We report simple methods of laser active Cr2+:ZnSe powder fabrication with average grain sizes of either ~10 or ~1 µm without crystal growth stage. Pure, uniformly mixed ZnSe and CrSe powders annealed at 1000 °C for 3 days in a sealed evacuated (~10-4 Torr) quartz ampoule exhibited middle-infrared laser action at room temperature under 1.56 µm excitation of D2 Raman shifted radiation of Nd:YAG laser. The output-input characteristic clearly demonstrated the threshold-like behavior of the output signal with the threshold energy level of 0.5 and 3 mJ in 2.9 mm spot for 10 and 1 µm grain sizes, respectively.
©2008 Optical Society of America
The Random laser is based on laser active scattering medium without any cavity as well as other optical elements. The localization arises due to interference of multiply scattered photons. The first detailed experimental study of random powder laser emission demonstrated by Markushev et al. was based on a Nd3+ doped (Na5La1-xNdx(MoO4)4 powder . Since then a variety of random powder lasers have been developed, however most of them emitting in the visible to near - infrared wavelength range [2–5]. Middle-infrared (Mid-IR) lasers are of great interest for numerous scientific and technological applications such as molecular spectroscopy, remote sensing, optical communication, infrared countermeasures, skin resurfacing and surgical scalpels [6, 7]. For example, the detection of atmospheric trace gases and pollutants is based on strong fundamental absorption bands –“molecular fingerprints”-located in the mid-IR spectral region (e.g. CH4:~3.3 µm; H2S:~2.7 µm; and NH3:2.3 µm).
Recent research advances in transition-metal (TM) doped chalcogenides have spurred considerable effort in the development of practical mid-IR sources. TM doped semiconductors have recently emerged as a new class of mid-IR gain materials for applications in solid-state lasers and optoelectronic devices. Effective room-temperature laser action has been reported for Cr2+:ZnS [8, 9], Cr2+:ZnSe [10–12], Cr2+:Cd1-xMnxTe , Cr2+:CdSe , and Fe2+:ZnSe [15,16] crystals.
Due to the low cost of fabrication, mirror-less cavity, and small size, the random lasers are attractive for many important applications, such as low coherent laser sources, wavelength domain markers, biological sensors and etc. (see Ref  for details). Recently, Cr:ZnSe and Cr:ZnS random lasers were demonstrated in [17–19]. In these experiments Cr:ZnSe and Cr:ZnS powders were made by a mechanical grinding of Cr doped single crystals grown by physical or chemical vapor transport. The powder grains were irregularly shaped and had an average size ranging from several hundreds of nm to tens of µm. In the gain-switched regime Cr:ZnSe and Cr:ZnS random lasing with 0.2-0.3 mJ pump threshold was demonstrated.
Here we report a simple method of TM doped II-VI laser active powders fabrication with average grain sizes of either ~10 µm or ~1 µm, where the crystal growth stage in powder preparation is completely eliminated. The basic spectroscopic properties of the prepared Cr2+ doped ZnSe powders and output characteristics of random lasers on their basis are presented.
2. Sample Preparation
Among a large number of Cr2+ -doped media the Cr2+:ZnSe and Cr2+:ZnS powders feature the most favorable combination of physical and spectroscopic properties. There are many methods for doping of ZnSe and ZnS with Cr2+ ions. Thermally activated diffusion of chromium may prove to be an effective way of doping the powders if the process can be sufficiently controlled . The fabrication of Cr2+:ZnSe powder involved two simple stages. At the first stage pure ZnSe and CrSe (with a concentration of Cr2+ ion 2.42×10-19 cm-3) chemicals with an average grain size of 40 µm were uniformly mixed by mechanical shaker. At the second stage the obtained material was sealed into evacuated (10-4 Torr) quartz ampoule and annealed at 1000 °C for 3 days. Because of a very small CrSe concentration (~0.1w%) in the starting material and a lack of visible remains of not reacted CrSe powder after annealing, no additional separation of ZnSe and CrSe powders after annealing was performed. As shown in Fig. 1(A), this treatment resulted in formation of chromium doped ZnSe microcrystalline powder with ~10 µm average grain size. A subsequent mechanical grinding of this powder reduced the average grain size to ~1 µm (Fig. 1(B)). In the powder samples, 75% of the analyzed particles were within 1–3 µm range for a small size powder, and 83% of the analyzed particles were within 5–15 µm range for a big size powder.
