Room-temperature laser on a ZnSe : Fe²⁺ polycrystal with undoped faces, excited by an electrodischarge HF laser

Lasers on a ZnSe : Fe²⁺ crystal with optical pumping have been intensively investigated for over 15 years [1–22]. These sources generate a powerful coherent radiation in the spectral range of 4–5 μm, which is important for many practical applications. The highest values of energy per pulse (4.9 J [14]) and of average generation power (35 W [20]) of a ZnSe : Fe²⁺ laser have been reached by cooling active elements to the temperature of liquid nitrogen. A ZnSe : Fe²⁺ single-crystal [14] and polycrystal doped by the diffusion method [20] were used as active elements. The crystals were pumped by an Er:YAG-laser operated in free running mode. In practical applications, laser operation at room temperature or under moderate cooling of an active element is preferable. The lifetime of upper laser level in ZnSe : Fe²⁺ becomes shorter as temperature increases. At room temperature it is ~360 ns [8, 17]. Hence, efficient generation can only be obtained under pumping by sufficiently short optical pulses. The characteristics appropriate for this purpose are specific to high-power non-chain electrodischarge HF lasers, which generate pulses with the duration of ~100–150 ns in the spectral range of 2.6–4.1 μm [23–26]. Pumping of ZnSe : Fe²⁺ crystals by HF lasers yielded the current highest generation energy and average power at room temperature of the active element. Maximal generation energies obtained with polycrystalline and single-crystal samples are respectively 253 mJ [21] and 1.2 J [18, 22]. The maximal average generation power (on single-crystal) was 2.4 W at the energy per pulse of 14 mJ [14].

Obtaining such high laser characteristics under pumping of ZnSe : Fe²⁺ crystals by HF lasers at room temperature is explained by an actually unlimited, in the context of the problem considered, energy of the pumping source itself. Under such pumping, the energy deposited into a crystal and, consequently, the generation energy could be increased by simply increasing the size (irradiation area) of the pumping radiation spot on a crystal surface (at the maximal density of energy limited by the surface breakdown threshold). However, the possibilities of further scaling up the energy of a laser on diffusion-doped ZnSe : Fe²⁺ polycrystals with a high (~10¹⁹ cm⁻³) concentration of Fe in a surface layer at a relatively small depth of doping (short active medium length) by using this simple method have been exhausted because a parasitic generation arises at large dimensions of the pumping spot [12, 21]. To overcome this difficulty it was suggested [21] to fabricate ZnSe : Fe²⁺ samples with several doping layers by the technology developed earlier in [27, 28] for active elements on a ZnSe : Cr²⁺ polycrystal. This technology provides fabrication of active elements with zero concentration of a dopant on the surface and the maximal concentration inside the sample (samples with undoped faces). In [21], this technology was provisionally termed 'internal doping'. In a multilayer sample, layers of ZnSe : Fe²⁺ alternate with ZnSe layers. The maximal concentration of Fe in each doped layer should reduce with the number of layers. Thus, the increase in the length of a laser active element attained due to a greater number of doping layers should be accompanied by a lower concentration of the dopant.

Characteristics of a laser on a single-layer ZnSe : Fe²⁺ sample with undoped faces were investigated for the first time in [21]. Despite relatively low generation energy (90 mJ) obtained in the experiment, the investigations have revealed the promising character of the 'internal doping' technology and suggest expediency of its further development. With this purpose, in the present work we study the generation characteristics of a ZnSe : Fe²⁺ laser on a sample with internal doping fabricated by the original technology that excludes contacts of deposited Fe film with oxygen and atmospheric moisture. The study is aimed at obtaining data needed for elaborating the technology of manufacturing multilayer samples with a required maximal concentration of iron ions in each layer and, finally, at increasing the generation energy and efficiency of the laser at room temperature.

The active element with undoped faces was fabricated as follows. A polycrystalline ZnSe sample of high quality, preliminarily grown by the method of chemical vapor deposition (CVD), was subjected to mechanical polishing on a diamond abrasive instrument toobtain a surface corresponding to the third grade of finish. After washing in an ultrasonic bath the sample was placed into a reactor for synthesising CVD-ZnSe. The CVD-reactor comprised the unit for feeding iron precursor vapour, equipped with an independent heater, and a high-accuracy system for controlling the evaporator temperature. Precursor Fe vapours were passed to the reactor in the flow of a carrier gas, namely argon. A metal iron film on the polished surface of ZnSe was produced according to the reaction of iron dichloride reduction by zinc vapours. The film thickness was determined by the time of deposition, which was calculated from preliminarily performed macro-kinetic investigations of the process of iron chemical deposition. A film of the required thickness having been obtained on the surface, the supply of iron precursor was stopped, and the reaction zone was filled by hydrogen selenide diluted with argon for depositing ZnSe. Zinc selenide was synthesised by the method described in [29]. The process of growing ZnSe layers lasted for 48–72 h (depending on the required deposition thickness). As the result of the procedure described, a three-layer structure ZnSe–Fe–ZnSe with a thickness of up to 6–8 mm has been obtained. Then the sample was extracted from the CVD-reactor, mechanically treated to obtain the required geometrical form, and annealed by barothermic treatment in a gas-static unit at a pressure of 100 MPa and temperature of 1250 °C for 72 h.

