Recent progress in fabrication and mid-IR lasing of transition metal doped II-VI single crystal and thermo-diffusion doped polycrystalline and hot-pressed ceramic gain media as well as nano and micro-crystalline laser active powders, powders in the liquid suspension, polymer-film, thin film waveguides and chalcogenides glass composites is reported.
©2011 Optical Society of America
Starting from the first lasing of ruby crystal, transition metal doped (TM) materials were considered promising as gain media for solids state lasers. Tunable oscillations of different crystalline hosts doped with Cr3+, Co2+, and Ni2+ ions were demonstrated in the near IR spectral range . The most significant among this group of lasers is titanium-sapphire laser (Ti3+:Al2O3, or Ti:S) , revealing tunability bandwidth exceeding half of a central wavelength. This laser opened new horizons of tunable and ultra-short laser applications in scientific research, sensing, and medicine. Currently, Ti:S is the most widely used tunable solid state laser. It serves as a key element of at least two research programs that were awarded the Nobel Prize in chemistry (1999 ) and physics (2005 ).
Extensive research of TM doped II-VI semiconductor materials started more than 50 years ago [5,6]. Several research groups made significant contributions to spectroscopic characterization of these materials (see [7–9] and references therein). However, only in 1996, scientists from Lawrence Livermore National Laboratory  were the first to suggest and demonstrate that TM2+ doped wide bandgap II-VI semiconductor crystals are very advantageous for middle-infrared (mid-IR) lasing. Consequent active interest to these materials was inspired by the fact that mid-IR laser sources operating over the “molecular fingerprint” 2-15 µm spectral range are in great demand for molecular spectroscopy, non-invasive medical diagnostics, industrial process control, environmental monitoring, atmospheric sensing and free space communication, oil prospecting, and numerous defense related applications such as infrared countermeasures, monitoring of munitions disposal, and stand-off detection of explosion hazards. The authors of  have formulated the following features that make these materials so attractive for mid-IR laser applications:
- • II-VI compounds have a tendency to crystallize as tetrahedral coordinated structures. Tetrahedral coordination of the TM ions, as opposed to the typical octahedral coordination at the dopant site, gives approximately twice smaller crystal field splitting, placing the TM dopant transitions further into the IR.
- • Another key feature of these materials is that the heavy anions in the crystals provide a very low energy optical phonon cut-off that makes them transparent in a wide spectral region and decreases the efficiency of non-radiative decay, which gives 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 promising for ultra-broad mid-IR tunability as well as for ultra-short pulse generation.
Among different TM doped II-VI semiconductors, crystals doped with chromium and iron ions attract the most attention due to their unique spectroscopic characteristics. In the tetrahedral crystal field (Td) of the II-VI compounds, the ground term (5D) of the Cr2+ (3d4) and Fe2+ (3d6) ions is split into triplet 5T2 and doublet 5E. The transition between these levels is symmetry allowed, but all other transitions to the upper terms are spin forbidden. The absorption and emission bands corresponding to 5E↔5T2 transitions of some chromium and iron doped II-VI crystals are depicted in Fig. 1 .
Among all the II-VI chalcogenides ZnS crystals feature the maximum 5D term energy splitting due to the smallest inter-ligand distance. Therefore, the absorption and emission bands of chromium 5E↔5T2 transitions in ZnS are shifted to the shorter wavelengths in comparison with the bands of other II-VI semiconductors. On the other hand, crystals based on Cd and Te feature a shift of the 5T2 ↔5E transition to a longer wavelength. Cr emission in the ZnS, ZnSe, and CdSe hosts have a radiative life of ~6 μs and feature a high luminescence yield (0.8-1) at RT.
Iron ions have smaller crystal field splitting in comparison with chromium ions resulting in a long wavelength shift of absorption and emission bands. The upper level lifetime of Fe ions in ZnSe is ~380 ns at room temperature, and the radiative lifetime, estimated from the low temperature (T<100K) kinetics of luminescence, is 55-75 μs. Detailed spectroscopic parameters of the TM doped II-VI materials were summarized in [1,11]. Shortly after the first publication , different regimes of oscillation were demonstrated with Cr2+ and Fe2+ doped binary (e.g., ZnSe, ZnS, CdSe, CdS, ZnTe) and ternary (e.g., CdMnTe, CdZnTe, ZnSSe) chalcogenide compounds. The unique laser characteristics of the chromium and iron doped II-VI gain media (laser efficiency >70%, maximum CW power >18W, short pulse oscillation <80fs, maximum output energy >20mJ, and overall tunability over 2-5 μm, see below and [11–14]) explain the fact that sometimes these media are called “mid-IR analogs of Ti:S laser”. These lasers currently represent the simplest and most cost-effective route for high power, broadly tunable lasing over 2-5 µm range and are already commercially available . In this review recent progress in the development of chromium and iron doped materials and mid-IR lasers on their basis is presented.
