With the purpose of investigation on the visible emission properties of Tm<sup>3+</sup> ions, a Tm<sup>3+</sup>-doped Y<sub>2</sub>O<sub>3</sub> transparent ceramic was fabricated by sintering at 1800 °C for 20 hour with a vacuum degree of 1x10<sup>-3</sup> Pa. 3 at% ZrO<sub>2</sub> was introduced as the sintering aid and the average grain size was measured to be 22 μm. The optical transmittance of the ceramic achieved 76.3 % at 1 μm. The PL spectra at room temperature and low temperature were measured under 361 nm excitation. The ~453 nm emission bands were observed and the luminescence mechanisms were discussed. It was
found that the Tm: Y<sub>2</sub>O<sub>3</sub> transparent ceramic have the potential to be used in white LED packaging structure.
Absorption and photoluminescence spectra of Tm<sup>3+</sup> and Ho<sup>3+</sup> ions in LiYF<sub>4</sub> crystals have been investigated at various temperatures between 10 and 320 K. The photoluminescence is investigated under excitation with Xe-lamp and laser diode. In addition to blue up-converted emission of Tm<sup>3+</sup> and Ho<sup>3+</sup> and green up-converted Ho<sup>3+</sup> emission, anti-Stokes emission bands are observed at 687 and 703 nm under excitation in the <sup>3</sup>H<sub>4</sub> state of Tm<sup>3+</sup> with 785 nm laser diode. These bands are observed above 200 K, and their intensities increase exponentially with increasing temperature. They are attributed to endothermic Tm<sup>3+</sup> emission due to the transition to the <sup>3</sup>H<sub>6</sub> ground state from the upper <sup>3</sup>F<sub>3</sub> state which is thermally populated from the <sup>3</sup>H<sub>4</sub> state. Discussion is given on the optical process of green up-converted Ho<sup>3+</sup> emission which is generated by the 785 nm laser diode excitation.
Photovoltaic effect has been studied on single- and multi-layer OLED devices based on phosphorescent PtOEP and Ir(ppy)<sub>3</sub> molecules, together with the electroluminescence (EL). The incident photon to current efficiency (IPCE) spectra of these PtOEP and Ir(ppy)<sub>3</sub> devices are similar to the absorption spectra of PtOEP and Ir(ppy)<sub>3</sub> molecules, respectively. This indicates that the photovoltaic effect is caused by the optical excitation of PtOEP and Ir(ppy)<sub>3</sub> molecules. The single-layer devices show weak EL luminance and low EL efficiency. Although the EL efficiency of the multi-layer Ir(ppy)<sub>3</sub> OLED device is high, the IPCE value is quite low, e.g. 0.013% in the single-layer device at the absorption peak wavelength of 386 nm. The same is true for PtOEP OLED, e.g. 0.0425% at the absorption peak wavelength of 371 nm. Such a low IPCE is considered to be due to low carrier mobility of PtOEP and Ir(ppy)<sub>3</sub> molecules. It was found that the multi-layer PtOEP and Ir(ppy)<sub>3</sub> OLED devices with emitting layer of guest-host system show much less efficient photovoltaic effect than the single-layer devices without guest-host.
The saturable absorption behavior of F<sub>2</sub><sup>-</sup> color centers in LiF is studied by picosecond laser pulse excitation (wavelength λ<sub>L</sub> = 1054 nm, duration Δ<i>t</i><sub><i>L</i></sub> = 6 ps). The color center number density, <i>N</i><sub>0</sub>, the ground-state absorption cross-section, σ<sub><i>L</i></sub>, and the single-photon excited-state absorption cross-section, σ<sub><i>ex,L</i></sub>, are determined by numerical fitting the energy density dependent energy transmission. A decrease of energy transmission at high excitation energy densities (<i>w</i><sub>0</sub><sub><i>L</i></sub> > 1 J cm<sup>-2</sup>) is attributed to two-photon absorption of other color centers (N<sub>1</sub> and N<sub>2</sub> bands of F<sub>4</sub> centers) present in the sample.
Luminescence spectra of Tm<SUP>3+</SUP> ion have been investigated for YVO<SUB>4</SUB> and LiNbO<SUB>3</SUB> crystals by excitation with 798 nm laser diode of high powers in a range from 0.6 W up to 10 W. In addition to the one-photon excited infrared emission bands, emission bands at 700 and 1208 nm are observed in YVO<SUB>4</SUB> by high power excitation and the similar emission bands are observed at 702 and 1216 nm in LiNbO<SUB>3</SUB>. It is observed that each of the 700 and 1208 nm emission intensities in YVO<SUB>4</SUB> has quadratic pump-power dependence in a pump power range of 0.6 - 4 W and cubic dependence in 4 - 10 W range. Same result is observed for the 702 and 1216 nm emission in LiNbO<SUB>3</SUB> but the change from the quadratic to cubic dependence appears at 5 W. Discussion is made on the luminescence process for these up-conversion.
