Two-octave-wide (3-12 µm) subharmonic produced in a minimally dispersive optical parametric oscillator cavity.

We report a subharmonic (frequency-divide-by-2) optical parametric oscillator (OPO) with a continuous wavelength span of 3 to 12 µm (-37dB level) that covers most of the molecular rovibrational "signature" region. The key to obtaining such a wide spectral span is the use of an OPO with a minimal dispersion-through the choice of intracavity elements, the use of all gold-coated mirrors, and a special "injector" mirror. The system delivers up to 245 mW of the average power with the conversion efficiency exceeding 20% from a 2.35 µm Kerr-lens mode-locked pump laser.

High power single-mode delivery of mid-infrared sources through chalcogenide fiber.

Mechanically robust and low loss single-mode arsenic sulfide fibers are used to deliver high power mid-infrared sources. Anti-reflection coatings were deposited on the fiber facets, enabling 90% transmission through 20 cm length fibers. 10.3 W was transmitted through an anti-reflection coated fiber at 2053 nm, and uncoated fibers sustained 12 MW/cm2 intensities on the facet without failure. A Cr:ZnSe laser transmitted >1 W at 2520 nm, and a Fe:ZnSe laser transmitted 0.5 W at 4102 nm. These results indicate that by improving the anti-reflection coatings and using a high beam quality mid-infrared source, chalcogenide fibers can reliably deliver ≥10 W in a single mode, potentially out to 6.5 µm.

Cr:ZnS laser-pumped subharmonic GaAs optical parametric oscillator with the spectrum spanning 3.6-5.6 µm.

Using a subharmonic optical parametric oscillator (OPO) based on orientation-patterned GaAs, we produced a broadband instantaneous output that spans 3.6-5.6 µm at 50-dB level (4.4-5.2 µm at 3 dB level), has 110 mW of average power, and is suitable for producing wideband-frequency combs in the mid-infrared. The OPO was synchronously pumped by a compact Kerr-lens mode-locked femtosecond Cr:ZnS oscillator with the central wavelength 2.38 µm and pulse repetition frequency 175 MHz.

Mid-IR photoluminescence of Fe(2+) and Cr(2+) ions in ZnSe crystal under excitation in charge transfer bands.

Spectroscopic characterization of Fe:ZnSe(Cr:ZnSe) crystals under visible excitation into the charge transfer bands of Transition Metal ions were studied. The excitation efficiencies of mid-IR photoluminescence between (5)T(2)((5)E) and (5)E((5)T(2)) states via direct relaxation to the upper laser levels and via metastable upper (3)T(1) were investigated. It was demonstrated that the latter route is the dominant process for Cr(2+) ions and could provide sufficient pump rate for mid-IR lasing. The pump efficiencies via direct relaxation to the upper laser levels were estimated to be <2% for both ions under 532 nm excitation wavelength.

Crystalline Cr²âº:ZnSe/chalcogenide glass composites as active mid-IR materials.

We propose new transition metal (TM)-doped ZnSe/As2S3:As2Se3 composite materials for mid-IR fiber lasers. The composites are the suspension of crystalline micro- and nanosized TM²âº:ZnSe or TM²âº:ZnS powders in chalcogenide glasses with the refraction index matching. Mid-IR room-temperature lasing of Cr²âº:ZnSe/As2S3:As2Se3 microcomposite material is demonstrated at the 2.4 µm wavelength.

Mid-IR laser oscillation in Cr2+:ZnSe planar waveguide.

We demonstrate 2.6 µm mid-infrared lasing at room temperature in a planar waveguide structure. Planar waveguides were fabricated using pulsed laser deposition (PLD) by depositing chromium doped zinc selenide thin films on sapphire substrate (Cr2+:ZnSe/sapphire). Highly doped Cr2+:ZnSe/Sapphire thin film sample was also used to demonstrate passive Q-switching of Er:YAG laser operating at 1.645 µm.

10-watt, pure continuous-wave, polycrystalline Cr2+:ZnS laser.

