Han-Bae Bang

Synthesis and Nonlinear Optical Properties of Novel Y-type Polyimides with Highly Enhanced Thermal Stability of Second Harmonic Generation

ABSTRACT 2,3-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-4′-nitrostilbene dianhydride ( 4 ) was prepared and reacted with 4,4′-oxydianiline and 4,4′-diaminobenzanilide to yield unprecedented novel polyimides 5–6 containing 2,3-dioxynitrostilbenyl groups as NLO-chromophores, which constituted parts of the polymer backbones. The resulting polyimides 5–6 were soluble in polar solvents such as DMSO and DMF. Polymers 5–6 showed a thermal stability up to 300°C in TGA thermograms with T g values from DSC thermograms in the range of 135–141°C. The SHG coefficients (d 33) of poled polymer films were around 6.14 × 10−9 esu. The dipole alignment of poled polymer films exhibited exceptionally high thermal stability even at 30°C higher than T g and there was no SHG decay below 170–190°C due to the partial main chain character of polymer structure, which was acceptable for NLO device applications.

Synthesis Of Novel T-Type Polyurethanes Containing 2,5-Dioxynitrostilbenyl Group As A Nlo Chromophore and Their Properties

2,5-Di-(2′-hydroxyethoxy)-4′-nitrostilbene(2)was prepared and condensed with 2,4-toluenediisocyanate, 3,3′-dimethoxy-4,4′-biphenylenediisocyanate and 1,6-hexamethylenediisocyanate to yield novel T-type polyurethanes3–5containing the NLO-chromophore 2,5-dioxynitrostilbenyl group, which constitutes a part of the polymer backbone. Polymers3–5were soluble in common organic solvents such as acetone and DMSO. Polymers3–5showed a thermal stability up to 280°C in TGA thermograms withTgs of 79–159°C in DSC thermograms. The SHG coefficients(d33)of poled polymer films were around 5.4 × 10−9esu, and they exhibited greater thermal stabilities of dipole alignments even at 10°C higher thanTgdue to the partial main chain character of the polymer structure.

Synthesis and Electro-Optic Properties of Novel Y-Type Polyurethanes having Dioxybenzylidenemalononitrile

3,4-Di-(2′-hydroxyethoxy)benzylidenemalononitrile(3)was prepared and condensed with 2,4-toluenediisocyanate and 3,3′-dimethoxy-4,4′-biphenylenediisocyanate to yield unprecedented novel Y-type polyurethanes(4–5)containing 3,4-dioxybenzylidenemalononitrile group as a NLO-chromophore, which is a part of the polymer main chain. The resulting polyurethanes4–5were soluble in common organic solvents such as acetone and DMF. Polymers4–5showed a thermal stability up to 280°C in TGA thermograms with Tgvalues from DSC thermograms in the range of 148–152°C. The SHG coefficients(d33)of poled polymer films were around 5.96 × 10−9esu. These poled polymer films exhibited greater thermal stabilities of dipole alignments even at 20°C higher than Tgdue to the partial main chain character of the polymer structure, which is acceptable for NLO device applications.

Development of an autocorrelator based on the organic polymer thin film

Ultrafast lasers have many applications mainly due to its two properties, the ultrashort pulse width and the ultrahigh intensity. Because the former is the main cause of the latter, it is very important to exactly measure the pulse width of the ultrafast laser. Currently, there are several different kinds of experimental methods to measure the ultrashort pulse width. Among those systems for this measurement, the autocorrelator using the second harmonic generation (SHG) is by far the most simple and basic method. This type of autocorrelators usually uses inorganic crystals, such as BBO, as the SHG medium. The thinner medium is necessary for analyzing the shorter laser pulses. However, the polishing process which is necessary for obtaining the optically good surfaces makes it difficult to reduce the thickness of medium as desired. We present an autocorrelator system which overcomes these shortcomings. Our system is based on the SHG using organic polymer. Polymers can be easily prepared in the form of thin film on the strong substrate through the process of spin casting. Thickness less than 1 m can be obtained without difficulties. Furthermore, due to its high nonlinearity, thin film of polymer can produce the bright second harmonic light. Polyurea was used as the second harmonic generation material of the autocorrelator because it has the pretty good transparency. An autocorrelator system based on the 397nm-thick poled polyurea thin film has been developed and used to measure the pulse width of a home-made Ti:sapphire laser oscillator. Then, the system was compared with that based on a 100 μm-thick BBO crystal, which is widely used. The pulse width of laser beam was measured to be 9.8 fs with the former. The value is believed to be more accurate than that of 7.2 fs measured with the latter.