The principal difference of this method from the method used earlier in [11–13] is lack of contact of newly deposited iron film with oxygen or air moisture. This protects iron from oxidation or hydrolysis and excludes penetration of oxygen into the ZnSe matrix in the following high-temperature annealing. As was established in [30], the presence of oxygen in a ZnSe matrix may elevate non-selective optical losses in the material in the spectral range of 2–9 μm.

The sample was fabricated in the form of a disk with a diameter of 20 mm and thickness of 6 mm. The distribution of Fe ion concentration over the sample depth was measured after performing laser experiments according to our method described in [31].

The scheme for examining characteristics of a ZnSe : Fe²⁺ laser is given in figure 1.

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The cavity of the ZnSe : Fe²⁺ laser with length 130 mm was formed by a concave mirror M₁ (a mirror with gold coating on a quartz substrate) with radius of curvature R  =  1 m and a plane outcoupling mirror M₂. The outcoupling mirror with an interference coating had the reflection coefficient of 40% at wavelengths of λ  =  4.1–4.8 μm. The beam of a non-chain electrodischarge HF-laser with the FWHM light pulse duration of τ  ≈  140 ns [32, 33] attenuated by a set of light filters F was focused by a spherical lens L to a spot of diameter d  =  6–9 mm on a sample surface (90% of the energy of radiation falling to the surface). The angle of pumping radiation incidence to the sample surface was ~20°. The crystal was mounted in a cavity in such a way that its polished faces (the angle between the faces was ~20'') were normal to the optical axis of the cavity. The energies of HF laser radiation incident to the surface, of radiation passed through the sample, and the generation energy of the ZnSe : Fe²⁺ laser were measured by calorimeters C₁, C₃ ('Molectron') and C₂ ('Gentec-EO') respectively.

Distribution of Fe ion concentration over sample depth is shown in figure 2 for a sample with undoped faces. For comparison, in figure 3 one can see the corresponding distribution for the sample with diffuse doping from two sides investigated earlier in [11–13].

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In figures 2 and 3 one can see that the maximal concentration of Fe ions in the sample with undoped faces is lower than in the sample studied earlier that was doped from two sides. The concentration of Fe ions in the sample with internal doping can be varied by changing the thickness of the film deposited onto the substrate surface and is easily calculated in the case of fabricating ZnSe : Fe²⁺ elements with two or more internal doping layers. As mentioned, in multilayer samples the maximal ion concentration in doping layers should fall with increasing number of layers.

The transmission of the sample with undoped faces versus the density of HF laser energy on a surface shown in figure 4 has been measured in the conditions of ZnSe : Fe²⁺ laser generation. The generation spectrum of an HF laser comprises many lines pertaining to a single pulse, and the transmission of material is a strongly nonlinear parameter with respect to the density of laser energy. Hence, the dependence in figure 4 is a more informative characteristic than ordinary transmission spectra of materials taken by using spectrometers at a low energy density of test radiation.

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As one can see in figure 4, the transmission exhibits a typical saturation feature: at a high density of pumping radiation it reaches the value of ~30%. For comparison, the transmission of the sample with diffuse doping from two sides investigated in [11–13] at HF laser wavelengths in the presence of generation varied from 4% to 7.1% when the energy density of radiation falling to the surface changed from 0.2 J cm⁻² to 2.8 J cm⁻² [12].

Dependence of the density of generation energy of a ZnSe : Fe²⁺ laser versus the parameter E_abs/S (the density of generation energy is defined as E/S where E is the generation energy, S is the area of pumping radiation spot on a sample surface, and E_abs is the energy absorbed in the sample) obtained at various diameters of pumping radiation spot is shown in figure 5.

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One can see that points corresponding to various diameters of pumping spot fit a single curve well. The linear part of the curve corresponds to the laser slope efficiency of η_slope  =  45%. Note that the same value of the slope efficiency at room temperature has been obtained in [18] for a single-crystal sample used as an active element.

Laser generation energy versus energy absorbed in a sample is shown in figure 6 for the case of the diameter of pumping radiation spot d  =  8.1 mm. In the conditions of our experiments, at this value of the diameter of pumping radiation spot the obtained generation energy was maximal at 298 mJ, with efficiency with respect to the absorbed energy of η_abs  =  40%.

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Thus, the technology for fabricating polycrystalline samples with undoped faces described above resulted in a noticeable increase in the efficiency and generation energy of the laser as compared to the corresponding values obtained earlier [21] both for the sample doped from two sides (253 mJ, η_slope  =  33%) and for the sample with undoped faces (90 mJ, η_slope  =  32%). The maximal diameter of the pumping spot (d  =  8.1 mm), at which the dependence of generation energy on the absorbed energy is linear over the whole range of absorbed energy variation, has also increased (in [21] the value of this parameter was 7.5 mm). However, already at d  =  9 mm the linear character was broken and the generation energy could not be increased above 200 mJ despite a large reserve of HF laser energy.

Investigations performed in the present work have shown that the technology used for fabricating an active element with undoped faces, which excludes an iron film newly deposited on a substrate from a contact with air moisture and oxygen, yields a higher efficiency and energy of ZnSe : Fe²⁺ laser. It seems reasonable to carry out investigations aimed at further improving this technology in order to develop multilayer ZnSe : Fe²⁺ samples and obtain better output characteristics of the laser at room temperature.

The work was supported by the RSF grant № 15-13-10028 in the part concerning development and manufacturing ZnSe : Fe²⁺ samples and by the grants RFBR № 15-02-06005 and № 15-08-02562 in the part of developing experimental methods and carrying out investigations.