Timothy Londergan

From molecules to opto-chips: organic electro-optic materials

Recent advances in polymeric electro-optic materials and device fabrication techniques have significantly increased the potential for incorporation of these materials and devices into modern high bandwidth (fiber and wireless) telecommunication, information processing, and radar systems. Charge transfer π-electron chromophores characterized by molecular first hyperpolarizability (second order optical non-linearity) values approaching 3000×10 –30 esu have been synthesized. Elucidation of the role of intermolecular electrostatic interactions in inhibiting the efficient translation of molecular optical non-linearity to macroscopic electro-optic activity has permitted systematic modification of materials to achieve electro-optic coefficients approaching 100 pm V –1 . Improvements in the optical loss of polymeric materials at wavelengths of 1.3 and 1.55 µm have been effected. Mode matching of passive transmission and active electro-optic waveguides has been addressed, permitting a dramatic reduction in insertion loss. The putative ability of polymeric electro-optic materials to be efficiently integrated with very large scale integration semiconductor electronic circuitry and with passive optical circuitry has been demonstrated. Several devices of varying degrees of complexity have been fabricated and evaluated to operational frequencies as high as 150 GHz. The operational stability of polymeric devices is very competitive with devices fabricated from lithium niobate and gallium arsenide.

The effect of ligand conjugation length on europium complex performance in light-emitting diodes

Abstract Novel europium complexes containing either symmetrical or unsymmetrical diphenanthryl β-diketone ligands were synthesized and characterized. The ligand conjugation length has a profound effect on the spectral properties of the complexes and the device performance of the complexes as emitting dopants in light-emitting diodes. The unsymmetrical ligand has a longer conjugation, which red-shifts the absorption of the complex. The longer wavelength absorption facilitates the choice of host material that can efficiently transfer energy to the complex. However, the longer conjugation also results in a less efficient energy transfer from the ligand to the central Eu3+ ion.

Efficient emission from a europium complex containing dendron-substituted diketone ligands

A new europium (Eu) complex with dendron-substituted diketone ligands was synthesized and found to exhibit a photoluminescence efficiency of 45%. Double-layer light-emitting diodes based on polymer matrices doped with the Eu complex were fabricated. An electroluminescence external quantum efficiency of 0.80% was achieved when a copolymer containing side-chain carbazole and 1,3,4-oxadiazole groups was used as the matrix. The results are analyzed in the context of Forster energy transfer.

Achieving large electro-optic response: DH-type chromophores in both crosslinked systems and linear high-Tg systems

Drawing on the success of the DH6-polycarbonate system, new derivatives based on the DH bridge structure, bis-1,2-(dibutoxythiophene)vinylene have been synthesized and tested. The property-structure function has been investigated, showing that location of the trifluorovinylether (TFVE) crosslinker on the chromophore has an impact on electro-optic (EO) activity as well as stability. A crosslinked polymer, IMOH_PCT-R and a linear high glass transition (Tg) analog, IMOH_HT, in combination with various DH derivatives are discussed. The temporal stability data of these systems show comparable results for both crosslinked and linear polymers, furthering the argument that the Tg of the EO material is the critical parameter in determining device lifetime.

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).