The Fe:Zn(1-x)Mg(x)Se (x = 0.19, 0.27, and 0.38) solid solutions spectroscopic properties were investigated and laser oscillations were achieved for the first time. The increase of the magnesium concentration in the Fe:ZnMgSe crystal was shown to result in an almost similar long wavelength shift of both absorption and fluorescence spectra of about 60 nm per each 10% of magnesium. With the Fe:ZnMgSe crystal temperature decrease, the fluorescence spectrum maximum shifts towards shorter wavelength resulting mainly from strong narrowing of the longest wavelength fluorescence line. Laser radiation wavelength dependence on the magnesium concentration as well as on temperature was observed. The Fe:ZnMgSe x = 0.38 laser oscillation wavelength increased from 4780 nm at 80 K to 4920 nm at 240 K using the optical resonator without any intracavity spectrally-selective element. In comparison with the Fe:ZnSe laser operating in similar conditions, these wavelengths at both temperatures were shifted by about 500 nm towards mid-IR region.
© 2016 Optical Society of America
New laser applications as pollutant measurement, free-space communications, range finding, defense or medicine require the radiation in the mid-infrared (mid-IR) 4 - 6 µm region . Up to now, lasers based on gas (CO), semiconductor (lead-salt) , quantum cascade lasers , or nonlinear effects as OPO  have been used in this region. Nevertheless, for some applications these lasers are not suitable due to their low output energy, complexity, and costs. Thus compact and reliable laser sources are still desired and therefore new active materials are being investigated . For the 4 – 5 μm region, the divalent iron ion (Fe2+) in the ZnSe matrix was found to be good active material to obtain a tunable laser generation in the mid-IR due to a broad and intensive fluorescence band covering this spectral range. This material characteristics and fundamental laser properties have already been published [6–10].
The Fe:ZnSe absorption band is ranging from ~2.5 to 5 μm with the maximum around 3 μm . At room temperature, the fluorescence spectrum is broad covering the region 3.5 – 5.5 µm with the maximum located around 4.5 µm . However, cooling down this active material to 80 K resulted in significant fluorescence spectrum narrowing and shift towards shorter wavelengths with the maximum around 4.1 μm [9,12].
As far as the fluorescence lifetime is concerned, the 5T2 → 5E mid-IR transition in the Fe2+:ZnSe crystal suffers from strong nonradiative quenching  and therefore the lifetime is shortened down to 300 ns [11,13] at room temperature imposing additional specific demands on the pumping sources. The Fe2+ fluorescence lifetime increases significantly up to ~100 µs with the temperature decrease down to ~100 K [9,11] allowing to pump this active material by longer pulses generated by free-running lasers as well as to achieve higher optical efficiency. The first Fe:ZnSe laser was pumped by an Er:YAG laser at 2.698 µm and the output wavelength tuning with temperature from 3.98 µm at 15 K to 4.54 µm at 180 K was observed . The highest slope efficiency was 8% with the output energy of 4.5 µJ. The output energy scaling can be performed by increasing the pump energy, pulse duration, and the size of the active volume in the Fe:ZnSe crystal. The output energy of 0.4 J at 4.16 µm was demonstrated at the pump energy of 1.3 J corresponding to the slope efficiency of 44%. In this case the Fe:ZnSe active crystal was cooled to 77 K and pumped by an Er:YAG laser at 2.94 µm with a pulse duration of 250 µs . Furthermore, the output energy of 4.9 J near 4.1 µm was achieved in the case of the liquid-nitrogen-cooled Fe:ZnSe laser active element pumped by an Er:YAG laser at 2.94 µm with a pulse duration of 1 ms and absorbed energy of ~10 J .
As for the Fe:ZnSe laser output wavelength tunability most broadband tunability from 3.77 to 4.40 µm at liquid nitrogen temperature under Er:YAG pumping using an intracavity CaF2 prism was demonstrated in . The laser oscillation wavelength tuning by temperature is also possible within the range from 4.16 to 4.65 μm for the 77–250 K temperature range but with a significant drop in the generated energy and efficiency for temperatures above 200 K .
