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This paper summarizes recent improvements of output characteristics of polycrystalline Cr:ZnS/Se master oscillators in Kerr-Lens-Mode-Locked regime. We developed a flexible design of femtosecond polycrystalline Cr:ZnS and Cr:ZnSe lasers and amplifiers in the spectral range 2–3 μm. We obtained few-optical-cycle pulses with multi-Watt average power in very broad range of repetition rates 0.08–1.2 GHz. We also report on efficient nonlinear frequency conversion directly in the polycrystalline gain elements of ultra-fast lasers and amplifiers. In this work we also report on recent progress in spinning ring gain element technology and report to the best of our knowledge the highest output power of 9.2 W Fe:ZnSe laser operating in CW regime at 4150nm.

We report on CW, acousto-optically tuned Cr:ZnS mid-IR laser operating over 1890 - 2785 nm spectral range with 40% slope efficiency with respect to the pump power in the maximum of tuning curve. The tuning range over 1890 - 2785 nm was demonstrated by tuning the radio frequency (RF) applied to a TeO 2 acousto-optical filter between 30.3 MHz and 20.12 MHz. A maximum output power of 2.8 W was achieved at 2.23 μm under 10.7 W of pump power. The linewidth of the Cr:ZnS laser measured at central wavelength was < 0.8 nm.

Degenerate (subharmonic) optical parametric oscillators (OPO) show great promise for the generation of broadband mid-infrared (MIR) frequency combs. Their main features are low pump threshold, dramatic extension of the spectrum of the pump laser, and phase locking to the pump frequency comb. Here we report on obtaining instantaneous spectrum ranging from 2.85 to 8.40 μm at -40 dB level from a subharmonic OPO pumped by an ultrafast Cr2+:ZnS laser. Our experimental setup includes a free running Kerr lens mode locked 2.35 μm Cr2+:ZnS laser, with 62-fs time-bandwidth limited pulse duration, 630-mW average power, and 79 MHz repetition rate that synchronously pumps a ring-cavity orientation-patterned (OP-GaAs) based OPO. A 0.5-mm-long OP-GaAs crystal has a quasi-phase-matching (QPM) period of 88 μm and is designed to provide a broadband parametric gain at OPO degeneracy. A 0.3-mm-thick ZnSe wedge inside the cavity was used to minimize group velocity dispersion. Spectral span of 1.56 octaves in the MIR that we achieved can be further improved by fabricating an in-coupling dielectric mirror with (i) broader reflectivity range and (ii) with compensation of the residual group velocity dispersion. The broad spectrum achieved, 2.85 - 8.40 μm (2320 cm-1 wide instantaneous span), overlaps with a plethora of fundamental molecular IR resonances and can be used for frequency comb spectroscopic detection applied to such fields as remote sensing, study of fast combustion dynamics and medical diagnostics, to name a few.

Temperature influence on spectroscopic characteristics is crucial for many aspects of laser engineering including output noise, single frequency oscillation, and thermal bistability. We report on the spectroscopic characterization of chromium and iron doped ZnSe gain element media at temperatures ranging from 77K to 389K. Heating of Cr:ZnSe resulted in the absorption peak shifting to a shorter wavelength from 1.806 μm at 77K to 1.753 μm at 389K. It also resulted in broadening of the absorption band from Δλ=260 cm-1nm to Δλ=373 cm-1nm and decreasing of the absorption cross section by 69%. Similar characterization was done for Fe:ZnSe laser material. The cooling of the Fe:ZnSe crystal from room temperature to 77K resulted in a 32% increase of the absorption coefficient at 2.94 μm which is usually used as a pump source. We also studied the absorption of the electrical free-carriers in n-type Al:ZnSe crystals in visible and mid- IR absorption spectral ranges. Diffusion of Al into ZnSe samples was achieved by annealing at 1000°C during 4 days in Al vapors. It was demonstrated that free-carriers absorption of Al:ZnSe samples with resistivity σ=100-150 Ω×cm resulted in an increase of the absorption coefficient at 2.4 μm up to 2.5 cm-1.

Many organic molecules have strong and narrow absorption features in the middle Infrared (mid-IR) spectral range. The ability to directly probe absorption features of molecules enables numerous mid-IR applications in non-invasive medical diagnosis, industrial processing and process control, environmental monitoring, etc. Thus, there is a strong demand for lasers operating in mid-IR spectral range. Transition metal (TM) doped II-VI semiconductors such as Fe/Cr:ZnSe/S are the material of choice for fabrication of mid-IR gain media due to favorable combination of properties: a four level energy structure, absence of excited state absorption , broad mid-IR vibronic absorption and emission bands. Despite the significant progress in post-growth thermal diffusion technology of TM:II-VI fabrication there are still some difficulties associated with diffusion of certain TM’s in these materials. In this work we address the issue of poor diffusion of Fe in ZnSe/S polycrystals. It is well known that with the temperature increase the diffusion rate of impurity also increases. However, simple application of high temperatures during the diffusion process is problematic for ZnSe/S crystals due to their strong sublimation. The sublimation processes can be suppressed by application of high pressures. Hot isostatic pressing was utilized as the means for simultaneous application of high temperatures (1300°C) and high pressures (1000atm, 3000atm). It was determined that diffusion coefficient of Fe was improved 13 and 14 fold in ZnSe and ZnS, respectively, as compared to the standard diffusion at 950°C. The difference in diffusion coefficients can be due to strong increase in the grain size of polycrystals.

This paper summarizes recent improvements of output characteristics of polycrystalline Cr:ZnS/Se master oscillators in Kerr-Lens-Mode-Locked regime: 1.9 W average power at 41 fs pulse duration, 24 nJ pulse energy and 515 kW peak power with efficiency of 19% with regards to 1567 nm pump power from linearly polarized Er-fiber laser. A simple design of mid-IR fs Cr:ZnS MOPA enabled power scaling to 6.8 W at 79 MHz repetition rate. This was accompanied by a 2 fold spectral broadening to 600 nm at -10 dB level, pulse compression from 44 to

Progress in fabrication and mid-IR lasing of Cr and Fe thermal-diffusion and radiation enhanced thermal diffusion doped II-VI binary and ternary polycrystals is reported. We demonstrate novel design of mid-IR Fe:ZnSe and Cr:ZnSe/S solid state lasers with significant improvement of output average power up to 35W@4.1 μm and 57W@2.5 μm and 20W@2.94 μm. We report significantly improved output characteristics of polycrystalline Cr:ZnS/Se lasers in gain-switched regime: 16 mJ at 200 Hz, pulse duration 5 ns with tunability over 2400-3000 nm as well as Kerr-Lens-Mode-Locked regime in terms of average power (up to 2 W), peak power and pulse energy (0.5 MW and 24 nJ, respectively), and pulse duration (less than 29 fs).

