B. Bose-Einstein condensation 70 IX. Future Directions 75 Acknowledgments 76 References 76 ''Laser Cooling and Trapping of Atoms,'' J. Opt. Soc. Am. B 6, No. 11, November 1989. Two more recent reviews (Metcalf and van der Straten, 1994; Adams and Riis, 1997) update a decade of developments since the early work recounted in the J. Opt. Soc. Am. B special issues. 2 For an introduction to current research in alkali-atom Bose-Einstein condensation see the special issue on Bose-Einstein condensation in the Journal of Research of the National Institute of Standards and Technology 101, No. 4, July-August 1996.

A systematic study of the degenerate two-photon absorption (2PA) spectra of seven azoaromatic compounds in dimethyl sulfoxide solution is reported, which employed the Z-scan technique with femtosecond laser pulses from the bottom of the azo compound absorption bands up to 1100 nm. The 2PA peaks for pseudostilbene-type azo compounds (Disperse Orange (DO) 3, Disperse Red (DR) 13, DR1, DR19, and DR19-Cl) were observed at twice the peak wavelength of the linear absorption. However, such peaks were not observed for other azo compounds (PAMINO and DIAMINO) because of the symmetry of these molecules. A resonance enhancement of the 2PA cross-section was observed for all compounds. The 2PA peak and the nonlinear resonance enhancement behavior could be adjusted with a model based on perturbation theory. Such knowledge can be a guideline to the understanding of the 2PA process in azoaromatic compounds.

No abstract