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We report absorption, fluorescence, and thermometric studies of both (α) cubic and (β) hexagonal phase KYF4:Yb/Er nanoparticles. The cubic phase of KYF4:Yb/Er nanoparticles was achieved using a hydrothermal approach, and the cubic to hexagonal phase transformation was successfully realized by controlling the annealing temperature at 500 °C. Both nanoparticle systems exhibit similar maxima in the Yb3+ absorption spectra at 10 268 cm−1 (974 nm). The 2H11/2, 4S3/2, and 4F9/2 → 4I15/2 transitions are recorded in the Er3+ upconversion fluorescence spectrum using 974 nm laser excitation. The green and red fluorescence of the hexagonal phase were approximately 100 and 2000 times more intense, respectively, than that of the cubic phase. The red to green ratio (R/G) was 50:1 and 2:1 for β-KYF4:Yb/Er and α-KYF4:Yb/Er nanoparticles, respectively. A three-photon process is involved in green upconversion for β-KYF4:Yb/Er, as described by 2F5/2 (Yb3+) + 4F9/2 (Er3+) → 2F7/2 (Yb3+) + 4G11/2 (Er3+). Using the fluorescence intensity ratio technique from thermally coupled Er3+2H11/2 and 4S3/2 levels, a very high thermal sensitivity of 1.76 and 1.49% K−1 at ambient temperatures was achieved for α- and β-phase materials, respectively. [ABSTRACT FROM AUTHOR]

Under ideal conditions, the optical path for the two counter-propagating beams in a square ring laser cavity is expected to be entirely reciprocal. This property, together with the absence of any moving parts in the gyro, makes ring lasers a very useful rotation-sensing device. For a typical aircraft application, a sensor stability of the order of 0.01 ° /h and a resolution of 1 ppm is required. The demands for inertial rotation sensing in space geodesy are three orders of magnitude higher. Therefore, the perturbations from the presence of the active laser gain medium inside the cavity cannot be ignored. While these perturbations can be sufficiently contained in aviation gyros due to the much lower requirements, they cause a notable bias in large ring laser gyroscopes for the observation of the instantaneous rotation rate of the Earth. In this paper, we report on an improved model for bias stability from the presence of the laser gain medium in the gyro cavity of the large ring laser "G" at the Geodetic Observatory Wettzell. Typical values between 5 and 10 ppB are obtained over several months. [ABSTRACT FROM AUTHOR]

We report Zeeman infra-red spectroscopy of electronic-nuclear levels of 5I8 → 5I7 transitions of Ho3+ in the C4v(F–) centre in CaF2 with the magnetic field along the 〈111〉 direction of the crystal. Transitions to the lowest 5I7 state, an isolated electronic doublet, and the next group of states, a pseudo-quadruplet consisting of a doublet and two nearby singlets, exhibit strongly non-linear Zeeman splittings and intensity variations. Simulated spectra based upon a crystal-field analysis give an excellent approximation to the data, illustrating the strong predictive ability of the parametrized crystal-field approach. Anti-crossings in the hyperfine splittings, the basis of quantum information storage in rare-earth doped insulating dielectrics, are also predicted. [ABSTRACT FROM AUTHOR]

We model the dynamic behaviour observed for impurity-trapped excitons in SrF2:Yb2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF2. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states. [ABSTRACT FROM AUTHOR]

Single-component ring lasers have provided high-resolution observations of Earth's rotation rate as well as local earthquake- or otherwise-induced rotational ground motions. Here, we present the design, construction and operational aspects of ROMY, a four-component, tetrahedral-shaped ring laser installed at the Geophysical Observatory Fürstenfeldbruck near Munich, Germany. Four equilateral, triangular-shaped ring lasers with 12 m side length provide rotational motions that can be combined to construct the complete vector of Earth's rotation from a point measurement with very high resolution. Combined with a classic broad-band seismometer, we obtain the most accurate 6 degree-of-freedom ground motion measurement system to date, enabling local and teleseismic observations as well as the analysis of ocean-generated Love and Rayleigh waves. The specific design and construction details are discussed as are the resulting consequences for permanent observations. We present seismic observations of local, regional and global earthquakes as well as seasonal variations of ocean-generated rotation noise. The current resolution of polar motion is discussed and strategies how to further improve long-term stability of the multicomponent ring laser system are presented. [ABSTRACT FROM AUTHOR]

We report Zeeman infra-red spectroscopy of electronic-nuclear levels of 5I8 → 5I7 transitions of Ho3+ in the C4v(F–) centre in CaF2 with the magnetic field along the 〈111〉 direction of the crystal. Transitions to the lowest 5I7 state, an isolated electronic doublet, and the next group of states, a pseudo-quadruplet consisting of a doublet and two nearby singlets, exhibit strongly non-linear Zeeman splittings and intensity variations. Simulated spectra based upon a crystal-field analysis give an excellent approximation to the data, illustrating the strong predictive ability of the parametrized crystal-field approach. Anti-crossings in the hyperfine splittings, the basis of quantum information storage in rare-earth doped insulating dielectrics, are also predicted. [ABSTRACT FROM AUTHOR]

Hyperfine structure is observed on several infrared transitions to the 5I7 and 7I6 multiplets of the Ho3+ C4v center in CaF2. Particularly complex and detailed hyperfine structures are observed for transitions to the Y3 level of the 5I7 multiplet, which include the appearance of ΔI2 ≠ 0 transitions. The hyperfine lines of this level are satisfactorily accounted for by strong mixing of close-lying levels of this multiplet by the perpendicular hyperfine interaction, which leads to relaxation of the ΔI2 = 0 selection rule and a redistribution of the hyperfine intensities over many transitions. An excellent correspondence is found between the measured and simulated spectra. [ABSTRACT FROM AUTHOR]

Hyperfine structure is observed on several infrared transitions to the 5I7 and 7I6 multiplets of the Ho3+ C4v center in CaF2. Particularly complex and detailed hyperfine structures are observed for transitions to the Y3 level of the 5I7 multiplet, which include the appearance of ΔI2 ≠ 0 transitions. The hyperfine lines of this level are satisfactorily accounted for by strong mixing of close-lying levels of this multiplet by the perpendicular hyperfine interaction, which leads to relaxation of the ΔI2 = 0 selection rule and a redistribution of the hyperfine intensities over many transitions. An excellent correspondence is found between the measured and simulated spectra. [ABSTRACT FROM AUTHOR]

The local mode lineshape for substitutional H− ions in CaF2 shows a broadened, two peak structure when La3+ co-doping up to 6 mol% is included. Pump-probe measurements of the local mode lifetime show a decrease in the anharmonic decay rate as a function of disorder. This phenomenon appears well correlated with four-wave mixing measurements which display a multiexponential behaviour having long lasting components with a homogeneous dephasing time greater than 90 ps. [Copyright &y& Elsevier]

A single H− site with axial symmetry is observed for LiYF4 crystals while two distinct H− sites of axial and rhombic symmetry are observed for KY3F10 crystals. Infrared pump–probe measurements yield 10 K lifetimes of 2.7 and 14 ps for the axial H− sites in LiYF4 and KY3F10, respectively. The temperature dependence of relaxation processes for these H− modes is modelled on a combination of anharmonic phonon decay and elastic-phonon scattering processes. [Copyright &y& Elsevier]