Mid-IR solid-state lasers based on Cr^2+ and Fe^2+ ions
##Laser Physics	1. Historical background
1. Introduction	Divalent states of chromium and iron ions have been known in tetrahedral sites in many crystalline hosts including crystals from the spinel group and III-V/II-VI wideband semiconductors. The lowest ^5D term of Fe^2+ (3d^6) and Cr^2+ (3d^4) ions is split by the tetrahedral crystal filed into ^5T2 and ^5E multiplets. A ^5T2 multiplet is the ground state for Cr^2+ ions, while a ^5E multiplet is the ground state for Fe^2+ ions in a tetrahedral site. The first laser oscillation at the transition between ^5T2 and ^5E multiplets in Fe:InP crystal was reported at 3.53 mm in 1983 [1]. In this publication, the authors used optical excitation (hn > Egap) to demonstrate laser oscillation on the intra-shell transition of Fe^2+ ions in InP crystal at temperature T=2K. However, progress in laser development based on these ions has been achieved after the publication reported in 1996 by scientists from Lawrence Livermore National Laboratory [2]. In this publication, the authors formulated the major features that make Transition Metals (TM) doped II-VI wideband gap semiconductors (ZnSe, ZnS, ZnTe, CdS, CdSe, CdTe) so attractive for middle infrared (MIR) laser applications: · Tetrahedral coordination of the TM ions in II-VI semiconductors gives approximately two times smaller crystal field energy splitting in comparison with typical octahedral coordination sites, placing the TM dopant transitions further into the infrared (IR). · The crystal’s heavy anions enable a very low-energy optical phonon cut-off. This makes II-VI crystals transparent in a wide spectral range and decreases the rate of non-radiative relaxation, giving promise of a high yield of fluorescence at room temperature (RT). · Strong electron-phonon coupling of the TM ions results in significant broadening (up to 50% of central wavelength) of their amplification band making TM:II-VI gain media of great interest for ultra-broad MIR tunability as well as for ultra-short pulse generation. · Among a large group of TM ions Cr^2+ (3d^4) and Fe^2+ (3d^6) ions are the most attractive for laser applications. These chemically stable divalent dopant ions have high absorption and emission cross-sections at ^5T2 « ^5E transition and the absence of excited state absorption. · Spectroscopic and laser characteristics of TM:II-VI materials are practically identical in single crystal and in polycrystalline forms of the II-VI hosts The authors also reported first room temperature the lasing of a Cr:ZnSe crystal first reported. According to crystal field theory, energy splitting between ^5T2 and ^5E levels is proportional to d^−5 (where d is the distance between ligands and transition metals). This explains why the shortest and the longest spectral positions of the absorption/emission bands are displayed in ZnS (d = 2.34 A˚ ) and CdTe (d = 2.81 A˚ ) crystals, respectively. The energy splitting of the Fe^2+ (3d^6) ions are smaller than Cr^2+ (3d^4) in the same hosts. The figure below shows RT emission bands of Cr^2+ and Fe^2+ ions in II-VI semiconductors
	RT emission cross-section in arbitrary units for (a) Cr:ZnS, (b) Cr:ZnSe, (c) Cr:CdSe, (d) Fe:ZnS, (e) Fe:ZnSe, (f) Fe:CdSe, and (g) Fe:CdTe
	The figure below summarizes historical progress in the development of high-power CW lasers and shows that output power approximately doubled during a half-year period adopted from the review published in 2018. Unfortunately, there is no upgrade of these plots in recent years, however, it could be changed in the case of demand of the CW with higher output power.
	Progress in high power CW TM:II-VI lasers: Cr^2+:ZnS (green stars), Cr^2+:ZnSe (red stars), Fe^2+:ZnSe (blue pentagons). The dashed lines indicate exponential growth
Links	ü Materials and Spectroscopic properties of Cr2+ and Fe2+ doped II-VI semiconductors at ^5T2«^5E Transition ü Milestones in the Development of Cr^2+-doped II-VI Lasers ü Milestones in the Development of Fe^2+-doped II-VI Lasers
References	Ø P. B. Klein, J. E. Furneaux, and R. L. Henry , "Laser oscillation at 3.53 μm from Fe2+ in n‐InP:Fe", Appl. Phys. Lett. 42, 638-640 (1983) https://doi.org/10.1063/1.94057 Ø L. D. DeLoach, R. H. Page, G. D. Wilke, S. A. Payne, and W. F. Krupke, “Transition metal-doped zinc chalcogenides: spectroscopy and laser demonstration of a new class of gain media,” IEEE J. Quantum Electron. 32, 885–895 (1996). Ø R. H. Page, K. I. Schaffers, L. D. DeLoach, G. D. Wilke, F. D. Patel, J. B. Tassano, S. A. Payne, W. F. Krupke, K. T. Chen, and A. Burger, “Cr2+ -doped zinc chalcogenides as efficient, widely tunable mid-infrared lasers,” IEEE J.Quantum Electron. 33, 609–619 (1997).