Markus P. Hehlen

Uniform upconversion in high-concentration Er 3+ -doped soda lime silicate and aluminosilicate glasses

Uniform upconversion in erbium-doped silicate glasses is investigated as a function of glass composition, concentration, and fabrication method. Comparisons of upconversion coefficients are made among soda lime silicate and aluminosilicate bulk glasses and soda lime silicate waveguides. Comparisons are also made with studies performed by other researchers. The results indicate that both the composition and the preparation method of the glass affect the value of the upconversion coefficient, with as much as a factor-of-4 variation observed at fixed Er(3+) concentration. Values of the upconversion coefficient are found to be consistent with the Förster-Dexter microscopic model.

Optical spectroscopy of the dimer system Cs3Yb2Br9

Single crystals of Cs3Yb2Br9 were grown using the Bridgman technique. Well‐resolved absorption, luminescence, excitation, and Raman measurements down to 5 K are reported and analyzed. The crystal‐field splitting of Yb3+ in Cs3Yb2Br9 was determined and can be rationalized in terms of a trigonally distorted YbBr63− octahedron. Under near‐infrared (NIR) cw‐laser excitation of the 2F7/2→2F5/2 transitions, Cs3Yb2Br9 shows up‐conversion luminescence in the green spectral region. Conversely, NIR 2F5/2→2F7/2 luminescence can be excited at twice the energy of the 2F7/2→2F5/2 transitions in the green spectral region. These effects are shown to result from cooperative processes within the Yb2Br93− dimers. The cooperative dimer‐absorption oscillator strength of 4.0×10−12 corresponds to the square of the NIR absorption oscillator strength of 1.9×10−6, as expected for weak coupling within the Yb2Br93− dimer. By a numerical deconvolution the cooperative dimer luminescence spectrum can be quantitatively correlated with t...

COOPERATIVE OPTICAL BISTABILITY IN THE DIMER SYSTEM CS3Y2BR9:10% YB3+

In single crystals of the dimer compound Cs3Y2Br9:10% Yb3+ below 31 K, both visible (VIS) and near‐infrared (NIR) luminescence intensities were found to exhibit hysteresis as a function of incident NIR intensity and temperature. The optical bistability is intrinsic to Cs3Y2Br9:10% Yb3+ and not a result of an external feedback. Lowering the temperature to 11 K strongly enhances the all‐optical switching behavior. The switching on VIS cooperative upconversion and NIR luminescence transitions occurs simultaneously and with opposite polarity reflecting the competition of both emission processes. On/Off switching ratios of up to 4.8 and 1.7 were observed for VIS and NIR luminescence intensities. Using NIR luminescence spectroscopy, differences in the internal sample temperature of up to 7 K were found between the upper and lower branches of the hystereses. A two‐level density‐matrix model is developed which includes ground‐ and excited‐state interactions and shows that the intrinsic bistability due to a local ...

Focus issue introduction: persistent phosphors

Persistent phosphors can emit light long after the excitation has ended. The field of persistent luminescence has strongly matured during the past decade, with considerable progress having been made in synthesis and characterization methods, the understanding of trapping and de-trapping mechanisms, and the application of these materials. This focus issue “Persistent Phosphors,” within the April 2012 issue of Optical Materials Express, features papers presented at the first International Workshop on Persistent Phosphors (Phosphoros 2011) held in Gent, Belgium.

Solid-state optical refrigeration to sub-100 Kelvin regime.

Since the first demonstration of net cooling twenty years ago, optical refrigeration of solids has progressed to outperform all other solid-state cooling processes. It has become the first and only solid-state refrigerator capable of reaching cryogenic temperatures, and now the first solid-state cooling below 100 K. Such substantial progress required a multi-disciplinary approach of pump laser absorption enhancement, material characterization and purification, and thermal management. Here we present the culmination of two decades of progress, the record cooling to ≈ 91 K from room temperature.

Crystalline electric field effects in Ce M In 5 ( M = Co , Rh , Ir ) : Superconductivity and the influence of Kondo spin fluctuations

We have measured the crystalline electric field (CEF) excitations of the

Stabilization of charge ordering in La1/3Sr2/3FeO3-delta by magnetic exchange.

\mathrm{Ce}M{\mathrm{In}}_{5}\phantom{\rule{0.3em}{0ex}}(M=\mathrm{Co},\mathrm{Rh},\mathrm{Ir})

Excited-state dynamics and optical bistability in the dimer system Cs3Lu2Br9: Yb3+

series of heavy fermion superconductors by means of inelastic neutron scattering. In each case, the CEF excitations are considerably broadened, due to Kondo hybridization of the localized

Ultrafast, cross-correlated harmonic imaging through scattering media

f

Measurement of solid-state optical refrigeration by two-band differential luminescence thermometry

-moments with the conduction electrons. Fits to a phenomenological CEF model reproduce the inelastic neutron scattering spectra and the high-temperature magnetic susceptibility. We also present calculations within the noncrossing approximation (NCA) to the Anderson impurity model, including the effect of CEF level-splitting for the inelastic neutron scattering spectra and the magnetic susceptibility. Our results indicate that the CEF level-splitting in all three materials is similar, and can be thought of as being derived from the cubic parent compound