Chromium-doped Gain Media
Chromium (chemical symbol: Cr) is a chemical element belonging to the group of transition metals. Chromium ions of different charge states are used as laser-active dopants of gain media:
- Cr2+ ions are mostly used in zinc chalcogenides such as Cr2+:ZnS, Cr2+:ZnSe, Cr2+:ZnSxSe1−x, and Cr2+:CdSe. Lasers based on these crystals can emit roughly between 1.9 and 3.5 μm and are typically pumped around 1.5–1.9 μm. Despite this huge emission bandwidth, they can have reasonably low threshold pump powers and can be diode-pumped. It is possible to passively mode-lock such lasers for generating pulses with durations well below 100 fs.
- Cr3+ ions are the active ingredients of ruby (chromium-doped aluminum oxide), the laser medium of the first laser, and alexandrite (Cr3+:BeAl2O4), an early tunable solid-state laser medium. Cr3+ ions are now mostly used in gain media such as Cr3+:LiSrAlF6 (Cr:LiSAF), Cr3+:LiCaAlF6 (Cr:LiCAF) and Cr3+:LiSrGaF6 (Cr:LiSGAF), emitting around 0.8–0.9 μm. (Such crystals are called colquiriites.) Passively mode-locked lasers based on such media can be used for pulse durations down to roughly 10 fs. Compared with titanium–sapphire lasers, such lasers can be much cheaper, because they use a red rather than a green pump source and can be operated with low pump powers, so that diode pumping is feasible. However, the output powers achievable are lower (partly because of thermal quenching effects at higher temperatures), the wavelength tuning range is smaller, and the minimum pulse duration is larger. Some rather new materials are Cr3+:LiInGeO4 (Cr:LIGO), Cr3+:LiScGeO4, and Cr3+:LiInSiO4 (Cr:LISO) [21, 23, 25]. Here, Cr3+ ions emit in a surprisingly long wavelength range between about 1.2 and 1.6 μm (which is more typical for Cr4+) and with a very large bandwidth.
- Cr4+ ions occur in media such as Cr4+:YAG, Cr4+:MgSiO4 (forsterite) and other silicates, and also in germanates, apatites and other crystal types. The emission range is e.g. ≈ 1.35–1.65 μm for Cr4+:YAG and 1.1–1.37 μm for Cr4+:MgSiO4. Pulse durations below 20 fs have been achieved e.g. with Cr4+:MgSiO4. Nd:YAG lasers are often used for pumping such Cr4+ lasers.
Due to the strong electron–phonon interaction in such gain media, chromium-doped lasers are called vibronic lasers and have a large gain bandwidth.
Note that some chromium-doped crystals, in particular Cr4+:YAG, are also used as saturable absorbers in Q-switched lasers.
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