We report a novel design of CW Cr 2+ :ZnS/ZnSe laser systems and demonstrate record output powers of 27.5 W at 2.45 μm and 13.9 W at 2.94 μm with slope efficiencies of 63.7% and 37.4%, respectively. Power scaling of ultra-fast Cr 2+ :ZnS/ZnSe Kerr mode-locked lasers beyond 2 W level, as well as the shortest pulse duration of 29 fs, are also reported. New development of Fe:ZnSe laser with average output power > 35 W at 4.1 μm output wavelength and 100 Hz pulse repetition rate (PRR) was achieved in a nonselective cavity. With intracavity prim selector, wavelength tunability of 3.88-4.17 μm was obtained with maximum average output power of 23 W. We also report new results on Tm-fiber pumped passively and actively Q-switched Ho:YAG laser systems. High peak power actively Q-switched Ho:YAG laser demonstrates stable operation with pulse energy > 50 mJ, 12 ns pulse duration, and 100-1000 Hz PRR which correspondents to more than 4 MW peak power. The actively Q-switched Ho:YAG laser system optimized for high repetition rate delivers 40 W average output power at 10-100 kHz PRR. The Ho:YAG laser with passive Q-switcher demonstrates constant 5 mJ output energy from 200 Hz to 2.23 kHz PRR with optical slope efficiency with respect to Tm-fiber laser of ~43%.

Detailed spectroscopic characterization of Cr:ZnSe (Cr:ZnS) crystals under visible excitation into the charge transfer bands of Cr2+ impurities was performed. Middle infrared photoluminescence of Cr:ZnS under this excitation exhibits shorter rise time (~150ns) than that previously observed in Cr:ZnSe (~4-10 μs). As a result the quantum yield of Cr:ZnS mid-IR photoluminescence under 10ns pulsed 416nm excitation into the charge transfer band was estimated at close to 100%, which contrasts with low (~14%) quantum yield measured in Cr:ZnSe under 532nm pulsed excitation, indicating the possibility of efficient excitation of the upper laser level of Cr:ZnS using this mechanism. The rise time can be caused by cascade relaxation from higher lying levels through the 3T1 metastable level, producing luminescence in the near-IR. Measurements of the temperature dependence of the middle- and near infrared photoluminescence signals are reported. These values indicated that more efficient pumping of Cr:ZnSe under 532nm excitation can be achieved at temperatures greater than 300 K. Results of high temperature laser experiments supporting this idea are presented.

5E mi d-IR transitions of iron ions. In addition, samples of ZnSe were similarly sealed in evacuated quartz ampoules in the presence of Praseodymium metal and annealed at 950°C under 43.99 R/s and 0 R/s and the diffusion lengths and Pr concentrations were compar HG 7KH -irradiation results in better intrusion of the iron ions from the metal film and increase of the diffusion length at ~25%, while 3UDVHRG\PLXPGLIIXVLRQLVGUDPDWLFDOO\HQKDQFHGE\ -irradiation during the annealing process.

We report on spectroscopic characterization of laser active powders based on iron doped II-VI ternary and quaternary semiconductors for mid-IR laser applications. Iron doped Cd 1-x Mn x Te, Cd 1-x MnxS, Cd1-xMn x Se, Cd 0.5 Mn0.5S 0.5 Se 0.5 , Cd1-xZnxTe compounds with x=0.5-0.25, were prepared by using thermo diffusion technique. The starting binary powders were mixed in the appropriate molar ratios, sealed in evacuated (10-3 Torr) quartz ampoules, and annealed at 800-1000oC for several days. Samples composition, integrity, and grain size were characterized by micro-Raman and Xray diffraction and revealed a variation of the crystal field parameters depending on powder composition. Fe 2+ photoluminescence was characterized by spectral band position (normalized with respect to the detection platform spectral sensitivity) and lifetime at different temperatures, enabling calculation of the absorption and emission crosssections. Practical utility of the developed powders was demonstrated by a room temperature random lasing of iron doped Cd 0.5 Zn 0.5 Te powders over 5620-6020 nm spectral range pumped by a 2.94 μm radiation of a Q-switched Er:YAG laser. In summary, the following has been accomplished: (1) It was demonstrated that laser active Fe 2+ doped ternary and quaternary II-VI materials can be produced by simple annealing of the commercially available binary powders omitting expensive and complicated crystal growth processes; (2) It is possible to effectively shift PL of Fe 2+ in II-VI host materials towards shorter or longer wavelength by varying composition, type and amount of the second cation in ternary II-VI materials; (3) Major spectroscopic characteristics of Fe 2+ doped II-VI ternary and quaternary compounds were obtained and their practical utility for mid-IR lasing demonstrated.