In order to achieve laser generation at room temperature, a gain-switched operation is required. Such mode can be obtained using ~3 µm pumping sources based on either Q-switched solid-state lasers or electric-discharge HF lasers generating short pulses with the duration comparable to the Fe2+ fluorescence lifetime. The first operation in this regime at room temperature was demonstrated in  where the Fe:ZnSe laser was pumped by a 2.92 µm, 2nd D2 Raman Stokes of a Nd:YAG laser. Using the optical resonator with the broadband mirrors, the central laser oscillation wavelength was in the range 4.4 – 4.5 µm [12,17]. The highest slope efficiency of 45% with the output energy of 390 mJ with respect to the absorbed energy of 940 mJ was obtained for the large active volume pumping of the Fe:ZnSe crystal by a HF pulsed laser at room temperature . In a different setup described in this paper, the output energy reached 1.2 J at the incident energy of 4.8 J. On the other hand, under Q-switched solid-state lasers pumping the room temperature Fe:ZnSe laser highest slope efficiency of 13% and the output pulse energy of 0.37 mJ was demonstrated in . The laser cavity with CaF2 prism allowed to obtain tunable oscillation in the 3.9 - 4.7 μm spectral range  and moreover in the range 3.9 – 5.05 µm . An intracavity prism enable to tune the output wavelength in the range ~4 – 5 µm [8,10].
An increase of the Fe2+ lifetime in ZnSe up to ~100 µs with the temperature decrease to 100 K [9,11] allows to pump this active material by longer pulses generated by free-running lasers as well as achieve higher optical efficiency. However, cooling down this active material to 80 K resulted in significant shift of the oscillation wavelength towards shorter wavelength (~4.1 µm) [9,12]. Therefore, other materials have to be investigated to develop laser systems operating at room or cryogenic temperatures within longer wavelength region of 4.5 – 5 µm.
Solid solutions approach is well known in the rare-earth (RE) doped solid state lasers development. Modification of the RE local environment in the solid solution compared to pure compound shifts electronic levels positions and thus changes the fluorescence spectrum shape and oscillation wavelengths. A similar approach can be used in the case of ZnSe matrix. Inserting magnesium into the ZnSe matrix up to 55 at. %  allows to change the spectroscopic and laser properties of Fe2+ ions in comparison with the pure ZnSe crystal [13,20–22] and allows to shift the generated wavelengths further to mid-IR. Laser oscillations at the central wavelength of 4.5 µm with the slope efficiency of 4.5% were demonstrated in  using room temperature Fe:Zn1-xMgxSe (x = ~0.19) active crystal placed in a cavity without additional spectrally selective element.
The Fe2+ energy levels positions are significantly affected by the surrounding crystal field. In case of ZnSe crystal with cubic structure, each Zn2+ ion (which is substituted by Fe2+ ions in doped crystal) is tetrahedrally coordinated by four Se ions located at the vertices of a tetrahedron. The fundamental energetic scheme of a Fe2+ ion in a tetrahedral crystal (as ZnSe) shows that the 5D ground state is split into the doublet 5E presenting the ground state and the triplet 5T2 being the first excited state – see Fig. 1 [10, 23,24]. According to the crystal-field theory for the tetrahedral compounds, the 5T2 upper state level is predicted to split by the first order spin-orbit interactions into three multiplets that are further split by the second order interactions and by the strong Jahn-Teller coupling.
The addition of Mg ions in concentrations higher than ~7% leads to formation of hexagonal wurtzite structure where each Zn ion is still coordinated by four Se ions located now at the vertices of a triangle pyramid. This leads generally to changes of lattice parameters  followed by “red shift” of absorption and fluorescence spectra maxima as was shown previously for Cr2+ ions [26,27]. Some basic Fe:ZnMgSe active material properties for x = 0.11 and 0.31 have been published in  and further for different concentrations by our group in the previous works [20,22,28].
