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Active medium
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Laser light is generated in the active medium of the laser. Energy is pumped into the active
medium in an appropriate form and is partially transformed into radiation energy. The energy
pumped into the active medium is usually highly entropic, i.e. very
disorganised, while the resulting laser radiation is highly ordered and thus has lower
entropy. Highly entropic energy is therefore converted into less entropic energy within the laser. Active laser media are available in
all aggregate states:
- solid (crystalline or
amorphous)
- liquid
- gaseous or as plasma
The way in which the energy is introduced is highly dependent on the aggregate
state. Many gaseous laser media can, for instance, be stimulated with the aid of gas
discharge. This facilitates the transfer of electrical energy to free electrons which, in turn, emit their energy on collision
with atoms or molecules. Solid state and liquid lasers can onlybe pumped
optically; radiation from an ordinary lamp or another laser is absorbed by the
active medium and the energy is re-emitted at longer
wavelengths.
Atoms and molecules are normally present in the so-called ground
state. This is a stable condition - atoms in the ground state cannot release energy. Atoms can exist in other states inwhich there is more energy present than in the ground state - a supply of energy is needed to
raise the atoms to these higher states. The energy can be provided by other
particles, particularly free electrons, or light quanta (photons). The atoms
can return to lower energy levels (e.g. to the stable ground
state) from this ‘excited’ state by releasing energy.
The surplus energy can be passed on to another particle, e.g. an electron or another
atom, or a photon which is then emitted - the energy of the photon is equivalent to the
difference in energy between the higher and lower states. This de-excitation can be
spontaneous or stimulated by other photons. Stimulated emission is a basic requirement
for lasing.
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The energy contained in a quantum of light or photon is
equivalent to Planck’s constant
multiplied by the frequency of
light. The frequency is, in turn, inversely proportional to the wavelength of the
light. The infrared light emitted by a CO2 laser at a wavelength of 10.6 m thus corresponds to an energy
difference of 0.0117 eV while the 193 nm ultraviolet light from an ArF excimer laser is
equivalent to an energy difference of 6.42 eV.
The amount of energy which atoms can accept or release is very specific - the energy spectrum
usually consists of discrete and continuous sections. Discrete energy levels exist just above the
ground state; they become closer to each other with increasing energy and eventually cross over
to a continuum. Continuous radiation is emitted, for instance, when an electron recombines with a positive
ion. The emitted wavelength is then not only dependent on the atomic energy level in
which the electron is captured, but also on the kinetic energy of the electron before
recombination - the kinetic energy is not restricted to any particular values. Continuous spectra
can also be produced when several discrete levels with finite energy uncertainty overlap as is the
case in solids and, above all, in liquids.
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