Mixed electric-magnetic second-order nonlinear optical response of helicenes

The mixed electric-magnetic second-order nonlinear optical responses of oriented films of helicenes have been computed ab initio using the random phase approximation method and compared to the pure electric-dipole counterpart. It turns out that the mixed electric-magnetic responses can be of the same order of magnitude as the pure electric-dipole counterpart when there is no donor/acceptor (D/A) substituent or these D/A pairs are weak, i.e., when the pure electric-dipole response is small. When adding strong D/A substituents, the pure electric-dipole response increases substantially and much more than its mixed electric-magnetic counterpart. Consequently, the ratio between the mixed electric-magnetic and pure electric responses decreases. Although there is no general rule, the mixed responses evolve as a function of substitution quasi similarly to the pure electric contribution. This study confirms therefore the possibility of tuning the mixed electric-magnetic response by employing appropriate chiral molecules.     

57Fe Mössbauer spectroscopic analysis of a welding wire's coating

The coating layer of a steel welding wire has been removed and studied by⁵⁷Fe Mössbauer spectroscopy at different temperatures and at 5 K in an applied magnetic field of 60 koe. The subspectrum of the metallic phase is typical for a low-carbon steel. The coating phase cannot be identified as any known modification of iron oxide or oxyhydroxide. The spectral features rather suggest that it consists of an amorphous ferric material similar to a ferric gel.     

Bacteriorhodopsin: a natural (nonlinear) photonic bandgap material

The angular dependence of the hyper-Rayleigh scattered light intensity from a suspension of bacteriorhodopsin has been analyzed. The observation calls for combination of the second-order nonlinear hyperpolarizability of the retinal chromophore with the linear refractive index of the apo-protein matrix. The structuring of the small nonlinear chromophore in the large linear matrix imparts properties of a photonic bandgap to bacteriorhodopsin.     

Quasi-phase-matching in chiral materials

The second-order nonlinear optical coefficients associated with chirality differ in sign for the two mirror-image forms (enantiomers) of a chiral material. Structures comprised of alternating stacks of the enantiomers can therefore be used for quasi-phase-matched frequency conversion, as we demonstrate here by second-harmonic generation from Langmuir-Blodgett films of a helicenebisquinone. Such structures could lead to new types of frequency converters in which both the second-order nonlinear response and quasi-phase-matching arise from the chirality of a material rather than its polar order.     

Calix[4]arenes as molecules for second order nonlinear optics

Abstract

The synthesis of a novel class of molecules for second order nonlinear optics, i.e. calix[4]arenes with extended π-systems, is described. These compounds are obtained via Wittig-Horner reactions of the formylated calix[4]arenes 5 and 6 to give the stilbene derivatives 7–9, or by diazotization of calix[4]arene, 1, followed by alkylation to give the phenylazocalix[4]arenes 11 and 12. The molecular second order nonlinear optical properties (β_z) of these calix[4]arenes have been measured by electric field-induced second harmonic generation. The influence of different acceptors as well as the influence of the different conformations of the calix[4]arenes on β_z values were determined. Surprisingly, the wavelength of the charge-transfer band λ_max is lower when β_z increases upon increasing the number of acceptors.     

Nonlinear optical properties of polymeric materials and polymer films: Recent developments and future trends

An overview of recent developments in the field of nonlinear-optical properties of polymers is presented. The emphasis is placed on frequency doubling, the electro-optic effect, and photorefractive properties of polymers.