Forsterite Crystals Research Articles

Optical properties of forsterite crystals grown by the Czochralski method

Optical properties of forsterite crystals grown by the Czochralski method

Nonradiative energy transfer from the impurity ion into the host lattice forCr4+-doped forsterite laser crystal

A theoretical model explaining the nonradiative energy transfer from a photoexcited impurity ion into the lattice in a laser crystal is presented. The energy-transfer mechanism consists of electronic energy transfer to local vibrations that then dissipate their energy to lattice (phonon) modes of the same energy creating a nonequilibrium phonon population. The model explains the experimental temporal profiles of nonequilibrium optical phonons probed by time-resolved Raman scattering in Cr-doped forsterite laser crystal. The electronic transition time and relaxation lifetimes for phonon and for local modes are $\ensuremath{\sim}3,$ 4, and 8 ps, respectively.

Electron paramagnetic resonance study of at position in forsterite

In an electron paramagnetic resonance study of a synthetic single crystal of forsterite, , one spectrum of with maximum local symmetry -1 was observed. This spectrum arises from substituted for at the position. The second-order constants of the spectrum are and . All the fourth-order spin-Hamiltonian constants are given: they are much larger than generally observed in silicates. The zero-field splitting of the EPR spectrum is larger than the energy of the hyperfrequency field in the Q band; this case is near the limit of possibility of calculating the fourth-order constants of the spin Hamiltonian. It is shown that the substitution of for does not strongly modify the local structure and that there is no local charge compensation.

In situ oxygen isotope analysis with an excimer laser using F 2 and BrF 5 reagents and O 2 gas as analyte

In situ oxygen isotope analysis of silicate minerals has been validated by interlaboratory calibration using an excimer laser (KrF fill gas, 248 nm), F 2 gas fluorinating reagent, and O 2 gas as mass spectrometer analyte. A value of δ 18O SMOW = 5.82%. (±0.03, 4 analyses) was obtained for almandine UWG-2 compared to a recommended value of 5.8%0 (Valley et al., 1995). The interhalogen BrF5 was tested by comparing in situ analyses made alternatively with either BrF 5 or F 2 gas on crystals of almandine, epidote, forsterite, tourmaline, and zircon. Results for different pressures of BrF 5 (20 to 40 torr) in the reaction chamber show that δ 18O values decrease by 1 permil for an increase of 10 torr in BrF 5 pressure. In strong contrast, δ 18O values measured with purified F 2 gas are precise and accurate over a range of from 15 to 150 torr F 2 gas pressure. Use of F 2 gives higher yields of O 2 than with BrF 5. Values of δ 17O and δ 18O measured in situ on a variety of silicates all plot on the terrestrial mass fractionation line, provided that NF 3 contamination is eliminated from the O 2 gas analyte.

Experimental determination of the nonlinear refractive index in an operating Cr:forsterite femtosecond laser

The characteristics of a femtosecond forsterite Kerr-lens mode-locked laser pumped synchronously by an Nd:YAG laser are reported. Transform-limited femtosecond pulses of 60-fs (FWHM) duration with average TEM 00 output powers of 200 mW at 1.25 μm were generated. Experimental studies of power stability and self-starting regime are described and a new experimental method to determine the nonlinear refractive index of the chromium-doped forsterite crystal (which is either the laser rod) is reported. We infer a value of 6 × 10 −16 cm 2/W at 1.25 μm, making possible Kerr-lens mode-locking.

Spectroscopic properties of Cr-doped and Cr,Li-doped synthetic forsterite crystals

Cr-droped and Cr,Li-doped forsterite crystals were grown and their optical properties were investigated. It was shown that when only Cr is doped, Cr3+ is substituted at the site of low crystal field, and the energy level 2E lie above the 4T2 level, while 4T2 is just above 2E when Cr and Li are codoped. The difference was rationalized by a deformation of the Cr substituted site with the introduction of Li.

Self-starting mode-locked femtosecond forsterite laser with a semiconductor saturable-absorber mirror

We report on a self-starting mode-locked femtosecond Cr:forsterite laser pumped by a diode-pumped Nd:YVO(4) laser. The mode locking is initiated by a semiconductor saturable-absorber mirror (SESAM). We also present the measured group delay of the forsterite crystal and the SESAM.