Up-conversion by Yb<SUP>3+</SUP> sensitization in Er3+ doped YVO4 crystals has been studied under excitation with 976 nm laser diode. Up-converted (available in paper) emission bands are observed at 490, 547, 554, 660 and 670 nm at room temperature, together with down-converted Yb<SUP>3+</SUP> emission at 1010 nm and Er<SUP>3+</SUP> emission at 1470 - 1630 nm. Excited state absorption from the <SUP>4</SUP>I<SUB>11/2</SUB> state of Er<SUP>3+</SUP> to the <SUP>4</SUP>F<SUB>7/2</SUB> state is observed in the Yb<SUP>3+</SUP> emission spectrum. It is confirmed, from quadratic dependence on pump intensity, the up-conversion is induced by two-photon excitation process.
Color centers are lattice vacancies which are trapping one or more electrons or holes in ionic crystals. Some of them have been known to be high-gain active materials in tunable solid state lasers. However, most color centers (e.g. FA(II) color center in KC1 crystal) give rise to laser oscillation when the laser-active crystals are cooled with liquid nitrogen . For practical use, it is requested to be able to operate the laser at room temperature (RT) because it is easy to adjust the optical alignment and it is unnecessary to maintain the crystals at liquid nitrogen temperature not only during operation but also after operation to avoid the thermal bleaching of the color centers. The RT stable color center lasers have been achieved predominantly using F2, F2, F2 and F3 color centers in LiF[2,3,4]. Although the RT LiF:F2 color center laser oscillation has been studied by several scientists, all the lasing characteristics have not been clarified.
Up-conversion of red light with wavelength of 660 nm in Tm<SUP>3+</SUP>-doped BaY<SUB>2</SUB>F<SUB>8</SUB> powder results in the two violet luminescence bands with peaks at 417 and 430 nm and two blue luminescence bands with peaks at 455 and 470 nm. The two violet bands are observed to be stronger than the blue bands. The blue luminescence is also observed by pumping with 993 nm light. The up-conversion is explained by a multiple excited state absorption process.
A reliable pulsed LiF:F<SUB>2</SUB><SUP>+</SUP> color center laser with a two-mirror cavity has been constructed. A near-infrared laser has shown a stable operation at room temperature and remarkable amplitude by pumping with the second harmonic of pulsed Nd<SUP>3+</SUP>-doped YAG laser. The 532 nm radiation gives rise to create the F<SUB>2</SUB><SUP>+</SUP> centers by ionization of the F<SUB>2</SUB> centers in a process of two-photon excitation. The LiF:F<SUB>2</SUB><SUP>+</SUP> laser has a broadband with peak at 930 nm and half width of 40 nm. The slope efficiency is 8% and the pump threshold is 10 mW.
An F<SUB>2</SUB><SUP>+</SUP> color center laser operation has been performed using a two-mirror cavity at room temperature by photo-ionizing of the F<SUB>2</SUB> color centers in LiF crystal. The ionization was undertaken with the 532 nm second harmonic radiation of pulsed Nd<SUP>3+</SUP>-doped YAG laser. We observed the F<SUB>2</SUB><SUP>+</SUP> broadband (i.e. free-running mode) laser oscillation with a peak at 930 nm and compared with the broadband LiF:F<SUB>3</SUB><SUP>+</SUP> color center laser oscillation.
Optical fiber-to-thin film optical waveguide couplers have been studied experimentally as the base for in-line fiber- optic active components. Laser active crystals have been used in a planar structure as active substrate or active core for fiber-optic active components design.
In this paper, the absorption and fluorescence spectra of Tm<SUP>3+</SUP>:LiNbO<SUB>3</SUB> crystals at room temperature were reported for the first time. The energy levels and experimental absorption oscillator strengths were obtained from the absorption spectrum. According to Judd-Ofelt theory, the intensity parameters (Omega) <SUB>(lambda</SUB> ) ((Omega) <SUB>2</SUB> equals 3.30 multiplied by 10<SUP>-20</SUP> cm<SUP>2</SUP>, (Omega) <SUB>4</SUB> equals 1.37 multiplied by 10<SUP>-20</SUP> cm<SUP>2</SUP>, (Omega) <SUB>6</SUB> equals 0.84 multiplied by 10<SUP>-20</SUP> cm<SUP>2</SUP>) were worked out by a least-square-fitting procedure. With these values, electric dipole oscillator strength, radiative transition probability, radiative lifetime, branching ratio and integrated emission cross-section were calculated. The potential use of Tm<SUP>3+</SUP>:LiNbO<SUB>3</SUB> crystal as a laser material was also discussed.
LiF crystal with F<SUB>3</SUB><SUP>+</SUP> color center has been demonstrated to be a nonlinear material for 460 nm light and generate a phase conjugate wave by the degenerate four-wave mixing method. It is shown that the nonlinearity of the F<SUB>3</SUB><SUP>+</SUP> centers is caused by the saturation of optical absorption.