What is the Excimer Laser? Construction and Working. 3 Applications.

Excimer Laser

An excimer laser is the type of molecular laser – Isolated atoms and ions in ionic laser have discrete energy levels. However, molecules have energy bands. These bands have electronic, vibrational, and rotational energy levels.

Each electronic energy level has a large number of vibrational energy levels and each vibrational level has a large number of rotational energy levels,

The excimer laser is the powerful light source in the ultraviolet (UV) region of the electromagnetic spectrum.

The term excimer is the short form of excited dimer means a molecule formed by combining two identical noble gas ( i.e. Argon, Krypton, Xenon ) atoms in the excited state. Examples of excimers laser are Ar*₂ Kr*₂ Xe*₂.

However,  the excimer term is now extended to noble gas halides. Typically excimer laser uses noble gases and halides ( chlorine, Fluorine) combination called exciplex ( i.e excited complex ), and this combination exists in the excited state. Example ArCL*, KrCL*, XeCL*, ArF*, KrF* and XeF*. Thus, in excimer lasers, exciplex is used as the active or lasing medium. 

Construction and working

Active medium ( or Excimer laser gain medium )

The main component of an Excimer laser is a medium in the form of a noble gas ( Argon, Krypton, Xenon ) molecules called active medium. The main characteristics of the active medium are as follows:

• It must have a pair of energy levels separated by a certain amount of energy. The energy level having energy is known as an upper energy level or higher excited energy level and the energy level having low energy is known as low energy or ground state.
• It must allow a population inversion between two energy levels.

Pumping device

 it is an external source of energy that provides the necessary energy to the active medium to produce a state of population inversion essential for lasing action.

A resonant cavity

 Population inversion is achieved to amplify the signal by ( or Photon ) stimulated emission. However, in practice, most of the atoms in the excited state emit spontaneously and do not contribute to the overall output.

Only a few atoms in the excited state emit via stimulated emission and the hand’s overall gain of the output is small. Therefore, we require a positive feedback mechanism to make most of the atom in the excited state emit via stimulated emission for contributing to the current output.

A device used to you have a positive feedback mechanism for Maximum coherent output is known as a resonator or resonant cavity.

Thus, a resonant cavity or resonator is a feedback device that makes the photon to move back and forth through the active medium. In this process, the number of photons emitted due to stimulated emission are multiplied.

A resonant cavity consists of a pair of plane or spherical mirrors placed parallel to each other at the end of the active medium. One of the mirrors is a fully reflecting mirror and the other is a partially transmitting mirror. The laser output is taken out through the partially transmitting mirror which is also called the output coupler mirror.

Population inversion in Excimer laser

Population inversion in Excimer laser is achieved by electric discharge pumping. In this case, voltage is applied across the electrodes of the gas discharge tube which is filled with a low-pressure gas mixture known as the gain medium. 

The applied voltage produces an electric field within the tube. This electric field accelerates electrons within the gas. These electron collides with the gas atom or gain medium and excite their atom to higher energy levels or excited energy levels.

If the atom in the lower-lying energy level makes the transition to the excited state faster than the atom in the higher-lying energy level makes the transition to the lower-lying energy levels, then the population of atom in the higher energy level is more than the population of atom in lower energy levels. Hence, population inversion in gases is achieved.

The arrangement to produce excimer laser is shown in the figure.

Construction of Excimer laser

It consists of a tube formed by two electrodes connected to the discharge voltage source. The active medium is placed between these two electrodes.

The arrangement is enclosed in a resonant cavity formed by two planes and parallel mirrors M1 and M2. Mirror M1 is a perfectly reflecting mirror, while mirror M2 is a partially reflecting mirror through which pulse laser comes out. 

Working of Excimer Laser

Gas discharge at high pressure in the laser tube is initiated. The noble gas and halide placed in the laser to form the examiner molecule. The excimer molecule has table excited electronic states and weakly Bound ground state as shown in the figure. 

The upper or excited energy level is populated by a three-body collision involving noble gas positive ion, halogen negative ions, and a buffer gas like Helium or Neon. When the high energy compound disassociates, a photon is released whose energy is equal to the ground state of the noble gas halogen ( say KrF ) molecule. 

The wavelength of light emitted by an excited excimer molecule is amplified in the resonant cavity. Thus, a discrete high energy pulse of duration 10 ns to several nanoseconds in the ultraviolet region (UV) is produced. The wavelength of excimer laser depends on the composition of the active medium ( i.e noble gas + halides).

Most commonly excimer laser are ArF (193 nm), KrF (248 nm) and XeCL (308nm). 

Applications of Excimer Laser

Excimer laser are used in 

• The vision correction system is ophthalmology.
• Microlithography.
• Micromachining. 

FAQ on Excimer Laser