The isobaric heat capacity of monocrystalline Ca2GeO4 (olivine structure) has been measured between T = (6 and 350) K by precision adiabatic vacuum calorimetry for the first time. The experimental results have been used to calculate the standard (p(o) = 0.1 MPa) thermodynamic functions C-p,m(o)/R, Delta H-T(6)m(o)/RT, Delta S-T(6)m(o)/R and Phi(o)(m)/R = Delta S-T(6)m(o)/R= Delta H-T(6)m(o)/RT (where R is the universal gas constant) of Ca2GeO4 (cr) over the range from T = (6 to 350) K. The thermal expansion coefficient (alpha) of this synthetic mineral has been determined from low-temperature X-ray measurements over the range from T = (100 to 300) K. Isochoric heat capacity C-v,C-m/R has been calculated using this low-temperature X-ray measurements and available compressibility data. The "acoustical" and "optical" components of the isochoric heat capacity have been estimated. The "optical" component of the one are calculated from the vibrational frequencies (v) as measured by Raman spectroscopy with different polarization at room temperature and at 1 bar. (C) 2014 Elsevier Ltd. All rights reserved.

Femtosecond pulses as short as 365 fs were generated from a Cr4+: Cunyite laser using a combination of a broadband semiconductor saturable absorber mirror (SESAM), chirped mirrors, and passive mode locking.

Fluorescence decay dynamics of photo-excited p-element (Bi, Pb, Sb, Sn)-doped and Al-codoped germanate glasses have been studied over a wide range of time scales and intensities upon excitation with 800 nm and 532 nm short light pulses. Decay of fluorescence contains several components with different spectral and temperature dependencies in the range from 10 ns to 1000 mu s. The temporal evolution of a fluorescence spectrum is discussed. The model of the relaxation of excitation is proposed to explain the experimental data. (C) 2009 Optical Society of America

We have shown that the broadband near-IR (NIR) fluorescence recently discovered in Bi-doped glasses is not specific due solely to Bi ions. Glasses doped with different 6p (Bi, Pb) and 5p (Sn, Sb) ions exhibit very similar behavior characterized by four major spectral peaks observed in two-dimensional excitation-emission plots and the lifetime of metastable level of about 400 micros. Our results challenge the existing models of optical centers in Bi-doped glasses. Point defect optical centers caused by the presence of 6p (Bi, Pb) and 5p (Sn, Sb) ions are proposed for the explanation of NIR emission in these laser materials.

Optical properties of neodymium self-activated nanocrystals grown in porous glass have been investigated. Mechanisms responsible for the opacity of pores glasses impregnated with solutions have been studied. Highly transparent glasses with nanocrystals grown in pores have been manufactured. Absorption, fluorescence, and fluorescence decay dynamic measurements were used to identify the phases grown in pores. We have shown that the major phase synthesized in the pores are crystal-like structures similar to that in bulk crystals. Optical properties of nanosize crystals differ from those in bulk single crystals because of specific size effects. (c) 2007 Optical Society of America.

Quasi-continuous wave (cw) laser action has been demonstrated by direct diode pumping of a new extremely broadband Cr(3+)-doped crystal. In contrast to previous works, where large-frame pump lasers have been employed, we have shown that low-power direct diode pumping of LiInGeO(4) is feasible, opening up the way for realizing compact and efficient laser sources for telecommunication applications.

The optical properties of transparent chromium-doped glass-ceramics with the chemical composition similar to that of cunyite (Cr4+:Ca2GeO4) laser crystal were investigated. Room and low temperature absorption, excitation, fluorescence and Raman spectroscopy, as well as lifetime measurements were used to develop a model for the optical centers in the glass and nanocrystals. Parent (as quenched) glass does not exhibit any fluorescence; after the heat treatment Cr4+ fluorescence band appears at ∼1280nm. Optical properties of nanocrystals formed in the glass-ceramics differ from those in the bulk single crystal Ca2GeO4. By comparison of Ca2GeO4 – like glass-ceramics with the Ca2GeO4 bulk single crystals and Mg2SiO4 – glass-ceramics we concluded that the nanocrystal phase is a modified Ca2GeO4 structure. Broadband fluorescence and a high quantum efficiency of new glass-ceramics make the new Cr-doped glass-ceramics promising medium for fiber lasers and amplifiers.

Synthesis and devitrification behavior of Cr-doped CaO–GeO2–Li2O–B2O3(Al2O3) glasses have been studied. A range of glass compositions was found to yield transparent glass-ceramics after devitrification. The size of crystallites is below 1μm. Glass-ceramic samples exhibit 1050–1600nm broad-band emission with a maximum around 1260nm, very similar to the emission of Cr4+:Ca2GeO4 bulk crystals. X-ray diffraction measurements indicate that the structure of crystallites exhibiting near infrared emission in glass-ceramics may be assigned to Cr4+:Ca2GeO4 with increased lattice parameters.

Synthesis, crystal growth, and spectroscopic characterization of Cr: LiInGeO4 belonging to olivine structure crystal family are described. Single crystals of this material were grown by flux method, using a Bi2O3-based solvent. Cr:LiInGeO4 is characterized by broad-band fluorescence in 950–1600nm wavelength range and 7μs room-temperature lifetime. The new material is to be very promising for tunable laser applications covering the important optical communication, eye safe, 1.2–1.6μm wavelength range.

