TITANIUM-SAPPHIRE LASER AS AN EXCITATION SOURCE IN 2-PHOTON SPECTROSCOPY

The passively mode-locked titanium:sapphire laser provides new opportu nities for acquiring two-photon spectral data in the near-infrared, a region not commonly accessible to synchronously pumped dye lasers. Thi s source generates pulses with peak powers near 100 kW at average powe rs over 1 W and is capable of yielding two-photon signals roughly two orders of magnitude larger than is possible with synchronously pumped dye lasers. However, the multimode output of this laser exhibits signi ficant temporal and spectral pulse profile variations as the laser wav elength is tuned, As a consequence, peak powers of the titanium:sapphi re laser can vary independently from average power across the tuning r ange. This wavelength dependence, coupled with the quadratic dependenc e of the two-photon signal upon the instantaneous power of the laser, precludes simple average power correction of nonlinear spectral band s hapes. Here, we investigate the key properties of the titanium:sapphir e laser as an excitation source for two-photon spectroscopy. We also i dentify a chemical reference suitable for obtaining source-corrected e xcitation spectra in the near-infrared using a double-beam, ratiometri c approach; this is based on a source-independent two-photon excitatio n spectrum for the laser dye coumarin-480 that has been obtained with a single-frequency titanium:sapphire laser. From these data, correctio n factors are generated for correction of multimode source data.