Organic Dye Laser Threshold

Abstract

This paper describes the results of an investigation of the interrelated physical parameters that determine laser threshold for three organic dyes: rhodamine B, rhodamine 6G, and fluorescein. Not only are these dyes (rhodamine 6G in particular) among the most widely used and important laser dyes, but insights into the threshold condition derived from our studies of these particular dyes should be generally applicable to future dye laser research. Our detailed characterization of threshold for these dyes and our novel insights result from the use of triplet state spectral data in a thorough and quantitative way for the first time, coupled with the physically realistic approximation that the triplet population is proportional to the singlet excited state population. For the case of self-tuning of the laser emission wavelength, solutions to the threshold equations are presented that establish relations between all the parameters affecting lasing: critical inversion, emission wavelength, extrinsic losses, dye concentration, length of active medium, and triplet to excited singlet population ratio. For reasonably high extrinsic cavity losses, the critical inversion is a simple power law function of extrinsic loss, and the emission wavelength is self-tuned in such a manner that the ratio of extrinsic losses to intrinsic singlet absorption losses is substantially constant. There exists a region of low, but still physically realizable, cavity losses, where laser action is determined exclusively by intrinsic characteristics of the dyes (triplet absorption, singlet emission, and absorption). Asymptotic limits on the maximum wavelength of emission and on the minimum critical inversion appear in this region, beyond which there is no point in reducing extrinsic cavity losses. The fraction of triplet state molecules also determines long wavelength cutoffs for an externally tuned laser. Methods are outlined for measuring the ratio of triplet state to excited singlet state populations for dyes with known triplet absorption spectra and for determining semiquantitatively the triplet effects in dyes where this information is not known. A final interesting conclusion is that molecular modifications of rhodamine 6G cannot be expected to improve its threshold characteristics as a laser dye by much more than a factor of 4.

Optofluidic dye lasers

“…Therefore, laser dyes often have strong and wide absorption bands in the UV and visible region as shown in Fig. 1 (Peterson et al 1971). The emission spectra of dye molecules are Stokes-shifted to longer wavelength and form almost mirror images of the absorption bands due to the so-called Frank-Condon principle (Svelto 1998).…”

“…1 Stimulated singlet state absorption and emission cross sections for Rhodamine 6G in ethanol. Reprint with permission from Peterson et al (1971) electronic states. T 1 and T 2 are the first and second excited triplet states.…”

“…A more thorough treatment can be found in Peterson et al (1971). In general, the pump threshold can be obtained by numerically solving the rate equations.…”

A versatile dye-laser generator-amplifier system for intense, tunable picosecond pulse generation

“…The absorption crosssections are obtained from transmission measurements. The stimulated emission cross sections are derived from backward fluorescence measurements [23][24][25][26][27] (excitation source: tungsten lamp; detection with Tracor DARRS diode array system). Other dye parameters are listed in Table 1.…”

Tunable microfluidic dye laser

“…Single mode operation and wavelength tunability is commonly achieved by actuated hulk optical components inside the laser cavity. The lasing wavelength'can also he tuned by changing the concentration of dye molecules in the solvent [4].…”