Recent Progress in Transition-Metal-Doped II–VI Mid-IR Lasers

810 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 3, MAY/JUNE 2007

Recent Progress in Transition-Metal-Doped II–VIMid-IR Lasers

Sergey B. Mirov, Vladimir V. Fedorov, Igor S. Moskalev, and Dmitri V. Martyshkin

(Invited Paper)

Abstract—Recent progress in transition metal (TM)-doped II–VI semiconductor materials (mainly Cr2+:ZnSe) makes them thesources of choice for laser when one needs a compact continuouswave (CW) system with tunability over 1.9–3.1 µm, output powersup to 2.7 W, and high (up to 70%) conversion efficiency. The uniquecombination of technological (low-cost ceramic material) and spec-troscopic characteristics make these materials ideal candidates fornew regimes of operation such as microchip and multiline lasing.This paper reviews these nontraditional Cr-doped middle-infrared(mid-IR) lasers as well as describes emerging Fe2+:ZnSe lasershaving potential to operate at room temperature (RT) over thespectral range extended to 3.7–5.1 µm. In addition to being wide-band semiconductors, effective RT mid-IR lasing TM-doped II–VImedia also hold potential for direct electrical excitation. This papershows the initial steps toward achieving this goal by studying Cr2+-Co2+- and-doped quantum dots. We have demonstrated a novelmethod of TM-doped II–VI quantum dots fabrication based onlaser ablation in liquid environment. TM-doped II–VI quantumdots demonstrated strong mid-IR luminescence. It opens a newpathway for future optically and electrically pumped mid-IR lasersbased on TM-doped quantum confined structures.

Index Terms—Doped quantum dots, laser, middle-infrared(mid-IR), tunable.

I. INTRODUCTION

TRANSITION metal (TM2+, e.g., Cr2+ or Fe2+) dopedbinary and ternary chalcogenides crystals represent rela-

tively new class of solid state gain media with strong and ul-trabroad absorption and emission bands in the middle-infrared(mid-IR) range of optical spectra. In 1996, scientists from theLawrence Livermore National Laboratory [1] were the first toshow that among different types of crystalline gain materials,TM2+-doped wide bandgap II–VI semiconductor crystals couldbe very special for mid-IR lasing. These TM2+-doped II–VIcompounds have a wide bandgap and possess several impor-tant features that distinguish them from other oxide and fluoridelaser crystals. These features are as follows.

1) The heavy anions in the crystals provide a very low-energyoptical phonon cutoff that makes them transparent in awide spectral region and decreases the efficiency of non-radiative decay, which gives a promise of a high yield offluorescence at room temperature (RT).

Manuscript received November 19, 2006; revised March 13, 2007. This workwas supported by the National Science Foundation under Grant ECS-0424310,Grant EPS-0447675, and Grant BEC-0521036.

The authors are with the Center for Optical Sensors and Spectroscopies,Department of Physics, University of Alabama at Birmingham, Birmingham,AL 35294, USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/JSTQE.2007.896634

2) The II–VI compounds tend to crystallize as tetrahedrallycoordinated structures, as opposed to the typical octahe-dral coordination at the dopant site. Tetrahedral coordina-tion gives smaller crystal field splitting, placing the dopanttransitions further into the IR.

Active interest in TM-doped II–VI compounds, inspired by[1], was explained by the fact that these media are close mid-IR analogs of the titanium-doped sapphire (Ti-S) in terms ofspectroscopic and laser characteristics, and it was anticipatedthat TM2+-doped chalcogenides will lase in the mid-IR with agreat variety of possible regimes of oscillation, similar to theTi-S laser. Indeed, shortly after the pioneering publications [1],[2] gain-switched, pulsed lasing of Cr:Cd1−xMnxTe [3], [4],Cr:CdSe [5], [6] at RT and Fe:ZnSe at 15–180 K [7] was suc-cessfully demonstrated. Then, the major attention of researcherswas focused mainly around Cr2+:ZnSe, where regimes of oper-ation were significantly improved by the first demonstrations ofdirect diode excitation [8]–[10], continuous wave (CW) lasingwith efficiency exceeding 60% [10], [11], power levels in excessof 1.8 W [12], and range of tunability over 2000–3100 nm [13]pulsed lasing with output power up to 18.5 W [14], a rangeof tunability over 1880–3100 nm [15], active [16] and pas-sive mode-locking [17], [18], first demonstration of sub-100-fsmode-locked operation [19], first microchip [20]–[23] and disklaser [24] operations, single-frequency operation [25], randomlasing [26], multiline and ultrabroadband operation in spatiallydispersive cavities [27], and lasing via photoionization transi-tions [28].

The first CW operation of Cr2+:ZnS in external cavity andmicrochip regimes reported in [21], demonstration of outputpowers in excess of 0.7 W [29], [23], diode-pumped oper-ation [29], [23], as well as demonstration of semiconductorsaturable absorber mirror (SESAM) passively mode-locked op-eration with output power of 125 mW and pulse duration of1.1 ps [30] showed that Cr2+:ZnS is an interesting alternative toCr:ZnSe gain material especially for applications when tuningrange shifted to shorter wavelengths (∼100 nm) relatively toCr:ZnSe is required. Moreover, having spectroscopic character-istics similar to Cr:ZnSe [1], [2], [31], [32], Cr:ZnS has betterhardness, higher thermal conductivity, higher thermal shock pa-rameter, and lower thermal lensing, providing potentially higherpower handling capability [32].

Chromium-doped cadmium chalcogenides crystals, havingspectroscopic characteristics similar to Cr:ZnSe, are attractivefor applications when emission shifted to longer wavelengths(up to 3400 nm) is required. Under pulsed 2.05-µm excitation,

1077-260X/$25.00 © 2007 IEEE

MIROV et al.: RECENT PROGRESS IN TRANSITION-METAL-DOPED II–VI MID-IR LASERS 811

TABLE ISTATE-OF-THE-ART IN Cr:ZnSe, Cr:ZnS, AND Fe:ZnSe LASERS

output power of 0.82 W at 27% conversion efficiency [6] as wellas remarkably broad tuning range over 2.45–3.4 µm [33] havebeen achieved with the use of Cr2+:CdSe. However, relativelypoor thermal properties of CdSe and Cd1−xMnxTe prevent theiruse in CW lasers. Cr:Cd1−xMnxTe CW lasing was obtained withoutput power below 6 mW [34], and Cr:CdSe CW operationwas not obtained so far. There are only a few RT solid statelasers operating beyond 3.4 µm. Pulsed, tunable over 3.98–4.54 µm, operation of Fe:ZnSe with output energy of 12 µJ at130 K [7] stimulated research of these crystals. Recently, theoutput characteristics were significantly improved. The outputenergies in excess of 180 mJ were obtained for Fe:ZnSe at85 K and in excess of 140 mJ for thermoelectrically cooledFe:ZnSe [34]–[37], and the first gain-switched lasing at RT wasrealized in [38], [37].

