LASING ion MEDIUM

     The lasing medium in an laser is a rare gas such as argon or krypton that has been ionized; that is, it has one or more electrons removed from the outer shell. Argon and krypton are the most common ion lasers, although neon, xenon, and a few other gases will laser in an ion form as well. Ionized species exhibit different energy levels than neutral species do and the degree of ionization (the number of electrons removed) affects these levels.

     As an example of the mechanics of a typical ion laser, consider singly ionized argon (denoted Ar⁺‏), the active species of the most common ion laser by far. Ions are created by discharging a current of up to 40 A through low-pressure (< 1 torr) argon gas. Discharges may be pulsed, as the earliest lasers were, but most ion lasers are CW, so a continuous current of 40 A is required (which leads to complex tube and power supply designs). Ar‏ has numerous energy levels in two bands nine ULLs centered around 36 eV and two LLLs around 33 eV. Many transitions share an upper or lower energy level. Overall, 10 laser transitions exist in the violet-to-green region of the spectrum.

        Ions are pumped to the ULL by a variety of methods, some by decay from a higher level (the expected route for a four-level laser) or directly to the ULL by electron impact in a process resembling that of a metal-vapor laser. In the case of argon, the neutral (non-ionized) configuration of the atom is 1s² 2s² 2p⁶ 3s² 3p⁶ and when ionized (which requires 15.76 eV of energy) the ground state for the ion (Ar‏⁺) becomes 1s² 2s² 2p⁶ 3s² 3p⁵. The argon ion can now be promoted to a higher-energy state in which one electron assumes a 4s state. There are two such states possible with different spins (1/2 and 3/2), and these serve as lower lasing levels. Even-higher energy states are possible, in which an electron enters a 4p state there are numerous states here that serve collectively as upper lasing levels (in the visible argon laser spectrum seven tightly clustered ULLs are involved). Four distinct pathways for pump energy have been identified which serve to excite ions to the ULL, including the direct pumping of ions from the neutral ground state to the ULL, a two-step process in which argon atoms are ionized to the ion ground state and later pumped to the ULL from there, and two processes in which the ULL is excited via decay from levels above the ULL.

       Having made a transition from one of the ULLs to a LLL, the ion decays almost immediately to the ion ground state (still 15.76 eV above the ground state of the neutral argon atom) by a radiative process in which a 74-nm extreme UV photon is emitted. This decay process is fast a requirement to maintain a large population inversion but the extreme UV photon created in the process poses problems since it can damage optical windows and degrade tube materials. Windows must be fabricated from special crystalline quartz to withstand the constant bombardment of extreme-UV radiation. The energy levels involved, transitions, and depopulation process are shown in Figure 1.1.

      Krypton gas may be used in an ion laser as well with various wavelengths, covering the entire visible spectrum from violet to red. Physically, krypton laser tubes are similar to argon tubes with the exception that krypton lasers require a large ballast volume. Krypton, however, is less efficient than argon, so output powers are lower than those for a comparable argon laser (the most powerful lines of the krypton laser are only one-fifth the power of the most powerful argon lines). The wider range of visible wavelengths available, however (including a

Figure 1.1. Argon-ion laser energy levels and transitions.

powerful red line and a yellow line, both lacking in the argon laser spectrum), make this laser a popular choice for entertainment purposes.

       Table 1.1 lists the common visible wavelengths of argon and krypton ion lasers and typical output power for a comparably sized single-line (wavelength-selected) laser using each gas. With broadband optics it is possible to have several lines oscillate simultaneously, but in many cases these levels share energy levels, so competition between lines can occur which prevents all possible lines from lasing simultaneously. In the case of the argon laser used in this example, only six of the 10 possible lines will lase simultaneously when broadband optics are employed. Krypton lasers are generally not used in multiline mode but rather, with optics, to select the red (647.1 nm) line alone, both the red and yellow (568.2 nm) lines, or white-light mode, in which three or four lines are allowed to oscillate. By selecting only required lines, the output power of the already weak krypton laser is preserved.

TABLE 1.1. Comparison of Argon and Krypton Laser Output.   