We demonstrate, for the first time, 10 W, Er-fiber laser pumped, pure CW, thermally diffusion doped, polycrystalline Cr(2+):ZnS laser operating at 2380 nm. We also show Littrow-grating, "single-knob", wavelength tuning of the laser spanning 1940-2780 nm spectral range with the maximum output power of 7.4 W near the central wavelength of 2400 nm and above 2 W over 1970-2760 nm wavelength range. The laser performs with 40% real optical- and 43% slope efficiency, and shows no output power roll-off up to the highest available incident pump power of 27 W.

Tunable, single-frequency, and multi-watt continuous-wave Cr2+:ZnSe lasers.

We demonstrate CW rapidly-tunable (4.5 microm/s), highpower (150 mW), single-longitudinal-mode (120 MHz) single-crystalline Cr(2+):ZnSe laser; CW widely-tunable (2.12-2.77 microm), multi-watt (2 W over 2.3-2.7 microm), polycrystalline Cr(2+):ZnSe laser; CW multi-watt (6 W, at 2.5 microm), highly efficient (48% real efficiency) polycrystalline Cr(2+):ZnSe laser; CW multi-watt (3 W, at 2.5 microm), highly efficient (41% real efficiency) ultra-compact polycrystalline Cr(2+):ZnSe laser; and CW hot-pressed ceramic Cr(2+):ZnSe laser.

Mid-infrared Cr2+:ZnSe random powder lasers.

We report simple methods of laser active Cr2+:ZnSe powder fabrication with average grain sizes of either approximately 10 or approximately 1 microm without crystal growth stage. Pure, uniformly mixed ZnSe and CrSe powders annealed at 1000 masculineC for 3 days in a sealed evacuated (approximately 10(-4) Torr) quartz ampoule exhibited middle-infrared laser action at room temperature under 1.56 microm excitation of D(2) Raman shifted radiation of Nd:YAG laser. The output-input characteristic clearly demonstrated the threshold-like behavior of the output signal with the threshold energy level of 0.5 and 3 mJ in 2.9 mm spot for 10 and 1 microm grain sizes, respectively.

Hot-pressed ceramic Cr(2+):ZnSe gain-switched laser.

The technology of hot-pressed Cr(2+):ZnSe ceramic preparation is reported. Comparative gain-switched lasing of hot-pressed ceramic and CVD grown Cr(2+):ZnSe samples with slope efficiencies up to 10% and output energies up to 2 mJ were demonstrated. This is a milestone in the development of future large scale mid-IR laser systems based on ceramic TM(2+):II-VI materials. This evolving technology has excellent potential as media for practical low cost, high power mid-IR laser applications.

3.9-4.8 microm gain-switched lasing of Fe:ZnSe at room temperature.

We report a room temperature Fe:ZnSe laser in gain-switched operation and tuning over the 3.9 - 4.8microm spectral range. Mid-IR emission of Fe2+:ZnSe was studied under three regimes of excitation: ordinary optical (2.92mum) excitation of 5T2 excited state of Fe2+; excitation via 5E level of Cr co-dopant (1.56mum); and excitation via photo-ionization transition of Fe2+ (0.532mum). The energy transfer from Cr2+ (5E level) to Fe2+ (5T2 level) under 1.56microm wavelength excitation was observed and resulted in simultaneous RT emission of Fe:Cr:ZnSe crystal over ultra-broadband spectral range of 2- 3 and 3.5-5mum. We also report the observation of mid-IR emission at 3.5- 5mum induced by 2+->3+->2+ ionization transitions of the iron ions in Fe:ZnSe.

Fluorescence assay for monitoring Zn-deficient superoxide dismutase in vitro.

