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Silver (Ag) plasmonic structures on the surface of Сe and Yb co-doped yttrium aluminium garnet fluorescent epitaxial films were prepared by the magnetron sputtering technique. The nucleation and growth of Ag nanoparticles (NPs) were controlled by a thermal annealing process. Annealing of the samples at T ≥ 350 °C results in growth of Ag NPs size. A broad absorption band peaking in the range from 440 to 550 nm was observed due to the dipole mode of the surface plasmon resonance (SPR) of Ag NPs. Photoluminescence emission and excitation spectra were measured for samples having different concentrations, sizes and shapes of Ag particles. Plasmon-enhanced Ce 3+ luminescence was observed for samples with particles bigger than 50 nm. On the other hand luminescence quenching was observed for samples with high Ag concentration and smaller NPs size. Presented luminescence results suggest that there are two competitive effects, namely enhancement and quenching, affecting the luminescence in the presence of Ag NPs. The enhancement of Ce 3+ luminescence is mainly attributed to local field effect i.e. the SPR of Ag NPs causes an intensified electromagnetic field around them, resulting in enhanced optical transitions of Ce 3+ ions in the vicinity of Ag NPs. The quenching effect, in the presence of Ag NPs, suggests an energy transfer from Ce 3+ ions directly to them. The competition between the plasmonic enhancement and the quenching effect is discussed for samples with different nanoparticles concentrations and sizes.

Fundamental understanding of the Stokes and anti-Stokes emissions in luminescence materials and their mechanism is very important for different technological applications. In this article, we report the dual mode, down/upconversion (DC/UC), luminescence in a LiAlSi2O6 (β-spodumene) single host along with its mechanistic investigations. A series of emission tunable Li1-xAlSi2O6:xEr3+ (LAS:xEr3+; 0 ≤ x ≤ 0.07) and Li0.99-yAlSi2O6:0.01Er3+,yYb3+ (LAS:0.01Er3+,yYb3+; 0.001 ≤ y ≤ 0.007) phosphor powders were prepared by solid state reaction method at 1300 °C. The formation of single-phase phosphors is verified by XRD technique and other structural properties are investigated by FT-IR, UV-Vis-NIR spectra. The phosphors produced NIR DC emission at 1535 nm (under 380 and 980 nm excitation) and an intense red UC emission at 655 nm (under both 980 and 808 nm excitations). The dual mode emission intensity of the LAS:0.01Er3+ is further improved by co-doping Yb3+ ions via energy transfer from Yb3+ to Er3+. The red UC emission intensity of LAS:0.01Er3+,0.005Yb3+ phosphor under 980 nm excitation is ~10 times stronger than the singly doped LAS:0.01Er3+ phosphor. In addition, the phosphors can also be efficiently excited by 808 nm laser and hence can be used for biological application since 808 nm NIR irradiation exhibits higher light penetration depth without inducing overheating. The mechanism of energy transfer from Yb3+ to Er3+ by considering different emission pathways viz: ground state absorption (GSA), excited state absorption (ESA), energy transfer upconversion (ETU), back energy transfer (BET) and cross relaxation (CR) are discussed in detail. Based on the dual mode emissions, the LAS:xEr3+ and LAS: xEr3+,yYb3+ (x = 0.1 and 0.001 ≤ y ≤ 0.007) phosphors could be a potential candidate not only for the biological application as bio-probe but also as energy convertors for solid-state lighting, displays, and solar cell applications.

Low temperature infrared absorption spectra of yttrium orthoaluminate and lutetium orthoaluminate bulk crystals and liquid phase epitaxy grown single crystalline films doped with Ce 3+ are presented. In the region of intra-configurational 4 f – 4 f transitions the spectra indicate the existence of at least three different Ce 3+ related centers. The dominant center is associated with Ce substituting yttrium or lutetium. The two additional centers are most probably related to so called antisite positions of rare-earth ions in orthoaluminates, i.e., ions on Al sites. The concentration of antisite defects is lower in epitaxially grown layers of yttrium orthoaluminate than in the bulk crystals, in spite of the higher doping concentration. In contrast, in lutetium orthoaluminate additional Ce 3+ centers are also present in epitaxial layers at quite high concentration. Crystal field analysis based on the exchange charge model exhibits excellent agreement of the calculated transition energies with experimental data for the dominant Ce 3+ center. The linewidths of the 4f-4f transitions in both types of crystals are different depending on the final level of transition. This is explained as a result of electron-phonon coupling, enhanced by the matching inter-level spacing of the 2 F 7/2 state with phonon energies of the examined materials.