Table I summarizes the most important state-of-the-art outputcharacteristics of the Cr:ZnSe, Cr:ZnS, and Fe:ZnSe lasers. Asshown in Table I, Cr- and Fe-doped II–VI lasers could be viablecompetitors in the 2–5 µm spectral range to the conventionalsemiconductor lasers and laser systems based on frequency con-version techniques.

TM2+-doped II–VI lasers combine a unique versatility of theion-doped solid state lasers with the engineering capabilities ofsemiconductor lasers, paving the route to the future “nontradi-tional” regimes of oscillation. This paper explores several impor-tant directions of further Cr2+ and Fe2+ laser development in-

cluding microchip, hot-pressed ceramic material fabrication andlasing, multiline and ultrabroadband lasing in spatially disper-sive cavities, power scaling, broadly tunable single-frequencyoperation, Fe:ZnSe lasing at elevated temperatures, and promis-ing future paths for achieving TM:II–VI mid-IR lasing underelectrical excitation.

II. CR-DOPED MID-IR LASERS

A. Cr:ZnSe and Cr:ZnS Microchip Lasers

The unique combination of technological (low-cost ceramicmaterial) and spectroscopic characteristics (ultrabroadband gainbandwidth, high στ product, and high-absorption coefficients)make these materials ideal candidates for microchip lasing. First,microchip laser experiments on both Cr2+:ZnS and Cr2+:ZnSechemical vapor transport (CVT) grown crystals were performedand reported in [20]–[23].

After growth, doped Cr2+:ZnS and ZnSe crystals were pre-pared by a two-stage method. During the first stage, undopedsingle crystals were synthesized by a CVT reaction from the gasphase using an iodine gas transport scheme in a quartz tube of20-mm diameter and 200-mm length placed in a two-zonefurnace. At the second stage, chromium was introduced intothe crystalline host by thermal diffusion carried out in sealedampoules under a pressure of 10−5 torr and temperature of1000 ◦C over 5–20 days. Some crystals, at the second stage


Fig. 1. Output–input characteristic of the Cr2+:ZnSe CW microchip laserunder Er-fiber laser excitation-focused pump beam arrangement.

of chromium introduction into the crystalline host, were dopedby thermal diffusion from a chromium thin film deposited by apulse laser deposition (PLD) method on a thin wafer of II–VImaterial. The mirrors were directly deposited on the parallelpolished facets of a thin wafer of laser material. Two differentpump arrangements were utilized. The first was with couplingoptics between the fiber and the microchip. The second pumparrangement was without any coupling optics and was arrangedby microchip laser mounting at a close (∼20µm) distance fromthe tip of the Er-fiber pump laser.

The maximum absorption coefficients of the studied crystalsat RT were α = 10 cm−1 (at λ = 1690 nm) and α = 12 cm−1

(at λ = 1770 nm) for Cr2+:ZnS and Cr2+:ZnSe crystals, re-spectively. Passive losses for both crystals estimated from aFindley–Clay analysis were below 2%. The crystals were pol-ished flat and parallel (parallelism of ∼10′′) to 1.0- and 2.5-mmthickness, respectively. Input and output dichroic mirrors had0.01% and 3.5% transmission over 2300–2500-nm spectralregion, respectively.

In a focused pump beam arrangement, a laser threshold of120 mW and a slope efficiency of 53% with respect to absorbedpump power were realized for the Cr2+:ZnS microchip laser.High, close to the theoretical limit of 65%, slope efficiency ofthe microchip laser indicates the crystal to be of high qualitywith low loss. The maximum output power of Cr2+:ZnS mi-crochip laser reached 150 mW at a slope efficiency of 43%. Theoutput spectra in free-running laser operation covered the 2280–2350 and 2550–2620 nm spectral ranges for ZnS and ZnSemicrochip lasers, respectively. The maximum output powerof the Cr2+:ZnSe microchip laser was reported to be 0.5 W(see Fig. 1).

Thermal effects provided cavity stabilization within the mi-crochip crystals and are responsible for the slightly nonlinearcharacter of output versus input laser performance. In the sec-ond pump arrangement, when the microchip lasers were directlycoupled to the fiber tip, laser thresholds of 150 mW and slopeefficiencies of 36% with respect to absorbed pump power wererealized for Cr2+:ZnS microchip laser. The maximum outputpower of the Cr2+:ZnS microchip laser was virtually unchangedin either of the pump arrangements.

We attempted to arrange a mode control of the microchiplasers by means of coupled cavity arrangement, with an addi-tional external etalon, made of a plane-parallel Si plate, coupledto the microchip cavity. The etalon, with a thickness of 0.4 mm,

was placed adjacent to the microchip resonator. In the coupledcavity experiments, the number of axial modes was decreasedup to three to four modes. This can be further decreased to asingle longitudinal mode oscillation in a coupled cavity config-uration using a narrowband output coupler as it was successfullydemonstrated for Tm:YLF in [39].

Gain-switched microchip laser experiments were performedwith CVT grown ZnS and ZnSe crystals, however, the bestresults were obtained with ZnSe [22], [23]. The ZnSe crystalused was 5.5-mm thick with polished but uncoated parallel facesand had a coefficient of absorption of k = 6 cm−1 at 1.77 µm.Pumping was via the 1.56-µm output of a D2 Raman-shiftedNd:YAG laser operating at 10 Hz with a pulse duration of about5 ns and 1.5-mm beam diameter. Threshold input energy wasfound to be 7 mJ. A maximum slope efficiency of 6% withrespect to pump energy and a maximum output energy of 1 mJwere obtained. The laser output spanned over 2470–2490 nmspectral range.