3. Experimental Details
3.1 X-ray diffraction
The X-ray diffraction (XRD) study of fabricated 10 µm grain size Cr2+:ZnSe powder was performed using (θ-2θ) angle X-ray diffraction (Philips X-Pert MPD, The Netherlands) with a Cu K-alpha anode. A spectrum was taken from 20° to 90° (2θ) at the step size of 0.02 degrees. Initial powder Cr doped ZnSe sample had zinc-blend structure, as seen in Fig. 2. The most prominent ZnSe diffraction peaks located at 2θ=27.5, 45.5, 53.3, 65.9, 72.6, and 83.5° coincide with (111), (220), (311), (400), (331), and (422) planes reflection of cubic ZnSe.
3.2 Optical characterization
The experimental setup for laser spectroscopic characterization of the fabricated Cr:ZnSe powders is depicted in Fig. 3. The mid-IR photoluminescence (PL) spectra, PL kinetics and lasing spectra were measured under 1560 nm D2-Raman shifted Nd:YAG laser excitation with the pulse duration of 5 ns. The PL or stimulated emission signal was collected with a 76 mm lens (CaF2) and detected by Acton Research Spectra-Pro 300i Spectrograph - liquid nitrogen cooled InSb detector (EGG Judson J10D-M204-R04M-60) combination with a time resolution of 0.5 µs. The PL lifetime was measured by a Tektronix TDS 5104 Digital Phosphor Oscilloscope and the PL intensity was processed through a boxcar averager. All the measurements were performed at room temperature.
4. Results and discussions
PL and laser experiments were performed with Cr2+:ZnSe powder placed in a sealed from one side glass tube with the inner diameter of 5 mm. The pump beam diameter was 2.9 mm. Measured PL spectrum of the Cr:ZnSe powder was in a good agreement with a well documented 5E→5T2 chromium emission spectrum in the bulk ZnSe host. Three basic experiments on pump energy dependences of PL kinetics, output emission, and spectral linewidth (in glass tube and on a glass slide) of 10 and 1 µm powder samples prepared from the same Cr:ZnSe powder were performed for demonstration of the Cr2+:ZnSe powder lasing. The decay profiles of Cr2+:ZnSe 10 and 1 µm powders measured at 2350 nm at room temperature (RT) are shown in Fig. 4(A, B), respectively. In our experiments we kept pump energy density below optical damage threshold. Optical damage was monitored by the appearance of a broadband (including visible spectral range) fluorescence of the powder.
The RT PL lifetime of 1µm size powders was 3.5 µs, which is slightly shorter than the ~5 µs lifetime typical for Cr2+ in the bulk crystals. However, the 10µm size powders’ lifetime was 5.3 µs at RT, which is close to that of the bulk samples. One of the reasons of the smaller size powder decay time shortening can be a more pronounced crystal defects and parasitic surface states resulting in a non-radiative quenching of the chromium excitation. The 10µm and 1µm size emission kinetics of the Cr2+:ZnSe powders were measured at (i) 0.3 mJ, (ii) 0.5 mJ, (iii) 0.7 mJ, and (iv) 1.2 mJ; (i) 2 mJ, (ii) 3 mJ, and (iii) 5 mJ 1560 nm pump energies, respectively. As one can see from the inserts (C, D) in Fig. 4, the emission kinetics demonstrate different behavior for pump energy below and above laser threshold. For low pump energy, one can see that kinetics demonstrate single exponential decay. For pump energy above laser threshold one can see multi exponential decay with a short laser spike. It results in a shortening of the signal decay time . Because the temporal response of our detector was about 0.5 µs we could not study the temporal dynamics of the lasing pulses.