Besides the solid-solutions approach there is also a possibility to fabricate Fe2+-doped ternary and quaternary semiconductors from powder using thermal annealing of mixtures. Compounds as CdMnTe, CdMnS, CdMnSe, ZnMnS, and ZnCdTe were prepared and random lasing based on laser active scattering media without any cavity was observed for the Fe:Zn0.5Cd0.5Te sample at 5.9 μm .
The aim of this work is to investigate spectroscopic and laser properties of Fe2+ doped Zn1-xMgxSe (for x = 0.19 to 0.38) single crystal solid solutions. The development of such solid solution can help to obtain new oscillation wavelengths and expand tuning range in mid-infrared spectral region. This article presents the influence of the magnesium content (x) in Zn1-xMgxSe solid solution on spectroscopic and laser properties of Fe2+ ions in wide temperature range.
2. Active media characterization
A set of Zn1-xMgxSe crystals with different Zn–Mg ratio described by Mg content x (0, 0.19, 0.27, 0.38) doped with divalent iron Fe2+ were synthesized using Bridgman technique. The crystals were cut from the bowl and two surfaces were laser-quality polished. These crystal faces were without any antireflection coatings.
For all synthesized crystals an effort has been made to keep the same Fe2+ doping in the melt at the level of ~ 5x1017 cm−3. Though measured absorption at 2.94 μm (excitation wavelength of Er:YAG laser) gives the following values of Fe:Zn1-xMgxSe crystal absorption coefficient k: x = 0 (Fe:ZnSe), k = 3.8 cm−1; x = 0.19, k = 2.3 cm−1; x = 0.27, k = 2.1 cm−1; and x = 0.38, k = 2.5 cm−1.
2.1 Absorption spectra
The absorption spectra of Fe2+ ions in the Zn1-xMgxSe crystals were measured using the FTIR spectrophotometer Infralum FT-08 in the spectral region from 2000 to 6000 nm. For low (down to 14 K) temperature measurements a sample was put into a closed-cycled helium cryostat (CFA-101) with the helium compressor (CH-202). For measurements within 77 – 300 K range the liquid nitrogen cooled cryostat (Janis VPF-100) equipped with uncoated CaF2 windows was used.
The low temperature (14 K) absorption spectrum was measured for two Fe:Zn1-xMgxSe crystals with the lowest (x = 0) and the highest (x = 0.38) magnesium concentration. In the case of x = 0 (ZnSe), three absorption maxima resulting from the spin-orbital (HSO) interaction and one maximum resulting from the additional Jahn-Teller interaction (HJT) according to the simplified energy level diagram of Fe2+ ions in the tetragonal (D2d) crystal field in the presence of the Jahn–Teller interaction HJT [10,23] were observed. These low temperature absorption maxima correspond to the wavelengths of about 2900, 3110, 3400, and 3610 nm that have been also observed in [10,11]. It should be noted that the low temperature (4 K) absorption spectrum of Fe2+ ions in the ZnSe crystal from  has also demonstrated the existence of quite similar three local absorption bands (much stronger structured) around 2900, 3200, and 3400 nm. Though the most long wavelength maximum around 3650 nm was not shown (or was missed) in the 4 K absorption spectrum.
The normalized absorption spectrum for the Fe:Zn1-xMgxSe (x = 0.38) sample at different temperatures (14, 77, and 300 K) is shown in Fig. 2. The Gaussian-shaped curves positions used for the decomposition of the absorption spectra at different temperatures were tried to be kept close to the positions of maxima in the low temperature (14 K) spectrum. In Fig. 2 an example of such decomposition at room temperature is shown.
