Large Third-order Susceptibility Research Articles

Electronic Structures and Nonlinear Optical Properties for Cross-Conjugated Polyene

ABSTRACTFor several cross-conjugated molecules, molecular excited states and third-order nonlinear susceptibilities γ are calculated by semi-empirical molecular orbital method. The calculated γ are compared with each other and the reason for the differences are discussed in terms of the orbital shapes which contribute mainly to γ. The reason that long polyene chains have large γ, is ascribed to the large transition moment which is explained by the signs of the LCAO coefficients. On the other hand, it is found that the cross-conjugated molecules in which two similar-length chain cross at the center is not good for large third-order susceptibility.

Rapid radiative decay and enhanced optical nonlinearity of excitons in a quantum well.

An exciton has a macroscopic transition dipole moment because it is a coherent excitation over the whole crystal. The interaction of this exciton with a radiation field, which results in a polariton in a bulk crystal, brings about the rapid radiative decay of the exciton in low-dimensional systems due to breakdown of the translational symmetry. This large decay constant at the same time makes the excitons deviate from ideal bosons so that we have a large third-order optical susceptibility enhanced by the macroscopic transition dipole moment under near-resonant excitation. The nonlinear optical phenomena are expected to have a fast response time of a picosecond in GaAs quantum wells and a subpicosecond in CdS quantum wells through the short lifetime of excitons.

Structure-property predictions for new planar forms of carbon: Layered phases containing s p2 and s p atoms

Structure, thermodynamics, and electronic properties are predicted for a new low energy phase of carbon which contains planar sheets equally occupied by sp2 and sp carbon atoms. The isolated planar sheets have the same planar symmetry as do the layers in graphite (p6m) and can be formally viewed as resulting from the replacement of one-third of the carbon–carbon bonds in graphite by –C 3/4 C– linkages. This material, called graphyne, is predicted to have a crystalline state formation energy of 12.4 kcal/mol carbon, which appears to be much lower than for any carbon phase which contains acetylenic groups as a major structural component. Based on the major structural reorganization required for graphitization and the observed high temperature stability of known model compounds, high temperature stability is predicted for graphyne. While graphyne will have similar mechanical properties as graphite, it is predicted to be a large bandgap semiconductor (Eg=1.2 eV) rather than a metal or semimetal. Based on this bandgap and the known behavior of related conjugated polymers having linear structures, interesting nonlinear optical properties (including a large third-order susceptibility) are expected. Property aspects are also predicted for other previously uninvestigated carbon phases which are structurally related to graphyne. Finally, structural features of alkali metal charge–transfer complexes of graphyne, which are expected to be metallic, and of related carbon phases are predicted.

Near-infrared dye having a large ultrafast third-order susceptibility

Efficient degenerate four-wave mixing of 160-psec infrared light pulses at lambda = 1.064 microm in solutions of a new dye that absorbs between 1.0 and 1.6 microm is reported. A Kerr-like third-order susceptibility of X((3))(xyyx) = 8 +/- 3) x 10(-12) esu with a decay time of less than 50 psec is observed in various solvents.

Non-linear susceptibilities of cyanine dyes. Application to frequency tripling

We have studied third-harmonic generation in a cyanine solution as a function of the dye concentration and we have shown that the dye increases the conversion efficiency mainly by its own non-linearity and also by phase-matching. The very large third-order susceptibilities of cyanines are plotted versus the length of the conjugated chain.