Recent efforts have demonstrated efficient Cr2+ :II-VI chalcogenide (e.g. ZnSe, ZnS) broadly tunable (1.9-3.3μm) lasers under direct intra-shell Cr2+ optical excitation. We report on the spectroscopic study of Cr2++:ZnSe/ZnS under visible excitation into the charge transfer band of Cr2+ ions. Polycrystalline samples prepared by thermal diffusion method were studied. Middle-infrared (mid-IR) photo-luminescence (PL) of Cr2+ ions was compared under continuous wave (CW) direct 1532nm (5T2→5E) excitation and under 532nm excitation into charge transfer band. The quantum yield of Cr:ZnSe mid-IR luminescence under CW green excitation was estimated as close to 100% at room temperature. To estimate Cr excitation rate via charge transfer band under short pulse excitation, mid-IR PL kinetic measurements were performed with the use of 532nm picosecond and nanosecond pumping. Mid-IR PL kinetics of Cr:ZnSe under pulsed green excitation exhibit a relatively slow growth reaching a peak at ~5-10μs for nanosecond and picosecond excitations, respectively, while PL kinetics in Cr:ZnS reveal shorter measured rise time (

Cobalt doped II-VI wide band semiconductors (e.g. ZnSe, ZnS, CdSe) are promising media for infrared (IR) laser applications. They could be utilized as effective passive Q-switches for cavities of Alexandrite as well as Nd and Er lasers operating over 0.7-0.8, 1.3-1.6, and ~2.8 μm spectral ranges. We report spectroscopic characterization of Co:ZnSe and Co:ZnS crystals. Absorption cross-sections were measured for 4 A 2 (F) → 4 T 1 (P), 4 A 2 (F) → 4 T 1 (F), and 4 A 2 (F) → 4 T 2 (F) transitions with maximum absorption at 768(726), 1615(1500), 2690(2740) nm for ZnSe(ZnS) crystals, respectively. The calculated absorption cross-sections of the above transitions were estimated to be 64(56)×1019, 7.5(7.8)×1019, and 0.52(0.49)×1019 cm2 for ZnSe(ZnS) crystal hosts. In addition to the above applications the cobalt ions could be utilized for excitation of Fe 2+ ions via resonance energy transfer process. Tunable room temperature lasing of Fe 2+ doped binary and ternary chalcogenides has been successfully demonstrated over 3.5-6 μm spectral range. However, II-VI lasers based on Fe 2+ active ions don’t feature convenient commercially available pump sources (e.g. some Fe doped crystal hosts require pump wavelengths longer than 3 μm). Therefore, the process of energy transfer from Co 2+ to Fe 2+ ions could enable utilization of commercially available visible and near-infrared pump sources. We report a spectroscopic characterization of iron-cobalt co-doped ZnS and ZnSe crystals over 14-300K temperature range. Mid-IR laser oscillation at 3.9 μm(3.6 μm) via energy transfer in the Co:Fe:ZnSe (Co:Fe:ZnS) co-doped crystals was demonstrated under cobalt excitation at 4 A 2 (F) → 4 T 1 (P) (~0.7μm) and 4A2(F) → 4T1(F) (~1.56 μm) transitions.

Power scaling of mid-infrared laser systems based on chromium and iron doped zinc selenide (ZnSe) and zinc sulfide (ZnS) crystals is being advanced through the integration of surface relief anti-reflection microstructures (ARMs) etched directly in the facets of the laser gain media. In this study, a new ARMs texture fabrication process is demonstrated for polycrystalline ZnSe and ZnS material that results in a significant increase in pulsed laser damage resistance combined with an average reflection loss of less than 0.5% over the wavelength range of 1.9-3.0μm. The process was utilized to fabricate ARMs in chromium-doped zinc selenide (Cr 2+ :ZnSe) materials supplied by IPG Photonics and standardized pulsed laser induced damage threshold (LiDT) measurements at a wavelength of 2.09μm were made using the commercial testing services of Spica Technologies. It was found that the pulsed LiDT of ARMs etched in ZnSe and Cr 2+ :ZnSe can match or even exceed the level of a well-polished surface, a survivability that is many times higher than an equivalent performance broad-band thin-film AR coating. The results also indicate that the ARMs plasma etch process may find use as a post-polish damage mitigation technique similar to the chemical immersion used to double the damage resistance of fused silica optics. ARMs etched in Cr 2+ :ZnSe were also evaluated by IPG Photonics for survivability under continuous wave (CW) laser operation at a pump laser wavelength of 1.94μm. Catastrophic damage occurred between power levels of 400-500 kilowatt per square centimeter for both as polished and ARMs textured samples indicating no reduction in CW damage resistance attributable to surface effects.

Mid-IR lasers based on Cr 2+ and Fe 2+ ions in II-VI tetrahedral crystal hosts have been proven useful as tunable solid-state lasers operating over the 1.9-6 μm spectral range. However, the development of new effective gain media promising for lasing over 3-4 μm and with wavelengths longer than 6 μm is still under progress. In this paper we report spectroscopic characterization of Titanium doped AgGaS 2 and Iron doped MgAl 2 O 4 crystals with coordination number four. Polarized absorption and luminescence spectra of titanium doped AgGaS 2 crystals were measured at room and low temperatures. It was shown that Ti ions in the AgGaS 2 crystal were in both 1+ and 3+ valence states substituting silver and gallium sites, correspondingly. Ti 3+ ions in AgGaS 2 and Fe 2+ in MgAl 2 O 4 crystals feature broad absorption bands with a maximums near 2μm. These ions have no second excited state absorption bands and could potentially be used as effective saturable absorbers for passive Q-switching of 2.1 μm holmium laser cavities. Broad emission covering the 3-6 μm spectral range was observed under 1.9 μm excitation into Ti 3+ absorption band. A luminescence signal between 3 and 6 μm was also observed in the Fe:MgAl 2 O 4 crystal.

Currently, a majority of Volumetric Bragg gratings (VBG) use photorefractive glasses with a transmission band between 0.3 and 2.7 μm. We have proposed and realized VBGs for mid-IR spectral range based on LiF color center crystals (LiF:CC). γ-irradiated LiF:CC crystals feature strong absorption bands in the visible and near-IR spectral range, where selective color center photo-bleaching allows for the LiF refractive index modification. The absence of active absorption in LiF:CCs at wavelengths longer than 1.3 μm results in a VBG that is stable under mid-IR irradiation. Our calculations predict that ~60% diffraction efficiency over 1-6 μm spectral range could be realized in ~1 cm long VBG. To verify this calculation, we fabricated periodic structures in LiF:CC crystals with 24 and 12 μm spacings by CCs photo-bleaching using femtosecond Ti:sapphire laser pulses. Periodic structures exhibit diffraction in multiple orders in the Raman-Nath regime at 0.532, 0.632, and 1.56 μm. The first order diffraction efficiencies were stronger for a visible radiation due to a bigger refractive index variations and additional amplitude modulation. The demonstrated diffraction at 1.56 μm is a clear manifestation of a phase grating in LiF and serves as a proof of feasibility of these LiF:CC crystals for mid-IR VBG applications.