Influence of thermal effects in Kerr-lens mode-locked femtosecond Cr 4+:forsterite lasers

We report the characterization of a synchronously-pumped Kerr-lens mode-locked (KLM) femtosecond Cr 4+:forsterite laser, generating 66 fs pulses at 1240 nm, with ≈ 200 mW output power. The influence of the induced thermal lensing on the fast Kerr modulation is analysed by a simple model, which shows that the conventional criteria employed for KLM optimization of Ti:sapphire must be revised when considering the strong thermal effects induced in the Cr 4+:forsterite crystal. Indications from our simple model agree with the experimental results.

Nucleation-dominated crystallization of forsterite in the Ubehebe Peak contact aureole, California

The morphology and number of forsterite crystals in the Ubehebe Peak, California, contact aureole vary systematically as a function of metamorphic grade. From their first appearance to as close as ∼150 m from the intrusive contact, forsterite crystals are large (5–20 mm) and have a tabular habit ( a ≈ c ≫ b ). In contrast, forsterite near the contact is equigranular and much smaller (<1 mm in diameter). The number of crystals per mole of forsterite increases from 3.5 × 104 at the forsterite-in isograd to more than 1.5 × 108 near the contact. This trend is interpreted to result from an increase in the ratio of nucleation rates with respect to growth rates with proximity to the intrusion. The change in morphology from tabular to equigranular is explained by kinetic surface roughening. The nucleation and growth information gained from this study highlights the important role of nucleation kinetics in the crystallization of forsterite at the Ubehebe Peak aureole.

Efficient laser operation of heavily doped Cr4+:forsterite

Pulsed laser operation of a new high Cr4+ concentration forsterite crystal is reported. Slope efficiencies of 13% and 5% have been obtained for 1064 and 532nm pumping. Results are compared with the performance of three other forsterite crystals. A threefold enhancement in operating efficiency is obtained over standard low dopant-level material. Tuning ranges have also been investigated.

Forsterite powder prepared from water-soluble hybrid precursor

Partially polycondensed y-aminopropylsilanetriol (APSTOL) was used as the silica precursor to prepare magnesium orthosilicate in the form of crystalline forsterite. Aqueous solutions of APSTOL and magnesium acetate were mixed and dried forming a water-soluble product that was converted into forsterite by heating to 1,000°C. The thermal evolution of the system was studied using TGA, FTIR, UV-VIS spectrophotometry, and XRD techniques, especially for the acetate decomposition and transformation of CSiO 3 -tetrahedra into the orthosilicate anions. The combustion of organics and the crystallization of forsterite proceed at temperatures 350 and 870°C, respectively. At intermediate temperatures, the system contains amorphous silicates with oligomeric and polymeric anions, which gradually depolymerize as temperature increases. The resulting crystalline product contains less impurities (periclase and enstatite) than samples prepared using other wet-chemistry techniques.

Femtosecond Cr:forsterite laser with mode locking initiated by a quantum-well saturable absorber

We studied a mode-locked Cr:forsterite laser pumped by a diode pumped Nd:YVO/sub 4/ laser. Both the Kerr lens mode locking and the semiconductor saturable absorber initiated mode locking have been demonstrated. Using our measured dispersion data of the forsterite crystal, together with our dispersion compensation technique, we obtained 20-fs pulses for the pure Kerr lens mode locking and 36-fs pulses for semiconductor saturable absorber initiated mode locking, respectively.

Magnesium site exchange in forsterite; a direct measurement by high-temperature25Mg NMR spectroscopy

High-resolution 25Mg NMR data are reported for a single crystal of pure forsterite (Mg2Si04) at temperatures to about 1400 °C, which allow rates of exchange of Mg cations among specific sites in the structure to be obtained. Results are consistent with estimates of hopping frequencies derived from macroscopic diffusivity data, suggesting that this approach holds potential for tracing microscopic diffusion pathways in even very refractory minerals.

