Takitaro Morikawa

Local structure analysis of an optically active center in Er-doped ZnO thin film

The local structure of an optically active center in erbium-doped zinc oxide (ZnO:Er) thin film produced by a laser ablation technique and its optical activation process are investigated by Er LIII-edge x-ray absorption fine structure analysis using a synchrotron radiation as an x-ray source. In as-ablated ZnO:Er thin film, Er has an approximately five-fold coordination of O surrounded by eight other O atoms as second-nearest neighbors. The high-order coordination of O decreases the Er-related photoluminescence (PL) intensity due to an undesirable crystal field for 4f radiation transition. After annealing in O2 ambient, the local structure of Er changes to a pseudo-octahedral structure with C4v symmetry, similar to the optically activated Er-doped Si (Si:Er), resulting in strong PL. The bond lengths of Er–O are evaluated, and differences in the optical activation processes between ZnO:Er and Si:Er thin films are discussed.

Highly erbium-doped zinc–oxide thin film prepared by laser ablation and its 1.54 μm emission dynamics

Erbium-doped ZnO (ZnO:Er) thin films were fabricated by the KrF excimer laser ablation technique, which is a useful and simple technique to dope Er atoms on the order of 1020 cm−3 into a host material. As-prepared ZnO:Er films showing strong c-axis orientation with a hexagonal crystalline structure indicate a low electrical resistivity of 6.4×10−3 Ω cm. The sharp and intense photoluminescence (PL) at 1.54 μm originating from the intra-4f transition in the Er3+ ions as well as PL in UV region from the ZnO host were observed even at room temperature. Significant distinction arising from the different Er emission centers responsible for the 1.54 μm emission cannot be found in the temperature dependence between the ZnO:Er and Si:Er film as a reference, except for the PL spectrum feature and main PL peak position. This result suggests the existence of Er emission centers in ZnO:Er and Si:Er films that are different from each other. The details of Er-related 1.54 μm emission dynamics of ZnO:Er films have been i...

1.54 μm emission dynamics of erbium-doped zinc-oxide thin films

Erbium-related 1.54 μm emission dynamics of Er-doped ZnO thin films has been investigated for the different excitation conditions. The excitation was achieved either by exciting indirectly Er3+ ions due to an electron–hole-mediated process or exciting directly discrete energy levels of Er3+ ions. There is no change in the 1.54 μm emission spectrum feature in spite of the different excitation conditions, whereas dramatic change can be seen in the rise time of 1.54 μm emission. The shorter rise time of 1.54 μm emission observed for indirect excitation implies an excitation efficiency superior to direct excitation of Er3+ ions.

Fabrication of ruby sensor probe for the fiber-optic thermometer using fluorescence decay

Ruby single crystals have been grown successfully from the melt droplet on the sapphire crystal fibers as the sensor head directly coupled with optical fiber for the thermometer applications using fluorescence decay. In this fabrication process, droplets of slurry of ruby powders are formed on the end of the sapphire crystal fiber and are dried, sintered, and then melted and solidified in a floating-zone furnace. The photoluminescence (PL) lifetime of the ruby sensor head varies from τ=3.8 to 2.5 ms with the Cr3+ concentrations from 0.07 to 1.0 at. %. The ruby crystal with a Cr3+ concentration of 0.35 at. % is found to be the most suitable sensor head having the longest PL lifetime. The temperature coefficient of the fiber-optic thermometer equipment using the directly coupled ruby sensor head with a Cr3+ concentration of 0.35 at. % is −12.4 μs/K.

Thermally Stimulated and Photostimulated Luminescence from Long Duration Phosphorescent SrAl2 O 4 : Eu , Dy Crystals

The traps in long duration phosphorescent SrAl 2 O 4 :Eu 2+ , Dy 3+ crystals have been evaluated using thermally stimulated luminescence and photostimulated luminescence (PSL) techniques. Afterglow phosphorescence is not observed at temperatures lower than 150 K. Traps with depths of E t,TSL = 0.0024, 0.46, and 0.49 eV are detected by thermally stimulated luminescence techniques. Photostimulated luminescence evaluations reveal that trapped carriers can be released optically using an infrared laser with E t,PSL = 0.55 eV. The trap at E t,PSL = 0.55 eV detected by the PSL may be correlated with the shallow trap levels at E t,TSL = 0.0024 eV.

