Studies of the Off-Resonant Nonlinear Optical Properties of an Organic Molecular Crystal and Specific Nonconjugated Conductive Polymers

Abstract

In this dissertation, the second order nonlinear optical properties of an organic molecular crystal and the third order off-resonant nonlinear optical properties of specific nonconjugated conductive polymers have been studied in detail. Presently available electro-optic devices are based on long propagation of light through bulk crystals or waveguides. Usually more than 1 cm propagation length is necessary to produce a sufficiently large phase change at an acceptably low electric field. This is due to the relatively low electro-optic coefficients that are usually observed in commercial materials such as lithium niobate (r33 = 30 pm/V). Hence to compensate for the low electro-optic coefficients, the interaction length is increased. The organic molecular salt, 4 -dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) has been proven to be an excellent candidate for an electro-optic material for many significant and novel applications. A single-pass thin-film electro-optic modulator with a large intensity modulation (20%) at 720 nm for a low ac field (1 V/µm, at 4 kHz) using a 3-µm-thick single-crystal film of DAST (red phase) has been demonstrated. Owing to the exceptionally large electro-optic coefficient and excellent optical quality of the films, large modulation depths despite an interaction length of a few microns were observed. The magnitude of the electro-optic coefficient is found to be r11 = 530 pm/V at 720 nm and 445 pm/V at 750 nm. The electro-optic coefficient of DAST at a technologically important wavelength (1.55 microns) was determined to be r11 = 200 pm/V. Electro-optic modulation was also demonstrated at high speed (up to 1.5 GHz) and the response was found to be flat over the higher frequency range. A Novel Pockels cell device based on the single-pass field induced birefringence configuration was designed, fabricated and successfully demonstrated with significant modulation and a comparative study was done. A fiber coupled electro-optic device based on the same configuration also was designed, fabricated and successfully demonstrated. Quadratic electro-optic measurements on a novel nano-optical polymer based on nonconjugated conductive polymers were done for the first time. Both the nonconjugated polymers 1,4-cis-polyisoprene and poly(ß-pinene) investigated showed exceptionally large nonlinearities. The nonlinear absorption coefficient of poly(ß-pinene) was determined using a modified z-scan technique and was found to be 2.6 cm/MW and a time resolved measurement involving field-induced birefringence for the same material was performed. The experiment was also conducted using PTS-polydiacetylene as the standard. The large optical nonlinearities observed have been accredited to the nature of the charged sites formed upon doping as well as the confinement of the charge in the sub-nanometer domain. The optical, mechanical and electrical properties of another nonconjugated conductive polymer, doped styrene-butadiene rubber were studied.