Organic Dye Lasers

Abstract

AbstractFluorescent organic dyes are efficient laser materials in the visible and near infrared. If a solution of such a dye is placed in a laser cavity and its fluorescence excited by a sufficiently fast‐rising light source, such as a giant‐pulse laser or a specially designed flash‐lamp, laser emission sets in, when the pump power has passed the threshold level. The time‐behaviour, spatial and spectral characteristics of these dye lasers have been studied. Of special interest is the dependence of the laser wavelength on the concentration of the dye solution, which may be explained by a four‐level‐laser scheme, as derived from the known absorption and fluorescence spectra of the dyes.Since the spectral band‐width of the dye laser emission may become several nm, when pumped high above threshold, two methods of band‐width restriction and wavelength selection have been investigated. In the first method the cavity length is reduced to a few microns, so that only one transmission maximum of the Fabry‐Perot resonator lies within the fluorescence band of the dye. In the second method the dye solution is excited in a Brewster angle cuvette without any reflecting surfaces. This results in a superradiant emission consisting of one single line, the band‐width of which is limited by the resolution of our spectrograph (7 Å). While a great variety of dyes exhibit laser emission when excited by a giantpulse laser, so far only a few dyes of the xanthene family and some substituted aminopyrene sulfonic acids show laser action with flash‐lamp excitation. Even with these dyes the laser emission ceases before the pump power has dropped below threshold level. In the case of a 10−4 molar solution of rhodamine 6 G in methanol the laser action ceases after 400 nsec, whereas the pumping power remains above threshold for at least 1100 nsec. This behaviour may be explained by radiationless transitions from the first excited singlet to the tripelt state with a concomitant depletion of the ground state.