Room temperature iron doped II-VI lasers have demonstrated broad band tunability between 3.5 and 6 μm with efficiency ~40%. However, these lasers require pump sources with a wavelength ~3 μm which could be selected only from a few available. Cobalt ions in the II-VI materials have strong absorption bands at 4 A 2 → 4 T 1 ( 4 P) and 4 A 2 → 4 T 1 ( 4 F) transitions located at ~0.75 and 1.5 μm, respectively. A number of different laser sources (including diode lasers) could be potentially used for cobalt excitation followed by energy transfer to iron ions. Here we report materials fabrication and study of energy transfer in Co:Fe:ZnS(ZnSe) crystals. Iron-cobalt co-doped samples were prepared using a two-stage post-growth thermal diffusion procedure with Fe concentrations of 8.5-19x10 18 cm -3 . Kinetics and photoluminescence spectra reveal energy transfer under cobalt excitation at 4 A 2 - 4 T 1 ( 4 F) transition by 1.56 μm radiation and 4 A 2 - 4 T 1 ( 4 P) transition by 0.7 μm radiation. Analysis shows effective energy transfer from 4 T 1 ( 4 F), 4 T 2 and 4 T 1 ( 4 F) Co 2+ energy levels to 5 T 2 excited level of Fe 2+ ions the first realization of Fe 2+ ions lasing at 3.6 μm and 3.8μm via Co-Fe energy transfer. Demonstrated effective Co 2+ →Fe 2+ energy transfer process could result in utilization of a more convenient laser pump sources for the Fe 2+ :II-VI lasers.

The anti-EGFR antibody, cetuximab, was labeled with IRDye 800CW fluorescent dye and conjugated to gold nanorods (GNR). GNR with aspect ratio of ~ 4 and plasmon resonance peak at ~785 nm were fabricated for use in these experiments. The IRDye:cetuximab:nanorod conjugate treatment with NIR light selectively heated the GNR and was sufficient to treat cancers. Excitation induced fluorescence of the IRDye 800CW enabling real-time imaging. We characterized and optimized the parameters for the conjugation of the GNR to cetuximab to facilitate active targeting of the nanorods to the site of the tumor. This combination of selective targeting, imaging, and photothermal treating of malignant cells is a viable approach for a variety of squamous cell carcinomas.

We report an optimization of Fe:ZnSe crystals fabrication, as well as a fourfold increase of the output energy of the gain-switched middle-infrared Fe:ZnSe laser pumped by the radiation of Q-switched Cr:Er:YSGG (2.8μm) laser. Lasing was studied over 236-300K temperature range. In Fabry-Perot cavity with 18% OC reflectivity the maximum output energy reached 4.7 mJ @ 4.3μm and 3.6 mJ @ 4.37μm at 236K and 300K, respectively and was limited only by available pump energy. Threshold was about 8 mJ and was practically unchanged over studied temperature range. The laser slope efficiencies decreased from 19% to 16 % with an increase of temperature from 236 to 300K.

We report a comprehensive study of gamma-irradiation on optical, electrical, and laser characteristics of pure and transition-metal doped single and polycrystalline ZnS and ZnSe. Polished pure, Cr-doped, and Ag, Au, Cu, Al, In, and Mn co-doped ZnS and ZnSe crystals after absorption and electro-conductivity characterization were gamma-irradiated at doses of 1.37x10 8 , and 1.28x10 8 rad at +10 and -3°C, respectively. Dynamic RT absorption studies, electro-conductivity measurements and mid-IR lasing were performed for different exposition times of crystals at RT. Cr:ZnSe and Cr:ZnS lasers based on identical gamma-irradiated and non-irradiated crystals featured a very similar pump thresholds, slope efficiencies, and output powers.

Compact solid-state laser systems based on chromium doped II-VI semiconductor materials (ZnS, ZnSe, CdSe) with tunability over 2–3.6 μm, output power exceeding 10W, and efficiency up to 70% were demonstrated recently. A further increase of the output power requires a thorough thermal management of the active element. Fiber geometry of gain element is very promising among other different approaches to control beam quality and thermal lensing. The proposed transition metal doped ZnS:ZnSe/As2S3:As2Se3 composite materials with index matching of II-VI and V-VI components represent a new way for design of mid-infrared laser active fibers. Chalcogenide glasses have wide transparency range in mid-IR, enable fiber geometry, and their refractive index can be varied from n=2.1 to n=2.5 matching refractive index of ZnS (n=2.26) and ZnSe (n=2.44) crystals and eliminating scattering losses. The II-VI compounds provide a tetrahedral coordination of the chromium ions required for mid-IR lasing. We report the first mid-IR laser active Cr:ZnSe/As2S3:As2Se3 composites fabrication and room-temperature lasing at 2.4 µm. The Cr:ZnSe/As2S3:As2Se3 composites were prepared by annealing of the appropriate compounds under vacuum and by casting and drying of Cr:ZnSe microparticles suspension in As2S3:As2Se3 propylamine solution. All samples demonstrated mid-IR photoluminescence typical for Cr 2+ ions in ZnSe host. High optical gain and low passive losses in Cr:ZnSe/As2S3:As2Se3 composite material were demonstrated in random lasing experiments.

Middle infrared (mid-IR) chromium-doped zinc selenide (Cr:ZnSe) bulk lasers have attracted a lot of attention due to their unique combination of optical and laser properties facilitating a wide range of potential scientific, industrial, and medical applications. Utilization of thin film waveguide geometry enabling good thermal management and control of beam quality is a viable pathway for compact chip-integrated optical laser design. Cr:ZnSe thin films are also promising as saturable absorbers and mode-lockers of the cavities of solid state lasers operating over 1.3-2.1 μm. We recently reported the first successful demonstration of mid-IR Cr:ZnSe planar waveguide lasing at 2.6 μm under gain-switched short-pulse (5 ns) 1.56 μm excitation as well as the passive Q-switching of the cavity of a fiber-pumped Er:YAG laser operating at 1645 nm using a highly doped Cr:ZnSe thin film. PLD grown Cr:ZnSe waveguide were fabricated on sapphire substrates (Cr:ZnSe/sapphire) with chromium concentration of 1018-1019 cm-3. Further development of mid-IR lasing in the Cr:ZnSe planar waveguide under continuous wave excitation were investigated. In addition, deposition of Cr:ZnSe-based thin film structures on n-type GaAs substrates were also investigated for possible mid-IR electroluminescence.

