Transition-metal-doped Laser Gain Media
A number of solid-state laser gain media are doped with transition metal ions. Those have optical transitions involving the electrons of the 3d shell. Table 1 gives an overview of the most common transition metal ions and their host media.
|Ion||Common host media||Typical emission wavelengths|
|titanium (Ti3+)||sapphire||0.65–1.1 μm|
|divalent chromium (Cr2+)||zinc chalcogenides such as ZnS, ZnSe, and ZnSxSe1-x||1.9–3.4 μm|
|trivalent chromium (Cr3+)||ruby (Al2O3), alexandrite (BeAl2O4); LiSAF, LiCAF, LiSAF, and similar fluorides||0.7–0.9 μm|
|tetravalent chromium (Cr4+)||YAG, MgSiO4 (forsterite) and other silicates||1.1–1.65 μm|
|divalent iron (Fe2+)||ZnSe, ZnS, CdSe||4–5 μm|
More exotic ions for lasers are cobalt (Co2+) and nickel (Ni2+).
A common property of transition metal ions is that the corresponding absorption and laser transitions have a very broad bandwidth, leading in particular to a very large gain bandwidth. This results from the strong interaction of the electronic transitions with phonons (→ vibronic lasers), which is a kind of homogeneous broadening. Nevertheless, the transition cross-sections can be reasonably high – of the same order as those of rare-earth-doped laser gain media having a much smaller transition bandwidth.
Laser-active transition metal ions are basically always used in crystals rather than glasses as host media, since crystals offer a higher thermal conductivity and the additional inhomogeneous broadening from glasses would hardly be useful.
The most important lasers based on transition-metal-doped gain media are titanium–sapphire lasers and various lasers based on chromium-doped laser gain media such as Cr4+:YAG or Cr3+:LiSAF. Less common are lasers based on media such as Co2+:MgF2, Co2+:ZnF2 and Ni2+:MgF2. They are particularly used for mode-locked lasers, generating ultrashort pulses, and for broadly tunable lasers.
|||R. Scheps, “Cr-doped solid-state lasers pumped by visible laser diodes”, Opt. Mater. 1, 1 (1992), DOI:10.1016/0925-3467(92)90011-B|
|||E. Sorokin et al., “Ultrabroadband infrared solid-state lasers”, J. Sel. Top. Quantum Electron. 11 (3), 690 (2005), DOI:10.1109/JSTQE.2003.850255 (a review mainly concerning Cr2+ and Cr4+ lasers)|
|||S. B. Mirov et al., “Recent progress in transition-metal-doped II–VI mid-IR lasers”, J. Sel. Top. Quantum Electron. 13 (3), 810 (2007), DOI:10.1109/JSTQE.2007.896634|
|||V. V. Fedorov et al., “3.77–5.05-μm tunable solid-state lasers based on Fe2+-doped ZnSe crystals operating at low and room temperatures”, IEEE J. Quantum Electron. 42 (9), 907 (2006), DOI:10.1109/JQE.2006.880119|
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