It is also possible to doubly or triply ionize a species. Doubly ionized argon (Ar^2+‏), produced by a higher current density than required for Ar‏, produces several powerful lines in the UV region between 275 and 364 nm, so this laser is an important source of continuous UV output. For doubly ionized argon, however, the ULLs are located about 27 eV above the ion (Ar²⁺‏) ground state, which is far above the Ar⁺‏ or neutral ground state. The ULLs for the species are hence 71 eV above ground, so enormous amounts of pump power are required for this laser, with tube currents of 70 A common. Also, efficiency is much lower than for the singly ionized species with power output on the UV lines 20-fold less than the expected multiline visible output. Krypton can be doubly ionized as well but is of even lower efficiency than doubly ionized argon and not commonly available.

      Aside from argon and krypton, other noble (inert) gases can be utilized in an ion laser. Xenon-ion lasers, emitting on several lines in the blue, green, and yellow regions of the spectrum, have been demonstrated to operate in pulsed mode. This laser was once popular for precision laser trimming (using the powerful output on several green lines) but has been superseded by more efficient frequency-doubled YAG lasers. Xenon-ion lasers are uncommon today and are usually found only in research labs. Neon, another noble gas, is more efficient than doubly ionized argon at producing UV output. With ULLs centered around 53 eV above ground state, the energy-level situation in a neon-ion laser is more favorable than in a doubly ionized argon (Ar^2‏+) laser with comparable UV output wavelengths, so a neon-ion laser operates at lower currents than does a doubly ionized argon laser. Commercially, there are few manufacturers offering lasers of this type.

          Many other gases will operate as ion lasers, such as oxygen and nitrogen. Most were discovered by accident when they were present as impurities in laser tubes intended to lase argon or krypton (in many cases they were left over from an incomplete cleaning of the tubes and electrodes). It is interesting to note, however, that the fact that the argon ion lases at all was discovered by accident in 1964 when argon was tried as a buffer gas for a mercury-vapor ion laser (the first ion laser discovered). An excellent account of the development of ion lasers can be found in “Ion lasers: the early years” by W.

         Many materials in addition to gases will operate as an ion laser when vaporized. The aforementioned mercury vapor was the first ion to lase. Buffered with helium at low pressures (and with a high vapor pressure at room temperature), a mercury ion produces pulsed laser output at 615 nm (in the red) and 568 nm (in the green). A number of other metals lase in a similar manner, including selenium, cadmium, silver, gold, nickel, copper, and many others, although many metals require elevated temperatures to increase the vapor pressure and hence the number of atoms available in the plasma. Of the 41 elements that will lase as an ion, only one other than the noble gases (e.g., argon, krypton) became a commercially available laser: cadmium. The cadmium-ion laser, called the helium–cadmium (HeCd) laser, operates much more like a HeNe laser with much lower currents (around 100 mA) and temperatures (around 250^oC) than argon or krypton ion lasers. It is thus generally not classed as an ion laser (since it is so different from other ion lasers) despite the fact that the active lasing species is indeed ionic (although one could argue that the ruby or YAG laser is an ion laser, too). Like the HeNe laser, the primary excitation mechanism is transfer from excited helium to the active lasing species. HeCd lasers produce output in the UV at 325 nm, blue at 441.6 nm (the strongest output), and on four other lines in the green and red. These are commercially available from a number of manufacturers and are an important source of continuous UV light.

       In any ion laser a high-current density (measured in amperes per square centimeter) in the gas discharge is required to excite the ions to the level required. High currents (up to 40 A for an Ar⁺‏ laser) and a small-diameter (between 0.5 and 2 mm) bore are used in plasma tubes to increase current density since the output power increases as the square of current density. In the case of a 1-mm-diameter bore with a 40-A discharge current, the current density is over 1200 A/cm², an extremely large value compared to a typical current density of 0.16 A/cm² for a small HeNe tube. Doubly ionized species (Ar²⁺‏) require even current densities. These high current densities create a host of problems for designers of ion laser tubes since the plasma runs at an extremely hot temperature and can easily erode tube structures such as electrodes and tube materials.

        Finally, revisiting Figure 1.1, it is obvious that the efficiency of the argon laser is poor since almost 36 eV of energy must be injected into an argon atom to produce a photon with an energy of about 2.5 eV. The dynamics of a doubly ionized laser (as well as the krypton laser) are even worse: doubly ionized argon (Ar^2+‏) has a ground state 43 eV above ground and upper lasing levels about 28 eV above that! Overall, about 71 eV of energy is required to produce a single photon with 4 eV of energy.