We report methods of fabrication and laser-spectroscopic characterization of mid-IR gain media based on micron size Cr2+:ZnSe/ZnS powders, as well as Cr2+:ZnSe/ZnS doped fluorocarbon polymer films, and perfluorocarbon liquids. All samples demonstrated strong mid-IR luminescence over 2000-3000nm spectral range under optical 1700nm excitation. The random lasing of the doped liquids and polymer films was realized with pump energy density of 100 and 15mJ/cm2, respectively. Previously we have demonstrated mid-IR electroluminescence of Cr:ZnSe with n-conductivity provided by thermal diffusion of Al and Zn. However, the formation of conductivity was accompanied by compensation of the Cr2+ optical centers and relatively weak chromium electroluminescence. In this paper we report study of the Cr2+ compensation in the crystals co-doped with donor and acceptor impurities. Optical and electrical characterization of Cr:ZnSe crystals with Ag, Cu, Al, In, and Zn co-dopants were studied to optimize mid-IR electroluminescens of the Cr2+ ions. The best results were obtained with p-conductive Ag:Cr:ZnSe samples featuring a low 600 Ωcm resistivity. First mid-IR electroluminescence in presumable p-type Ag:Cr:ZnSe was demonstrated, which could prove valuable for developing laser diodes that function in this spectral region.

Here we report a new method for transition-metal (TM) doped II-VI Quantum Dots (QD) fabrication and first mid-IR (2-3 μm) lasing at 77K of Cr 2+ :ZnS QD powder (~ 27 nm grain size). Cr 2+ :ZnS nanocrystalline dots (NCDs) were prepared using laser ablation. The mid-IR photoluminescence (PL) and lasing were studied. The dependence of PL spectrum profile on pump energy demonstrated a threshold behavior accompanied by the appearance of a sharp stimulated emission band around 2230 nm. The stimulated emission band is shifted to the longer wavelength with respect to the spontaneous emission and corresponds to the peak of the Cr:ZnS gain spectrum. This was also accompanied by a considerable lifetime shortening.

Recent progress in transition metal doped II-VI semiconductor materials (mainly Cr2+:ZnSe) makes them the laser sources of choice when one needs a compact system with continuous tunability over 2-3.1 mm, output powers up to 2.7 W, and high (up to 70%) conversion efficiency. The unique combination of technological (low-cost ceramic material) and spectroscopic characteristics make these materials ideal candidates for "non-traditional" regimes of operation such as microchip and multi-line lasing. This article reviews these non-traditional Cr-doped mid-IR lasers as well as describes emerging Fe2+:ZnSe lasers having potential to operate at room temperature over the spectral range extended to 3.7-5.1 mm. In addition to effective RT mid-IR lasing transition metal doped II-VI media, being wide band semiconductors, hold potential for direct electrical excitation. This work shows the initial steps towards achieving this goal by studying Cr2+, Co2+, and Fe2+ doped quantum dots. We have demonstrated a novel method of TM doped II-VI quantum dots fabrication based on laser ablation in liquid environment. TM doped II-VI quantum dots demonstrated strong mid-IR luminescence. It opens a new pathway for future optically and electrically pumped mid-IR lasers based on TM doped quantum confined structures.

We demonstrate an Er-fiber-laser-pumped, CW, high-power, single-longitudinal-mode Cr2+:ZnSe laser, tunable in the 2- 3 mm spectral region. The laser is operating in a single longitudinal mode regime with a linewidth of 80-130 MHz over a 120 nm tuning range around 2.5 mm, and delivers up to 150 mW of output power. The laser design is very compact and is based on Kogelnik/Littman cavity configuration with the total optical length of the folded cavity of 10 cm. The narrow-linewidth output spectrum can be quickly scanned over a 10 nm spectral range with a repetition rate of 220 Hz by a piezo-controlled tuning mirror which allows for an extremely fast wavelength tuning of the output spectrum over a large number of absorption spectral lines of trace gases of interest. As a test experiment, we performed a Dopplerlimited- resolution intracavity laser absorption spectroscopy of ro-vibrational transitions of the n3 and n1 bands of H2O with minimum detectable absorption coefficient of ~3x10-7 cm-1, which corresponds to 9 parts per billion by volume water vapor detection limit. The laser is currently in active stage of development and its further optimization will allow for full 2-3 mm fast tuning range and Watt-level output powers. This laser is being designed as a seeding source for an OPG-OPA-based, highly sensitive trace-gas sensor system for real-time detection of gas traces of biological pathogens and explosives in the molecular fingerprint mid-IR spectral region of 2-10 mm.

The use of Fe:ZnSe polycrystals as passive Q-switches for the Er:Cr:YSGG laser operating at 2.8mm is introduced. Fe:ZnSe samples with 1-7cm-1 coefficients of absorption were prepared using thermal diffusion of iron in CVD grown polycrystalline ZnSe. A flashlamp pumped Er:Cr:YSGG laser with a variable (40 - 80% reflectivity) output coupler (OC) was used as a test bed for passive Q-switching. Using a 90% initial transmission Fe:ZnSe placed at the Brewster angle we obtained a single giant pulse lasing with a pulse duration of ~65 ns and a maximum output of 13 mJ under 30 J of flashlamp pump. Multi-pulse (19 pulses) output was obtained with 85 mJ total output energy at a pump energy of 30 J. The saturation curve of Fe:ZnSe was measured. Fitting this data with a theoretical model results in absorption crosssection of 0.56 × 10-18 cm2, which is close to the value of the absorption cross-section obtained from spectroscopic measurements (0.85 × 10-18 cm2 at 2.8 mm).