III. HOT-PRESSED CERAMIC Cr:ZnSe LASERS

Currently, the state of the art in TM:II–VI laser systems relieseither on single crystal, or vapor-grown polycrystalline materi-als. Both of these materials have particular drawbacks in termsof crystal doping. The commonly used doping methods are inmelt [40], vapor growth [41], [42] or after growth thermodiffu-sion doping [43]. Under atmospheric pressure, ZnSe sublima-tion occurs at a temperature above ∼400 ◦C, which is lower thanthat of the melting point [41]. Therefore, to use melt growthtechniques, in addition to a high temperature (1515 ◦C), it isnecessary to apply a high pressure (75 × 102 KPa) [42]. Hightemperature melt growth is often accompanied by uncontrolledcontamination. This contamination can lead to undesirable andparasitic absorptions. Control of the amount of Cr2+ ions incor-porated in the crystal is difficult when using vapor growth tech-niques [41], [43]. Doping of host crystals/polycrystals that arealready grown allow for another method of TM incorporation,i.e., thermal diffusion. This technique utilizes thermally acti-vated diffusion of TM ions into the II–VI crystals [43]. However,this technique has several drawbacks as well. These drawbacksinclude nonuniform doping, large concentration gradients, andthe poor repeatability of the procedure. These problems withthe control of dopant concentration are not appropriate for fab-rication of low-cost, large-scale samples for high-energy mid-IRapplications. Thus, another method of crystal growth is requiredthat will allow for large-scale crystal production in a timely andefficient manner. In the 1960s, researchers hypothesized that adense polycrystal of a pure material will be optically indistin-guishable from a single crystal of the same composition [44].Since then many advances have been made in Nd:YAG ceram-ics [45]–[47]. It has been recently demonstrated that hot-pressedceramics are a promising and viable “alternative route” for thesynthesis of large-scale mid-IR laser media based on chromium-doped ZnSe [48], [49]. Hot-Pressed Cr:ZnSe laser materialscan be made quickly with any dopant concentration and manyvaried geometries. The preparation of the powder for hot press-ing was performed by mixing pure ZnSe and a preliminary


prepared mixture of ZnSe–CrSe (1 mol%). This final mixturewas subjected to grinding in a “Fritch” spherical agate mortar.The grinding yields particles with a size less than 10 µm. Beforethe hot-pressing, the samples were first briquetted at RT witha pressure of 60 MPa. The internal diameter of the press-formwas ∼15 mm. The mass of the powder used to prepare a pel-let was chosen such that the final height of the pellet will be20–25 mm. These sample pellets, containing 0.01, 0.03, 0.05,and 0.1 mol% of CrSe, were further hot-pressed. Pyroliticallydensed graphite was used for preparation of the press-form andpunch. Initial heating was performed in a resistive heating fur-nace, and the temperature reached up to 900 K. Further heatingwas achieved by applying a direct electric current through thesample. This current heated the samples to 1400–1500 K. Inconjunction with the heating, the samples were subjected to ax-ial compression with a pressure of 30–35 MPa. After 10–15min of this hot-pressing, samples were cooled to RT. Then, finalcutting and polishing was performed. The final samples had adiameter of 15 mm and a height of 10.5 mm.

X-ray diffraction (XRD) measurements demonstrated that thehot-pressed ceramic samples are indeed polycrystalline ZnSein the cubic crystal form. A broad absorption band centeredat 1.78 µm indicates that chromium is indeed in the opticallyactive 2+ state. Photoluminescence measurements show that thedecay constant corresponding to the ceramic samples (7.8 µs)is in good agreement with that for the conventional thermallydiffused polycrystalline samples [50].

In the gain-switched laser experiments, the 10-mm Cr:ZnSehot-press ceramic sample with absorption coefficient of∼1.6 cm−1 was used. The laser cavity consisted of a single flathigh-reflectivity mirror (99.5%) placed less than 1 mm from thesample and the Fresnel reflection (R ∼ 18%) from the secondface of the ceramic sample as an output coupler. The ceramiccrystals were pumped at the Brewster angle from the front sur-face with a pump spot diameter of 6.5 mm. The pumping wasachieved using the first Stokes (1.907 µm) of the H2 Raman-shifted single-frequency Nd:YAG laser. The input–output char-acteristic of the gain-switched laser is shown in Fig. 2 [48], [49].The slope efficiency of 5% and the maximum output energy of2 mJ at repetition rate of 10 Hz and 5-ns pulse duration wereachieved. The spectral output was centered at 2450 nm withfull-width at half-maximum (FWHM) of ∼30 nm.

Laser studies of the hot-pressed ceramic sample under CWEr-fiber excitation were performed in a Kogelnik cavity with10% and 5% output couplers [see Fig. 2(B)]. For the first time,CW lasing of hot-pressed ceramic Cr:ZnSe was achieved withthe maximum output power of 0.25 W with 20% efficiency withrespect to absorbed power.

The described results demonstrate the feasibility of the mid-IR laser systems based on hot-pressed ceramics.

A. Multiline and Ultrabroadband Cr:ZnSe Laser

There are many practical applications where a source of radi-ation combining spatial coherence and high intensity with con-tinuous or discreet ultrabroadband, multiwavelength spectrumis required. Since the 1970s, there have been many attempts

Fig. 2. Output–input characteristics of the hot-pressed ceramic Cr2+:ZnSelasers operating in (A) gain-switched and (B) CW regimes for 10% (i) and 5%(ii) output couplers.

to build multiwavelength and ultrabroadband laser sources. Allthese attempts are based on the idea to suppress mode compe-tition in the cavity, which is responsible for output spectrumnarrowing.

There are two possible ways for implementation of this idea:mode separation in temporal or spatial domains (see review [51]and references herein). Cr:ZnSe crystals featuring ultrabroad-band gain bandwidth are ideal candidates for ultrabroadband andmultiline lasing in spatially dispersive cavities. First demonstra-tion of a CW, multiwavelength, ultrabroadband, broadly tunablemid-IR (2200–2800 nm) laser source based on 1.55-µm Er-fiberlaser pumped, polycrystalline Cr2+:ZnSe gain medium, utilizedin a Littrow-mounted grating spatially dispersive cavity wasdone in [27], [52].

A schematic diagram of the experimental setup is shown inFig. 3. The multiwavelength, ultrabroadband laser cavity con-sists of four major elements: a broadband plane input mirror, alaser crystal, an intracavity focusing lens, and a diffraction grat-ing, installed in the Littrow mount configuration. The intracavitylens and the diffraction grating provide a spatial dispersion ofdifferent frequency components of the laser gain profile in theactive medium, thus, enforcing each frequency to be amplifiedin its own region of the laser crystal. As a result, the mode com-petition, natural for the conventional lasers, is eliminated, andthe laser operates at many wavelengths simultaneously (see [52]for more details).


Fig. 3. Schematic diagrams of the Cr2+:ZnSe multiline laser.

Fig. 4. (A) Tuning of the dual-wavelength spectrum by simultaneous trans-verse motion of the pump beams across the Cr2+:ZnSe laser crystal. (B) Tuningof the wavelength spacing by changing the transverse distance between the pumpbeams.

The described spatially dispersive Cr2+:ZnSe laser can op-erate at many wavelengths simultaneously, producing any pre-assigned output spectral composition as well as a continuousultrabroadband spectrum within 2–3 µm spectral range.

In the dual-wavelength laser shown in Fig. 3, the pump radi-ation is split into two beams, and each of them pumps its ownregion of the laser crystal. The transverse positions of the beamsin the gain element, together with the parameters of the intra-cavity lens and diffraction grating, determine the lasing wave-lengths and provide a simple way of the laser tuning by meansof the transverse motion of the pump beams across the lasercrystal. The simultaneous tuning of the output wavelengths isshown in Fig. 4(A), where two wavelengths, separated by 50 nm,are tuned over a 600-nm spectral range. The individual tuningof the spectral lines in the dual-wavelength mode of operationis shown in Fig. 4(B). In this case, the wavelengths are tunedseparately such that the wavelength separation is consequentlychanged from 30 to 560 nm.