A method has been developed for selective detection of the zinc-deficient form of Cu, Zn superoxide dismutase (SOD1) in vitro. Zinc-deficient SOD1 mutants have been implicated in the death of motor neurons leading in amyotrophic lateral sclerosis (ALS or Lou Gerhig's disease). Thus, this method may have applicability for detecting zinc-deficient SOD1 mutants in human ALS patients samples as well as in a transgenic mouse model of ALS and in cultured motor neurons. We determined previously that structural analogs of 1,10 phenanthroline, which react specifically with Cu(I), react with the active Cu(I) of SOD1 when zinc is absent, but not when zinc is also bound, as evidenced by the fact that the reaction is inhibited by pretreatment of the enzyme with zinc. We report herein that bathocuproine, or its water-soluble derivative bathocuproine disulfonate, react with zinc-deficient SOD1 to form a complex which fluoresces at 734 nm when excited at 482 nm. Fluorescent intensity is concentration dependent, thus we propose to use fluorescent confocal microscopy to measure intracellular levels of zinc-deficient SOD1 in situ.

Erbium fiber laser-pumped continuous-wave microchip Cr(2+):ZnS and Cr(2+):ZnSe lasers.

Efficient continuous-wave (cw) lasing of Cr(2+):ZnS and Cr(2+):ZnSe crystals in external hemispherical cavities and in a microchip configuration under Er-fiber-laser pumping at room temperature are reported. The key result is what is believed to be the first successful demonstration of cw Cr(2+):ZnS and Cr(2+):ZnSe microchip lasers with maximum output powers of 63 and 100 mW at 2320 and 2520 nm, with slope efficiencies of 53% and 20%, respectively.

Polarized Raman spectroscopic studies of tetragonal lysozyme single crystals.

Polarized Raman spectra have been obtained for tetragonal lysozyme single crystals of different relative quality. The Raman band at 507 cm-1, which corresponds to the totally symmetric stretch vibration of the gauche-gauche-gauche (ggg) disulfide bridges of the protein, has been shown to possess different polarization characteristics compared with the gauche-gauche-trans (ggt) disulfide bridge band at 528 cm-1. The relative intensities of the ggg and ggt bands in the polarized Raman spectra have been numerically estimated for a number of tetragonal lysozyme single crystals, the X-ray diffraction data of which are available from the Protein Data Bank. On the basis of comparison between the experimental and calculated polarization characteristics of the disulfide Raman lines, the following main conclusions have been drawn. The orientation of the protein molecules correlates with the average orientation of their ggg disulfide bridges. This in turn can be described by the rhoggg value which reflects the average orientation of the S-S bonds with respect to the Z crystallographic axis and can be determined from polarized Raman spectra. Crystals of better quality are characterized by a better alignment of the protein molecules with respect to the Z axis, a smaller perturbation of the protein molecules in the crystal lattice and a somewhat higher interlattice water content.

Multiline, superbroadband and sun-color oscillation of a LiF:F(2)(-) color-center laser.

We report experimental and theoretical studies of the temporal, spectral, and spatial features of a superbroadband laser. The results obtained show that the superbroadband room-temperature operable LiF:F(2)(-) color-center laser can provide low-coherence, high-intensity laser radiation with a spectral width of 1400 A centered at 1.14 mum and 700 A in the visible range from green to red and exhibit good spatial collimation with a divergence of between 5 and 6 mrad. Oscillation of all the lines of a superbroadband spectrum is completely synchronous and occurs almost simultaneously with the pump pulse, exhibiting 4-9-ns pulse delay at 20-ns pump pulse duration. Second-harmonic generation of superbroadband oscillation spectrum was realized with an overall efficiency of 10%.

Mode-locking operation of a pulsed Nd:YAG laser with F(2) BF:LiF color-center crystal in a dual configuration.

Passive stable mode locking of a pulsed Nd:YAG laser was achieved with a long-decay-time saturable absorber in a coupled-cavity longitudinal scheme. In this configuration the saturable absorber (LiF:F(2) ) also behaves as an active medium in intracavity pumping and is the load for the Nd laser. This arrangement provides proper lasing conditions for the F(2) color centers so that the cavity photon lifetime of the color centers is much shorter than the cavity round-trip time of the Nd laser. In this way the Nd pulses are simultaneously Q switched and mode locked. The pulses show a stable temporal profile, with an envelope of 160 ns and a train of mode-locked pulses of less than 200-ps duration.