Stephen A. Payne

Infrared cross-section measurements for crystals doped with Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/

The absorption and emission cross sections of the transition between the ground spin-orbit multiplet and the lowest excited multiplet were measured for Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/ ions in a variety of crystalline hosts. The materials that were investigated include LiYF/sub 4/, BaY/sub 2/F/sub 8/, Y/sub 3/Al/sub 5/O/sub 12/, LaF/sub 3/, KCaF/sub 3/, YAlO/sub 3/, and La/sub 2/Be/sub 2/O/sub 5/. The absolute magnitudes of the emission cross sections were determined from the absorption spectra, with the aid of the principle of reciprocity. The calculated radiative emission lifetimes derived from these measured cross sections agree well with the measured emission decay times for most materials. The potential use of these rare-earth-doped materials in pulsed laser applications requires that the ground state exhibit adequate splitting to minimize the detrimental effects of the ground state thermal population, and also that the emission cross section be sufficiently large to permit efficient extraction energy. The systems based on Ho/sup 3+/ in the eightfold coordinated sites of LiYF/sub 4/, BaY/sub 2/F/sub 8/, and Y/sub 3/Al/sub 5/O/sub 12/ appear to be the most promising. >

Evaluation of absorption and emission properties of Yb/sup 3+/ doped crystals for laser applications

The emission and absorption properties of numerous host crystals doped with Yb/sup 3+/ ions have been studied. The hosts which have been selected include LiYF/sub 4/, LaF/sub 3/, SrF/sub 2/, BaF/sub 2/, KCaF/sub 3/, KY/sub 3/F/sub 10/, Rb/sub 2/NaYF/sub 6/, BaY/sub 2/F/sub 8/, Y/sub 2/SiO/sub 5/, Y/sub 3/Al/sub 5/O/sub 12/, YAlO/sub 3/, LuPO/sub 4/, Ca/sub 5/(PO/sub 4/)/sub 3/F, LiYO/sub 2/, and ScBO/sub 3/. Spectral determinations have been made of the resonant absorption and emission cross sections between 850 and 1100 nm, and the emission decay times of the upper laser level have been measured. The emission cross sections have been evaluated using the absorption cross section and principle of reciprocity, and again using the Fuchtbauer-Ladenberg formula. Agreement between the two methods is within 20% for most materials. The results are discussed in the framework of requirements for an effective diode-pumped Yb/sup 3+/ laser system. Ca/sub 5/(PO/sub 4/)/sub 3/F:Yb is predicted to exhibit the most useful laser properties and is expected to be far superior to Y/sub 3/Al/sub 5/O/sub 12/:Yb in many key microscopic parameter values. >

Transition metal-doped zinc chalcogenides: spectroscopy and laser demonstration of a new class of gain media

The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

Cr/sup 2+/-doped zinc chalcogenides as efficient, widely tunable mid-infrared lasers

Transition-metal-doped zinc chalcogenide crystals have recently been investigated as potential mid-infrared lasers. Tetrahedrally coordinated Cr/sup 2+/ ions are especially attractive as lasants on account of high luminescence quantum yields for emission in the 2000-3000-nm range. Radiative lifetimes and emission cross sections of the upper /sup 5/E state are respectively /spl sim/10 /spl mu/s and /spl sim/10/sup -18/ cm/sup 2/. The associated absorption band peaked at /spl sim/1800 mm enables laser-diode pumping of the Cr/sup 2+/ systems. Laser demonstrations with ZnS:Cr and ZnSe:Cr (using a MgF/sub 2/:Co/sup 2+/ laser pump source) gave slope efficiencies up to 30%. Excited-state-absorption losses appear small, and passive losses dominate at present. Tuning experiments with a diffraction grating produce a tuning range covering at least 2150-2800 nm. Laser crystals can be produced by Bridgman growth, seeded physical vapor transport, or diffusion doping. Zinc chalcogenide thermomechanical properties of interest for medium-to-high-power operation compare favorably with those of other host materials, except for the larger refractive-index derivative dn/dT.

Strontium and barium iodide high light yield scintillators

Europium-doped strontium and barium iodide are found to be readily growable by the Bridgman method and to produce high scintillation light yields.

