Nonlinear optical properties are a sensitive probe of the electronic and solid-state structure of organic compounds and as a consequence find various applications in many areas of optoelectronics including optical communications, laser scanning and control functions, and integrated optics technology. Because of their strongly delocalized π electronic systems, polymeric and non-polymeric aromatic compounds show highly nonlinear optical effects. Nowadays, polymer chemists are able to tailor specific materials properties for various applications. Some organic substances with π electronic systems exhibit the largest known nonlinear coefficients, often considerably larger than those of the more conventional inorganic dielectrics and semiconductors, and thus show promise for thin-film fabrication, allowing the enormous function and cost advantages of integrated electronic circuitry. The electronic origins of nonlinear optical effects in organic π electronic systems are reviewed, with special emphasis being given to second-order nonlinear optical effects. Methods for measuring nonlinear optical responses are outlined, and the critical relationships of the propagation characteristics of light to observed nonlinear optical effects and to solid-state structure are discussed. Finally, the synthesis and characterization of organic crystals and polymer films with large second-order optical nonlinearities are summarized.

Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities

Conjugated oligomers represent a prominent class of compounds from a viewpoint of their theory, synthesis, and applications in materials science. Push-pull substitution with an electron donor D at one end of the conjugation and an electron acceptor A at the other end results in them having outstanding optical and electronical properties. This Review highlights fundamental synthetic strategies for the preparation of such oligomers with n repeat units (n=1, 2, 3, 4, ..) and the rules that govern their linear and nonlinear optical properties (absorption, frequency doubling and tripling). The unification of chemical, physical, and theoretical aspects with an interdisciplinary image of this class of compounds is attempted herein.

Conjugated oligomers with terminal donor-acceptor substitution.

The higher second‐harmonic efficiency, in powder, of a new organic molecular crystal N‐(4‐nitrophenyl)‐(L)‐prolinol (NPP) is reported. Electronic polarizability of NPP molecules relates to that of other para‐nitroaniline‐like structures such as that of previously reported N‐(2,4‐dinitrophenyl) methyl alaninate (MAP), and the increase in crystalline nonlinear efficiency by one order of magnitude above the latter is ascribed to an ‘‘optimized’’ crystalline structure rather than to minor changes of the molecular hyperpolarizability. The simultaneous chiral and hydrogen‐bonding character of the prolinol electron‐donating group leads to a quasioptimal angle, with respect to quadratic phase‐matched nonlinear interactions, between the molecular transition dipole moments and the twofold axis of the monoclinic P21 crystal structure. Based on a simple oriented gas description of the quasiplanar structure of the crystal, two possible mutually exclusive second‐harmonic phase‐matched configurations are evidenced and s...

Chirality and hydrogen bonding in molecular crystals for phase‐matched second‐harmonic generation: N‐(4‐nitrophenyl)‐(L)‐prolinol (NPP)

We have developed an infinite dilution extrapolation procedure for determining the molecular second order optical susceptibility β using dc induced second harmonic generation. We show how the effects of solute–solute and solvent–solvent interactions are minimized by this procedure, and how several local field models used in the procedure affect the calculation of β. Using hexane as a solvent for nitrobenzene, we have determined the best experimental value in solution for the intrinsic β of nitrobenzene of (1.11±0.17)×10−30 esu.

Measurements of molecular second order optical susceptibilities using dc induced second harmonic generation

Magnetic-field-induced microwave absorption in sintered high-Tc copper oxide superconductors is found to be proportional to the surface area of samples and not to their volume, establishing that absorption takes place near the sample surface. The surface resistance and reactance increase with magnetic field as a result of energy loss through fluxons driven by superconducting currents. When the magnetic field is modulated, absorptive and dispersive signals arise from modulation of the complex surface impedance through processes induced by the changing magnetic fields.

https://iopscience.iop.org/article/10.1209/0295-5075/5/5/015/pdf

Microwave Magneto-Surface Impedance of High-Tc Superconductors

New experimental features of the field-modulated microwave absorption in high-Tc ceramic superconductors are presented. The proposed model for the loss mechanism can explain the complicated field and temperature dependences of the absorption signals.

https://iopscience.iop.org/article/10.1209/0295-5075/10/6/015/pdf

Mechanisms of Field-Dependent Microwave Absorption in High-Tc Ceramic Superconductors

Second order hyperpolarizabilities of a group of para‐disubstituted benzenes have been determined from the measurements of the dc electric field induced second harmonic generation in liquid samples. Four donor groups and four acceptor groups were used to provide different combinations of donor–acceptor disubstitution. The results show clearly that it is possible to establish a sequence of donors (acceptors) with increasing donating (accepting) effectiveness of each other. It has been observed that the increase of the second order hyperpolarizabilities is followed in general by a red shift of the first electronic absorption peak. The interpretation in terms of the intramolecular charge transfer has been presented.

The regularities observed in the second order hyperpolarizabilities of variously disubstituted benzenes

Microwave-absorption measurements using a conventional electron-spin resonance spectrometer provide a novel leadless technique for the detection of Josephson junctions and their size in superconductive Ba-Y-Cu-O compounds.

Size of Josephson junctions in Ba-Y-Cu-O compounds

By studying the variation of the microwave absorption of superconductive GdBa2Cu3O9-y samples with an ESR apparatus, we have detected the influence of the magnetic field on the intergrain Josephson currents on the surface of the sample. The onset of superconductivity is easily detectable as a sharp signal rise. The observed evolution of the signal intensity below Tc is a useful indicator of the improvement of the superconducting properties of the sample. Quenched nonsuperconductive samples showed no signal at all.

https://iopscience.iop.org/article/10.1209/0295-5075/4/12/011/pdf

Microwave study of Josephson junctions in Gd−Ba−Cu−O compounds

EPR detection of the flux distribution in ceramic high-Tc superconductors

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