Jeremy Peppers

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.

Fe-doped II-VI mid-infrared laser materials for the 3 to 8 μm region

We report on recent progress in development of new gain media for tunable (3-8 μm) mid-IR lasers as well as Fe:ZnS/Se lasers operating in CW (>1.5W), gain-switched (>1 mJ@7ns@lkHz) and long-pulse (>0.5 J@20μs) regimes.

Chromium doped ZnSe and ZnS gain media for optically and electrically pumped mid-IR lasers

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.

Mid-IR spectroscopy of Fe:ZnSe quantum dots

We report spectroscopic characterization of Fe:ZnSe quantum dots (for 2% of Zn/Fe molar ratio) fabricated by microemulsion hydrothermal synthesis. Mid-IR photoluminescence of the 5E↔5T2 transition of Fe2+ ions over 3.5-4.5 μm spectral range was observed in Fe:ZnSe quantum dot samples and kinetics of luminescence have been characterized at temperatures of 30-300 K under direct (2.788 μm) mid-IR excitation and indirect (0.355 μm) photoionization excitation. The radiative lifetime (τrad) was estimated from these measurements to be 48 µs while lifetime at room temperature was measured to be 440 ns. This agrees closely with the behavior of bulk material.

Spectroscopic characterization of Ti3+:AgGaS2 and Fe2+:MgAl2O4 crystals for mid-IR laser applications

Mid-IR lasers based on Cr²⁺ and Fe²⁺ 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 AgGaS₂ and Iron doped MgAl₂O₄ crystals with coordination number four. Polarized absorption and luminescence spectra of titanium doped AgGaS₂ crystals were measured at room and low temperatures. It was shown that Ti ions in the AgGaS₂ crystal were in both 1+ and 3+ valence states substituting silver and gallium sites, correspondingly. Ti³⁺ ions in AgGaS₂ and Fe²⁺ in MgAl₂O₄ 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 Ti³⁺ absorption band. A luminescence signal between 3 and 6 μm was also observed in the Fe:MgAl₂O₄ crystal.

Mid-IR electroluminescence of Cr:ZnSe crystals co-doped with donor and acceptor impurities

Studies of divalent Chromium ions compensation in ZnSe crystals after thermo-diffusion of Al and Ag co-dopants are presented. We report 2-3mm Mid-IR electro-luminescence in the Al:Cr:ZnS and Ag:Cr:ZnSe bulk samples.

Fe-doped Binary and Ternary II-VI Mid-Infrared Laser Materials

We report on recent progress in development of new iron-doped binary and ternary mid-IR gain media for 3-8 μm spectral region and laser operation in CW, gain-switched and long-pulse regimes.

Mid-IR laser oscillation via energy transfer in the Co:Fe:ZnS/Se co-doped crystals

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 ⁴A₂→⁴T₁(⁴P) and ⁴A₂→⁴T₁(⁴F) 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 8.5-19x10¹⁸ cm^-3. Kinetics and photoluminescence spectra reveal energy transfer under cobalt excitation at ⁴A₂-⁴T₁(⁴F) transition by 1.56 μm radiation and ⁴A₂-⁴T₁(⁴P) transition by 0.7 μm radiation. Analysis shows effective energy transfer from ⁴T₁(⁴F), ⁴T₂ and ⁴T₁(⁴F) Co²⁺ energy levels to ⁵T₂ excited level of Fe²⁺ ions the first realization of Fe²⁺ ions lasing at 3.6 μm and 3.8μm via Co-Fe energy transfer. Demonstrated effective Co²⁺→Fe²⁺ energy transfer process could result in utilization of a more convenient laser pump sources for the Fe²⁺:II-VI lasers.

Laser spectroscopy of highly doped NV- centers in diamond

In this paper, prospects of using diamond with NV− centers as a gain medium have been studied. Spectroscopic characterization of NV− centers in diamond as well as absorption saturation and pump-probe experiments have been carried out. Absorption and emission cross-sections were estimated to be 2.8 × 10-17 cm2 and 4.3 × 10-17 cm2 at the maximum of absorption and emission bands, respectively. It was observed from emission spectra under pulse excitation that some NV− are photoionized to NV0 centers with ZPL at 575 nm. Room temperature luminescence lifetime of NV− centers was measured to be 12ns, which is close to the previously reported lifetime in bulk diamond (~13ns). Saturated transmission was only about 11% of calculated values even at energy fluence much higher than the saturation flux. Two excited state absorptions (ESAs) with different relaxation times (“fast-decay” and “slow-decay with relaxation times of ~500 ns and several tens of microseconds, respectively) were revealed in transmission decay kinetics at 632 nm. Kinetics of transmission at 670 nm was dominated by “slow-decay” ESA process. Kinetics of dk/k0 in shorter wavelength were strongly dominated by “fast-decay” ESA process. These results definitively indicate that stimulated emission of NV− centers is suppressed by photoionization and ESAs and the possibility of diamond lasers based on NV− centers is low.

Recent progress in mid-IR materials and lasers based on Cr and Fe doped chalcogenides (Conference Presentation)

II-VI chalcogenides (e.g. ZnSe/S) doped with transition metal (TM) ions such as Cr, and Fe are arguably the materials of choice for fabrication of effective mid-IR gain media. TM:II-VI materials feature a favorable blend of laser spectroscopic parameters: a four-level energy structure, absence of excited state absorption, close to 100% quantum efficiency of fluorescence (for Cr doped II-VI media), broad mid-IR vibronic absorption and emission bands. This talk summarizes progress in fabrication of high quality Cr:ZnS/Se and Fe:ZnS/Se by cation vacancy and cation interstitial enhanced post growth thermal diffusion. We also describe recent breakthrough on recrystallization and effective doping of ZnS ceramics under hot isostatic pressing resulting in a large cm-scale monocrystalline domains formation and an increase of the Fe diffusion coefficient by three orders of magnitude. We report recent advances in high-power Cr:ZnS/Se and Fe:ZnSe laser systems, enabling a wide range of tunability (1.8-5.0µm) with output power levels of up to 140 W near 2500 nm, 32 W at 2940 nm, and 35 W at 4300 nm with corresponding optical efficiencies of 62%, 29%, and 35%. Current improvements of output characteristics of polycrystalline Cr:ZnS/Se oscillators in Kerr-Lens-Mode-Locked (KLM) regime are reported: up to 2 W output power at 75-1200 MHz repetition rate, up to 2 cycle pulse duration (16 fs) with efficiency of 20-25% with regards to Er-fiber laser pump power. The effects of efficient up-conversion of mid-IR fs pulses in the laser medium as well as supercontinuum generation are demonstrated. Further extension of mid-IR spectral coverage to 3-8 m is demonstrated by Cr:ZnS KLM laser pumped degenerate (subharmonic) parametric oscillators (OPOs) based on based on quasi-phase matching in orientation-patterned gallium arsenide, and random phase matching in polycrystalline ZnSe.