Normalized absorption spectra of Fe:Zn1-xMgxSe crystals at room temperature in ~2500-6000 nm spectral region for different Mg concentrations x are presented in Fig. 3. In the Fe:ZnMgSe crystals, the Fe2+ absorption spectrum falling edge is seen to be shifted towards longer wavelengths with the magnesium content increase covering the wavelength range up to 5600 nm while the leading edge position remains almost unchanged being advantageous for pumping of such material by available sources as Er:YAG at 2.94 µm. For further analysis the absorption spectra were decomposed into four Gaussian lines according to the technique described above (as an example this decomposition is shown in Fig. 2 for x = 0.38). The positions of four Gaussian curves maxima used for the Fe:Zn1-xMgxSe 300 K absorption spectra decomposition for all measured magnesium concentrations x are shown in Fig. 4. As follows from this figure the positions of all Gaussian curves wavelength maxima are increasing almost linearly with the magnesium concentration (value of x). Though the slope of the curves for three absorption maxima corresponding to shorter wavelengths (HSO – blue points in Fig. 4) are seen to be much smaller than that for the Gaussian line with the longest wavelength maximum (HJT - red points in Fig. 4).
2.2 Fluorescence spectra
The Fe:Zn1-xMgxSe fluorescence spectra, lifetime, and oscillation spectra were measured under excitation by 2.94 µm Er:YAG laser radiation. A laboratory-designed Er:YAG laser  was operated in an electro-optically Q-switched mode. The maximal output pulse energy was 15 mJ with the pulse duration of 110 ns (FWHM) at the repetition rate of 1 Hz. The beam was directed to the investigated Fe:ZnMgSe sample without any focusing optics and the irradiated spot area was 0.024 cm2. The fluorescence spectra were measured using a grating monochromator Oriel 77250 (grid 77301) using an IR photovoltaic detector Vigo PVI-6.
An example of the measured room temperature fluorescence spectra for the Fe:Zn1-xMgxSe x = 0 (ZnSe) and x = 0.38 are shown in Fig. 5. The measured spectra are seen to be influenced by the ambient air, mainly intensive CO2 absorption line at ~4250 nm. The fluorescence spectrum measured in  at 34 K has shown three local short wavelength fluorescence peaks around 3900 nm separated nearly equidistantly by about 90 cm−1 which we measured to become practically undistinguishable at 77 K. So for room temperature each fluorescence spectrum was decomposed using only two Gaussian curves as shown in Fig. 6 for the sample with x = 0.38 as an example. This figure presents the positions of both decomposed Gaussian curves maxima for the whole investigated range of magnesium concentration x. It can be seen that similarly to the absorption spectrum the shift of both fluorescence maxima positions is nearly linear with the increasing value of x. The short-wavelength part of the fluorescence spectra is weakly affected by the magnesium concentration increase (as well as in the case of absorption) while the long-wavelength part of these spectra is significantly shifted towards longer-wavelengths. This shift can be approximated by the linear dependence (4670 + 575 x) nm or approximately 60 nm per each 10% of magnesium concentration in the solid solution.
2.3 Absorption and fluorescence spectra temperature dependence
The Fe:Zn1-xMgxSe (x = 0.38) crystal absorption spectrum temperature dependence measured in the liquid nitrogen cooled temperature controlled cryostat is shown in Fig. 7. As follows from this figure the absorption spectrum shape changes with temperature decrease quite similar to that for increasing of Mg concentration value x (see Fig. 3). The main changes were observed at the long wavelength part of the spectrum while the short wavelength edge remained practically unchanged. The absorption spectrum at 80 K decomposed into four Gaussian lines is also shown here. The position of three short wavelength lines are seen to be practically unchanged with respect to room temperature spectrum. Only slight narrowing and redistribution of lines intensities is observed. At the same time, the most long wavelength line is strongly affected with maximum position shifted towards shorter wavelengths and drastic linewidth decrease.
The temperature dependence of the Fe:Zn1-xMgxSe (x = 0.38) crystal fluorescence spectra is shown in Fig. 8. Even at 80 K, the fluorescence spectrum remains broad enough at the long-wavelength part around 5 µm. This is a serious benefit compared to the Fe:ZnSe crystal, where the long-wavelength part above ~4100 nm was observed to decrease significantly with lowering temperature strongly shortening possible tuning range .