Oxygen activity dependence of the chromium (IV) population in chromium-doped forsterite crystals grown by the floating zone technique

The lasing of Cr-doped forsterite, Cr:Mg 2SiO 4, at 1.2 μm has been attributed to the presence of Cr 4+. The goal of this work was to determine the Cr 4+ Cr 3+ ratios of crystals grown at different oxygen partial pressures and to compare them with a model based on point defect thermodynamics. Cr:forsterite crystals were grown by the floating zone technique with gas atmospheres of 100% Ar, 99% Ar + 1% O 2, 80% Ar + 20% O 2, 33% Ar + 67% O 2, and 100% O 2 at 1 atm total pressure. The Cr 4+ Cr 3+ ratios, as well as the distribution of the Cr 3+ cations on the M1 and M2 sites, were determined by electron paramagnetic resonance techniques. The fraction of Cr present as Cr 4+ ions as a function of the oxygen activity of the growth environment was well described by the model.

The role of active ion concentration in tuned chromium forsterite oscillators

We report a comparative study of the tuned oscillator performance of chromium forsterite crystals with a range of crystal characteristics. Emphasis is placed on results obtained for the recently developed high dopant level materials. Tuning range, conversion efficiency and peak emission wavelength are analysed for prism-tuned and Littrow grating cavities. Diagnostic spectral measurements are carried out following second harmonic generation in KTP. Linewidth is measured for the Littrow cavity, and the potential for narrow bandwidth lasers is assessed.

Chromium-activated forsterite crystal as the passive Q switch in the cavity of an Nd:YAG laser and as the active medium in an Nd:YAG—Cr:forsterite laser system

An investigation was made of the generation and self-modulation of the λ = 1.064 and 1.25 μm radiation in an Nd:YAG—Cr:forsterite laser system. The advantages of this system over those with traditional methods were demonstrated. The feasibility of self-control of the rate of pumping of the Cr:forsterite crystal was revealed and recommendations on optimisation of the system were developed.

Criterion for selecting phonons and local vibrations involved in the nonradiative relaxation of photoexcited Cr4+ ions in forsterite

The energy of Cr4+ local modes and the energy and dynamics of Mg2SiO4 phonon modes involved in the nonradiative relaxation of photoexcited impurity Cr4+ ions in forsterite crystal were measured. The experimental results suggest that resonance in energy between local vibrations of the impurity ion and lattice (phonon) modes of the host crystal plays a crucial role in selecting the phonons which participate in the nonradiative relaxation of the photoexcited ions in crystals.

Femtosecond forsterite Kerr-lens mode-locked laser pumped synchronously by an Nd3+:YAG laser

A femtosecond laser (pulse duration 30 fs, centre of the emission spectrum at λ = 1260 nm) with a chromium-doped forsterite crystal was developed. This laser operated on the basis of Kerr-lens mode locking. Synchronous pumping with an Nd3+:YAG picosecond laser resulted in forced generation of femtosecond pulses characterised by an enhanced stability of the output power, which was 5–7 times better than the stability of the pump laser. The output radiation of the forsterite laser was converted efficiently to the second harmonic (central wavelength 630 nm) in a thin nonlinear BBO crystal. The average power of the second-harmonic radiation was 15 mW.

Temporal and kinetic studies of chromium forsterite oscillators with 1064 nm laser excitation

Temporal characteristics and kinetics of the gain-switched operation of a chromium forsterite oscillator are presented for fundamental Nd:YAG laser excitation. Three temporal excitation regions have been studied with the pump laser running in fixed Q and Q-switched modes. Measured temporal behaviour as well as threshold and efficiency measurements are compared with results obtained using 532 and 732 nm excitation. The laser performance of three different chromium forsterite crystals is compared. A threshold of 0.04 J and a slope efficiency of 26% have been measured for one crystal. In one case, 10 ns gain-switched pulses were emitted with a 35 ns delay from the pump pulse, the shortest yet reported with a simple chromium forsterite laser system.

Chromium doped forsterite ring laser

Operation of a Cr 4+: Mg 2SiO 4 crystal in a ring cavity is studied with different dopant levels. Threshold energies of 13 mJ, 10.6 mJ and 11 mJ were measured for Mitsui low-doped, Russian and Mitsui highly doped crystals respectively. Slope efficiencies of 0.9%, 1.1% have been obtained for the Mitsui low-doped and Russian crystals. An efficiency of 3.6% was recorded for the highly doped chromium forsterite crystal from Mitsui. The peak output wavelength is 1275 nm. These results are compared with these crystals in the simple cavity.