Dye-Laser Fluorescence of 4-Methylumbelliferone and Related Coumarins

Abstract

During tha eight years since phthalocyanine solution was used by Sorokin and Lankard in the original dye laser¹ and DTTC bromide and iodide were used in the work of Schafer, Schmidt and Volze², laser emission has been reported³ from close on 400 combinations of laser dye and solvents covering a wavelength range extending from 340 to 1175 nm. Many hundreds or even thousands of attempts to induce laser action in a vast number of available chemicals have proved unsuccessful, either by pulsed or continuous laser pumping, or by flashlamp techniques. In fact, relatively few combinations have established themselves as highly efficient, reliable and entirely satisfactory dye laser media. The compound rhodamine 6G is probably the most widely used in the range 540–640 nm. However, some ten different solvents and a variety of excitation techniques have been necessary to maintain maximum fluorescence intensity over this range. A t shorter wavelengths an acidic solution of 4,6- dimethyl-7-methylamino-coumarin in ethanol was observed to lase⁴ over the range 430–530 nm and a particular solution of 4-methylumbelliferone to lase from 391 to 567 nm⁵. This offered the possibility of achieving narrow linewidth tunability using dye lasers based on single solutions. It represented a significant advance on the first experiments² in which it was possible by changing the solvent, varying the solute concentration or changing the reflectivity of the resonator mirror, to Shift the wavelengths by up to 60 nmm in the infra-red region.