A dye
laser is a laser
which uses an organic dye as the lasing medium,
usually as a liquid
solution.
Compared to gases
and most solid state lasing media, a dye can
usually be used for a much wider range of wavelengths,
often spanning 50 to 100 nanometers or more. The wide bandwidth makes them
particularly suitable for tunable lasers and pulsed lasers. The dye
rhodamine 6G, for example, can be tuned from 635 nm (orangish-red) to 560 nm
(greenish-yellow), and produce pulses as short as 16 femtoseconds. Moreover, the dye can be replaced by another type in order to generate an even
broader range of wavelengths with the same laser, from the near-infrared to the
near-ultraviolet, although this usually requires replacing other optical
components in the laser as well.
Dye lasers were independently discovered by P. P. Sorokin
and F. P. Schäfer (and colleagues) in 1966.
In addition to the usual liquid state, dye lasers
are also available as solid state dye lasers (SSDL). SSDL use
dye-doped organic matrices as gain medium.
The dyes used in these lasers contain rather large organic
molecules which fluoresce. The incoming light excites the dye molecules into
the state of being ready to emit stimulated radiation, the singlet state. In this state, the molecules emit light via fluorescence, and the dye is transparent to the lasing
wavelength. Within a microsecond, or less, the molecules will change to their triplet state. In the
triplet state, light is emitted via phosphorescence, and the molecules absorb the lasing wavelength,
making the dye opaque. Liquid dyes also have an extremely high lasing
threshold. Flashlamp pumped lasers need a flash with an extremely short
duration, to deliver the large amounts of energy necessary to bring the dye
past threshold before triplet absorption overcomes singlet emission. Dye lasers
with an external pump laser can direct enough energy of the proper wavelength
into the dye with a relatively small amount of input energy, but the dye must
be circulated at high speeds to keep the triplet molecules out of the beam
path.
Since organic dyes tend to decompose under the
influence of light, the dye solution is normally circulated from a large
reservoir.[11] The dye solution can be flowing through a cuvette, i.e., a glass container, or be as a dye jet,
i.e., as a sheet-like stream in open air from a specially-shaped nozzle. With a dye
jet, one avoids reflection losses from the glass surfaces and contamination of
the walls of the cuvette. These advantages come at the cost of a
more-complicated alignment.
Liquid dyes have very high gain as laser
media. The beam needs to make only a few passes through the liquid to reach
full design power, and hence, the high transmittance of the output coupler. The high gain also leads to high losses, because
reflection from the dye cell walls, or flashlamp reflector, will dramatically
reduce the amount of energy available to the beam. Pump cavities are often coated, anodized, or otherwise made of a material that will not
reflect at the lasing wavelength while reflecting at the pump wavelength.[10]
Some of the laser dyes
are rhodamine
(orange, 540--680 nm), fluorescein (green, 530--560 nm), coumarin
(blue 490--620 nm), stilbene (violet 410--480 nm), umbelliferone
(blue, 450--470 nm), tetracene, malachite
green, and others.[21][22]
While some dyes are actually used in food coloring, most dyes are very toxic,
and often carcinogenic.[23]
Many dyes, such as rhodamine 6G, (in its chloride form), can be
very corrosive to all metals except stainless steel. Although dyes have very
broad fluorescence spectrums, the dye's absorption and emission will tend to
center on a certain wavelength and taper off to each side, forming a tunability
curve, with the absorption center being of a shorter wavelength than the
emission center. Rhodamine 6G, for example, has its highest output around 590
nm, and the conversion efficiency lowers as the laser is tuned to either side
of this wavelength.
A wide variety of solvents can be used, although
some dyes will dissolve better in some solvents than in others. Some of the
solvents used are water,
glycol,
ethanol,
methanol,
hexane,
cyclohexane,
cyclodextrin,
and many others. Solvents are often highly toxic, and can sometimes be absorbed
directly through the skin, or through inhaled vapors. Many solvents are also
extremely flammable. The various solvents can also have an effect on the
specific color of the dye solution and, thus, on the lasing bandwidth
obtainable with a particular laser-pumping source.
Adamantane is added to some dyes to prolong their life.
Cycloheptatriene
and cyclooctatetraene (COT) can be added as triplet
quenchers for rhodamine G, increasing the laser output power. Output power of
1.4 kilowatt at 585 nm was achieved using Rhodamine 6G with COT in
methanol-water solution.
Rhodamine
6G Chloride powder; mixed with methanol;
emitting yellow light under the
influence of a green laser
