Organic dyes typically used dissolved in an organic solvent. The dye solution is circulated continuously through the laser chamber of the dye laser, in which it is excited (pumped) by flash lamps or by another laser.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 362 (https://doi.org/10.1351/pac200779030293)