Journal of Nonlinear Optical Physics & Materials

A novel dmit^2- salt: (hexadecyltrimethylammonium)bis(4,5-dithiolato-1,3-dithiole-2-thione)copper, abbreviated as HTCu, is synthesized and characterized for third-order optical nonlinearity by Z-scan technique at a wavelength of 1064 nm with laser duration of 30 ps. Z-scan spectra reveal a negative Kerr coefficient at 1064 nm and no nonlinear absorption was observed under this condition. The nonlinear refraction coefficient n₂ and the second hyperpolarizability γ have been determined to be -1.51 × 10^-11 and 1.36 × 10^-30esu, respectively, suggesting HTCu is a promising material for all-optical switching.

Conical emission is investigated for Ti:sapphire femtosecond laser pulses propagating in water. The colored rings can be observed in the forward direction due to the constructive and destructive interference of transverse wavevector, which are induced by the spatio-temporal gradient of the free-electron density. With increasing input laser energy, due to filamentation and pulse splitting induced by the plasma created by multiphoton excitation of electrons from the valence band to the conduction band, the on-axis spectrum of the conical emission is widely broadened and strongly modulated with respect to input laser spectrum, and finally remains fairly constant at higher laser energy due to intensity clamping in the filaments.

We investigate modal phase matching in lithium niobate step-index waveguides for second harmonic generation. We predict doubling bandwidths as large as 110 nm and show that temperature tuning can compensate for any deviation from the designed film thickness.

We report an investigation of luminescent and tunable photonic crystal structures formed by the infiltration and inversion of opal templates with high index and luminescent materials. Protocols are reported for the deposition of these materials and the properties of the resulting structures investigated by conventional structural and optical measurements. The properties of multi-layered and backfilled structures are reported and demonstrate the potential to modulate and statically tune the luminescence from photonic crystals.

We use the method of Adomian to investigate the solution of the nonlinear equation giving the evolution of a plane wave in a medium exhibiting linear, two- and three-photon absorption. The second-order solution is in good agreement with numerical simulations. An effective two-photon absorption coefficient is conveniently defined to fit experimental data.

Poled polymers are currently of great interest in second-order nonlinear optical (NLO) applications such as electro-optic (EO) modulation, optical switching, etc. The EO active chromophores doped in polymer matrices generally have large dipole moment. The strong electrostatic interaction between molecular dipoles causes orientation relaxation after poling, decrease of poling efficiency, phase separation, and etc. at high doping concentration. A new type of NLO chromophore with low ground-state dipole moment has been designed and synthesized. It was doped in PMMA polymer matrix. The electrostatic interaction between chromophore molecules in this doped film is reduced greatly. This provides a way to increase the doping level in guest-host poled polymer system. At a very high loading level of 52% wt. (N = 6.41 × 10²⁰/cm³), which is over 2 times of the theoretically predicated value for the spherical shape molecules, reasonable film quality and dipole orientation stability have been achieved.

In this paper, we report our investigations on thick holographic recording material of the phenanthrenequinone doped poly(methyl methacrylate) (PQ:PMMA) photopolymer. The design strategy and fabrication technique for making thick polymer samples with negligible shrinkage and good optical quality are presented. The physical mechanism for holographic recording in PQ:PMMA material is described, and methods for improving are proposed. Based on these methods, photopolymer samples with different compositions are fabricated and experimentally characterized. The results show that by modifying compositions, the material sensitivity and dynamic range for volume holographic recording have been improved.

Fiber optic parametric amplifiers (FOPA) based on triangular photonic crystal fiber (PCF) is analyzed theoretically. Equations about parametric unsaturated gain and gain bandwidth in normal or abnormal dispersion regimes are derived. Numerical simulations are performed for single-pump and two-pump FOPAs, respectively. Comparisons with FOPAs based on dispersion-shifted fiber (DSF) and highly nonlinear fibers (HNLF) are carried out. The results show that, due to the excellent dispersion profile and nonlinearity of PCF, single-pump FOPA has higher peak gain and two-pump FOPA presents higher flat gain over wider bandwidth.

The optical nonlinearity in liquid crystals doped with 2,5 azo-substituted anthraquinone (ASAQ) dye is studied. Dynamic holographic grating formation is observed under conditions of low power laser light and no external fields. This new dye is an interesting alternative to methyl red (MR) dopant for liquid crystals (LCs) as materials for single-layer optically addressed spatial light modulators (OASLMs). The thickness and concentration dependence are explored in ASAQ-doped LCs. Stability and suitability for device applications are researched. In MR-doped LC systems the performance of the devices deteriorates quickly, while ASAQ devices are remarkably stable.

The cyano and nitro groups were chosen as acceptor groups, and the substituent amino or ether groups as donor groups to the matrix. Polyurethane (PU) was modified by glycerol to increase the content of chromophore and to improve the stability of the nonlinearity. T_g and T_m were raised and solubility, film-forming ability and other physical properties were improved. The average functional groups of the reactant can be adjusted to ≥ 2 with polygroups and monogroups mixture. These poled polymers show high second-order optical nonlinearity and would have potential application in frequency-doubling or electro-optical controlling devices.

A nonlinear optical (NLO) microscope has been developed based on recording the second harmonic generation (SHG) signals of the B₂- and B₄-phases of bent shaped liquid crystalline (LC) materials. We observed that the microstructures of the polar order can be accurately characterized by 2D scanned images implementing this device. Specifically, we have investigated two bent-core compounds, which have either B₄ or B₂ phases at room temperature and exhibit SHG activity. The NLO microscope was calibrated according to the Maker-fringes method allowing the in situ evaluation of the relative SHG efficiency. The developed NLO microscopy device may serve as an important tool to characterize liquid crystalline features at a microscopic scale for both fundamental and applied research.