Gary D. Wilke

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.

Optical properties and laser demonstrations of Nd-doped sol-gel silica glasses

Abstract Nd-doped sol-gel silica glasses possessing optical quality that was adequate for a simple demonstration of laser action have been prepared. The highest quantum yield and longest emission lifetime obtained were 55% and 425 μs, respectively, for an Nd doping level of 0.36 × 1020 cm−3. The optical scattering loss was found to be on the order of 4.5%/cm. The sol-gel samples were prepared using a novel combination of techniques, including, for example, the employment of a particulate silica filler material to ensure crack-free drying, and the addition of propylene oxide to reduce the gelation time.

Effects of OH content, water vapor pressure, and temperature on sub-critical crack growth in phosphate glass

Abstract The effects of temperature and water vapor pressure on the rate of sub-critical crack growth in meta-phosphate laser glasses containing different OH concentrations (128 and 773 ppmw) are reported. The crack velocity was measured using the double-cleavage-drilled-compression method. When plotted as a function of stress intensity, the samples have the classic region I, II and III crack growth properties similar to that reported for silicate glasses. The glass containing the larger OH content has a 10-fold greater crack velocity in region I; crack velocities is region II are the nearly the same for both glasses. The crack velocities are analyzed using a chemical kinetic and mass-transport limited reaction rate model. At temperatures >150°C and water vapor pressures >10 mmHg, crack tip blunting is observed and the glass containing the larger OH content is more prone to blunting.

Optical spectroscopy of the new laser materials, LiSrAlF6:Cr3+ and LiCaAlF6:Cr3+

Abstract We have obtained the absorption and emission spectra and the emission lifetimes, of Cr 3+ -doped LiCaAlF 6 and LiSrAlF 6 . The spectral observations can be understood by carefully accounting for the small non-octahedral components of the crystal potential field existing at the substitutional Al site. The polarization properties of the absorption spectra are due to a static t 2u -type component of the field, although the odd-parity dynamical component also contributes significantly to the oscillator strength. An analysis of the spin-orbit components of the 4 T 2 state reveals the importance of a Jahn-Teller distortion in the e g coordinate. Finally, the overall configurational displacement between the ground and excited states is found to be larger in LiSrAlF 6 , compared to LiCaAlF 6 .

Auger upconversion losses in Nd-doped laser glasses

Abstract We have measured the Auger upconversion constant, γ, of Nd 3+ ions in phosphate, silicate, and fluoride glasses. The Auger process involves two Nd 3+ ions initially in the 4 F 3 2 metastable state that interact such that one ion is promoted to a higher lying state, while the other is forced to the 4 I J levels of the ground state. γ values ranging from 2.6 to 8.1 × 10 -17 cm 3 s -1 were measured, depending on the host medium and neodymium concentration. The product of the emission lifetime and the Auger coefficient was found to be in the range of γτ = (1 − 2) × 10 -20 cm 3 for the materials investigated. This suggests that Auger upconversion is an important problem for excited state densities approaching (5 − 10) × 10 19 cm -3 .

Laser properties of a new average-power Nd-doped phosphate glass

The Nd-doped phosphate laser glass described herein can withstand 2.3 times greater thermal loading without fracture, compared to APG-1 (commercially available Average-Power Glass from Schott Glass Technologies). The enhanced thermal loading capability is established on the basis of the intrinsic thermomechanical properties (expansion, conduction, fracture toughness, and Youngs modulus), and by direct thermally induced fracture experiments using Ar-ion laser heating of the samples. This Nd-doped phosphate glass (referred to as APG-t) is found to be characterized by a 29% lower gain cross section and a 25% longer low-concentration emission lifetime. Other measurements pertaining to the concentration quenching, thermal lensing, and saturation of the extraction are also described in this article. It is note-worthy that APG-t offers increased bandwidth near the peak of the 1054 nm gain spectrum, suggesting that this material may have special utility as a means of generating and amplifying ultrashort pulses of light.

Index-of-refraction change in optically pumped solid-state laser materials.

Degenerate four-wave mixing measurements were made on several Nd(3+)-doped laser materials by resonantly pumping the Nd(3+) ions. The dominant contribution to the signal under these conditions is associated with the difference in polarizabilities of the Nd(3+) ions in the metastable state versus the ground state. Absolute measurements of the signal strengths imply a laser-induced polarizability change of the order of 0.01-0.05 A(3). The results indicate that, for the case of a single-shot laser amplifier system, the beam distortion induced by this population induced change in the refractive index is comparable with the distortion arising from thermal effects.

Optical properties and nonradiative decay of Sm2+ in CaF2-YF3 and CaF2-LaF3 mixed crystals

The authors have obtained the absorption and emission spectra for the Sm2+ impurity in the mixed fluorides, Ca1-xYxF2+x and Ca1-xLaxF2+x. From these spectra they conclude that the Sm2+ ions tend to be coordinated by one or more extra F- ions in addition to the normal eightfold coordination expected in CaF2. Measurements of the emission lifetime and intensity as a function of temperature lead them to propose that the luminescence quenching of Sm2+ in the mixed fluorides is due to a competition between ionisation of the 4f55d level with relaxation into the 5D0 (4f6) emitting state. In addition, the thermal activation energy for Sm2+ emission quenching in various alkaline-earth halides is consistent with the calculated optical separation of the 4f55d level from the conduction band of the host, lending further support to the ionisation mechanism of emission quenching.

Transient gratings by 4f → 5d excitation of rare earth impurities in solids

Abstract We have generated transient population gratings in CaF2 : Eu2+ and LiYF4 : Nd3+ by utilizing 355 nm excitation from a pulsed laser source. The scattering from the gratings was observed with the aid of a helium-neon laser operating at 632.8 nm. The grating is the result of a 4f → 5d excitation mechanism for both crystals. For the case of CaF2 : Eu2+, the grating is absorptive in origin, and is due to the excitation of the 4f7 → 4f65d transition by the crossed pump beams. A unique situation is encountered for the LiYF4 : Nd3+ system, in which the grating is created via a two-step two-photon excitation process, described by the transition sequence: 4I9/2(4f3) → 4D3/2(4f3) → 4f25d. Following excitation into the 4f25d manifold, a portion of the Nd3+ ions rapidly relax into the high-lying 2G9/2(4f3) state. The grating arises from the difference in the optical properties of the 2G9/2 and 4I9/2 states.