Учебно-методическое пособие по чтению специальной литературы для студентов 1 курса физического факультета. Дроздова И.В - 26 стр.

                                         26

     happen only to upper-state atoms, and absorption can happen only to lower-
     state atoms. For stimulated emission to dominate absorption, therefore, more
     atoms must be in the upper state than in the lower state. This unusual situation
     is called population inversion and can be achieved by supplying energy
     (“pumping” the laser) and carefully selecting the active medium. Typical
     pumping schemes include the use of light from flash lamps or other lasers,
     collisions of the atoms with electrically accelerated electrons in a gas discharge
     tube, excitation with energetic particles from nuclear reactions, chemical
     reactions, and direct electrical input to semiconductor. Continuous lasing is
     harder to achieve than pulsed lasing.

     It is necessary to ensure that the stimulating light is sufficiently strong.
     Stimulated emission then occurs in a time interval that is short compared to the
     spontaneous lifetime of the existed state. This situation is achieved by keeping
     a fraction of the laser light trapped between two mirrors enclosing the active
     medium. Domination of stimulated emission over spontaneous emission
     becomes more difficult to achieve as the spontaneous lifetime becomes
     shorter. Because shorter spontaneous lifetimes are associated with states that
     emit radiation of higher frequencies, it is difficult to make an ultraviolet-
     emitting laser, and an X-ray laser was not successfully demonstrated until
     1984. Despite this complexity of construction, however, ultraviolet lasers, or
     excimers, have gained widespread use in industry. Emitting ultraviolet light
     when a halogen and rare gas atom combine temporarily, they are used in
     applications ranging from glass etching and photolithography to sterilization of
     wines.

     Atoms initially in a lower state are raised to the upper state by energy from a
     flash lamp or some other pumping source. Some of these atoms emit light
     spontaneously in random directions. Light traveling perpendicular to the
     mirrors stays within the active medium long enough to stimulate emission from
     other atoms, whereas light traveling in other directions is soon lost. The light
     amplified by stimulated emission is now more intense and more likely to
     stimulate further emission. Some light reaching the output mirror is transmitted
     to form the laser beam; some is reflected back through the medium to continue
     the stimulated-emission process.

4.   The fundamental principles underlying the operations of the maser and laser
     were established long before these devices were successfully demonstrated:
     stimulated emission was proposed by Albert Einstein in 1916, and population
     inversion was discussed by V.A. Fabrikant in 1940. These fundamental ideas,
     followed by two decades of intensive development of microwave technology,
     set the stage for the maser, an ammonia maser, constructed in 1954 by J.P.
     Gordon, H.J. Zeiger, and Charles H. Townes. Over the next 6 years many
     workers, including Nickolai G. Basov, Aleksander M. Prokhorov, Arthur L.
     Schawlow, and Townes, made important contributions that helped to extend