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Transition-metal-doped Laser Gain Media

Definition: laser gain media which are doped with transition metal ions

German: Übergangsmetall-Lasermedien

Categories: optical materials, laser devices and laser physics, optical amplifiers

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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 ZnS_xSe_1-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

Table 1: Common transition metal ions and host media.

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.

Suppliers

The RP Photonics Buyer's Guide contains 17 suppliers for transition-metal-doped laser gain media.

Bibliography

[1]	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
[2]	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)
[3]	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
[4]	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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