2.4 Fe:Zn1-xMgxSe fluorescence lifetime temperature dependence
The Fe:Zn1-xMgxSe fluorescence lifetime was measured in the same arrangement within the temperature range from 140 K up to room temperature and the results are shown in Fig. 9. Similar to the Fe:ZnSe crystal the lifetime increase with the temperature decrease was observed (but the slope is seen to be lower). Fe2+ ions lifetime in Zn1-xMgxSe solid solutions was found to become shorter for higher Mg ion concentration. As it was observed, even small addition of magnesium into the ZnSe crystal results in sufficient shortening of the Fe2+ lifetime, though further decrease with the magnesium concentration increase is much slower. At 270 K, the lifetime decreases from ~1 µs for x = 0 (ZnSe) to 130 ns for x = 0.19 and further down to 100 ns for x = 0.38. In the case of Zn1-xMgxSe crystal (x = 0.19) the longest lifetime of 10 µs at 150 K was obtained compared to ~80 µs for Fe:ZnSe (x = 0). Such strong lifetime shortening should be the price for the red shift of fluorescence spectrum resulting in much stronger nonradiative quenching.
2. Laser oscillations
Changes in the oscillation wavelength within temperature range from 80 up to 240 K were investigated for the Fe:Zn1-xMgxSe crystals with different magnesium concentrations value x and the results for x = 0, 0.19, and 0.38 are shown in Fig. 10. A 70 mm long stable laser cavity formed by a flat dichroic pumping mirror (R ~100% @ 4 – 5 μm and T = 97% @ 2.94 μm) and a concave output coupler (R ~95% @ 4.3 – 5 μm, r = 500 mm) was used. Both mirrors were placed outside the cryostat. The only difference were Fe:Zn1-xMgxSe crystals which were changed one by one in the copper holder mounted on the cold finger of the liquid nitrogen cooled cryostat. The Fe:Zn1-xMgxSe lasers were pumped by the Q-switched Er:YAG laser described in section 2.2 (output pulse energy 15 mJ, pulse duration 110 ns, repetition rate 1 Hz). Almost linear increase in the central oscillation wavelength with the increasing temperature was observed for samples with the value of x = 0.19 and 0.38 (Fig. 10). In the case of x = 0.19, the oscillation wavelength increased from 4550 nm at 90 K to 4750 nm at 230 K. For x = 0.38, the oscillation wavelength increased from 4780 nm at 80 K to 4920 nm at 240 K. The example of the oscillation spectrum measured at 80 K in the Fe:Zn1-xMgxZe crystal (x = 0.38) shown in Fig. 8 is seen to match well the fluorescence maximum of the long wavelength Gaussian line. In both cases, the output energy was ~200 µJ at 80 K and it was linearly decreasing with the temperature increase up to 240 K. The Fe:ZnSe oscillation wavelength is shown in Fig. 10 for comparison – the oscillation wavelength increased from 4150 nm at 90 K to 4380 nm at 245 K which differs from results reported in  where oscillation wavelength was changing from ~4700 nm at 85 K till 4170 nm at 255 K and are much closer to the shift demonstrated in . This difference in measured Fe:ZnSe oscillation wavelength dependence can be caused by strong effect of the CO2 gas absorption line at ~4250 nm when cavity mirrors are outside the vacuum chamber. This absorption causes a dual line oscillation spectrum for temperatures above 180 K (Fig. 10) placed on both sides of the CO2 absorption line. It should be noted that when the cavity mirrors were placed inside the liquid nitrogen cooled cryostat the similar to Fe:Zn1-xMgxSe crystal close to linear increase of oscillation wavelength from 4150 nm to 4570 nm was obtained.
