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S2
T2
S1
T1
So
The dye is a large molecule
with a large number of
closely spaced vibrational
states – essentially a
continuum of states. The
pump pulse populates the
singlet states S1 and S2. Fast
internal conversion
(radiationless transitions in
which energy id disspated
thermally) occurs down to the
lowest state of S1 which
lases down to an excired
vibrational state of the
ground state So. Care needs
to be taken maintian
population inversion by
ensuring internal conversion
to the triplet state does not
deplete S1 to quickly
High power fixed-frequency
laser pulse is split to pump
both dye laser cell (~40%)
and amplifier cell (~60%)
Pump
Laser
Grating is rotated
gradually to tune the
resonant cavity and so
scan across the required
frequency range
Dye
amplifier
Dye Cell
Beam expanding
telescope
Output
coupler
Rhodamine 6G
http://physics.nist.gov/Divisions/Div842/Gp1/laser.html
http://www.fineartradiography.com/hobbies/lasers/dye/
http://en.wikipedia.org/wiki/File:Coherent_899_dye_laser.jpg
http://technology.niagarac.on.ca/sop/SOP-DyeLaser.html
http://plasma.physics.ucla.edu/pages/lif.html
Solid-State LasersThe gain medium in a solid-state laser is an impurity center in a
crystal or glass. Solid-state lasers made from semiconductors are described below. The
first laser was a ruby crystal (Cr3+ in Al2O3) that lased at 694 nm when pumped by a
flashlamp. The most commonly used solid-state laser is one with Nd3+ in a Y3Al5O8
(YAG) or YLiF4(YLF) crystal or in a glass. These Nd3+ lasers operate either pulsed or
cw and lase at approximately 1064 nm. The high energies of pulsed Nd3+:YAG lasers
allow efficient frequency doubling (532 nm), tripling (355 nm), or quadrupling (266 nm),
and the 532 nm and 355 nm beams are commonly used to pump tunable dye lasers.
Dye Lasers
The gain medium in a dye laser is an organic dye molecule that is dissolved in a solvent.
The dye and solvent are circulated through a cell or a jet, and the dye molecules are
excited by flashlamps or other lasers. Pulsed dye lasers use a cell and cw dye lasers
typically use a jet. The organic dye molecules have broad fluorescence bands and dye
lasers are typically tunable over 30 to 80 nm. Dyes exist to cover the near-uv to nearinfrared spectral region: 330 - 1020 nm.
Semiconductor Lasers
Semiconductor lasers are light-emitting diodes within a resonator cavity that is formed
either on the surfaces of the diode or externally. An electric current passing through
the diode produces light emission when electrons and holes recombine at the p-n
junction. Because of the small size of the active medium, the laser output is very
divergent and requires special optics to produce a good beam shape. These lasers
are used in optical-fiber communications, CD players, and in high-resolution molecular
spectroscopy in the near-infrared. Diode laser arrays can replace flashlamps to
efficiently pump solid-state lasers. Diode lasers are tunable over a narrow range and
different semiconductor materials are used to make lasers at 680, 800, 1300, and
1500 nm.
In the following description, which ignores triplet states, it is necesary
to remember that energy differences are related to frequency
differences via Planck's energy equation. In Fig. 1, the ground
electronic state is S0 and the first excited state is S1. Molecules are
excited from the ground state to either S1 or S2. If excited to S2 the
molecules rapidly decay, in a radiationless manner, to S1. From the
lower level of S1 transitions can take place to any level at the ground
state manifold. It is this energy decay range that gives origin to
tunability. This is explained in greater details, with corresponding
equations, in [14]. Initially organic lasers used a liquid gain medium
were the organic dye molecules are diluted in a solvent such as
ethanol or methanol [1-3]. In a solid-state dye laser the organic dye
molecules are uniformly distributed in a highly homogenous polymer
matrix. In a tunable polymer laser the gain medium is said to be a dyedoped polymer. An example of such polymer is a highly pure form of
poly(methyl-methacrylate) (PMMA).
In the case of semiconductor gain media, or diode gain media, the
tunability range depends approximately on the separation of the
conduction and the valence bands, minus the energy band gap, of the
semiconductor.