A series of the Sr 1.9 ZnW 1−x Mo x O 6 :0.05Eu 3+ ,0.05Li + phosphors was synthesized by high-temperature solid-state reaction method. The structure, ultraviolet-visible diffuse reflectance spectra, photoluminescence and decay properties of the phosphors were investigated as a function of the W/Mo ratio. All prepared mixed compounds have a cubic structure. The experimental spectroscopic results show the excitation bands of these phosphors is in the near ultraviolet region; the red-shift of these bands is observed upon increase of the Mo concentration. The strong red and weak orange-red emissions centered at 614 and 595 nm, respectively, were observed, which indicates that Eu 3+ occupies the non-centrosymmetric site. Co-doping with Mo can enhance the luminescent intensity and the optimal concentration was determined to be x = 0.4. Combination of the first-principles calculations with the experimental spectroscopic measurements confirmed an indirect band gap character for the prepared phosphors and allowed for estimation of their band gap value. The obtained results show that the Sr 2 ZnW 1−x Mo x O 6 phosphors have considerable potential for the near ultraviolet excited white light emitting diodes.

This investigation is mainly devoted to the studies on enrichment of green emission of Ho 3+ ions in PbO SiO 2 glasses containing 1.0 mol% of Ho 2 O 3 by co-doping with different concentrations of Bi 2 O 3 (0–9.0 mol%). The optical absorption spectra, photoluminescence spectra, photoluminescence decay, IR and Raman spectral studies on the above mentioned glasses have been performed as a function of Bi 2 O 3 concentration. The emission spectra of Bi 2 O 3 free glasses exhibited green and red emissions due to 5 S 2 + 5 F 4 → 5 I 8 and 5 F 5 → 5 I 8 transitions of Ho 3+ ions, respectively, when excited at 450 nm. The spectra also exhibited NIR emission due to the 5 I 7 → 5 I 8 transition at 2.0 μm. The intensity of green emission is found to be significantly enhanced (nearly three times) when the glasses are mixed with 5.0 mol% of Bi 2 O 3 . The reason for this enhancement has been identified as being due to the energy transfer from Bi 3+ to Ho 3+ ions. The predominant participation of Bi 3+ ions in modifying positions and due to the de-clustering of Ho 3+ ions are found to be the other reasons. The overall enhancement of emission has further been explained with the help of rate equations. The quenching of luminescence emission observed in the glasses containing more than 5.0 mol% of Bi 2 O 3 is attributed to the reason that in these glasses the Bi 3+ ions largely participate in the network forming positions and increase the phonon losses. These arguments have been quantitatively explained with the help of IR and Raman spectral studies.

Studies of ambient-pressure and high-pressure behaviour of photoluminescence (PL) as well as the phase transition pressure from wurtzite to cubic structure for a series of Zn 1−x Mg x O alloys as a function of the Mg-content x (0 ≤ x ≤ 0.34) are presented. The measured PL peak energy is shown to increase approximately linearly with increasing magnesium content from 3.36 eV to 4.11 eV, and the phase transition pressure from wurtzite to cubic structure decreases with increasing x from 12.6 GPa to 5.9 GPa. The phase transition is accompanied by a strong decrease of the near band-gap PL intensity. The value of the PL peak pressure coefficient d E PL /d p changes non-monotonically. It varies between 18.6 meV/GPa and 25.2 meV/GPa, and seems to depend on the layer thickness rather than on the layer composition, exhibiting strong scattering for several alloys with similar magnesium content. This behaviour can be due to the stabilizing influence of sapphire substrate, as is indicated both by the pressure coefficients and by the values of pressure of phase transition from wurtzite to rock salt structure in Zn 1−x Mg x O layers with the same or similar composition and different layer thicknesses.

