Fraunhofer Institute for Laser Technology ILT
Fraunhofer Institute for Laser Technology ILT

Active Medium

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.

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 µm thus corresponds to an energy
 of 0.117 eV while the 193 nm ultraviolet light from an ArF excimer laser is equivalent to an energy  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.