This photo shows the fluorescence of Coumarin 102 dye (dissolved in methanol) as a laser beam (445 nm, 1.3 W cw) passes through.

The end of the laser can be seen in the right-hand side of the photo. The image shows how the blue laser beam is invisible as it passes through the air, but its path becomes beautifully clear as it traverses the dye solution.

The laser drives electronic transitions in the dye, resulting in broad-band fluorescence. The beam is considerably attenuated as the dye is strongly absorbing at this wavelength, with almost all of the energy absorbed by the solution.

We will use this dye as the gain medium in our pulsed dye laser, pumping it at 355 nm to generate pulsed laser radiation at 486 nm, to drive the 1s -> 2p transition in positronium atoms.