Cobalt doped ZnSe and ZnS crystals have been studied to determine their effectiveness for passive Q-switching for 700-800nm spectral range (Alexandrite laser). Samples were prepared using Bridgeman technique for single-step growth of Co doped crystals as well as after growth thermal diffusion of Co in undoped crystals. ZnS:Co:Cr crystals, which have been produced using the Bridgeman technique, show maximum initial absorption coefficients of 17 cm -1 at 725nm. Experimental results are reported on effective thermal diffusion of Co 2+ in ZnSe and ZnS polycrystals and thermal diffusion constants of cobalt ions in ZnSe and ZnS are estimated. The nonlinear saturation properties of cobalt doped ZnSe and ZnS crystals have been investigated experimentally. The induced transparency measurements were performed using electro-optically Q-switched, alexandrite laser radiation at 731, 741, and 778 nm with a pulse duration of about 70 ns. The induced transmission measurements were analyzed using a four-level absorber model and the absorption cross sections have been estimated at both 731nm and 741nm to be 9.5 × 10 -18 cm 2 and 8.2 x 10 -18 cm 2 , respectively. Absorption cross sections calculated from saturation measurements at 4 A 2 → 4 T 1 (4P) transition are in agreement with results earlier reported for mid-infrared spectral region 4 A 2 → 4 T 1 of Co 2+ ions. The described Co-doped crystals are very promising as passive Q-switches for alexandrite laser resonators. Co 2+ centers feature high cross section of saturation and their absorption bands are nicely matched to the spectral emission of the tunable alexandrite laser. An efficient ZnS:Co:Cr passive Q-switching of the alexandrite laser cavity was realized with output energy of 15 mJ and 50 ns pulse duration.

We report the study of middle-infrared electroluminescence of n-type, Cr doped bulk ZnSe crystals. n-type, Cr-doped ZnSe samples were prepared in three stages. At the first stage, the undoped polycrystalline ZnSe samples were grown by chemical vapor deposition. During the second stage, the doping of 1 mm thick ZnSe polycrystalline wafers was performed by post-growth thermal diffusion of Cr. Finally, Cr:ZnSe wafers were annealed with Al 2 Se 3 and ZnSe powders in sealed vacuumed ampoules at 950 qC for 96 hours. Comparison of the absorption spectra of the crystals before and after thermal diffusion with Aluminum indicates the preservation of the desired Cr 2+ ions. Ohmic contacts for electrical measurements were formed by polishing the facets and wetting the surface of the crystals with In. The best crystals demonstrated conductivity of up to 10-100 ohm* cm. The electroluminescence measurements were taken using synchronous detection methods with an InSb detector. A pulse generator output (100V) at 5 kHz and a lock-in amplifier were used to distinguish luminescence signals from other possible noise sources. We report the observation of middle-infrared (2-3 P m and 8 P m) and visible (~600 nm) electroluminescence of n-type Cr doped bulk ZnSe crystals. Keywords : Electroluminescence, middle-infrared, Cr :ZnSe, n-type ZnSe, ohmic contacts.

The first room temperature stable tunable color center (CC) distributed feedback (DFB) laser is described. The laser utilizes stabilized F 2 +** centers in LiF (LiF:F 2 +** ) as a gain medium. Tunable DFB lasing was achieved in the near IR region (882 - 962 nm) with a lasing linewidth of less than 0.2 cm -1 . The lasing threshold was found to be 1.2 mJ, while the slope efficiency with respect to pump energy was found to be as high as 3%.

It is demonstrated that pulsed laser deposition is a promising “alternative route” for synthesis of middle infrared laser media based on chromium doped ZnS crystalline thin films with a precisely controllable concentration of dopant. The deposition rate and thickness of the thin films synthesized in our experiments varied from 0.017 to 0.109 nm per pulse and 200 nm up to 12 µm, respectively, depending on the laser energy density, number of pulses, and target-substrate distance. Cr concentration in the target mate rial and grown thin film measured by different techniques were very close to each other for a dynamic range of Cr concentration from ~ 10 19 to 3.5 × 10 20 cm -3 . Thin film features luminescence band which is similar to the band in bulk crystal (slightly blue-shifted). The emission lifetime of Cr 2+ : ZnS films with Cr 2+ concentration of ~2 × 10 19 cm -3 was measured to be ~3 µs. The emission lifetime was shortened to 1 µs for 1.8 × 10 20 cm -3 and to 0.67 µs for 3.5 × 10 20 cm -3 concentration of chromium due to the concentration quenching. Spectroscopic study shows that Cr 2+ :ZnS thin films synthesized by pulsed laser deposition are promising

This paper is focused on a design of flexible laser systems capable to provide spatial transformation of the pump radiation into the spectral domain of the output laser oscillation using specially designed “spatially dispersive” laser cavity. These systems also provide ultrabroadband or controlled spectral linewidth of the output laser oscillation. The theoretical analysis based on gaussian approximation for the designed laser cavities with “spatial dispersion” was used to provide maximum spatial resolution of the spatial-spectral transformation. The transformations of the spatial distribution of the pump radiation into the spectral domain of the output laser oscillation were experimentally demonstrated in the gain-switched LiF:F 2+** and LiF:F 2- lasers with total efficiencies of up to 20% and output pulse spectrum width wider than 140 nm, centered at 0.96 m and 1.14 m, correspondingly. As a result of the optimization of the angular dispersion of the output radiation, the simultaneous phase-matching for second harmonic generation in the single nonlinear crystal was realized for the whole oscillation spectral range. This technique allows to use a nonlinear frequency conversion for nonlinear transformation of the beam spatial distribution. Due to this, the ultra broadband (>100 nm) or multiline (20 lines) second harmonic and sum frequency oscillations were demonstrated in a LiIO

