Haipeng Li

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.

Size-, electric-field-, and frequency-dependent third-order nonlinear optical properties of hydrogenated silicon nanoclusters

We investigated the electronic properties and second hyperpolarizabilities of hydrogenated silicon nanoclusters (H-SiNCs) by using the density functional theory method. The effects of cluster size, external electric field and incident frequency on the second hyperpolarizability were also examined, respectively. We found that small H-SiNCs exhibit large second hyperpolarizability. With the increase of the number of silicon atoms in H-SiNCs, the frontier molecular orbital energy gap decreases, attributed to the enhancement of the second hyperpolarizability. Interestingly, we also found the electric-field-induced gigantic enhancement of the second hyperpolarizability for H-SiNCs due to the change of electron density distributions. In addition, our results demonstrate a significant dependence on the frequency of incident light.

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.

Melting Properties of Medium-Sized Silicon Nanoclusters: A Molecular Dynamics Study

The structures and melting properties of the medium-sized silicon nanoclusters have been comparatively studied using the molecular dynamics method. Structural and thermodynamic parameters are used to characterize the melting properties of the clusters. The size dependence of the melting temperature of silicon nanoclusters is determined using the computation results. Different from the homogeneous melting of bulk silicon, melting of silicon nanoparticles proceeds over a finite temperature range due to surface effects, which shows the heterogeneous melting of nanoclusters. We found that the melting starts at the cluster surface and progressively shifts into the core region. This study provides a fundamental perspective on the melting behaviors of semiconductor silicon nanoclusters at the atomistic level.

Polarization insensitive wide-angle triple-band metamaterial bandpass filter

Summary form only given. Metamaterials have attracted tremendous amount of interests owing to their tempting electromagnetic responses. Based on the metamaterials, many novel practical electromagnetic devices have been realized in the microwave, terahertz, and optical frequencies. Recently, a new type of metamaterial filter has attracted growing attention. Based on the method of hybrid multiple resonances, the metamaterial filter possesses a notable spectral-filtering capability with broad bandwidth and excellent band-edge transitions. In this letter, we report the design, fabrication, and measurement of a polarization insensitive wide-angle triple-band metamaterial bandpass filter (BPF) in the microwave frequency. The proposed BPF consists of two identical metal resonant units which have three concentric square slots separated by a dielectric layer. Experimental results show that the BPF has three distinctive transmission bands centered at frequencies 6.22 GHz, 9.46 GHz and 12.14 GHz with transmission rates of -0.40 dB, -0.71 dB and -1.40 dB, respectively, agreeing well with simulation results. By introducing the substrate integrated waveguide (SIW), it is shown that the filter is valid to a wide range of incident angles for both transverse electric (TE) and transverse magnetic (TM) polarizations. The triple-band metamaterial BPF is a promising candidate as new filters and radomes owing to its multiband transmissions, polarizations insensitive, and wide-angle responses.

Mach-Zehnder interferometer for auto-collimated beams in Non-channel photonic crystals

We propose a auto-collimating Mach-Zehnder interferometer (MZI) for refractive index measurement and optical on/off switch in non-channel photonic crystals (PhCs) by filling the lattice with fluid of different refractive index. The designed MZI, consisting of non-channel waveguides, optical bends and beam splitters for auto-collimated beams, is actively controlled by changing the refractive index of background material in one of its two optical paths. Equi-frequency contour (EFC) analysis and finite-difference time-domain (FDTD) simulations are used to quantify the optical properties of auto-collimation based bends, beam splitters and MZI. Simulation results show that the output intensities exhibit sinusoidal curves as a function of refractive index, and the π− phase shift is obtained as the background refractive index changes as small as Δn=0.16. The novel interferometer demonstrates a great potential to act as a sensitive refractive index sensor or as an intensity modulator in optical circuits.

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.

Thermally tunable microring resonator for self-collimated beams in photonic crystals

We demonstrate an ultra-compact thermally tunable microring resonator based on self-collimation effect in photonic crystals. Thermo-optical effect and the index of refraction changes due to temperature in microring resonator are used to manipulate dispersion quantity. Finite-difference time-domain method (FDTD) is used to investigate the characteristics of the microring resonator. The new design exhibits an ON-OFF contrast with an extinction ratio of more than 12 dB, an ultra-compact footprint of 3.3times3.3 mum2, and tuning efficiency of 0.08 nm/degC when it operates at the optical communication wavelength lambda =1.55 mum. The design presented may find applications in areas including ultra-compact optical switch and wavelength filer in integrated optical circuits.