2. Material properties
The material properties of the most important Cr2+ and Fe2+ doped II–VI semiconductors are summarized in Table 1 .
Among all the II–VI semiconductors, ZnS and ZnSe crystals have the widest bandgaps and reveal the most favorable thermo-optical characteristics. The use of other materials is motivated by a smaller crystal field splitting of the TM ions, resulting in additional emission red shifting in comparison with ZnS and ZnSe hosts. For example, tunable laser oscillation with a wavelength longer than 3 μm is very important for spectroscopic and sensing applications, since many organic compounds have characteristic vibronic transitions in this spectral range. For this reason, Cr:CdSe laser systems tunable up to 3.6 μm are of special interest. It should be noted that in comparison with the best oxide crystal hosts, the II–VI semiconductors feature strong thermal lensing (dn/dT = 49 × 10−6 and 8.6 × 10−6 for ZnS and YAG, respectively ). Therefore, proper thermal management is very important for power scaling of Cr and Fe doped chalcogenide lasers.
3. Fabrication techniques and laser characteristics
3.1. Physical vapor transport growth
Physical vapor transport (PVT) growth of TM doped II-VI materials was studied by several research groups [17–19]. In recent years, major progress in the PVT crystal development and laser characterization was reported by the research group from P.N. Lebedev Physical Institute of the Russian Academy of Sciences . The crystal growth takes place in a quartz ampoule in a H2 or He atmosphere. The TM doping is performed with the use of a separate source with TM selenide or sulfide compounds. The authors of  reported a successful crystal growth with Cr and Fe concentrations of 1017 - 1019 cm−3 and demonstrated laser oscillations of Chromium doped ZnSe, CdSe, CdS, and Iron doped ZnSe CdSe, and ZnS crystals. The reported crystal losses (0.02-0.07 cm−1) at the oscillation wavelength are similar to the losses in the gain materials prepared by other methods. These crystals reveal good laser slope efficiency with respect to the absorbed pump power or energy; however, the final output parameters were similar or worse than that reported for crystals prepared by the post growth diffusion method. The major reason for this is that the doping during PVT crystal growth is very difficult in terms of achieving homogeneous impurity distribution with predictable concentration, especially when one needs heavily doped single crystals. This crystal growth method is also difficult for realization when one needs more than one co-dopant.
Using gain elements grown by PVT, Cr:CdSe  and Fe:ZnS  tunable lasing over the 2.26-3.61 and 3.49-4.65 μm spectral ranges has been demonstrated. These lasers are promising for sensing and spectroscopic applications because they cover the 3-4 μm spectral range, overlapping with strong absorption lines of hydrocarbons. Cr:CdSe laser demonstrated RT operation with 1.5W output power and 53% slope efficiency with respect to the absorbed power of the Tm-fiber pump laser. In free-running regime Cr:CdSe laser featured 17mJ of output energy at 2.65 μm with a slope efficiency 63% under long (300 μs) pulse excitation by Tm:YAP laser. An additional extension of the tunability to the mid-IR spectral range (4.6-5.9 μm) was also demonstrated by using Fe:CdSe crystal .
3.2. Melt growth technique
Melt growth methods are used for growing large size II-VI single crystals. To suppress sublimation of II-VI compounds during Czochralski growth, it is necessary to simultaneously apply high pressure and high temperature (e.g. ~75x102 KPa, and 1515 °C for ZnSe growth), which make uncertain the commercial viability of mass production of these crystals by the Czochralski method. The Bridgman technique is a popular method of producing II-VI semiconductor crystals. The method involves heating polycrystalline material in a container above its melting point and slowly cooling it from one end where a seed crystal is located. A single crystal is progressively formed along the length of the container. High temperature melt growth is often accompanied by uncontrolled contamination. This contamination can lead to undesirable and parasitic absorptions.
