Atomic transitions which emit or absorb visible light are generally electronic
transitions, which can be pictured in terms of electron jumps between
quantized atomic energy levels.
The frequency that is emitted when an electron makes the downward
transition is the same as the frequency absorbed by this two-level system.
This can be generalized to the multiple energy levels of atoms.
The emission spectra of atoms are the series of frequencies emitted by those
atoms in gaseous form. If these same gases were cool, the same series of
frequencies would be selectively absorbed.
Absorption
Spontaneous
emission
Stimulated emission
hν
hν
ΔE
hν = ΔE
hν
hν
hν
Spontaneous absorption - an electron transit from a lower
energy level to a higher one by absorbing a photon.
Spontaneous emission - an electron spontaneously emits a
photon to transit from a higher energy level to a lower one.
Stimulated emission - photons incident into the matter to
stimulate the electrons to transit from a higher energy level to
a lower one and to emit a photon. The incident photon and
their emitted counterparts have the same wavelength and
phase; this wavelength corresponds to the energy difference
between the two energy levels. A photon stimulates an atom to
emit another photon, and hence two identical photons are
resulted.
The stimulated emission of light is the crucial quantum
process necessary for the operation of a laser.
Population Inversion
Population inversion occurs when a system (such as a group of atoms
or molecules) exists in a state with more members in an excited state than in
lower energy states. The concept is of fundamental importance in laser
science because the production of a population inversion is a necessary step
in the workings of a standard laser.
A population inversion cannot be achieved with just two levels
because the probability for absorption and for spontaneous emission is
exactly the same. The lifetime of a typical excited state is about 10-8 seconds,
so in practical terms, the electrons drop back down by photon emission about
as fast as you can pump them up to the upper level. The achievement of a
significant population inversion in atomic or molecular energy states is a
precondition for laser action. Electrons will normally reside in the lowest
available energy state. They can be elevated to excited states by absorption,
but no significant collection of electrons can be accumulated by absorption
alone since both spontaneous emission andstimulated emission will bring
them back down.
The excited state of the system must be a metastable state, that is, the
state in which the electrons remain longer than usual, because this will cause
more stimulated emissions to occur than spontaneous emissions.
Three-level lasers
Four-level lasers
Ruby laser – the first laser
Mirror
Spontaneous emission Mirror
Ef
Ei
Mirror
Stimulated emission
Ef
Ei
Mirror
Mirror
Feed-back by the cavity
Ef
Ei
Mirror
Mirror
Stimulated emission
Ef
Ei
Mirror
Mirror
Feed-back by the cavity
Ef
Ei
Mirror
After several round trips…
Mirror
Mirror
Ef
Ei
Laser beam
Photons with:
- same energy : Monochromatic
- same direction of propagation : Spatial coherence
- all in synchrony: Temporal coherence
1. Coherent. Different parts of the laser beam are related to
each other in phase. These phase relationships are maintained
over long enough time so that interference effects may be seen
or recorded photographically. This coherence property is what
makes holograms possible.
2. Monochromatic. Laser light consists of essentially one
wavelength, having its origin in stimulated emission from one
set of atomic energy levels.
3. Collimated. Because of bouncing back between mirrored
ends of a laser cavity, those paths which sustain amplification
must pass between the mirrors many times and be very nearly
perpendicular to the mirrors. As a result, laser beams are very
narrow and do not spread very much.
Major commercial lasers
laser type
molecular fluorine excimer
argon fluoride excimer
krypton fluoride excimer
xenon chloride excimer
organic dye (tunable)
helium-cadmium
argon ion
semiconductor (gallium nitride)
krypton ion
helium-neon
semiconductor (gallium-aluminumindium-phosphorus)
titanium-sapphire (tunable)
ruby
alexandrite (tunable)
semiconductor (gallium-aluminumarsenic)
wavelength (micrometres [μm])
0.157 (ultraviolet)
applications
photolithography for microelectronics
eye surgery, photolithography for
0.192 (ultraviolet)
microelectronics
photolithography for microelectronics,
0.249 (ultraviolet)
machining of electronic and medical
parts
machining of electronic and medical
0.308 (ultraviolet)
parts
0.320-1.000 (ultraviolet to near infrared) scientific research, dermatology
fluorescence measurements, mastering
0.325; 0.442 (ultraviolet; blue)
of CDs and DVDs
0.275-0.303; 0.330-0.360; 0.450-0.530 biomedical instruments, high-speed
(ultraviolet; blue-green)
printers
0.400-0.415 (violet)
next-generation optical disc players
0.330-0.360; 0.420-0.800 (ultraviolet;
light shows
blue to near infrared)
0.543; 0.6328; 1.150 (green; orange;
interferometry, holography, precision
near infrared)
measurement
DVD players, bar-code scanners, laser
0.630-0.680 (red)
pointers
0.680-1.130 (red to near infrared)
scientific research
0.694 (red)
eye surgery, tattoo removal
0.720-0.800 (near infrared)
hair removal, skin resurfacing
0.750-0.900 (near infrared)
CD players, fibre-optic communications
laser type
wavelength (micrometres [μm])
solid-state yttrium-aluminum-garnet
1.064 (near infrared); 0.532, 0.355,
0.256 harmonics
semiconductor (indium-galliumarsenic-phosphorus)
chemical (oxygen-iodine)
erbium
chemical (hydrogen fluoride)
chemical (deuterium fluoride)
carbon dioxide
1.200-1.600 (near infrared)
applications
drilling, welding, surgery, remote
sensing, mass spectrometry, range
finders
fibre-optic communications
1.315 (near infrared)
airborne weapons
1.535-1.560 (near infrared); 2.940 (short fibre-optic communications, surgery
infrared)
and dentistry
2.600-3.000 (short infrared)
space-based weapons
3.500-4.000 (short-middle infrared)
airborne weapons
9.000-11.000 (thermal infrared)
industrial cutting and drilling, surgery
angioplasty
cancer diagnosis
cancer treatment
cosmetic dermatology such as scar revision, skin resurfacing, laser hair
removal, tattoo removal
Dermatology, to treat melanoma
Lithotripsy,
laser mammography
medical imaging
microscopy
ophthalmology (includes Lasik and laser photocoagulation)
optical coherence tomography
prostatectomy
plastic surgery, in laser liposuction
Surgery, to ablate and cauterize tissue