We report on spectral and time-resolved photoluminescence ͑PL͒ studies performed on Eu-doped GaN prepared by solid-source molecular-beam epitaxy. Using above-gap excitation, the integrated PL intensity of the main Eu 3ϩ line at 622.3 nm ( 5 D 0 → 7 F 2 transition͒ decreased by nearly 90% between 14 K and room temperature. Using below-gap excitation, the integrated intensity of this line decreased by only ϳ50% for the same temperature range. In addition, the Eu 3ϩ PL spectrum and decay dynamics changed significantly compared to above-gap excitation. These results suggest the existence of different Eu 3ϩ centers with distinct optical properties. Photoluminescence excitation measurements revealed resonant intra-4 f absorption lines of Eu 3ϩ ions, as well as a broad excitation band centered at ϳ400 nm. This broad excitation band overlaps higher lying intra-4 f Eu 3ϩ energy levels, providing an efficient pathway for carrier-mediated excitation of Eu 3ϩ ions in The visible and infrared light emissions from rare-earthdoped GaN ͑GaN:RE͒ are of significant current interest for applications in thin-film electroluminescence ͑EL͒ devices. [1][2][3][4] For achieving red light emission, the 5 D 0 → 7 F 2 intra-4 f transition of trivalent Eu 3ϩ ions seems most promising. Intense red photoluminescence ͑PL͒ around 622 nm from GaN:Eu ͑as-grown and ion-implanted͒ has been reported from several research groups. [1][2][3][4][5][6][7][8][9] In addition, several EL device structures based on GaN:Eu have been demonstrated. [1][2][3][4][5] The optimization of present EL devices, however, requires a more detailed understanding of the incorporation, excitation, and emission properties of Eu 3ϩ ions in the GaN host matrix.Several studies have recently appeared focusing on the preparation and optical properties of GaN:Eu. 4 -11 Based on the comparison to RE ions in other III-V semiconductors ͑e.g., InP:Yb, 12 GaAs:Er 13 ͒, the most probable lattice location for Eu 3ϩ ions in GaN are ͑substitutional͒ Ga sites, which have C 3V symmetry. However, significant differences in the Eu 3ϩ PL properties have been observed depending on the material preparation. Monteiro et al. 7 studied Euimplanted GaN and Eu in situ doped GaN grown by metalorganic chemical vapor deposition. They observed significant differences in the Eu 3ϩ PL properties, including the number of emission lines associated with the 5 D 0 → 7 F 2 transition. Based on optical spectroscopy and Rutherford backscattering studies, the authors concluded that the local symmetry of the Eu 3ϩ ions has to be lower than C 3V symmetry. 7 Bang et al. 9 studied Eu-doped GaN prepared by gas-source molecularbeam epitaxy ͑MBE͒ and concluded, based on extended x-ray absorption fine-structure data, that Eu 3ϩ occupies Ga sites with C 3V symmetry. It was also suggested that more than one local environment of Eu 3ϩ ions may exist in the investigated GaN samples.In this letter, we present PL results on GaN:Eu prepared by solid-source MBE, which provide spectroscopic evidence for the existence of different Eu...

Nd 3 + -doped YF3 (YF3:Nd) nanoparticles with a size of ∼20 nm were synthesized by solvothermal decomposition of yttrium and neodymium trifluoroacetate precursors in oleylamine. Using the 4f-energy matrix diagonalization procedure various interaction parameters: Slater–Condon (F2, F4, and F6), spin-orbit (ξ), two body interaction (α, β, and γ), Judd parameters (T2, T3, T4, T6, T7, and T8), spin-other-orbit parameters (M0, M2, and M4) and electrostatically correlated spin-orbit interaction parameters (P2, P4, and P6), and the crystal-field parameters (Bqk) were evaluated. The potential of YF3:Nd as a laser host for 1052 nm emission was evaluated by quantitative analysis of the absorption, emission spectra, and fluorescence decay characteristics. Judd–Ofelt parametrization was employed to compute the radiative spectral parameters such as radiative transition probabilities, fluorescence branching ratios, stimulated emission cross sections, and quantum efficiencies of the observed bands in the fluorescence spectrum. Using the measured radiative properties, 75% quantum efficiency was obtained for the principal emission band at 1052 nm when the Nd dopant concentration was 0.25 mol %, with an emission cross section of 0.74×10−20 cm2. Analysis of the energy transfer kinetics showed that at low dopant concentrations of 0.25 mol % dipole-dipole interactions were dominant, whereas energy migration was the leading process at higher dopant concentrations. Quenching by OH impurities was found to be within the limit of optimum amplifier performance where multiphonon relaxation losses were negligible. Preliminary optical characterization showed that these nanocrystalline materials can be potentially used as optical amplifiers and in applications like infrared imaging, security and authentication.

The emission properties of rare earth (RE)-doped GaN are of significant current interest for applications in full color displays, white lighting technology, and optical communications. We are currently investigating the photoluminescence (PL) properties of RE (Er, Eu, Tm)-doped GaN thin-films prepared by solid-source molecular beam epitaxy. The most intense visible PL under above-gap excitation is observed from GaN:Eu (red: 622 nm) followed by GaN:Er (green: 537 nm, 558 nm), and then GaN:Tm (blue: 479 nm). In this paper, we present spectroscopic results on the Ga-flux dependence of the Er 3+ PL properties from GaN:Er and we report on the identification of different Eu 3+ centers in GaN:Eu through high-resolution PL excitation (PLE) studies. In addition, we observed an enhancement of the blue Tm 3+ PL from AlGaN:Tm compared to GaN:Tm. Intense blue PL from Tm 3+ ions was also obtained from AlN:Tm under below-gap pumping.