Despite these issues, TM:II–VI single crystals with good laser characteristics were reported by several groups. In  laser characteristics of Cr:ZnSe grown by the Bridgman technique in inert argon atmosphere were studied. 20mJ of output energy with slope efficiency 31% under long pulse excitation by Tm:YAP laser were reported. Laser oscillations in the chromium doped II-VI materials were also demonstrated in Cr:ZnMgSe ; CdSe , Cr:CdTe [26,27], and Cd1-xMnxTe [28,29] crystals grown by Bridgman techniques.
Iron doped II-VI crystals grown by Bridgman techniques were studied by many research groups (e.g [30,31].). However, there are only a few publications where the laser performance of iron doped Bridgman grown crystals were reported [24,32,33]. In  the first mid-IR laser oscillation of the Fe:ZnSe crystal at low temperature was reported. In [24,33] Fe:ZnSe lasers based on crystals grown by Bridgman and floating zone technique were compared. The maximum output energy was 1.2-1.3 mJ for both types of investigated crystals.
3.3. Thermal diffusion
The diffusion of the TM ions into II-VI semiconductors has been studied for more than 60 years (e.g., .). This technique utilizes thermally activated diffusion of transition metal ions into II-VI crystals. Thermal diffusion is usually realized from the TM metal film deposited on the crystal surface or from the vapor phase. In the first case, Cr or Fe films are deposited on the crystal surface, using pulsed laser deposition, thermal deposition, or magnetron spattering. At the second stage, thermal diffusion is carried out in sealed vacuumed (~10−5 Torr) ampoules at temperature of 900-1100°C over 7-20 days. In the vapor phase diffusion method, II-VI samples together with TM (Cr, Fe, Co, Ni) or TM compounds (CrS, CrSe, FeSe) are placed in the different parts of the ampoules. The ampoules are sealed at low pressure and annealed. The specific details of the thermal-diffusion process are reported in [35–37].
Thermal diffusion method in comparison with crystal growth is very cost-effective, simple, and has been used quite extensively. Its main drawbacks include qualitative nature of doping (hard to fabricate crystals with a pre-assigned concentration of dopant), non-uniform doping, large concentration gradients, degradation of optical quality of the crystals due to sublimation of Zn and Se sub-lattices, and, finally, the procedure has poor repeatability. Therefore, preparation of the large-scale samples with homogeneous TM ions distribution and low optical losses requires special technological arrangements. Scientists from the University of Alabama at Birmingham in collaboration with the IPG Photonics Corporation solved these issues and developed commercial, quantitative (accuracy of the pre-assigned concentration of dopant is better than 3%) and fast thermo-diffusion process of TM ions in II-VI polycrystals with suppressed sublimation in Zn/Cd and Se/S sublattices . In result, the fabricated crystals are uniformly doped through the thickness of up to 7 mm (see Fig. 2 ) and feature a low scattering loss of 1-2% per cm for samples with Cr concentration of 5x1018 cm−3. Consistently high optical quality of fabricated thermo-diffusion doped Cr:ZnSe/S and Fe:ZnSe polycrystals with low depolarization factor enabled the first polycrystalline based Cr:ZnSe femtosecond oscillator , ultra-broad tunability (1973 – 3339 and 1962 – 3195 nm for CW lasers based on polycrystalline Cr:ZnSe and Cr:ZnS, respectively ), and highest up-to-date output characteristics in CW (13W Cr:ZnSe , 10W Cr:ZnS ) and gain-switched regimes of operation (20mJ Cr:ZnSe , 4.7 mJ Fe:ZnSe ).
3.4. Hot press TM doped II-VI ceramics
The major advantage of laser ceramics is in advanced ceramic processing enabling affordable mass production and design flexibility of the laser elements (undoped ends, waveguiding structures, gradient of dopant concentration, etc) important for the development of efficient, high performance lasers. First laser active Cr:ZnSe ceramic was reported in [44,45]. In these publications Cr:ZnSe ceramic was fabricated by a multi-step process involving powder preparation and a multi-step heating and pressing procedures. The powder preparation for hot-pressing was performed by mixing pure ZnSe and preliminary prepared mixture of ZnSe-CrSe (1 mol %). This mixture contained particles smaller than 10 μm. Prior to hot pressing, the samples were first briquetted (cold-pressed) at room temperature under a pressure of 60MPa. Samples with different CrSe concentrations were then further hot-pressed at 1400–1500K at axial compression of up to 350MPa. After 10–15 minutes of sintering, samples were cooled to room temperature.