Multiwavelength ultrabroadband laser output can be obtainedby pumping the crystal with a single, highly elliptical, horizon-tally stretched pump beam. In this case, a broad continuousoutput spectrum is observed. An ultrabroadband operation ofthe laser, producing a continuous 135-nm wide spectrum cen-tered at 2500 nm and a 200-nm-wide (2400–2600 nm) multiline,tunable output spectra, consisting of up to 40 spectral lines wereachieved. A simultaneous tuning of a 20-lines ultrabroadbandspectrum over a spectral range of 2200–2800 nm was shown.The spectral tuning of the entire multiwavelength spectrum was

Fig. 5. Cr2+:ZnSe tunable, single-frequency laser based on Kogelnik/Littmancavity.

done by means of rotation of the diffraction grating in its disper-sion plane. In this configuration, approximately 20-line, 200-nmbroad spectrum tunable over a 600-nm spectral range wasachieved.

These lasers could find important practical application forfree-space optical communication, information coding, multi-analyte remote sensing, and numerous wavelength-specific mil-itary applications.

B. Tunable Single-Frequency Cr:ZnSe Laser

The first single-frequency Cr:ZnSe laser using a combinationof grating and two intracavity Fabry–Perot etalons was reportedin [25]. The laser produced∼10 mW of output power at 2460 nmand had a linewidth of less than 20 MHz.

In our experiments, an Er-fiber laser pumped single-frequencyCr:ZnSe laser with only one scanning element is reported. Thelaser design is very compact (the total optical length of thefolded laser cavity is 10 cm), as shown in Fig. 5, and is basedon a folded Kogelnik/Littman scheme utilizing two concavemirrors with R1 = 25 and R2 = 50 mm radii of curvature,600-g/mm gold-coated grating with reflectivity 50% at an angleof incidence 75◦, and thermodiffusion-doped Cr:ZnSe crystalabsorbing ∼70% of pump at 1.55 µm. Cr:ZnSe of the size4 × 8 × 1 mm3 was mounted at Brewster angle into a ther-moelectric cooling unit, providing temperature stabilization at20 ± 0.1◦C. The radiation of the single transverse mode linearlypolarized Er-fiber laser (IPG Photonics, ELM-10-LP) was fo-cused into the crystal by a 50-mm lens. The lasing wavelengthwas controlled with precision wavemeter EXFO WA-1500, andthe spectral structure and laser linewidth were monitored witha homemade scanning Fabry–Perot interferometer. The outputradiation had a linewidth of 120 MHz (measurement limit ofthe available Fabry–Perot interferometer) over a 120-nm tuningrange around 2.5 µm, and the maximum output power was upto 150 mW. The output spectrum of the laser is shown in Fig. 6.The tuning range of the laser in the single-frequency regime waslimited due to low diffraction efficiency of the grating at grazingangles.

Fig. 7 demonstrates preliminary results on practical utilizationof single-frequency Cr:ZnSe laser in Doppler-limited resolutionintracavity laser absorption spectroscopy of ro-vibrational tran-sitions of the v3 and v1 bands of H2O. The minimum detectable


Fig. 6. Interference pattern of the single longitudinal mode output spectrumfrom a 160-mm Fabry–Perot interferometer. The spectral resolution is limitedby the interferometer finesse of 7.7, which gives the upper limit on the laserlinewidth of 120 MHz.

Fig. 7. Weak ro-vibrational transitions of the v1 and v3 bands of water mea-sured with a single-frequency Cr:ZnSe laser at a scan speed of 4 µm/s.

absorption coefficient of ∼3 × 10−7 cm−1 has been achieved,which corresponds to nine parts per billion by volume of wa-ter vapor detection limit. It must be noted here that the outputwavelength of the laser was scanned by tilting the mirror aroundan axis lying in its plane with a piezo-driven homemade mirrorshaker with repetition rate of 220 Hz. Thus, the complete wave-length scan over about 9-nm range was performed in 1/440 s(2.3 ms). Simultaneous monitoring of the output laser spectrumwith a fixed-length Fabry–Perot interferometer showed that thelaser was operating in the single-mode regime during this entirewavelength scan, which allowed us to acquire the absorptionspectrum in Fig. 7.

C. Power Scaling of CW Cr:ZnSe Laser

Power scaling of the Cr:ZnSe laser output to multiwatt levelrequires a special geometry of the gain medium designed for themaximum suppression of thermal effects. In [14], utilization ofwater-cooled thick slab of ∼12-mm-long Cr:ZnSe resulted in18.5 W of output power under pulsed pumping with the 33 W ofTm:YALO laser radiation and ∼3 W at 7 W of pumping. This

record output power was achieved due to utilization of pumpduty cycle of ∼10−3, effective heat removal from the slab, andutilization of 1.94-µm pump radiation, enabling small quantumdefect in the gain medium. Another attractive approach wasdemonstrated in [53], where face-cooled disk design enabledpower scaling of Cr:ZnSe 1- and 0.5-mm disk laser outputs to4.2 W in 10-kHz repetition rate pulsed and 1.4-W CW regimes,respectively. It was revealed in [53] that for single face coolinggeometry, thermal lensing is proportional to disk thickness andabsorbed pump power, but disk temperature is proportional pri-marily to absorbed pump power and cannot be reduced by usingthinner disks. Attempts to reduce thermal lensing by reducingdisk thickness below 0.5 mm runs into a problem of insufficientpump absorption because utilization of heavily doped crystalsis not feasible due to high losses and increase in concentrationquenching [54].

To mitigate these problems, we utilize a different type of ge-ometry with the gain medium in the form of a thin and long par-allelepiped, which is pumped transversely and cooled throughtwo large polished faces. In this case, the Cr ions are concen-trated in a very thin (∼200µm) layer near one of the large crystalfacets and parallel to it. The crystal is wrapped into a 100-µmindium foil and fixed on a copper cold plate, which is cooled bywater or a thermoelectric (TEC) cooler. This method allows forvery efficient heat removal from the laser crystal and stronglyreduces the thermal lensing effects. With this approach, to ourknowledge, we have built, a multiwatt (2.7 W at 7.0 W pump),pure CW Cr2+:ZnSe laser for the first time, which is brieflydescribed in this section.

