasses.

Synthesis of new second-order nonlinear optical chromophores: implementing lessons learned from theory and experiment

Studies on the influence of chromophore geometry on electro- optic coefficient/loading density and poling efficiency reveal that chromophore optical loading density is largely defined by the shape of the center of chromophore, and bulky groups at the end of chromophore is not preferable for most efficient poling of chromophore dipole. An EO coefficient of 122 pm/V at 1.06 micrometers has been achieved as a result of the systematic chromophore geometry optimization. Even high EO coefficients are expected to realized in the near future. Practical Mach-Zehnder modulators have been fabricated using CLD-1/APC material and have shown good dynamic thermal stability (120 degree(s)C), low optical loss (1.67db?cm at 1.55 micrometers ), low modulation voltage (2.4 volt, 2cm modulation length), and high extinction ratio (26dB).

Systematic optimization of polymeric electro-optic materials

Chromophore-containing polymeric electro-optic materials must satisfy many requirements before they can be considered for use in applications at telecommunication wavelengths (1.3 and 1.55 microns). These include large macroscopic electro-optic activity, low optical loss, and stability (thermal, chemical, and photochemical). Such materials must be capable of being integrated with silica fiber optics and semiconductor electronics. We discuss design of chromophores not only for large hyperpolarizability but also for low optical loss and for thermal and photochemical stability. The processing of these materials to maximize electro-optic activity while minimizing processing- associated optical loss is discussed. Device structures appropriate for minimizing insertion loss are discussed, as is the fabrication of such dvices and three-dimensional active/passive optical circuits. The identification of new structure/function relationships provide design criteria for future improvements as well as permitting better definition of the performance limitations that can be expected for polymeric electro-optic materials prepared by electric field poling methods.

High E-O Coefficient Polymers Based on a Chromophore Containing Isophorone Moiety for Second-Order Nonlinear Optics

Organic second-order nonlinear optical (NLO) materials have been intensely pursued for the past two decades. The superior nonlinearity, along with the inherent ultrafast response and large laser damage threshold, suggests that organic NLO materials are ideal for electro-optic and telecommunications applications. A high-{micro}{beta} chromophore APII utilizing isophorone as {pi}-conjugation bridge was processed into both PMMA guest host and crosslinked (XL) polyurethane network with various loading densities. High electro-optic coefficients, r{sub 33} = 30 pm/V (in PMMA) and r{sub 33} = 32 pm/V (in polyurethane) at 1.06 {micro}m were obtained. Alignment temporal stability ranged from 90 to 120 C for XL polymer network. There is virtually no intrinsic absorption loss at 1.3 {micro}m (solution measurement). Also noteworthy is that high optimum loading densities of this chromophore are attainable in both cases without detectable chromophore aggregation due to intermolecular electrostatic interactions.