Change in photoluminescence from Er-doped TiO2 thin films induced by optically assisted reduction

Erbium-doped TiO2 (TiO2:Er) thin films with the anatase structure have been prepared on Si substrate by laser ablation. Sharp and intense Er-related emission in the visible region as well as in the IR region has been observed under over-band-gap excitation. The broad photoluminescence (PL) peaking at about 530 nm newly appears at low temperature. It has been understood that the broad PL is induced by an optically assisted reduction effect that is caused by both the H2O adsorption and the reduction process of TiO2 to Ti2O3 by UV illumination. In the IR region, Er-related emission consisted of one main peak located at 1.534 μm and many subpeaks located at around 1.54 μm can be observed even at room temperature. The drastic thermal quenching of the Er-related 1.54 μm emission is also considered due to the optically assisted reduction effect.

Fiber-optic thermometer using Cr-doped YAlO3 sensor head

Cr-doped YAlO3 phosphor crystals have been grown and characterized for applications in fiber-optic thermometers based on the temperature dependence of the photoluminescence (PL) lifetime. PL peaking at λ=735 nm is observed from the crystals following an excitation by a light-emitting diode at λ=520 nm. Long PL lifetime (τ=43.14 ms) is observed from Cr-doped YAlO3 at room temperature as compared with those of ruby (τ=4.2 ms) and spinel (τ=8.0 ms). The temperature coefficient (−0.075 ms/K) of Cr-doped YAlO3 is also larger than those of ruby (−0.010 ms/K) or spinel (−0.037 ms/K). YAlO3 is considered to be a sensitive sensor head material for fiber-optics thermometers.

Time response of 1.54 μm emission from highly Er-doped nanocrystalline Si thin films prepared by laser ablation

Er-doped nanocrystalline Si thin films have been controllably prepared over the Er density of 1019–1021 cm−3 using a prescribed amount of Er in a bulk target by laser ablation. Intense photoluminescence at 1.54 μm originating from intra-4f shell transitions in Er3+ ions has been observed. The increase of Er density cannot immediately result in a linear increase in Er3+-emission intensity. The time response measurement indicated that the change in the rise time of the Er3+ emission directly shows that Er3+ ions are excited by the energy transfer associated with the recombination of electron–hole pairs generated optically in the Si host. We found that the decrease of the excitation efficiency of Er3+ ions was responsible for the suppression of the Er3+-emission intensity in highly Er-doped nanocrystalline Si thin films.

The optically active center and its activation process in Er-doped Si thin film produced by laser ablation

The local structure of erbium-doped silicon produced by the laser ablation technique is investigated by Er LIII-edge x-ray absorption fine structure analysis. The combined analysis of extended x-ray absorption fine structure analysis and an x-ray absorption near-edge structure simulation based on multiple-scattering theory reveals the most probable atomic coordination of the optically active center; Er bonded with six oxygen atoms has a C4v symmetry. The optical activation process of this system is also discussed. The Si target with 10 wt% Er2O3 has two kinds of local structures, C-rare-earth Er2O3 grain and another Er phase incorporated in Si. The laser ablation homogenizes these phases, and deposits a new single-phase structure of the octahedron (Oh point group) on the substrates. In this phase, the optical transition probability is low due to the forbidden 4f transition of Er in the crystal field originating from the higher-order symmetry of O. After annealing, degradation of the symmetry from Oh to C4...

Long afterglow phosphorescent sensor materials for fiber-optic thermometer

Long duration phosphorescent SrAl2O4 phosphors doped with various auxiliary activators (Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) have been evaluated for the fiber-optic thermometer application. In this thermometer, temperature was measured based on the temperature dependence of the lifetime and/or intensity of the afterglow phosphorescence which was dominated by the thermal excitation of trapped carriers. Characteristics of the long phosphorescent fiber-optic thermometer are quite different from those of the conventional thermometer using fluorescent lifetime of the phosphors. Lifetime and intensity of the afterglow phosphorescence from the long afterglow phosphorescent sensor heads show large positive temperature coefficients in the narrow temperature region around room temperature. Sensitivity and temperature range of the long phosphorescent fiber-optic thermometer depend strongly on the trap depth and the trap density of phosphors. Since the trap depth and the trap density vary with a...