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We are engaged in a systematic study of the optical and laser properties of Cr 2ϩ -doped cadmium chalcogenides. Previously, we demonstrated quasi-continuous wave lasing from Cr 2ϩ -doped Cd 0.55 Mn 0.45 Te with slope efficiencies as high as 64% and a laser tuning range from 2,170-3,010 nm. In this paper, we report the first demonstration of lasing from Cr:CdTe at room temperature. Pulsed-laser operation was obtained with a free-running spectrum centered at 2,535 nm. The slope efficiency of the laser was low (ϳ1%) because of large parasitic losses at the laser wavelength. The spectroscopic properties of Cr:CdTe are favorable for laser applications because of a large emission cross section (ϳ2.5 ϫ 10 Ϫ18 cm 2 ) and a high emission-quantum yield (ϳ88%). In addition, CdTe can easily incorporate Cr ions either through melt growth or diffusion doping. Along with our results on Cr 2ϩ :CdTe, we report on the optical properties of several other Cr 2ϩ -doped II-VI semiconductors (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, Cd 0.9 Zn 0.1 Te, Cd 0.65 Mg 0.35 Te, Cd 0.85 Mn 0.15 Te, and Cd 0.55 Mn 0.45 Te) and compare them for applications as solid-state laser materials.

We report on the effect of high pressure on the room-temperature emission spectra and lifetimes of Cr +:GSGG (Gd3SC2Ga30») and Cr:GGG (Gd3Ga50»). In both systems we observed a dramatic change of the overall emission band shape upon increasing pressure, from a nearly structureless broadband ( T2~A 2 ) to a highly structured narrow band ( E~A 2 )~From the peak energy of the broadband emission, we estimated the pressure-induced blueshift of the T2~A2 transition to be 10 (+2) cm '/kbar. High-resolution measurements in the R-line region ( -700 nm) revealed that the E -+ A2 transition hardly shifts at low pressures ( &40 kbar), whereas at higher pressures ()60 kbar) a nearly linear redshift of 0.65 (+0.05) cm /kbar is observed. Besides pressure-induced spectral changes, an enormous increase in the emission lifetime with increasing pressure was found for both systems. In the case of Cr +:GSGG, the lifetime changed from 110 ps at ambient pressure to 4.4 ms at 125 kbar. For Cr'+:GGG, the lifetime increased from 168 ps to 7.3 ms for the same pressure range. The pressureinduced spectral and lifetime changes are described by a single configurational coordinate model that considers the effect of pressure on the thermal and spin-orbit coupling of the E and T2 states. A previously reported pressure-induced R-line-shift reversal in Cr +:GSGG and the effect of high pressure on the lifetime in Cr +: YAG are also discussed within the same framework.

Results of a photoluminescence (PL) and photoluminescence excitation (PLE) study of Er implanted GaN are presented. Upon optical excitation at 325 and 488 nm, we observed strong 1.54 μm Er3+ PL which remained temperature stable from 15 to 550 K. At 550 K, the integrated PL intensity decreased by ∼10% for above gap excitation (λex=325 nm) and ∼50% for below gap excitation (λex=488 nm) relative to its value at 15 K. The excellent temperature stability makes GaN:Er very attractive for high temperature optoelectronic device applications. PLE measurements were conducted to gain insight into the Er3+ excitation mechanisms in the GaN host. The PLE results show that Er3+ can be excited continuously over a broad wavelength region spanning from 425 to 680 nm. In addition, sharp PLE features were observed at approximately 495, 525, 553, 651, and 980 nm. The PLE spectrum suggests that optically active Er3+ ions can be excited either through carrier-mediated processes involving defects in the host or through resonant pumping into Er3+ 4f energy levels. With respect to these two excitation schemes, distinct Er3+ PL properties were observed for resonant and off-resonant Er3+ excitation indicating the presence of different subsets of Er3+ ions in GaN.

We present a spectroscopic overview of Cr +-doped Y3A150» (YAG), a new laser material for the near-infrared region (NIR). It is found that only 2% of the Cr ions are tetrahedrally coordinated Cr +.However, the optical spectra are dominated by this ion. The missing inversion symmetry for the tetrahedral site causes higher cross sections than are common for octahedrally coordinated ions. The experimental data include a survey of the absorption and emission features. Lifetime measurements, as well as piezospectroscopic experiments yield additional spectroscopic information. Our data suggest an energy-level assignment that is different from the one previously used to describe the spectrum. Instead of ascribing the strong NIR absorption band, centered at around 1000 nm, to the '8&('A2) 'E('T2) transition, we assign it to the '8&('A&)~'A2('T&) transition. This assignment is able to describe the polarization-dependent spectra, the piezospectroscopic results, and the results from earlier polarizationdependent saturation experiments.An analysis of the luminescence decay rate yields quantum eSciencies of around 49% at 10 K and 15% at room temperature, resulting in a peak emission cross section of 3.5X10 19 cm2