F. Auzel

Below Band-Gap IR Response of Substrate-Free GaAs Solar Cells Using Two-Photon Up-Conversion

We have developed a device based upon the concept of two-photon up-conversion to use a part of the IR photons otherwise lost by transparency in a GaAs cell. An ultra thin GaAs cell has been fabricated using the technique of epitaxial lift-off (ELO). This thin cell is placed on top of a 100 µ m thick vitroceramic doped with Yb3+ and Er3+. The two photon upconversion process involved here is based on sequential absorption and energy transfer of two IR photons from Yb3+ to Er3+, which then emit one photon in the green. This green light then produces a photoresponse in the GaAs cell. This cell coupled to the vitroceramic was lighted by an Ti-sapphire IR laser at 1.391 eV, a photon energy below the band gap of GaAs, with an input power able to reach ~1 W. The GaAs cell photoresponse increases quadratically with the input excitation. For an input excitation of 1 W at 1.39 eV on a 0.039 cm2 substrate-free GaAs cell, the measured efficiency was 2.5%.

Emission properties, oscillator strengths and laser parameters of Er3+ in LiYF4 at 2.7 μm

We reported already on the first observation of laser emission at several wavelengths (2.7–2.85 μm) for transitions between the 4I112 and 4I132 states of Er3+ in LiYF4 at room temperature under 0.8 μm diode laser and 0.514 μm argon laser excitation. In order to stabilise the action of CW lasing at 2.66 and 2.77 μm, the effect of polarisation on 4I112a4I132 emission is studied, and oscillator strengths from ground state and excited states are calculated to predict a useful cross relaxation energy transfer.

Up-conversion yield in glass ceramics containing silver

Abstract Small silver particles are known to increase the fluorescence yield in rare-earth-doped glasses. These particles can be grown easily in glass ceramics of general composition (PbF2, GeO2, YbF3, ErF3). We have studied the effect of the addition of silver on the up-conversion yield due to sequential energy transfer between Yb3+ and Er3+ ions. The origin and the information that can be obtained from this effect are discussed.

Absolute oscillator strengths of 5f-5f transitions of U4+ in ThBr4 and in hydrobromic acid solutions

Abstract Absolute oscillator strengths of 5f 2 transitions of U 4+ in ThBr 4 and in hydrobromic acid solutions are found to be about 10 -4 which is two orders of magnitude larger than those for 4f-4f transitions (lanthanide series) and one order of magnitude larger than 5f 3 ones (U 3+ ). Applicability of Judds theory to 5f 2 transitions is discussed with particular attention to the roles played by reduced matrix elements of odd rank tensor U λ and by the proximity of the 6d configuration.

Up conversion process in U4+-doped ThBr4 and ThCl4

Abstract Infrared excitation of tetravalent uranium in thorium tetrahalides produces a strong emission in the red region at room temperature. The experimental results show that a two-photon mechanism is involved. Below liquid nitrogen temperature the conversion efficiency increases and an extra green emission appears.

Phonon bottleneck effect in multiphonon non-radiative transitions of rare-earth ions

Abstract Fluorescence transients of resonantly excited RE multiplets were recorded as function of excitation density in different glasses. Multiphonon relaxation rates are reduced at high excited state concentration for large energy gaps. Such behavior is analysed in a statistical approach and explained on the basis of a theoretical model for the microscopic process. We suggest a phonon bottleneck effect on radiationless relaxation rate related to an accepting modes saturation. At high excited state density, the accepting modes are simultaneously filled by several ions lying in a phonon diffusion volume. The phonon diffusion length I c in the host deduced from our model is compared with the phonon mean-free path independently derived.

Electron-phonon coupling and self-quenching in transition metal vibronic laser materials

Abstract The self-quenching properties of otherwise well known tunable laser materials are investigated for the first time. Taking MgF 2 :Ni 2+ as principal example, several processes are advocated for which electron-phonon coupling and spin-orbit interactions are shown to play important roles.

Thermal behaviour of low temperature coherent emission in LiYF4:Er3+

Abstract In single crystal of LiYF 4 :Er 3+ with two unparallel unpolished faces, coherent emission at 2.72 μm is observed at low temperature from 9 K to 60 K when excited by cw krypton ion laser line at 6471 A. Several experimental fact such as square law behaviour, spectral narrowing, reduction of decay time, show that superfluorescence occurs under continuous excitation at low threshold.

Up-conversion of U4+ in ThBr4

Abstract Infrared excitation of tetravalent uranium in thorium tetrahalide single crystals gives strong emission in the visible range by up-conversion. The results show that a two photon mechanism is involved.

Application des transferts d’énergie résonnants a l’effet laser de verres dopés avec Er3+

SommaireEn regroupant sous forme de facteur de mérite les résultats obtenus dans une étude précédente [l], onchoisit deux verres particuliers de type germanate et fluorophosphate. Dans le cadre d’une réalisation pratique, on étudie divers transferts d’énergie. Le transfert d’énergie Yb3+ →Er3est utilisé pour améliorer le pompage. L’effet laser est alors obtenu dans un verre fluorophosphate avec un seuil d’une dizaine de joules à température ambiante.