The first lasing of the Cr:ZnSe hot-pressed ceramic in both CW and gain switched regimes was demonstrated in [44,45]. In gain-switched regime, under excitation with a radiation of H2 Raman shifted (1908 nm) Nd:YAG laser, the slope efficiency of 5% and maximum output energy of 2 mJ were achieved . CW lasing of hot-pressed ceramic Cr:ZnSe was achieved with a maximum output power of 0.25 W and 20% efficiency with respect to absorbed power of Er-fiber laser .
Hot-pressed ceramics are a promising and viable “alternative route” for the synthesis of large-scale mid-IR laser media based on transition metal doped chalcogenides. Hot-pressed chalcogenide gain media can be fabricated quickly, with any dopant concentration, and many varied geometries. However, the optical quality of the fabricated hot-pressed Cr:ZnSe ceramic material is still not adequate for effective laser operation and this technology requires a significant effort to reach maturity.
4. Laser active TM doped chalcogenide waveguide structures
II-VI materials are wide band semiconductors, and have been regarded as promising materials for visible diode laser application. Therefore, combination of electrical properties of the host materials with optical transitions of TM ions is very attractive for development of electrically pumped broadly tunable mid-IR lasers. TM:II-VI thin film technology is also of interest for integrated optical devices with electrical control of the lasing parameters as well as for development of compact laser systems. Spintronics is another important application area of the TM doped II–VI thin films. These promising practical applications stimulated study of the TM doped II-VI thin films structures.
Mid-IR photoluminescence associated with Fe2+ and Cr2+ transitions were reported for the II-VI (ZnS, ZnSe, ZnTe) thin film structures grown by molecular beam epitaxy (MBE) [46,47]; pulsed laser deposition (PLD) [48,49]; or RF magnetron spattering techniques. Electroluminescence of Cr2+ ions in ZnSe based thin film structures was reported by authors in several publications [50–52].
In recent publication , mid-IR planar waveguide lasing was demonstrated using a Cr:ZnSe film deposited on sapphire by PLD (Fig. 3.A). The planar waveguide structure was formed in the Cr:ZnSe layer due to a high refraction index in ZnSe (2.4) in comparison with refractive index of sapphire substrate (1.7). Figure 3.B shows the absorption spectrum of the Cr:ZnSe film deposited on the sapphire substrate. The interference pattern reveals a good homogeneous film with a thickness of 7.5 μm. The absorption dip near 1.7 μm results from the 5T2→5E transition of Cr2+ ions. The chromium concentration in the film calculated from absorption was as high as NCr = 6 ×·1019 cm−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 a pulse duration of 5 ns (see Fig. 3.C). As one can see from the measurements, at low pump energy the measured PL spectrum (Fig. 3.C curve i) is typical for the 5T2 → 5E chromium transition in the ZnSe host. Further increase of the pump energy above the threshold results in appearance of intensive, much narrower stimulated emission with a central peak (2550 nm) shifted to the longer wavelengths with respect to the PL peak (Fig. 3.C curve ii). This shift results from the trade-off between maximum emission cross-section at ~2400 nm and optical losses in the waveguide due to non-saturated chromium absorption. The pump energy-density laser-threshold was measured to be 0.11 J/cm2 which is approximately equal to saturation energy density Es = 0.1 J/cm2.
5. Laser Active TM doped II-VI powders
Studies of spectroscopic and laser properties of TM doped powders were stimulated by several research topics. The first research area where TM doped II-VI materials could be of interest is propagation and localization of electromagnetic waves in the disordered media. Impressive results were reported for scattering media based on GaAs semiconductor (photon mean free path l = 0.17 μm at 1.06 μm) .
The second research area is related to random laser physics. Random lasers are based on active scattering media without any optical elements when positive laser feedback arises from multiple photon scattering. Potentially the low-cost mirror-less random lasers could be attractive for such applications, as low coherent laser sources, X-ray and γ-ray lasers, low-noise imaging, new lighting systems, and optical coding . Combination of low passive losses, high refractive index, and high optical gain makes TM doped II-VI powders very attractive for these research areas.
Another research area where TM doped powders could be attractive is in crystal field engineering and fast prototyping of new TM:II-VI compounds. Spectroscopic characterization of TM doped binary, ternary, and quaternary powders prepared from raw materials without a crystal growth stage enables fast prototyping and simplifies development of the new laser media. TM doped laser active powders are also essential for fabrication of “green body” (raw compound) for active ceramics, targets for PLD grown laser active waveguides, as well as for development of composite TM:II-VI-glass fiber gain media.