Laser performance of LiSrAlF6:Cr3+

We have lased the new material, LiSrAlF6:Cr3+ (Cr3+:LiSAF). The single crystals were grown by the horizontal zone melting technique. The spectroscopic properties of Cr3+:LiSAF are similar to those of other low‐field Cr3+‐doped systems, although the emission cross section is strongly π polarized and is also somewhat larger than has been measured for other fluoride hosts. The free‐running lasing wavelength of Cr3+:LiSAF is 825 nm, and the tuning range extends from at least 780 to 920 nm. Using Kr laser pumping, we obtained slope efficiencies of 36% and 14% by utilizing output couplings of 4.8% and 0.8%, respectively. On the basis of these results, the extrapolated maximum efficiency of 53% is determined, to be compared to the quantum defect‐limited value of 78%. It is concluded that a moderate level of excited state absorption (ESA) loss is responsible for the reduced efficiency of the Cr3+:LiSAF system. This contrasts with the related results previously obtained for LiCaAlF6:Cr3+, where it was concluded th...

LiCaAlF/sub 6/:Cr/sup 3+/: a promising new solid-state laser material

LiCaAlF/sub 6/:Cr/sup 3+/ (Cr/sup 3+/:LiCAF) exhibits an intrinsic (extrapolated maximum) slope efficiency of 67%. For comparison, the intrinsic slope efficiencies of BeAl/sub 2/O/sub 4/:Cr/sup 3+/ (alexandrite), Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/ and ScBO/sub 3/:Cr/sup 3+/ were found to be 65, 28, and 26%, respectively. The tuning range of LiCaAlF/sub 6/:Cr/sup 3+/ was determined to be at least 720-840 nm. The conventional spectroscopic properties, such as the absorption, emission, and emission lifetimes as a function of temperature, are reported as well. >

Resonance transition 795-nm rubidium laser

Population inversion of the 2P 1/2 and 2S 1/2 levels and continuous-wave, three-level laser oscillation at 795 nm on the D1 transition of the rubidium atom has been demonstrated. Using a titanium sapphire laser as a pump source, we obtained a slope power efficiency of 54% relative to absorbed pump power, consistent with homogeneous broadening of the rubidium pump and laser transitions. The end-pumped rubidium laser performance was well described by use of literature spectroscopic and kinetic data in a model that takes into account ground-level depletion and a pump spectral bandwidth that is substantially larger than the collisionally broadened pump transition spectral width.

Quantum electronic properties of the Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/ laser

Few of the existing Cr/sup 3+/ vibronic lasers have achieved the slope efficiency and tuning range expected based on their known spectroscopic properties. To discover the cause of this behavior, the performance of chromium-doped gallium fluoride garnet, Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/, as a laser material has been investigated experimentally. The data reported include absorption and emission spectra, emission rates, quantum efficiency, laser wavelength tuning range, laser output slope efficiencies, and excited-state absorption spectra. Similar properties of the alexandrite laser material were studied for comparison. The results indicate that the performance of the gallium fluoride garnet laser is severely limited by Cr/sup 3+/ excited-state absorption (ESA). A model is presented to account for the unexpected nature of the ESA, which appears to be a common problem for all Cr/sup 3+/ vibronic lasers. Criteria are suggested for choosing Cr/sup 3+/ hosts for which the effects of ESA will be minimized. >

Laser demonstration of Yb/sub 3/Al/sub 5/O/sub 12/ (YbAG) and materials properties of highly doped Yb:YAG

We have demonstrated the first stoichiometric Yb/sup 3+/ laser based on Yb/sub 3/A/sub 5/O/sub 12/ (YbAG). The laser operated in pulsed mode with a highest possible duty cycle of 85%. A slope efficiency of 27%, with respect to absorbed energy, was measured and the free-running lasing wavelength was 1048 nm for a 10% duty cycle. In a systematic analysis, measurements of spectroscopic and materials properties of (Yb/sub x/Y/sub 1-x/)/sub 3/Al/sub 5/O/sub 12/ for nominal x values of 0.05, 0.1, 0.15, 0.18, 0.25, 0.5, and 1 are reported. We also present a formalism to calculate the intrinsic fluorescence quantum efficiency (free of radiation trapping) and the fraction of reabsorbed light, based on measurements of the bulk and intrinsic emission lifetimes and the fractional thermal loading. Our best YbAG sample has an intrinsic lifetime of 0.664 ms at 94% quantum efficiency and a thermal conductivity at room temperature of 0.072 W/(cm-K).