Organic materials with large two-photon absorption are desired for numerous photonics applications, such as optical limiting,upconversion lasing, three-dimensional data storage,and photodynamic therapy. Stilbazolium derivatives are interesting two-photon absorbers for these applications. In this work,the nonlinear transmissivities of trans -4-[4-(dimethylamino)styryl ]-1-methylpyridinium iodide (DASPI), trans -4-(4-aminostyryl)-1-methylpyridinium iodide (ASPI), trans -4-[2-(1-methylpyrryl)vinyl ]-1-methylpyridinium iodide (MPVPI), trans -4-(2-pyrrylvinyl)-1-methylpyridinium iodide (PVPI),and trans -4-styryl-1-methylpyridinium iodide (SPI)at 800,850,900,950 and 1000 nm have been studied respectively using 21 ps laser pulses. The two-photon induced fluorescence of these compounds at 872,900,and 940 nm has also been investigated.All of these compounds exhibit two-photon absorption and two-photon induced up-converted fluorescence in the near-IR wavelengths,and the two-photon absorption cross section and two-photon induced fluorescence intensity vary with the wavelength and with the chemical structure changes. These preliminary results suggest that it is possible to increase the two-photon absorption cross-sections by proper structure modifications

Organic materials with large excited state and/or two-photon absorption are desired for numerous device applications, such as optical limiting, two-photon upconversion lasing, three-dimensional data storage, and two-photon photodynamic therapy. Dialkylamino styryl thiazolium/benzoxazolium compounds are interesting excited state and two-photon absorbers for these applications. In this work, the nonlinear transmissivity of trans -2-[ p-(N -methyl- N -(hydroxyethyl)amino)styryl]- N -methylthiazolium iodide (MHAST), trans -2-[ p-(N -ethyl- N -(hydroxyethyl)amino)styryl]-N -methylthiazolium hexafluorophosphate (EHAST), trans -2-[ p-(N -methyl- N -(hydroxyethyl)amino)styryl]- N -methylbenzoxazolium iodide (MHASBO) at 532 nm has been studied using 6 ns laser pulses. The two-photon induced fluorescence of these compounds at 940 nm has also been investigated. At 532 nm, both MHAST and EHAST exhibit reverse saturable absorption, however, MHASB exhibit saturable absorption. At 940 nm, all of these compounds exhibit two-photon induced up-conversion fluorescence, and the fluorescence intensity varies when the chemical structure changes. These preliminary results suggest that the nonlinear absorption (excited state absorption or two-photon absorption) characteristics of these compounds vary at different wavelengths and vary when their chemical structure changes. Keywords : Excited state absorption, two-photon induced upconversion fluorescence, single photon fluorescence, dialkylamino styryl thiazolium/benzoxazolium compounds

We present a novel technical approach to build a multiwavelength/superbroadband laser source based on a combination of spectral and spatial domains. This approach provides a direct transformation of spatial domain of the pumping beam into spectral domain of the output oscillation. Several spatially dispersive laser cavities were designed and studied both theoretically and experimentally for optimal spatial to spectral transformation. This spatial to spectral transformation was demonstrated for LiF:F 2 + ** color center laser, single broad-stripe diode laser operating at 660nm and 1560 nm multi-stripe diode laser.

Laser properties and spectroscopic characterization of diffusion doped Cr 2+ : ZnS crystals synthesized by chemical transport reaction from gas phase are reported. Lasing was realized with a threshold of 170 J and slope efficiency of 9.5 % with respect to the 1.5607 -1m pump energy, in a hemispherical cavity. Low doped samples (3-4 cm at 1.7 m) of 1.7 mm thickness were utilized. The 1.5607 m excitation was realized with a D 2 Raman cell pumped in a backscattering geometry by the 1.064 m radiation of the single frequency Nd:YAG laser. Maximum output energy reached 100 J. Lasing in the hemispherical cavity was achieved with output couplers R 2.36 m =80 % and 90 % and radius of curvature 20 cm. Absorption cross section was estimated from spectroscopic measurements and was in a good agreement with saturation data ( abs. = 0.80x10 -18 cm 2 ) calculated with the modified Frantz-Nodvik equation for a four level slow absorber. Findlay Clay losses were found to be about 14%. Selective cavity experiments were performed in a hemispherical cavity with a CaF

A novel experimental set-up using laser-induced breakdown spectroscopy (LIBS) for environmental analyses of heavy metals is described in this paper. It is based on state-of-the-art spectroscopic equipment, advanced detectors, and laser atomizers: a 0.75 m spectrometer ARC-750, intensified TE- cooled 256 X 1024 CCD camera, probe with fiber optic guide for signal transportation, and Nd:YAG laser plasma atomizers with two different methods for sample delivery. In the first method the liquid solution containing the atoms to be investigated is drawn into the chamber of the nebulizer. The mixture passes through the nozzle, accompanied by argon gas along with formed aerosol, and enters the plasma plume, which is generated by the laser spark in argon. The second method is based on direct generating of the plasma in the water jet of a continuously circulating sample. LIBS testing of samples containing Al, Cd, Cu, Fe, Pb, Zn, and Cr ions was compared with results using atomic absorption spectrophotometry. Initial indications showed good agreement between these two methods. Detection levels of less than 100 ppb were observed for copper and chromium. The described spectroscopic system exhibits high sensitivity, accumulation of luminescence spectrum in real time; and high dynamic range for concentrations detection from 100 ppb to 1000 ppm.

An efficient room temperature all-solid-state laser system continuously tunable in the 0.2 - 10 micrometers spectral range has been developed. It is based on the alexandrite laser pumped LiF:F 2 +** color center laser system. The alexandrite - LiF:F 2 +** color center laser combination system has been shown to be a suitable drive source for a number of efficient nonlinear processes, including harmonic, sum-frequency and difference-frequency generation.

The presented work is part of a study investigating the surgical diagnostic potential of Raman spectroscopy for the malignant brain tumor, glioblastoma multiforme.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Laser atomic fluorescence (LAF) is a powerful spectroscopic tool in analytical applications when it is necessary to measure extremely low concentrations of atoms. The main purpose of this method is to determine the presence and absolute concentrations of different elements in materials. In this paper we will describe the basic principles of the method and how advances in lasers, fiberoptics and detectors allow for significant improvement and simplification of the LAF spectrometer.

