Dye laser
A laser whose gain medium is an organic dye, usually dissolved in a liquid, pumped by a flashlamp or another laser. A single dye tunes over roughly 30–80 nm, and a set of dyes covers about 320 nm to 1 µm; rhodamine 6G covers roughly 560–620 nm.
A dye laser uses a solution of fluorescent organic molecules, such as rhodamine 6G in ethanol, as its gain medium. The dye absorbs pump light and emits over a broad band on the long-wavelength side of its absorption, and a wavelength-selective element in the cavity picks the output wavelength anywhere in that band. One dye typically tunes over about 30–80 nm; rhodamine 6G, the most widely used, covers roughly 560–620 nm when pumped at 532 nm, and by changing dyes the same laser can reach from the near ultraviolet to about 1 µm. Dye lasers were the main tunable lasers of the visible from their invention in 1966 by Sorokin and Lankard and independently by Schäfer until solid-state sources took over most of their work in the 1990s.
Level structure
Each dye molecule has a singlet ground state S₀ and a first excited singlet S₁, each broadened into a quasi-continuum by many vibrational and rotational sublevels and by collisions with the solvent. Pumping lifts molecules to high vibrational levels of S₁, which relax within picoseconds to the bottom of S₁. Laser emission ends on upper vibrational levels of S₀, which also empty in picoseconds, so the dye behaves as a four-level laser with a lower laser level that is essentially empty. Because S₀ and S₁ have similar vibrational structure, the emission spectrum is roughly a mirror image of the absorption, shifted to longer wavelength, as in ordinary fluorescence.
The upper-state lifetime is a few nanoseconds, about 4 ns for rhodamine 6G, and the fluorescence quantum yield of good laser dyes is close to 1 (about 0.95 for rhodamine 6G in ethanol). The quantum defect is modest: a 532 nm pump photon carries 2.33 eV and a 590 nm laser photon 2.10 eV, a Stokes efficiency of
Triplet losses and flowing dye
Molecules in S₁ also cross over slowly to a triplet state T₁, which lives for microseconds to milliseconds and absorbs at the laser wavelengths. In continuous operation triplets accumulate and quench the gain, so the dye must be moved out of the pumped volume faster than the triplet population builds up. Continuous-wave dye lasers therefore use a free-flowing jet of dye, often in viscous ethylene glycol, crossing a tightly focused pump beam. A 10 µm pump spot crossed by a jet moving at 10 m/s renews the dye every 1 µs. Pulsed dye lasers with nanosecond pump pulses avoid the problem because the pulse ends before triplets build up, and use a flowing cuvette mainly to carry away heat and bleached molecules.
Pumping and cavity designs
Sources for optical pumping have included flashlamps, which give microsecond pulses with joules of energy, pulsed nitrogen and excimer lasers, frequency-doubled and tripled Q-switched Nd:YAG lasers, and continuous argon-ion or 532 nm solid-state lasers. Tuning uses a diffraction grating, a prism, or a birefringent filter plus etalons. Single-frequency ring dye lasers with active stabilization reach linewidths near 1 MHz and below, and these were the standard source for high-resolution atomic spectroscopy and early laser cooling.
The broad gain band also supports very short pulses. A 60 nm bandwidth at 600 nm corresponds to 50 THz, and a transform-limited sech² pulse with that bandwidth lasts
Colliding-pulse mode-locked dye lasers produced pulses below 100 fs in the early 1980s, and with external compression Fork and coworkers reached 6 fs in 1987, a record that stood for about a decade.
Where dye lasers are used now
Ti:sapphire lasers, optical parametric oscillators and tunable diode lasers have replaced dye lasers in most laboratories: they need no liquid handling and hold their performance for years. Dye lasers remain in use for pulsed-dye dermatology at 585–595 nm, where a yellow wavelength matched to haemoglobin absorption treats vascular lesions, in some spectroscopy and isotope work that needs high pulse energy anywhere in the visible, and in teaching.
Pitfalls
Dyes photodegrade, so output falls with accumulated pump energy and the solution needs periodic replacement; usable lifetimes vary strongly between dyes. Many dyes are toxic or suspected mutagens, and common solvents such as methanol and ethanol are flammable. Jet surface quality affects beam quality, and bubbles or particles in the jet cause intensity noise, so filtering and degassing matter.
Common questions
Why is rhodamine 6G so common?
It has a high fluorescence quantum yield, good photostability, strong absorption at 532 nm (a convenient pump), and gain near 590 nm in the yellow-orange, a region that solid-state lasers reached only through nonlinear conversion.
Can a dye laser run continuously?
Yes, if the dye flows through the pump spot quickly enough to prevent triplet build-up; jet-stream CW dye lasers delivered from milliwatts to a few watts.
What is a solid-state dye laser?
Dye molecules can be doped into a polymer or sol-gel host. These avoid liquid handling but photobleach faster, because the dye cannot be replenished by flow.
References: F. P. Schäfer (ed.), Dye Lasers, 3rd ed. (Springer, 1990); P. P. Sorokin and J. R. Lankard, IBM J. Res. Dev. 10, 162 (1966); R. L. Fork, C. H. Brito Cruz, P. C. Becker and C. V. Shank, Opt. Lett. 12, 483 (1987); A. E. Siegman, Lasers (University Science Books, 1986); J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed. (Springer, 2006).