The T dependence $\left(2\mathrm{}-400\hspace{0.5em}\mathrm{K}\right)$ of the electron paramagnetic resonance (EPR), magnetic susceptibility $\chi \mathrm{}\left(T\right),$ and specific heat $C_v\mathrm{}\left(T\right)\mathrm{}$ of the normal antiferromagnetic (AFM) spinel ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ and the spin-glass (SG) ${\mathrm{Zn}}_{1-x}{\mathrm{Cd}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ $\mathrm{}(x=0.05,0.10\mathrm{})\mathrm{}$ are reported. These systems behave as a strongly frustrated AFM and SG with $T_N\approx T_G\approx 12\hspace{0.5em}\mathrm{K}$ and $-400\hspace{0.5em}\mathrm{K}\gtrsim {\Theta }_{\mathrm{CW}}\gtrsim -500\hspace{0.5em}\mathrm{K}.$ At high-T the EPR intensity follows the $\chi \mathrm{}\left(T\right)\mathrm{}$ and the g value is T independent. The linewidth broadens as the temperature is lowered, suggesting the existence of short range AFM correlations in the paramagnetic phase. For ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$ the EPR intensity and $\chi \mathrm{}\left(T\right)\mathrm{}$ decreases below 90 and $50\hspace{0.5em}\mathrm{K},$ respectively. These results are discussed in terms of both nearest-neighbor ${\mathrm{Cr}}^{3+}$ $\mathrm{}(S=3/2\mathrm{})\mathrm{}$ spin-coupled pairs and spin-coupled tetrahedral clusters with an exchange coupling of $|J/k|\approx 35-45\hspace{0.5em}\mathrm{K}.$ The appearance of small resonance modes for $T\lesssim 17\hspace{0.5em}\mathrm{K},$ the observation of a sharp drop in $\chi \mathrm{}\left(T\right)\mathrm{}$ and a strong peak in $C_v\mathrm{}\left(T\right)\mathrm{}$ at $T_N=12\mathrm{}\hspace{0.5em}\mathrm{K}$ confirms, as previously reported, the existence of long range AFM correlations in the low-T phase. A comparison with recent neutron diffraction experiments, that found a near dispersionless excitation at 4.5 meV for $T\lesssim T_N$ and a continuous gapless spectrum for $T\gtrsim T_N,$ is also given.

We report the first observation of anti-Stokes laser-induced cooling in the ${\mathrm{Er}}^{3+}:{\mathrm{KPb}}_2{\mathrm{Cl}}_5$ crystal and in the ${\mathrm{Er}}^{3+}:\mathrm{CNBZn}$ (${\mathrm{CdF}}_2\mathrm{\text{−}}{\mathrm{CdCl}}_2\mathrm{\text{−}}\mathrm{NaF}\mathrm{\text{−}}{\mathrm{BaF}}_2\mathrm{\text{−}}{\mathrm{BaCl}}_2\mathrm{\text{−}}{\mathrm{ZnF}}_2$) glass. The internal cooling efficiencies have been calculated by using photothermal deflection spectroscopy. Thermal scans acquired with an infrared thermal camera proved the bulk cooling capability of the studied samples. The implications of these results are discussed.

In this work we report infrared to visible upconversion luminescence in the low phonon-energy host material ${\mathrm{KPb}}_2{\mathrm{Cl}}_5$ doped with ${\mathrm{Er}}^{3+}$ ions under ${}^4{I}_{9/2}$ excitation. The upconversion mechanisms are investigated by using steady-state and time-resolved laser spectroscopy. When the excitation wavelength is resonant with the ${}^4{I}_{9/2}$ level $\left(\approx 800\mathrm{nm}\right),$ the upconverted emission originates from the levels ${}^2{H}_{9/2}$ and ${}^4{S}_{3/2}.$ These upconverted emissions occur via energy-transfer upconversion processes. However, under nonresonant excitation at lower energies than ${}^4{I}_{9/2},$ the main emission results from the level ${}^2{H}_{9/2}.$ This latter upconverted emission can be attributed to excited-state absorption of the pump radiation. The proposed upconversion mechanisms responsible for the different emissions from the levels ${}^2{H}_{9/2}$ and ${}^4{S}_{3/2}$ are supported by both the time evolution of the upconversion luminescence after pulsed infrared excitation and the upconversion luminescence excitation spectra. Rate equation analysis has been used to identify and characterize the energy-transfer processes responsible for the observed fluorescence behavior.

A quantum mean field theory of the pyrochlore lattice is presented. The starting point is not the individual magnetic ions, as in the usual Curie-Weiss mean field theory, but a set of interacting corner-sharing tetrahedra. We check the consistency of the model against magnetic susceptibility data and find good agreement between the theoretical predictions and the experimental data. Implications of the model and future extensions are also discussed.

A critical assessment of the finite element (FE) method for studying two-dimensional dielectric photonic crystals is made. Photonic band structures, transmission coefficients, and quality factors of various two-dimensional, periodic and aperiodic, dielectric photonic crystals are calculated by using the FE (real-space) method and the plane wave expansion or the finite difference time domain (FDTD) methods and a comparison is established between those results. It is found that, contrarily to popular belief, the FE method (FEM) not only reproduces extremely well the results obtained with the standard plane wave method with regards to the eigenvalue analysis (photonic band structure and density of states calculations) but it also allows to study very easily the time-harmonic propagation of electromagnetic fields in finite clusters of arbitrary complexity and, thus, to calculate their transmission coefficients in a simple way. Moreover, the advantages of using this real space method in the context of point defect cluster quality factor calculations are also stressed by comparing the results obtained with this method with those obtained with the FDTD one. As a result of this study, FEM comes out as an stable, robust, rigorous, and reliable tool to study light propagation and confinement in both periodic and aperiodic dielectric photonic crystals and clusters.

Anti-Stokes cooling between room temperature and 77 K in a fluorochloride glass (CNBZn) and a fluoride glass (BIG) doped with 1 mol % of ${\mathrm{YbF}}_3$ has been demonstrated by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency for CNBZn glass which is ∼2.0% relative to the absorbed laser power at 1010 nm and 300 K falls about 20% at 77 K. The cooling efficiency for BIG glass was only ∼0.6% at room temperature. A model accounting for the photon-ion-phonon interaction is in good agreement with the observed temperature dependence of the cooling process and shows its relation with the vibrational properties of the material.