The laser is based on a standard Kogelnik cavity (as shownin Fig. 5, but with diffraction grating tuning mirror combina-tion substituted with a plane output coupler) with the sphericalmirrors of equal radii of curvature of 50 mm, and the planeoutput coupler placed at a distance of 120 mm from the foldingspherical mirror, resulting in ∼20-cm total cavity length. Thelaser is pumped by a single-mode Er-fiber laser (IPG Photonics,ELM-10-LP), which delivers the maximum of 7-W pump powerat 1.55 µm. The pump beam was focused into the Cr:ZnSe gaincrystal with a 80-mm lens resulting in a 50-µm pump beam spotsize to provide mode-matching with the calculated laser modebeam waist. The measured diameter of the output beam was∼0.8 mm and divergence was ∼0.5 mrad. In this series of laserexperiments, we have tested ten Cr:ZnSe samples with the sizesof 10 × 10 × 1.5 mm3 (Cr layer was parallel to 10× 10 mm2

surface). The average coefficient of absorption at 1.55 µm alongthe optimal path in the crystal was about 2.0 cm−1 (measuredwith the pump laser at low powers). In these experiments, weutilized two output couplers with 50% and 18% reflectivities.

All tested Cr:ZnSe active elements demonstrated same outputcharacteristics within 20% margin. The input–output character-istics for the best sample for the two output couplers are shownin Fig. 8. This pure CW 2.5-µm laser delivers up to 2.7 Wof output power at 7-W pump, which corresponds to 39% realefficiency. We believe that the laser performance can furtherbe significantly increased by using single crystalline ZnSe hostmaterial.


Fig. 8. CW Cr:ZnSe laser output power as a function of incident pump power.This is the highest power (2.7 W) demonstrated today for Cr:ZnSe CW laser.

IV. FE-DOPED MID-IR LASERS

Mid-IR transitions in the Fe2+:ZnSe crystal have multi-phonon quenching at RT. This has prevented RT lasing usingFe:ZnSe as a gain medium. The first tunable lasing of a Fe:ZnSecrystal in 3.98–4.5 µm spectral range was demonstrated in [7]for temperatures ranging from 15 to 180 K. After this pioneer-ing work, where laser effect was realized with output energy of5 µJ at 150 K, a much more efficient lasing of Fe:ZnSe has beenachieved in [35], [36] in the 85–255 K temperature range. For733-mJ pump energy of Er:YAG laser operating at 2.94 µm infree-running regime, the maximum output energy of Fe:ZnSelaser at 85 K reached 187 mJ. The maximum slope efficiency ofthe laser in nonselective cavity was 43%. The output spectrumof the Fe2+:ZnSe laser at T = 85 K was continuously tunedbetween 3.77 and 4.40 µm in a dispersive resonator with a CaF2

prism.High output energy of the Er:YAG-pumped Fe:ZnSe laser

was achieved in [36], [37] at 255 K when the ZnSe crystal wascooled with a two-stage thermoelectric unit. The output energyreached 142 mJ with 21% efficiency with respect to the incidentpump energy.

Fe:ZnSe has a large emission cross section σem = 2 − 3 ×10−18 cm2 at 4.4 µm, however, the luminescence lifetime ofFe:ZnSe at RT (370 ± 25 ns [37]) is too short for effectiveCW pumping but still longer than the typical pulse duration ofQ-switched lasers (1–100 ns). Hence, there is a good chanceto obtain RT lasing of Fe:ZnSe in a gain-switched regime withpump pulses shorter than 300 ns. The first RT gain-switched las-ing of Fe:ZnSe crystal at 4.4 µm in microchip configuration aswell as in a nonselective cavity under 2.92-µm excitation with apulse duration of 5 ns was demonstrated in [38]. The thicknessof crystal plate was 2 mm. The Fe ion absorption coefficient was10 cm−1. As a pump source for laser experiments, we used thesecond Stokes output of the D2 Raman-shifted Nd:YAG laserradiation with a pulse duration of ∼5 ns. The pump radiation at2.92 µm with a beam diameter of 1.5 mm excited the Fe:ZnSecrystal at the Brewster angle of incidence. The crystal tempera-ture remained at 300 K during all the laser measurements.

Selective cavity experiments were also performed in a Littrowmount configuration, and tunable oscillation of the Fe:ZnSecrystal over 3.9–4.8 µm spectral range was demonstrated atRT. The maximum output energy was of the order of severalmicro-Joules. This level of output energy is sufficient for injec-tion seeding of optical parametric oscillators (OPO) and opti-cal parametric amplifiers (OPA). First parametric amplificationof 4.4-µm microchip gain-switched Fe:ZnSe laser radiation inZnGeP2(ZGP)-based OPA was reported in [56]. The secondStokes 2.92-µm radiation, which had pump radiation of∼16 mJ,was split 50/50 to provide pumping of Fe:ZnSe microchip laserand ZGP optical parameter generator operation at 4.4 µm. Thesingle-pass amplification of the Fe:ZnSe microchip oscillationat 4.4 µm with factor of 2 was achieved with 2.92-µm pumpintensity of 20 MW/cm2 in 1-cm-long ZGP crystal.

Output energy and efficiency of Fe:ZnSe lasing at RT ingain-switched regime were further improved in [37], [70] whereoutput energy reached 0.4 mJ at 4.4 um with 20% quantumefficiency of lasing with respect to pump energy. The outputspectrum of the dispersive Fe2+:ZnSe laser was continuouslytuned in the spectral range of 3.95–5.05 µm.

Further improvements of Fe2+ lasing at RT require improve-ments in fabrication of high optical density ZnSe gain media aswell as for searching new low-phonon-energy hosts.

V. FUTURE ELECTRICALLY PUMPED TM:II–VIMID-IR LASERS

In addition to being wideband semiconductors, effective RTmid-IR lasing TM-doped II–VI media, also hold potential fordirect electrical excitation. The initial steps toward achievingthis goal were performed in [57], [58] by studying Cr2+ ionexcitation into the upper laser state 5E via direct electrical exci-tation as well as excitation and lasing via photoionization tran-sitions. It was also shown that molecular beam epitaxy (MBE)and PLD provide optically active chromium and iron in differ-ent chalcogenides and represent a viable route for fabricationof future optically, and possibly, electrically pumped waveg-uide laser structures, broadly tunable in the mid-IR spectralregion [59]–[63].

In [57], two promising roots for achieving TM:II–VI mid-IRlasing under electrical excitation were envisioned: 1) utiliza-tion of TM-doped quantum well II–VI heterostructures and 2)utilization of thin films based on TM-doped nanocrystallinequantum dots (NCD), or NCD dispersed in a conducive matrix.In the following section, we will describe our first steps towardfabrication and characterization of TM-doped NCD.

