Gang Tang

Theoretical studies on the anisotropy of the first hyperpolarizabilities of one- and two-dimensional charge-transfer molecules: role of solvent polarity

The effects of solvent polarity on the geometry and static first hyperpolarizability of one- and two-dimensional charge transfer molecules based on para-nitroaniline (PNA) and 1,3-diamino-4,6-dinitrobenzene (DADB) were studied using a polarizable continuum model (PCM) at the B3LYP/6-31+G(d,p) level of theory. The inclusion of solvent does not change the geometry significantly, but produces a change in the calculated results of the electronic structures and nonlinear optical properties. It is found that solvent polarity has an important influence on the first hyperpolarizability, whereas its influence is negligible on the nonlinearity anisotropy parameter both for PNA and DADB, due to the fact that the off-diagonal component and the diagonal component of the first hyperpolarizability possess similar solvent effect characteristics.

Solvent effects on polarizability and hyperpolarizability of spirobifluorene derivative.

We studied the solvent effect on the geometric structure and (hyper)polarizability of spirobifluorene derivative by using quantum chemistry calculations. We found that the geometric structure is slightly affected by the solvent polarity. However, solvent polarity significantly influences first hyperpolarizability and second hyperpolarizability respectively. In addition, the linear polarizability is less affected by the solvent than the hyperpolarizabilities. We also first studied the solvent effect on frontier molecular orbitals and found that the frontier molecular orbitals energy gap is strongly influenced by the surrounding solvent. Our results demonstrate frontier molecular orbitals energy gap and molecular hyperpolarizabilities can be tuned by a suitable choice of the solvent respectively. Such findings have crucial implications for organic nonlinear optical materials applications.

Theoretical study of the spectroscopic and nonlinear optical properties of trans - and cis -4-hydroxyazobenzene

We investigate the molecular structure, vibrational and electronic absorption spectra, and electronic hyperpolarizabilities of trans and cis isomers of 4-hydroxyazobenzene (HOAB) via density functional theory. Results show that the azo dye exhibits a high third-order nonlinear optical response and good optical transparency. Both the basis set and the functional are important influences on the results obtained when calculating the absorption spectrum and NLO response. We also study the effect of the solvent on the electronic absorption spectrum to assess the ability of the functional to reproduce the experimental spectrum in combination with a suitable solvent model. Our calculations show that the SMD model of Truhlar et al. handles the electrostatic and the non-electrostatic effects of hydrogen-bonding solvents on the absorption spectrum better than the traditional polarizable continuum model does. In addition, our results indicate that the dye trans-HOAB exhibits a high second hyperpolarizability and excellent optical transparency. Also, although the second hyperpolarizability of cis-HOAB is much lower than that of trans-HOAB, it is non-negligible when calculating the optical nonlinearity of HOAB under an optical pump. We also examine the effect of frequency dispersion on second harmonic generation. This study provides the basis for further research on the spectroscopic and nonlinear optical properties of novel azo dyes and other π-conjugated compounds.

Theoretical study on the spectroscopic and third-order nonlinear optical properties of two-dimensional charge-transfer pyrazine derivatives

ABSTRACT In this paper, density functional theory method was employed to study the electronic absorption spectrum and electronic static second hyperpolarisability of X-shaped pyrazine derivatives with two-dimensional charge-transfer structures. Computational results show that the push–pull electron abilities of the substituent groups and the length of the conjugated chains affect the electronic spectrum and static second hyperpolarisability of the pyrazine derivatives. As the push–pull electron abilities of the substituent groups or the length of the conjugated chains increases, the frontier molecular orbital energy gap decreases, resulting in increased second hyperpolarisability and redshift of the electronic absorption bands. The electronic absorption spectra of the pyrazine derivatives maintain good transparency in the blue light band. The electronic static second hyperpolarisability exhibits a linear relationship to the frontier molecular orbital energy gap. Particularly, increasing/decreasing the push–pull electron abilities of the substituent groups considerably affect the static second hyperpolarisability in long conjugated systems, which is important to the modulation of molecular organic nonlinear optical (NLO) properties. The studied pyrazine derivatives show large third-order NLO response and good transparency in the blue light band and are thus promising candidates as NLO materials for photonics applications.

New type of Y-branch power splitter and beam expander based on eliminating anomalous reflection effect

We propose a new type of Y-branch power splitter and beam expander with scales of microns in two-dimensional (2D) photonic crystals (PCs) by drilling air holes in a silicon slice. Its functionality and performance are numerically investigated and simulated by finite-difference time-domain (FDTD) method. Simulation results show that the splitter can split a TE polarized light beam into two parallel sub-beams and the distance between them is tunable by changing the parameters of the splitter, while the expander can expand a narrow beam into a wider one, which is realized in an integrated optical circuit. The proposed device is based on the avoiding of anomalous reflection effect and the coupling transmission of defect modes of the interfaces.