PHOTON PUMPING

When an ion is exposed to radiation with the precise energy corresponding to one of its transitions, it can be excited to the upper state. In some circumstances there is another level of intermediate energy to which the ion can decay which can create highly non-thermal distribution among the excited states, sometimes leading to population inversion between certain levels.

Photo-pumping of ions to create a population inversion is highly inefficient because most photon sources are broadband emitters of black body radiation, and only a small fraction of the energy within a very narrow range of wavelengths near the resonant absorption frequency of the transition can participate in pumping.

For this reason, most laboratory ion or atomic vapour lasers aren't photo-pumped with flashlamps or arclamps, they are instead excited by electron bombardment from a discharge.

Photon-pumping of ions is used only if a gas discharge lamp with a strong emission line corresponding to the same difference in energy as one of the transitions in the lasing medium.

Plot of absorption coefficient of ruby demonstrates the efficient transfer of pump power from the wide spectrum blackbody produced by flash-lamp to the chromium ions. The extremely narrow atomic absorption lines in gases eliminates photopumping as a laser pump mechanism.

Although history has shown otherwise, it was generally agreed that the first optical lasers would be gas irradiated with spectral lamps. Gould (1962) constructed a cesium vapour laser optically excited excited with a helium lamp long after the more practical lasers were commonplace. According to Lengyel (1971), because it functions by a chance coincidence of spectral lines, it is a freak, or a monument to prodigious amounts of effort and federal money expended for an objective that became almost meaningless before it was accomplished. This is why optical pumping of astrophysical lasers is not a viable population inversion mechanism.

Three conditions must be met before pumping by stellar radiation can be effective:

  1. The absorbing plasma must be optically thick to this line.
  2. The stellar atmosphere must emit copious emission lines.
  3. One of the emission line must fall exactly within the narrow absorption spectrum of the lasing ion.
  4. That particular absorption line of the lasing ion must have just the the right decay rates and transition probabilities to lead to population inversion between a pair of levels within the ions.
Each condition depends on the previous one being true, for example if condition one is not fulfilled, then conditions 2, 3 and 4 are irrelevant. Therefore the total probability that all of these conditions are satisfied simultaneously is obtained by multiplying the individual probabilities of satisfying each condition separately. The resulting total probability of obtaining highly a non-equilibrium distribution of excited states within an ion by photon pumping is extremely small.

However, three body recombination within a rapidly cooled plasma can easily lead to population inversion in many ions under various plasma conditions. Intense laser irradiation has been used to heat a plasma to high temperatures in order to strip away electrons and encourage collisional excitation, however strictly specking this is not photon-pumping, for any other rapid means of heating a plasma could be substituted instead.

In the very strong field of U.V. radiation surrounding a hot star electrons are stripped away from ions, and later recombine with the ions creating. Sometimes this can lead to population inversion, however since the plasma is optically thin, each UV photon may have to travel a great distance before being absorbed, allowing time for internal processes within the ions to come return the levels to an equilibrium distribution. The rapidly cooling stellar wind from a hot star is a much more efficient mechanism for recombination pumping, as we shall see later.

REFERENCES

  1. Lengyel,B.A.: 1971, Lasers, Second ed., Wiley-Interscience.
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