The main advantage of the Fe:Zn1-xMgxSe solid solutions is well seen from Fig. 10. Using Fe:Zn1-xMgxSe (x = 0.38) single crystal at 80 K the oscillations at 4800 nm can be obtained in the spectrally non-selective cavity. This wavelength was shown to be reached in the Fe:ZnSe crystal only in the spectrally selective cavity at room temperature .
The Fe2+ increased lifetime (up to about 10 μs at 80 K) makes it possible to obtain Fe:Zn1-xMgxSe crystals lasing under Er:YAG laser long pulse (Er:YAG laser was operating in the free-running mode with the overall pulse duration of ~200 μs, output pulse energy ~40 mJ) excitation for temperatures up to 140 K. Figure 11 shows the oscillation output energy dependence on temperature.
Moreover, one additional experiment was performed using a different set of resonator mirrors: a flat dichroic pumping mirror (R ~100% @ 5 – 5.5 μm and T = 76% @ 2.94 μm) and a concave output coupler (R ~96% @ 5.1 – 5.5 μm, r = 500 mm) allowing to obtain laser oscillations for the Fe:Zn1-xMgxSe (x = 0.19) crystal in the air at room temperature centered at the wavelength of 5100 nm with the linewidth of 175 nm at FWHM (see Fig. 12) without any additional selective cavity element. The maximum output energy in this case was 30 μJ with the slope efficiency about 1%.
3. Conclusion and further developments
This work investigated the spectroscopic and laser properties of the Fe:Zn1-xMgxSe (x = 0.19, 0.27, and 0.38) solid solutions. It was shown that magnesium concentration increase in the Fe: Zn1-xMgxSe crystal causes almost similar modification of both absorption and fluorescence spectra. In both these cases long wavelength part of the spectrum is significantly shifted (approximately 60 nm per each 10% of magnesium) towards longer wavelengths while the short wavelength edge position remains almost unchanged with x making these crystals well suitable for available Er:YAG lasers pumping. With higher magnesium concentration in the Zn1-xMgxSe crystal Fe2+ ions lifetime shortening was observed.
The fluorescence spectrum maximum shift to shorter wavelengths with the temperature decrease was shown to result mainly from strong narrowing of the longest wavelength fluorescence line. Nevertheless, the fluorescence spectrum for Zn1-xMgxSe solid solutions (x > 0) remains broad enough to cover the long-wavelength region around 5 µm at cryogenic (down to 77 K) temperatures. Fluorescence lifetime increase from hundreds of ns at 250 K up to tens of μs at 150 K with the temperature lowering was observed.
The lasing of the Fe:Zn1-xMgxSe single crystal solid solutions (x = 0.19 and 0.38) was demonstrated for the first time. No spectrally selective element was placed inside the laser cavity. Oscillation wavelengths dependence on the magnesium concentration as well as on the temperature was observed. The Fe:Zn1-xMgxSe x = 0.38 laser oscillation wavelength increased from 4780 nm at 80 K to 4920 nm at 240 K without any additional selective cavity element. For comparison, the Fe:ZnSe laser in the similar setup generated at much shorter oscillation wavelengths: it increased from 4150 nm at 90 K to 4380 nm at 290 K. This oscillation spectrum was additionally strongly affected by the CO2 gas absorption line around 4250 nm.
Moreover, using a different set of resonator mirrors allowed to obtain laser oscillations for the Fe:Zn1-xMgxSe (x = 0.19) crystal in the air at room temperature centered at the wavelength of 5100 nm with the linewidth of 175 nm. This wavelength is shifted more to the mid-IR region than is accessible using pure Fe:ZnSe active material even without any additional selective cavity element.
The results have proved that Fe:Zn1-xMgxSe solid solutions allow to modify effectively the spectroscopic properties of Fe2+ ions allowing to obtain lasing further in mid IR over the possibilities of Fe:ZnSe crystals.
Czech Science Foundation (grant 1505360S);
Russian Science Foundation (grant 14-22-00248).
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