The first chromium-doped ZnSe and ZnS random lasers were reported in powders prepared by mechanical grinding of Cr:ZnSe and Cr:ZnS crystals . Authors of [56–58] fabricated iron and chromium doped micron-size powders without a stage of crystal growing. The fabrication procedure was based on annealing of the powder components at 800-1000 °C in evacuated quartz ampoules for 1-6 days. Raman and XRD characterization of the powders demonstrated that the optimized annealing procedure results in the formation of new ternary and quaternary compounds .
Figure 4 depicts mid-IR random lasing of (Cr2+) and (Fe2+) doped ZnSe, ZnS, CdSe, and ZnCdTe powders over 2.2-5.9 μm spectral range. Random lasing was achieved under 1.5μm and 2.8μm ns-pulse excitation for chromium and iron doped powders, correspondingly. For chromium compounds, the laser thresholds were ~198 mJ/cm2, 8.1 mJ/cm2, and 73 mJ/cm2 for ZnS, ZnSe, and CdSe, respectively. Room temperature random laser oscillations at 2.4 μm were also obtained for Cr:ZnSe powders imbedded in perfluorocarbon liquid polymer solutions, and fluorocarbon polymer films .
Many applications require the utilization of nano-size TM doped II-VI powders. The majority of methods of TM doped II-VI nano particles syntheses are based on chemical approaches. Chemical methods use a variety of functional groups to stabilize nanoparticles and serve as capping agents. Various byproducts of chemical reactions may also interact with nanoparticles and their removal from the surface is tricky. Even a small amount of residual organic molecules used in nanoparticles synthesis may reabsorb and/or quench mid-IR luminescence.
An entirely physical approach of fabrication of TM doped II-VI nanoparticles was reported in [13,59]. To fabricate TM (Cr, Co and Fe) doped II-VI nanoparticles the laser ablation method was utilized. The variation of laser wavelength, pulse duration, ablation environment, and additional UV treatment produced nanoparticles with different sizes. To prepare 11-13 nm size nanoparticles, the TM:II-VI bulk samples were irradiated by the radiation of Nd:YAG fundamental harmonic (1064nm) with 30 ps pulse duration. Additional laser irradiation of the colloidal suspension by third harmonic of the same laser produced nanoparticles with an average size of 3 nm. The room temperature photoluminescence of fabricated Cr, Fe and Co doped ZnSe and ZnS nanocrystals spanned over the 2-6 μm spectral range. The authors of  also reported the first room temperature mid-IR random lasing due to impurity intra-shell transitions in Cr:ZnS nanocrystals. The observed difference in the spectral behavior of nano-powder lasing was explained by different scattering regimes in the 27 nm and 3 μm particle powders.
The fact that TM doped II–VI nanocrystals demonstrated strong mid-IR luminescence and random lasing may potentially open a new pathway for future optically and electrically pumped mid-IR lasers based on TM doped quantum confined structures .
6. TM2+ doped glassy compounds for active mid-IR fiber applications
Power scaling of TM:II-VI lasers requires thorough thermal management in the active element. Among different approaches to control beam quality and thermal lensing, a waveguide and fiber lasers are very promising for a variety of applications. Research in the fiber geometry of the gain medium is stimulated by the current progress achieved by the near-IR fiber laser systems. Nowadays, near-IR fiber lasers with CW output power >10kW are commercially available .
In spite of years of research, all the attempts to fabricate Cr:II–VI media in fiber geometry have not been successful. Among other materials, chalcogenide glasses are very promising hosts for passive mid-IR fiber applications due to their wide mid-IR transparency range and are commercially available from various vendors. Chemically stable divalent states of chromium and iron as well as their tetrahedral crystal-field coordination (enabling energy level structures suitable for effective tunable mid-IR lasing) are natural for II–VI crystal hosts, but they are not possible in the chalcogenide glass media. One of the approaches to solve this problem could be in development of TM:ZnSe(ZnS)/As2S3:As2Se3 laser active composite material suggested and reported in . In this composite material, II-VI component provides tetrahedral coordination of the TM ions and chalcogenide host makes fiber geometry possible. Recently, a refractive index values between n = 2.1 and n = 2.4 have been reported for the As2S3 chalcogenide glasses depending on the annealing temperature and time. This technology allows optimization of the fabrication procedure to provide refractive index matching between ZnS(n = 2.2)/ZnSe(n = 2.4) components and chalcogenide glasses , which is crucial for the reduction of the scattering losses from different components of the composite materials.