A. Why TM:II–VI NCD Are so Special for Mid-IR?

There are numerous publications related to preparation, lu-minescence properties, and potential applications of Mn2+:II–VI nanoparticles. The interest to this phosphor was stimulatedby Bhargava [64], whose most fundamentally interesting resultwas luminescence enhancement resulting from efficient energytransfer from the ZnS nanocrystals to Mn2+ ions facilitatedby mixed electronic states. Regarding Cr- and Fe-doped II–VInanocrystals, their photophysical properties have not yet been


addressed. It is believed that similar to Mn-doped ZnS nanocrys-tals, one can expect an efficient energy transfer from the low-dimensional II–VI structures to Cr2+ and Fe2+. Fast energytransfer from the low-dimensional host to Cr2+ and Fe2+ canbe qualitatively explained by the increase of the exciton oscilla-tor strength bound to the impurity center. First of all, quantumsize confinement should increase the oscillator strength of thefree exciton due to an increase of the electron–hole overlap fac-tor. Secondly, the oscillator strength of the exciton bounded tothe impurity center depends on the oscillator strength of the freeexciton and electron–hole exchange interaction term, which isalso supposed to be large due to the carrier’s confinement. Thus,one may expect a large enhancement of the oscillator strength ofthe exciton bound to the impurity embedded in nanostructuredmaterials with respect to bulk hosts. Another important issuethat was successfully proven by Tanaka [65] for Mn-doped II–VI nanocrystals relates to remarkable differences in thermalquenching of TM d-d photoluminescence in nanocrystals andbulk materials. First of all, the density of states for both elec-tron and phonons decreases with size. This is likely to resultin weaker electron–phonon coupling opening a pathway for de-velopment of a mid-IR light-emitting nanostructured materialwith high RT quantum efficiency of photoluminescence underintrashell IR optical excitation. Hence, Fe2+-doped nanostruc-tured materials might efficiently lase at RT. On the other hand,the increased overlap between the electron and hole wave func-tions decreases the exciton–phonon coupling. Analogously toMn in ZnS, such nanostructured materials are supposed to pro-vide much weaker thermal quenching of Fe2+ in nanocrystalsthan that of the bulk crystals. This analysis provides a back-ground for the remarkable differences that could be expected inexcitation of Cr2+ and Fe2+ ions in nanostructures with respectto the bulk crystals, and thus, the motivation for studies of thesenanostructures.

B. Fabrication of TM:II–VI NCD

The majority of methods of metal and semiconductornanoparticles synthesis is based on chemical approaches inwhich a variety of functional groups are used to stabilizenanoparticles and to serve as capping agents. The variousbyproducts of a chemical reaction may also interact withnanoparticles and are difficult to remove from the surface ofthe nanoparticles. An entirely physical approach of fabrica-tion of metal and semiconductor nanoparticles, such as laserablation, enables synthesis of pure uncapped nanoparticles. Re-cently, laser ablation was used to prepare various semiconductornanoparticles in liquid environment [66], [67]. However, forma-tion of semiconductor nanoparticles doped with TM ions underlaser ablation was not reported so far to the best of our knowl-edge. Here, we report the fabrication of II–VI NCD doped withTM (Cr, Co and Fe) ions by laser ablation method.

TM-doped ZnSe and ZnS nanoparticles were prepared usingmultistage process. Initially, polycrystalline ZnSe and ZnS sam-ples were grown using chemical vapor deposition (CVD) andwere TM (Cr, Co, Fe) doped by postgrowth thermal diffusion.During the thermodiffusion, the ZnSe and ZnS samples were

annealed in sealed evacuated ampoules with CrSe (CoSe, FeSe)or CrS powder for seven days at 950 ◦C. The average TM ionsconcentration was 1018–1019 cm−3.

At the first stage of NCD preparation, bulk polycrystallineTM-doped ZnS and ZnSe samples were ablated in pure distilledand deionized water by the fundamental harmonic radiation ofNd:YAG laser (EXPLA PL2143) with pulse duration of 30 ps,repetition rate of 10 Hz, and pulse energy of 10 mJ. Laserradiation was slightly focused on the surface of the bulk samplesto the spot size of 2 mm. The formation of nanoparticles wasobservable due to creation of colloidal suspension and colorationof the solution.

At the second stage of preparation, ZnS or ZnSe nanoparticlessuspension was sonicated in order to break large aggregates. Thesuspension was further irradiated by the radiation of the thirdharmonic (355 nm) of the Nd:YAG laser (EXPLA PL2143) withpulse duration of 30 ps, repetition rate of 10 Hz, and pulse energyof 15 mJ. The laser beam with 1-cm diameter was directed intothe optical cell without focusing. Initially, blurry nanoparticlessuspension became clear after 10 min of irradiation.

For mid-IR spectroscopic characterization, precipitatednanoparticles were extracted from aqueous solution, washedwith distilled-deionizer water and dried naturally under ambi-ent condition.

C. XRD Characterization of TM:II-VI NCD

The NCD samples were investigated using XRD to deter-mine the crystalline grain size range. This XRD measure-ments were performed using (θ − 2θ) angle XRD (Philips X-Pert MPD) with a Cu Kα anode. Spectra were taken from20◦ to 60◦ for 2θ. The XRD pattern shown in Fig. 9(A-a)corresponds to the initial Cr-doped polycrystalline ZnS sam-ple used for ZnS-1,2-naphthoquinone diazide (NQD) prepa-ration. The most prominent diffraction peaks located at 2θ =28.6◦, 47.5◦ and 56.4◦ coincide with (1 1 1), (2 0 0), (2 2 0),and (3 1 1) reflections of the zinc-blend structure. Cr:ZnSnanocrystals obtained after the first stage of laser irradiationexhibit XRD pattern [Fig. 9(A-b)] with several additional peakscharacteristic to the wurtzite ZnS (1 0 0), (1 0 1), (1 0 2), (1 1 0),and (1 0 3). The other peaks of the wurtzite structure (0 0 2),(1 1 0), and (1 1 2) overlap with main peaks of the zinc-blendstructure (1 1 1), (2 2 0), and (3 1 1). These results indicate thatafter the first stage of nanoparticles preparation, they have mixedwurtzite and zink-blend structure. From the width of the XRDpeak, mean crystalline size can be calculated using Scherrer’sequation: D = kλ/B cos(2θ), where λ is X-ray wavelength (forCu Kα, λ = 0.154 nm), 2θ is the diffraction angle, B is the mea-sured FWHM of the XRD peak, and k = 0.9. XRD pattern [seeFig. 9(A-c)] of ZnS NQD prepared after the second stage oflaser UV treatment exhibits significant peak broadening. Afterthe first and second stages of preparation, the mean crystal sizesof doped ZnS nanoparticles were estimated to be 11 and 3 nm,respectively.

