Dye Lases by Three-Photon Excitation

Kevin Robinson

Researchers at State University of New York in Buffalo have developed an organic dye that absorbs three photons of infrared radiation to produce stimulated emission at 553 nm. Because of its relatively high efficiency, the dye may have applications in laser up-conversion as well as in biomedical imaging.

Paras N. Prasad, executive director of the university's Institute for Lasers, Photonics and Biophotonics and lead researcher on the study, said that the dye is the product of years of research into the theoretical modeling and study of the relationship between structure and properties in nonlinear optical materials. It has a strong linear absorption peak at 430 nm, which gives it a three-photon absorption peak at 1.3 µm.

To study the dye, called APSS, the group exposed a 1-cm-long quartz cuvette of the substance dissolved in dimethyl sulfoxide to the 1.3-µm output of a custom-built optical parametric generator. At low pump powers, the dye displayed yellowish-green fluorescence, but as the peak power of the stimulation increased, it responded by lasing.

Prasad explained that the energy conversion is so efficient and the population inversion so large that there is no need for multiple passes to achieve stimulation emission. In other words, the dye lases without a cavity.

When pumped with 430-nm laser light, the dye exhibits similar characteristics but with down-conversion. Such 430-nm pumping is not desirable for telecommunications applications or for biomedical work because blue light is more damaging to living samples.

Applications for the dye could include imaging, diagnostics and the photodynamic therapy of deep-tissue tumors. In addition, the ability of the dye to up-convert laser light could lead to new UV lasers for weapons systems, ultrahigh-density data storage and semiconductor processing.

Prasad said that the researchers plan to focus on developing the dye for biological applications and to investigate the production of three-photon materials for use in 1.3- and 1.5-µm optical telecommunications.