Each year, more people at younger ages are diagnosed with primary brain tumors. Current histological discrimination between normal and diseased tissue occurs after tissue excision. A reliable optical biopsy for open craniotomy would optimize the amount and types of tissue removal by making an accurate evaluation before excision. The presented work is part of a study investigating the clinical diagnostic potential of Raman spectroscopy for gliomas. It has been shown that the optical properties of in vitro tissue are strongly dependent upon sample preparation. The investigation of the effects of time latency, paraformalin tissue fixation, and tissue perfusion with carbogen-bubbled cortical transport solution on their respective Raman spectra of brain tissue and tumors will be discussed, as well as their implications on the study of neurological tissue. The studies are conducted with in situ tissue samples from scid mice and 785 nm pulsed alexandrite laser excitation. Results illustrating positive qualitative and quantitative variations between Raman spectra of normal and malignant brain tissue will be presented.

We report on the development of a novel experimental set-up using laser atomic fluorescence for detection and concentration measurements of heavy metal atoms for environmental and biomedical analyses. This spectrometer is based on the application of tunable LiF:F2+** and LiF:F2- color center and alexandrite lasers with nonlinear converters for narrowband excitation of atomic fluorescence and the use of gated multichannel CCD detectors for fluorescence measurements. A standard graphite furnace module was used for sample atomization. The laser sources used provide narrowband selective laser excitation continuously tunable in the 200 - 400 nm range and are therefore suitable for resonant excitation of atomic transitions in practically all known heavy metal atoms. In the first experiments, water samples containing Cu, Pb and Fe impurities were studied and detection levels of less than 1 ppb were observed. Comparison of the results of atomic laser fluorescence analysis and traditional atomic absorption spectrometry showed good qualitative agreement between these two methods. It is projected that full optimization of our experimental set up will allow for improved detection levels of several orders of magnitude. Possible optimization and simplification of the spectrometer are discussed in the context of developing a portable instrument for field use.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A powerful photo- and thermostable, LiF:F 2 + color center laser pumped by radiation at 740 nm or at 683 nm is described. This is the first time an average power as high as approximately 1.4 W and an energy per pulse as high as approximately 100 mJ, with record real efficiency of 53 percent and 28 percent were achieved for a room temperature LiF:F 2 + color center laser. Such color center laser, tunable in the 820-1210 nm range, features a number of important advantages over pulsed lasers based on Ti or Cr doped crystals.

There are many problems with current environmental sampling and analysis programs. Foremost among these problems is the need to collect many samples in order to obtain the desired accuracy of the characteristics of interest. Other concerns involve inadvertent changes that may affect the sample characteristics between sample collection and analysis. The high cost of analyzing trace levels of organic and metallic toxicants is also restrictive. This paper summarizes sampling and analytical needs, based on current limitations of procedures and instrumentation, that may possibly overcome using laser-based procedures. These needs include current shortcomings in water and wastewater analyses, specifically problems with sample collection, preparation, and analysis. The use of inexpensive and fast instrumentation, especially if the procedures minimize sample preparation and can be used in-situ, will vastly improve environmental research and characterization efforts by enabling greatly increased sample sizes with better spatial and temporal resolution. Current research being jointly conducted by the Department of Civil and Environmental Engineering and the Department of Physics at the University of Alabama at Birmingham, Laboratory of Laser Spectroscopy of Solids at the General Physics Institute of the Russian Academia of Science, and Alabama Laser Technologies, is demonstrating he use of several configurations of laser-based instruments to cost- effectively analyze very low levels of toxicants. This National Science Foundation sponsored research effort has recently demonstrated the detection of copper and lead at low (mu) g/L levels using atomic laser fluorescence using a tunable color center laser, spectrometer, intensified TE- cooled CCD camera, and atomizing furnace. Fluorescence/Raman spectroscopic instrumentation with a He-Ne and/or Alexandrite laser, portable spectrograph and CCD camera has also been developed for qualitative characterization of water and solid waste samples.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe a coherence filtering technique based on degenerate four wave mixing (DFWM) in a thin nonlinear optical material. In contrast to previous works which used ultra-short laser pulses, we performed low-coherence filtering techniques through scattering media with broad- spectrum nanosecond pulses. In our first 'proof of principle' experiments we used a 100 micrometer thick layer of dye solution as a nonlinear optical material and investigated a one dimensional case for depth-resolved measurements through a scattering media consisting of a highly scattering suspension of dielectric microspheres in water. We also describe a technique to obtain instantaneous cross-sectional images (which can be depth scanned to obtain the third dimension) performed with a low-coherence nanosecond laser source on a liquid crystal doped with an infrared dye. Experimental results were obtained with room temperature LiF:F2- and LiF:F2+ color center lasers, and a Q-switched alexandrite laser. This technique can be used to provide instantaneous, single-shot, two-dimensional images of the internal structure of materials versus depth.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report here on three trends in Color Center Laser development.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The results are presented of theoretical and experimental studies of two-step photoionization and photochemistry of color centers in LiF crystals. The exact analytic expressions were obtained for the population of optic centers energy levels in the process of two-step photoionization within a framework of kinetic equations in three-level approximation. A transfer from cw to pulsed-periodic excitation was analyzed. An analytic dependence was obtained of the ionization cross-section on the incident light flux intensity. For the first time the cross-section was estimated of the photoionization from the excited level of F 2 color center in LiF crystal being pumped by 532 nm wavelength radiation. The processes were detected of mutual phototransformations of F 3 , F 3 + and N color centers in oxygen-free LiF crystals. It was demonstrated that the presence of F 3 and F 3 + color centers in crystals is necessary for selective accumulation of N color centers at high temperature (300 K) action of Nd:YAG laser third harmonic radiation. In LiF- OH crystals the processes of mutual phototransformations were detected in a CC ensemble with absorption bands maxima at wavelengths 380 nm, 450 nm, and within the range 500 - 570 nm. It was demonstrated that an effective destruction of the CC absorption band (380 nm) along with the formation of absorption bands with maxima within 500 - 570 nm takes place under high-temperature (300 K) action of Nd:YAG laser third harmonic radiation ((lambda) equals 354 nm). Reverse phototransformation with the recovery of the band at 380 nm takes place along with the known phototransformation F 2 (450 nm) yields F 2 + (654 nm) under the subsequent action of the same laser second harmonic radiation ((lambda) equals 532 nm).