Figure 5 shows the dependence of the averaged intensity of Cr2+:ZnSe powder emission at 2350 nm versus the pump energy. The output-input characteristics clearly demonstrate the threshold-like behavior of the output signal with the threshold energy level of 0.5 mJ (A) and 3 mJ (B) for 10 and 1 µm size powders, respectively. The threshold pump energy density of 10 µm size powder was as low as 7.4 mJ/cm2, which is a factor of more than 6 times smaller than that for the 1µm size Cr2+:ZnSe powder. In turn, the obtained threshold pump energy density for our small size powder was approximately the same with that reported in  for powder prepared by grinding of bulk crystal. However, the laser threshold for our 10 µm size powder was ~7 times. The higher laser threshold for small size powder could be explained by non-radiative excitation quenching, which is confirmed by a shorter decay time of the small size powder. Also, in the case of a small size powder, the bigger surface area could contribute to additional non elastic scattering and pump radiation absorption.
Figure 6 demonstrates PL spectra of Cr2+:ZnSe 10 and 1µm powders in the glass tube measured for different pump energies. As one can see in the Fig. 6, at low pump energy the measured PL spectra (curve 0.3 mJ (A) or 2 mJ (B)) were typical for the 5E→5T2 chromium transition in the bulk ZnSe host. The dependence of PL spectrum profile on pump energy demonstrated a threshold behavior accompanied by the appearance of a stimulated emission band around 2350 nm as shown in Fig. 6 (curve ii 0.5 mJ (A) and curve ii 3 mJ (B)). The stimulated emission bands are shifted to the longer wavelength with respect to the spontaneous emission and correspond to the peak of the Cr2+:ZnSe gain spectrum. Further increase of the pump energy (Fig. 6 (curve 1.2 mJ (A), or 5 mJ (B)) results in an increase of the stimulated emission intensity, which becomes much stronger than the PL signal. In our experiments the lasing spectra were accumulated and averaged for several minutes and, as one see from the Fig. 6 we did not observe narrow spectral spikes demonstrated for random laser with a coherent feedback . The emission spectra of 1 µm and 10 µm size powders obtained at the same experimental conditions are compared in Fig. 6(C).The emission bands of 1 µm size Cr-doped ZnSe powder is blue shifted with respect to the emission of 10 µm size powder sample. The blue shift of the Cr-doped ZnSe emission can be attributed to the following probable factors affecting the shape of Cr2+:ZnSe powder stimulated emission spectra. The shape of the stimulated emission spectrum can be perturbed by a strong surface absorption of OH group (~2.7 µm), which starts to be more prominent with the decrease of the average powder particle size . In addition, the shape of stimulated emission spectrum depends on elastic scattering responsible for the oscillation feedback in the random lasers. Finally, the “blue shift” could be also due to a strong spectral dependence of the back scattering cross section in the Mie region (kr≈1..10, where k is a wave vector, and r is a radius of the scatter).
Another question that we wanted to address was related to the nature of positive feedback of powder lasing. In our experiments, the most probable nature of positive feedback can be either scattering or whispering gallery modes and reflection of the capillary tube. To clarify between the two possibilities, we performed comparative laser experiments of the same 10 µm size powder sample placed either in the capillary tube or on the surface of the microscopic slide. The emission spectra from the surface of the powder sample for different pump energies are depicted in Fig. 6(A) and Fig. 7 for glass tube and microscopic slide arrangements, respectively. As one can see, the threshold pump energies are comparable (~2 times lower for the glass tube arrangement). This result shows that the dominant mechanism of the positive feedback of powder lasing is light scattering. In addition, we tried to identify the minimum mass of the 10 µm size powder sample (on the microscopic slide) required for lasing. It was revealed that reducing the mass of the powder from 200 to 10 mg results in only a slight increase of the threshold pump energy density from 10 to 15 mJ/cm2. Hence, the minimum masses supporting laser action of Cr:ZnSe powder can be much smaller than 10 mg, which is very important for practical applications.
We demonstrated a simple method of Cr2+:ZnSe powder fabrication and realized RT mid-IR random lasing under optical intra-shell excitation of chromium. Threshold dependence of the powder emission, significant line narrowing, and lifetime shortening are clear evidence of Cr:ZnSe powder lasing. Random powder lasing was achieved with a minimal threshold pump energy of 0.5 mJ (in a 2.9 mm spot) and a minimal mass of much smaller than 10 mg. This technology provides the cost effective route for fabrication of mid-infrared gain powder materials promising for scientific, technological, and defense related applications.
We acknowledge support from the National Science Foundation Grants No. ECS-0424310, EPS-0447675, and BES-0521036.
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