Ab initio Studies on Nonlinear Optical Properties of Organic Molecular Crystals and Their Band Gap Correction

• Main Authors: Chih-Kai Yang, 楊智凱
• Other Authors: 李明憲
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/06088612954181248735

碩士 === 淡江大學 === 物理學系碩士班 === 93 === The application of Nonlinear Optical Properties of Organic Molecular Crystals is very broad. Due to the multiplicity and adjustability of their molecules, organic molecular crystals become advantageous to materials design. In this thesis we focus on the zero-frequencyχ(2)of organic molecular crystals and do a series of research in order to build up systematic experience and to understand the mechanism for second harmonic generation(SHG) in these kind of materials. We first focus on widely studied urea and perform the predictions of β andχ(2) in single molecule and crystal respectively. The convergency of parameters in calculation has been carefully tested in order to build up the research experience in using plane wave pseudopotentials method to study these kinds of materials. We find the size of the selected super cell in molecular calculation is crucial besides the general parameters in calculation, such as numbers of unoccupied orbitals, k-point sampling and cut-off energy. After applying band gap correction to fix the band gap discrepancy between Kohn-Sham method and experiments, one can obtainχ(2) which shows good agreement with experiments. According to calculating urea , we launch a serial compared analysis between Organic Molecular Crystalsβand χ(2) to verify the search of the molecular crystalsχ(2) equals to βtotal value in general. In ten process calculating morlecular crystals: COANP、mNA、NPP、5NU、POM、MHBA、MDNB、PNP、L-PCA、FMA)NLO material, we realized most of molecular valuableβin crystals will be larger than the sum of each molecular, that is enhancing effects of calculating crystals. We could apply band resolve method to analyze the ten molecular χ(2), and we get one result contributing orbitals not only inherit from molecular but also contribute unique and new in unoccupied states area of high energy。 The above-mentioned proof enhancing effects derive from others systems. In view of band gap correction, we adapted the SUM-OVER-STATES method to obtain a closely correctχ(2)　; It is a very important point of the research. We probe into various kinds of different band gap correction method results.