Similar results were obtained in the case of ZnSe nanocrystalsprepared by 1064-nm laser ablation. Polycrystalline ZnSe target[see Fig. 9(B-a)] has XRD peaks at 2θ = 27.2◦, 45.1◦, and 53.4◦


Fig. 9. XRD diffraction pattern of Cr2+-doped (A) ZnS and (B) ZnSe NCDsafter first (b) and second (c) stages of preparations. The XRD patterns of the bulkZnS and ZnSe used polycrystals (a) are shown for comparison at the bottom.

corresponding to (1 1 1), (2 2 0), and (3 1 1) planes reflectionof cubic ZnSe. As shown in Fig. 9(B-b), ZnSe nanocrystals hadpeaks corresponding to wurtzite structure and peaks width wasbroadened due to the formation of relatively small nanocrys-tals. An average nanocrystal size after the first stage of fabri-cation was estimated to be ∼13 nm. It is worth noting that wehave not observed any additional peaks associated with ZnOor other impurities that could be produced at the stage of laserablation.

D. Optical Characterization of TM:II–VI NCD

After the first stage of preparation, absorption spectrum ofthe ZnS NQD reveals two minor absorption shoulders peakingat 295 and near 350 nm. After the second stage, the absorptionpeak at 350 nm disappears while the first peak is slightly blue-shifted to 290 nm. We assume that this band is associated withexcitonic absorption of ZnS NQD. The blue-shift of absorptionband indicates strong quantum size effect. The sizes of the NCDcould be estimated using the approximation equation [68] andis approximately equal to 3 nm, which is quite consistent withthe XRD analysis.

Spectroscopic characteristics of Cr- and Co-doped NCDs inmid-IR spectral region were studied under 1.6-µm laser excita-tion. RT fluorescence spectra of Cr:ZnS and Cr:ZnSe NCDs areshown in the Figs. 10(A) and 11(A). The luminescence spectraof bulk crystals obtained at the same experimental conditionsare shown on the same graphs for comparison. Luminescence

Fig. 10. (A) RT luminescence spectra of Cr-doped ZnS NCD (i) and bulk (ii)samples. (B) Kinetics of fluorescence of the chromium-doped ZnS NCD dotsmeasured at RT (iii) and 77 K (iv).

Fig. 11. (A) RT luminescence spectra of Cr-doped ZnSe NCD (i) and bulk (ii)samples. (B) Kinetics of fluorescence of the chromium-doped ZnSe NCD dotsmeasured at RT (iii) and 77 K (iv).

bands of ZnS and ZnSe NCDs were blue-shifted compared tothe bulk crystals as shown in the Figs. 10(A) and 11(A).

There are several possible explanations of this phenomenon.One of the most probable reasons affecting the shape of lumi-nescence spectrum is absorption of OH groups that possiblypresent on the surface of nanoparticles. A strong NCD lumines-cence signal suppression of wavelengths longer than 2600 nmcomparing with bulk crystals strongly supports this hypothesis.

Another fact that further supports influence of OH groupabsorption is that the luminescence maximum shift is moreprominent in ZnSe nanoparticles since luminescence band ofbulk ZnSe is located closer to the absorption band of OH group.Another cause of luminescence shift could be alteration of thestrength of the crystal field in nanoparticles.

Luminescence kinetics of ZnS and ZnSe NCDs at RT and 77 Kare given in Figs. 10(B) and 11(B), respectively. ZnS nanopar-ticles luminescence lifetime was smaller, down to 2.6 µs,


Fig. 12. (A) RT luminescence spectra of Co-doped ZnSe bulk (ii) and NCD(i) samples. (B) Kinetics of fluorescence of the Co-doped ZnSe NCDs measuredat RT (iii) and bulk sample measured at T = 77 K (iv).

comparing with 6 µs, which is typical to the bulk crystals. How-ever, at 77 K, NQD luminescence lifetime was 5 µs, which isclose to one in bulk sample. Thus, we conclude that the ma-jor factor responsible for shorter Cr2+ excited state lifetime innanoparticles is increased probability of nonradiative relaxation.

We have also observed shorter Cr2+ excited state lifetime inthe Cr:ZnSe NCDs.

Absorption spectra of Co2+ (d7) doped II–VI semiconduc-tors in mid-IR region have two bands associated with transi-tions 4A2 → 4T1 (∼1500 nm) and 4A2 → 4T2 (∼3000 nm) [1].4A2 → 4T2 transition is relatively weak since it is symmetry-forbidden [1]. Luminescence spectra of 4A2 → 4T2 transi-tion in Co:ZnSe quantum dots and bulk crystal excited to4A2 → 4T1 (∼1600 nm) at RT are shown in Fig. 12(A). Itcan be seen that Co2+ luminescence is strongly suppressed in2800-nm region, which is another confirmation of OH groupabsorption influence to the TM luminescence in this region.The lifetime of the excited 4T2 state at RT in bulk crystal was∼200 µs. Cooling of the sample down to 77 K led to increase inthe excited state lifetime to 1.1 ms [see Fig. 12(B)], which is ingood agreement with previously reported values [1]. We havenot observed buildup of luminescence due to population of 4T2

state, which is the evidence of fast excitation relaxation from4T1 to 4T2 states. Luminescence kinetics of Co:ZnSe nanopar-ticles at RT [see Fig. 12(B)] were similar to kinetics in bulkcrystal, and the decay time was 220 µs. Thus, we conclude thatin Co ions doped samples there was no increase of nonradiativerelaxations.

VI. CONCLUSION

There is a significant progress in the development of RT mid-IR lasers based on Cr2+-doped chalcogenides (mainly Cr2+:ZnSe and ZnS). These lasers have already become sources ofchoice for those who need a compact CW system with contin-uous tunability over 2–3.1 µm, output powers up to 2.7 W, and

high (up to 70%) conversion efficiency. This paper has attemptedto focus on specific regimes of operation of these lasers.

The unique blend of ultrabroadband gain bandwidth, highστ product, and high-absorption coefficients make these ma-terials ideal candidates for microchip regime of operation.Future improvements in technology of hot-pressed ceramicCr:ZnSe and ZnS materials could stimulate further cost reduc-tion of fabrication process and scale up the output power andenergy.

In addition, Cr2+-doped chalcogenide crystals featuringultrabroadband gain bandwidth are ideal candidates for ultra-broadband and multiline lasing in spatially dispersive cavities.These lasers can operate at many wavelengths simultaneously,producing any preassigned output spectral composition aswell as a continuous ultrabroadband spectrum within 2–3 µmspectral range.

This paper reviews the nontraditional Cr-doped mid-IR lasersas well as describes emerging Fe2+:ZnSe lasers having poten-tial to operate at RT over an extended spectral range of 3.7–5.1µm. Recent progress in Fe2+:ZnSe materials demonstrates thatlasers on their basis can operate in gain-switched regime at RTwith efficiencies of dozens of percents generating output en-ergies from milli-Joule level at RT till hundreds of milli-Joulesunder thermoelectric cooling of the crystal. Future progressin developing pure CW Fe2+ lasers depends on the successof searching new, less-quenched bulk materials. Utilization oflow-dimensional Fe2+:ZnSe and ZnS structures with reducedphonon density of states could suppress thermal quenchingof Fe2+ ions and make possible RT lasing over 3–5 µmspectral range under CW optical excitation.