The first laser oscillation of chromium doped Cr:ZnSe/As2S3:As2Se3 composite compound was demonstrated in  at 2.4 µm. Laser active Cr:ZnSe nano and micro size powders were used as starting materials. The Cr:ZnSe/As2S3:As2Se3 composites were prepared by using two different methods. The first type was prepared by annealing of the appropriate compounds under vacuum. The second type of sample was prepared by casting and drying of Cr:ZnSe powder suspension in As2S3:As2Se3 propylamine solution. The samples demonstrated mid-IR photoluminescence typical for Cr2+ ions in the tetrahedral coordination of the ZnSe host between 1.8 and 3.0 µm with characteristic photoluminescence kinetic lifetime of 3-5µs. The gain-switched random lasing of the Cr:ZnSe/As2S3/As2Se3 composite material was demonstrated under 5 ns pulse excitation at 1.9 μm.
The fabrication and spectroscopic characterization of Cr:ZnSe/As2S3 composite was recently reported in . As2S3 glasses containing Cr:ZnS and Cr:ZnSe crystals were prepared in three steps: (1) distillation of the As2S3 host glass in order to remove insoluble impurity particles, (2) dissolution of the semiconductor phase in molten glass and (3) subsequent annealing intended to relieve the internal stress in the glass and ensure effective diffusion of the dissolved semiconductor phase. Cr:ZnS(ZnSe) was dissolved at temperatures from 600 to 750 8C over a period of 1 - 5 h in a rocking furnace. The annealing temperature was varied from 200 to 280C, and the annealing time, from 12 to 65 h. The semiconductor content of the glass was varied from 0.1 wt % to 4 wt %. The researchers were able to fabricate 2-20 m long fibers with diameter 170-300 μm and demonstrate mid-IR photoluminescence over 1.8-3 μm spectral range. The optical losses of the Cr:ZnSe/As2S3 fibers in the 2-3 μm were approximately 3dB/m.
Recent progress in the fabrication and mid-IR lasing of transition metal doped II-VI single crystals, thermo-diffusion doped polycrystalline and hot-pressed ceramic gain media, nano and micro-crystalline laser active powders, powders in liquid suspension, polymer-film, thin film waveguides, and chalcogenide glass composites was reported. The unique blend of material (low phonon cut-off, broad IR transparency, high thermal conductivity, feasibility of high electrical conductivity), spectroscopic (ultrabroadband gain bandwidth, high στ product, high absorption coefficients, high quantum efficiency at RT), and technological parameters (low-cost, mass production technology of crystal fabrication by post-growth thermal diffusion of impurities), make these materials ideal candidates for broadly tunable mid-IR lasing in CW, gain-switched or mode-locked regimes of operation under optical (convenient pump sources - diode lasers and Er- and Tm-fiber lasers) and potentially under direct electrical excitation. Room temperature oscillation of TM doped II-VI materials was demonstrated at wavelengths longer than 6 μm and we believe that it could be further extended up to 10 μm . The major advantage of the TM:II-VI lasers in comparison with Quantum Cascade Lasers and active mid-IR fiber lasers are: high output power (>20W), high output energy (>20mJ), broad tunability (Δλ ~0.5λ0) and high peak power (>1GW). All these characteristics currently exceed limits of QCLs and mid-IR fiber lasers. Additionally, the laser systems based on TM:II-VI materials are more compact and flexible for energy scaling up in comparison with tunable sources based on nonlinear frequency conversion (such as OPO and DFG) and also demonstrate higher CW output power. Based on the above advantages of the TM:II-VI lasers in comparison with other mid-IR laser sources we believe that the major application areas are broadband and stand-off molecular sensing  where broad tunability and high output power of the laser source are required. Other possible areas could be material processing, medical applications, and infrared countermeasures  where high power/energy mid-IR radiation is essential.
We are grateful to our colleagues and collaborators: K. Schepler, P. Berry, and J. Goldstein, I. Sorokina, E. Sorokin, S. Trivedi, J. Williams, R. Camata, K. Graham, A. Gallian, C. Kim, and N. Myoung. The authors would like to acknowledge funding support from National Science Foundation (NSF)EPS-0814103 and ECCS-0901376. The work reported here partially involves intellectual property developed at the University of Alabama at Birmingham. This intellectual property has been licensed to the IPG Photonics Corporation. The UAB coauthors declare competing financial interests.
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