This Letter determines the ${\mathrm{Cr}}^{2+}\text{-}{\mathrm{Fe}}^{2+}$ ions energy transfer process parameters in the ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}\text{:}{\mathrm{Cr}}^{2+},{\mathrm{Fe}}^{2+}$ ($x=0.3$) crystal and demonstrates a dipole-dipole type of the ${\mathrm{Cr}}^{2+}\text{-}{\mathrm{Fe}}^{2+}$ ions nonradiative energy transfer. The intracavity ${\mathrm{Cr}}^{2+}\text{-}{\mathrm{Fe}}^{2+}$ pumping as well as pumping through ${\mathrm{Cr}}^{2+}\text{-}{\mathrm{Fe}}^{2+}$ energy transfer in the ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}\text{:}{\mathrm{Cr}}^{2+}$, ${\mathrm{Fe}}^{2+}$ ($x=0.3$) crystal under 1735 nm excitation by an YLF:Er laser was realized in a temperature range of 78 up to 150 K for the first time, to the best of our knowledge. Output pulse energy up to 20 μJ at $\sim 4400\mathrm{nm}$ under 5.5 mJ of 1735 nm absorbed pump energy was obtained for the ${\mathrm{Cr}}^{2+}\text{-}{\mathrm{Fe}}^{2+}$ ions energy transfer mode.

Millisecond-pulse laser operation of ${\mathrm{Fe}}^{2+}$ ions at 78 K is demonstrated in the ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}\text{:}{\mathrm{Fe}}^{2+},{\mathrm{Cr}}^{2+}$ ($x=0.3$) crystal under a Tm fiber 1940 nm laser pumping through a ${\mathrm{Cr}}^{2+}\to {\mathrm{Fe}}^{2+}$ energy transfer process for the first time, to the best of our knowledge. The laser slope efficiency was 1% with respect to absorbed pumping energy at 1940 nm. The laser central wavelength shift from 4450 nm at 78 K up to 4510 nm at 110 K was observed. Tunability from 4350 up to 4670 nm at 78 K was achieved using an intracavity tuning element.

A nonradiative Cr²⁺ → Fe²⁺ ions energy transfer process in Zn_1-xMn_xSe:Cr²⁺,Fe²⁺ (x = 0.05) single crystal with a Cr²⁺ to Fe²⁺ ions concentration ratio about 2.5:1 was investigated under short-pulse (200 ns) ∼1.73 µm excitation by YLF:Er laser. Fe²⁺ ions lasing in a temperature range from 78 K up to 200 K was achieved and the central oscillation wavelength was observed to increase from 4.1 µm to 4.4 µm, depending on the temperature of the active medium. Moreover, stable Fe²⁺ ions lasing under a “long” pulse excitation (1 ms, 10 Hz) by a ∼1.71 µm laser diode were demonstrated. The laser generated in a temperature range from 78 K up to 110 K at a central wavelength around ∼4.17 µm. The maximum mean laser output power of 4.1 mW with a slope efficiency of 2.3% with respect to the absorbed power was achieved. These results present a new possibility to develop a simple and compact diode-pumped coherent mid-infrared (4.1–4.3 µm) laser sources.

Two different mid-infrared (mid-IR) solid-state crystalline laser active media of Cr${}^{2+}$, Fe${}^{2+}$:Zn${}_{1-\mathrm{x}}$Mn${}_{\mathrm{x}}$Se and Cr${}^{2+}$, Fe${}^{2+}$:Zn${}_{1-\mathrm{x}}$Mg${}_{\mathrm{x}}$Se with similar amounts of manganese or magnesium ions of x ≈ 0.3 were investigated at cryogenic temperatures for three different excitation wavelengths: Q-switched Er:YLF laser at the wavelength of 1.73 $\mathsf{\mu }$m, Q-switched Er:YAG laser at 2.94 $\mathsf{\mu }$m, and the gain-switched Fe:ZnSe laser operated at a liquid nitrogen temperature of 78 K at ∼4.05 $\mathsf{\mu }$m. The temperature dependence of spectral and laser characteristics was measured. Depending on the excitation wavelength and the selected output coupler, both laser systems were able to generate radiation by Cr${}^{2+}$ or by Fe${}^{2+}$ ions under direct excitation or indirectly by the Cr${}^{2+}$→ Fe${}^{2+}$ energy transfer mechanism. Laser generation of Fe${}^{2+}$ ions in Cr${}^{2+}$, Fe${}^{2+}$:Zn${}_{1-\mathrm{x}}$Mn${}_{\mathrm{x}}$Se and Cr${}^{2+}$, Fe${}^{2+}$:Zn${}_{1-\mathrm{x}}$Mg${}_{\mathrm{x}}$Se (x ≈ 0.3) crystals at the wavelengths of ∼4.4 and ∼4.8 $\mathsf{\mu }$m at a temperature of 78 K was achieved, respectively. The excitation of Fe${}^{2+}$ ions in both samples by direct 2.94 $\mathsf{\mu }$m as well as ∼4.05 $\mathsf{\mu }$m radiation or indirectly via the Cr${}^{2+}$→ Fe${}^{2+}$ ions’ energy transfer-based mechanism by 1.73 $\mathsf{\mu }$m radiation was demonstrated. Based on the obtained results, the possibility of developing novel coherent laser systems in mid-IR regions (∼2.3–2.5 and ∼4.4–4.9 $\mathsf{\mu }$m) based on A${}^{\mathrm{II}}$B${}^{\mathrm{VI}}$ matrices was presented.

Novel Cr2+ and Fe2+ co-doped Zn1–xMnxSe (x = 0.3) crystal with Cr2+ to Fe2+ ions concentration ratio of about 1 : 2 (both doping ions concentration w