In addition to being wideband semiconductors, effective RTmid-IR lasing TM-doped II-VI media, hold the potential fordirect electrical excitation. This paper shows the initial stepstoward achieving this goal by studying Cr2+ and Co dopedquantum dots. We have demonstrated a novel method of TM-doped II–VI NCD fabrication based on laser ablation in liquidenvironment. This technique has considerable advantage overchemical synthesis of doped II–VI NCDs due to possibilityof doping nanocrystals with a variety of TM ions using laserablation of thermodiffusion-doped polycrystalline II–VI targets.To our knowledge, TM-doped II–VI NCD demonstrated strongmid-IR luminescence for the first time. It opens a new pathwayfor future optically and electrically pumped mid-IR lasers basedon TM-doped quantum confined structures.

Recently, we were able to demonstrate power scaling of CWCr:ZnSe laser output up to 6W at 48% real efficiency. Theoutput power was limited only by the available pump power ofthe Er-fiber laser.

ACKNOWLEDGMENT

The authors would like to thank A. Gallian and C. Kimfor helping with experiments. They would also like to thanktheir colleagues and collaborators V. Gapontsev, D. Gapontsev,N. Platonov (IPG Photonics Corporation), V. Badikov andD. Badikov (Kuban State University, Russia), E. Dianov andA. Zabezhailov (General Physics Institute, Russian Academyof Sciences), I. Kazakov, M. P. Frolov, Y. V. Korostelin,


V. I. Kozlovsky, A. I. Landman, Y. P. Podmar’kov, V. A. Akimov,A. A. Voronov (P. N. Lebedev Physics Institute, RussianAcademy of Sciences), and I. Sorokina and E. Sorokin (ViennaUniversity of Technology).

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Sergey B. Mirov was born in Moscow, Russia, onDecember 4, 1955. He received the M.S. degree inelectronic engineering from the Moscow Power En-gineering Institute, Technical University, Moscow,in 1978, and the Ph.D. degree in physics in 1983from the P. N. Lebedev Physical Institute, RussianAcademy of Sciences, Moscow.

He was a Staff Research Physicist at P. N. LebedevPhysical Institute. He was a Principal Research Asso-ciate and a Group Leader at the General Physics In-stitute, Russian Academy of Sciences, where he was

engaged in the physics of color centers formation under ionizing irradiation,photochemistry of the color center, laser spectroscopy of solids, developmentof the first room temperature operable commercial color center lasers, passiveQ-switches, and nonlinear filters for various types of neodymium lasers fromminilasers to powerful laser glass systems. Since 1993, he has been a FacultyMember in the Department of Physics, University of Alabama at Birming-ham (UAB), Birmingham, where he is currently a Professor of Physics andthe Co-Director of the Center for Optical Sensors and Spectroscopies. His cur-rent research interests include tunable solid state lasers, laser spectroscopy, andquantum electronics. He has authored or coauthored over 290 scientific publica-tions in the field of quantum electronics. He has published one book and severalbook chapters. He is the holder of thirteen patents.

Dr. Mirov received the USSR First National Prize for Young Scientists in1982 for the development of LiF color center saturable absorbers. He receivedthe Distinguished Research Award from the General Physics Institute in 1985and 1989, and from the P N. Lebedev Physical Institute in 1980. He receivedthe Snell Premium Award for the input in optoelectronics and development ofCr2+:ZnS mid-IR external cavity and microchip lasers from the Institute ofElectrical Engineers, London, U.K., in 2004. He is a member of the Optical So-ciety of America, the American Physics Society, and the International Societyfor Optical Engineering.

Vladimir V. Fedorov was born in Moscow,Russia, in 1961. He received the M.S. degree inphysical and quantum electronics from the MoscowInstitute of Physics and Technology, Dolgoprudny,Russia, in 1985, and the Ph.D. degree in physics fromthe General Physics Institute, Russian Academy ofSciences, Moscow.

He joined the General Physics Institute, RussianAcademy of Sciences, as a Research Fellow in 1987.His current research interests include coherent andlaser spectroscopy of doped solids, nonlinear optics,

color center physics, solid state lasers, and laser media based on semiconductormaterials with transition metals impurities. Since 2000, he has a been a ResearchAssociate in the Department of Physics, University of Alabama at Birmingham(UAB), Birmingham.

Dr. Fedorov received the Snell Premium Award from the Institute of Elec-trical Engineers, London, U.K., in 2004. He is a member of the Optical Societyof America and the International Society for Optical Engineering.


Igor S. Moskalev was born in Belarus, in 1973.He received the Bachelor’s degree in physics fromNovosibirsk State University, Novosibirsk, Russia, in1986, and the M.S. and Ph.D. degrees in physics fromthe University of Alabama at Birmingham (UAB),Birmingham, in 2002 and 2004, respectively.

From 1986 to 1998, he was a Research Assistantin the Institute of Laser Physics, Novosibirsk, wherehe was engaged in the development of a highly stable,single-frequency, diode-pumped Nd:YAG laser withintracavity frequency doubling. From 1998 to 1999,

he was a Research Assistant in Hong Kong University of Science and Tech-nology, where he studied algal motility using a modified laser particle imagevelocimetry (PIV) system. He joined the Department of Physics, University ofAlabama at Birmingham (UAB), Birmingham, in 2000, where he is engaged inthe development of novel laser systems based on semiconductor materials withtransition metals impurities.

Dr. Moskalev received the Snell Premium Award for the development ofCr2+:ZnS mid-IR external cavity and microchip lasers from the Institute ofElectrical Engineers, London, U.K., in 2004. He is a member of the OpticalSociety of America.

Dmitri V. Martyshkin was born in Russia in 1973.He received the Bachelor’s degree from NovosibirskState University, Novosibirsk, Russia, in 1996, andthe Master’s and Ph.D. degrees in physics from theUniversity of Alabama at Birmingham, Birmingham,in 2000 and 2004, respectively.

He is currently with the Center for Optical Sensorsand Spectroscopies, Department of Physics, Univer-sity of Alabama at Birmingham. His current researchinterests include application of laser spectroscopysuch as Raman, fluorescence, surface-enhanced Ra-

man spectroscopy (SERS), laser-induced breakdown spectroscopy (LIBS), cav-ity ring-down spectroscopy (CRDS) for biomedical applications, and devel-opment and characterization of novel materials designed for construction ofphotonic devices as well as for biomedical application.

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Recent Progress in Transition-Metal-Doped II–VI Mid-IR Lasers

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