The detailed spectroscopy of $\mathrm{Cr}:\mathrm{Li}\mathrm{Sc}\mathrm{Ge}{\mathrm{O}}_{4}$ and $\mathrm{Cr}:\mathrm{Li}\mathrm{In}\mathrm{Ge}{\mathrm{O}}_{4}$ is presented. Low temperature absorption, emission, and excitation spectroscopies reveal two types of optical centers: trivalent chromium in the distorted octahedral sites and tetravalent chromium in the tetrahedral sites. Energies of the transitions calculated from the exact atom positions using the angular overlap model are in a satisfactory agreement with the experimental data and describe the unusually low energy position of the ${\mathrm{Cr}}^{3+}$ metastable level. The spin-forbidden $^{2}E$ and $^{2}T_{1}$ transitions of ${\mathrm{Cr}}^{3+}$ appear as a Fano antiresonance in the absorption and excitation spectra. The temperature quenching of ${\mathrm{Cr}}^{3+}$ fluorescence is described in terms of the Struck and Fonger model. ${\mathrm{Cr}}^{3+}$ ions in highly distorted octahedral positions are active centers responsible for recently discovered ${\mathrm{Cr}}^{3+}$ laser action in the near-infrared range in these crystals.

We present a spectroscopic study of transparent forsterite nanocrystalline glass-ceramic doped with chromium, a promising active medium for near-infrared fiber-optic applications. Absorption, emission, excited-state absorption spectra, and continuous function decay analysis of luminescence decay reveal the presence of Cr(3+) and Cr(4+) centers in both glass and crystal phases. The optical behavior of Cr(3+) and Cr(4+) centers is discussed and compared with that in bulk forsterite crystals. (C) 2004 Optical Society of America.

The growth and optical properties of Cr4+-doped Li2CaSiO4 single crystals are described. Absorption spectroscopy together with calculations of energy levels shows distortion of tetrahedral sites with Cr4+ doping. The symmetry of the local environment was assigned to D2d; ligand field parameters were determined for both undistorted and distorted tetrahedral sites. Emission spectra show broadband fluorescence from 1000 to 1500 nm. The fluorescence spectrum is structureless at room temperature and exhibits sharp lines at low temperature. The temperature dependence of nonradiative relaxation shows a quantum efficiency ∼46%. From the experimental data and the best fit with the Struck and Fonger model, the following spectroscopic parameters were obtained: effective energies of odd- and even-parity phonons are hωodd=158 and hωeven=564 cm−1, respectively, Huang–Rhys parameter S=2.1, nonradiative constant Knr=1014 s−1, and the number of phonons bridging the energy gap is ∼19. Long lifetime (41 μs at 300 K) and high quantum efficiency make this crystal a new promising tunable laser medium for the near-infrared region.

Crystal growth and optical properties of Cr4+ centers in Li2TiGeO5 single crystals were investigated. Polarized absorption spectra at low and room temperature indicate a lowering of symmetry of the local environment of the Cr4+ ion from T-d to C-2 group of symmetry. The crystal field parameter Dq and the Racah paprameter B were determined to be Dq=930 cm(-1), B=510 cm(-1), and Dq/B=1.8. Emission spectra show wideband fluorescence from 1100 to 1600 nm with the maximum at 1200 nm. The lifetime of the metastable level decreases from 62 mus at 77 K to 6 mus at 300 K, indicating strong nonradiative quenching and a quantum efficiency of less than 10%. (C) 2003 American Institute of Physics.

The optical properties of Cr4+ centers in Ca2GeO4 (Cunyite) single crystals co-doped with Er3+ ions have been measured. Excitation of Cr4+ produces both Cr4+ fluorescence centered at 1280 nm and Er3+ fluorescence centered at 1550 nm. The cross-section of the 4I13/2–4I15/2Er3+ transition is estimated using Judd–Ofelt analysis to be 8×10−21cm2. The quantum efficiency of the energy transfer from Cr4+ to Er3+ is obtained based on measurements of the decay of fluorescence from Cr4+. The relative populations and gain coefficients of Cr4+ and Er3+ centers under pumping in the Cr4+ band are calculated with varying Er3+ concentrations. The co-doped Cunyite (Ca2GeO4) crystal can be used as an active optical medium for 1200 to 1600 nm region.

An electron paramagnetic resonance study of a synthetic single crystal of Cr-doped LiScGeO4 was carried out at the X- and Q-bands at 300 K and at the broad band (70-370 GHz) at 4.2 K. It was established that the EPR spectra with the magnetic multiplicity KM = 2 observed in all the frequency bands are due to the Cr3+ substituted for Sc3+ at the mirror symmetry octahedral site. The angular dependences of the two symmetry-related spectra of Cr3+ in the three crystallographic planes were fitted with the spin Hamiltonian (S = 3/2) of monoclinic symmetry. The zero-field splitting of the ground state energy levels was determined as 1.309(5) cm-1 which compares well with that for Cr3+ in forsterite and alexandrite crystals with similar olivine-like structure. Additional weak lines due to Mn2+ and Fe3+ at the mirror symmetry sites were also identified in the X- and Q-band spectra.