Natural Ruby Lasers

Natural rubies haven't been used in lasers, it is not a suitable laser material due to many factors:

Iron Impurities

When you shine a bright green or violet or ultraviolet light on a laser grade sample of ruby, it will fluoresce bright red at 693.4 nanometers. According to Nassau (1980), most natural rubies contain too many iron impurities which quench the red fluorescence from the chromium ions. I can just imagine myself sneaking in a portable UV blacklamp to shine on multi-million dollar rubies on public display in various museums vaults ;-)

Defects

According to Schawlow (1961) defects (imperfections) in synthetic ruby limits the effective lasing volume so that less than 10 % of the area at the surface of cylinder produces laser action. Although ruby manufacturing techniques have improved since then, natural rubies have many more defects and would probably have an even smaller effective lasing volume (if at all). Ruby crystal is non-isotropic i.e. the index of refraction and other properties depend on orientation. Any defects or grain boundaries could potentially split the beam into multiple components or have other deleterious effects which could preclude lasing.

Cylinders too Short

Ruby is the rarest form of corundum and the probability of finding defect free specimens longer than a certain length is low. The length of the amplifying medium is critical to laser design; mirrors are added at the ends to increase the 'effective' path through the gain medium. Below a certain threshold length, laser action is unlikely due to a combinations of factors including various loss mechanisms versus density of chromium ions and realistic optical pump power densities. The first ruby laser was 0.05 percent Cr (by weight) and during a single pulse of the flash lamp all the ions were raised to the excited metastable upper laser state. Since the laser medium was 'saturated' or maximally charged, in this case increased pump power would not have compensated for reductions in length. Shawlow (1961) was able to successfully increase the Cr ion dopant concentration by factor of 10 and still obtain laser action. However the pump power photon density must correspondingly be raised to produce a population inversion. Which raises another concern; thermal dissipation capabilities.

Power Dissipation

Natural ruby is often polycrystalline, and according to theory the thermal conductivity can be up to 10 times lower than for single crystals. Although ruby has one of the highest melting points, laser action begins to suffer even at moderately warm temperatures due to the thermal deactivation of the upper laser level by excessive photon collisions (sound quanta which carry heat) This thermal competition significantly reduces population inversions. Therefore the crystal must remain below a certain temperature for efficient laser action. The lower cooling rate due to the low thermal conductivity translates to a much lower pulse repetition rate. This allow the coolant to remove the heat generated by the non-radiative transitions from the pump absorption level (4T1,4T2) to the upper laser level (E).

Conclusion

Even if by luck you could obtain a sample of natural ruby which was relative free from defect and impurities and you managed to get it to lase, the output power would be much lower than theoretically possible by at least an order of magnitude. (due to the low pulse repetition rate, smaller dimensions etc...) You certainly could not get it to operate in continuous mode (CW).

Alternatives

The chromium ion within the sapphire crystal matrix can be used in a three level laser scheme. Contemporary lasers have drastically reduced pump power requirements because they use the more efficient four level laser scheme. With higher efficiency comes higher output power due to better cooling. Also continuous mode (CW) operations becomes practical and inexpensive. The neodymium ion operates as a four level laser based on perturbed 4f electrons, because this electron shell is well shielded the host material can be anything from crystalline YAG (Yttrium Aluminum Garnet) to glass. In chromium the 3d levels are less shielded which is the main reason why ruby lasers are prey to lattice defects.

The best solid state laser in term of efficiency, durability and tuneability is the titanium sapphire laser (Ti:Al2O3) pumped by diode laser(s). It seems that we have come full circle back to corundum (sapphire) as the ideal solid host material !

References

  1. Ruby: Variety of sapphire doped with chromium.
  2. Other names: corundum, aluminum oxide.
  3. Chemical Structure: Al2O3 + Cr
  4. Properties: The most common form is polycrystalline and because it has a of hardness 9 (diamond is 10) it is used in abrasives (emery paper etc...)
  5. Nassau,K.: 1980, Scientific American, Oct, p.124
  6. Shawlow,A.L.: 1961, Scientific American, Jun, p.52
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