A series of Yb3+-doped yttrium aluminum monoclinic Y4Al2O9 (Yb:YAM) single crystals with different Yb3+ ion concentrations of 0.1, 1, 5 and 10 at.% have been grown by means of micro pulling down method. Low temperature absorption measurements revealed that Yb3+ incorporates at several well defined centers. Site selective excitation and emission experiments allowed to locate the energy levels of the ground 2F7/2 and 2F5/2 manifolds for the major centers detected in the system. The temperature dependence of the transition energies and linewidths of the resonant transitions was measured from 10 to 300 K and was well described by one phonon near resonant processes. The effect of Yb3+ concentration on the fluorescence spectra structure of Yb:YAM has been also investigated. It was observed that with the increasing of Yb3+ concentration luminescence from the low energy site dominates the emission spectrum. The fluorescence dynamics recorded at temperature range from 10 to 300 K under selective excitation of each site was analyzed.

a b s t r a c t Studies of the absorption and temperature dependence of photoluminescence spectra and luminescence decay times of the intra-shell f-f transitions (2F5/2 ↔ 2F7/2) of Yb3+ ions in K2LaCl5:Yb3+ powders with 5, 10, 15 and 25% of ytterbium are presented. The spectroscopic properties of the powders with different ytterbium content are compared. Experiments were performed at the temperatures from 25 to 300 K. The strong emission around 982 nm has been observed under direct excitation of the luminescence center with 960 nm line of continuous wave Ti:sapphire laser pumped by Ar-ion laser. The temperature quench- ing effect of the luminescence was rather week, especially in the samples with higher concentration of ytterbium (15 and 25%). Additionally the probability of the f-f radiative transitions of the Yb 3+ ions in these powders was almost temperature independent for more heavily doped samples (with 15 and 25% of Yb) and only weakly temperature dependent for less doped samples (with 5 and 10% of Yb). These results reveal high thermal stability of the optical properties of the examined powders.

The thermoluminescence (TL) of calcium aluminate persistent luminescence materials doped with Eu 2+ and co-doped with R 3+ ions were studied between 20 and 325 K. The basic material, CaAl 2 O 4 :Eu 2+ , showed three TL bands between 150 and 300 K in the glow curve. Changing the R 3+ co-dopant to the adjacent element in the rare earth series affects significantly the thermoluminescence intensity and even the position of the glow curve maximum. The physical effect seemed to be removing the traps since the La 3+ , Y 3+ , and Lu 3+ ions suppressed the thermoluminescence. The Pr 3+ , Ho 3+ , and Dy 3+ co-doping enhanced mainly the low temperature traps and these materials have an intense but relatively short persistent luminescence at room temperature. The Nd 3+ and Tm 3+ ions enhanced the TL bands close to room temperature and are thus the most suitable co-dopants to induce intense and long persistent luminescence. The quenching of the thermoluminescence by Sm 3+ was concluded to be due to the presence of Sm 2+ that removes totally the traps.

Results of high-pressure luminescence spectroscopy of Cr 3+ doped MgO–2.5A1 2 O 3 non-stoichiometric green spinel under pressures up to 130 kbar at temperature T =10 K are reported. It is shown that Cr 3+ ions in this crystal form four major groups of centers. The broadband infrared luminescence is a result of the very large inhomogeneous broadening, which affects the intermediate strength-crystal field centers. Due to this broadening some of the Cr 3+ ions experience a very broad spectrum of the crystal field strength from a weak to strong crystal field scheme.

It is shown that the process of thermal relaxation of the photochromic effect in LaGaO 3 crystals proceeds due to transfer of the charges between defects involved in the effect and shallow traps. The crystals exhibit luminescence of Cr 3+ ions, which are an unintentional dopant. However, the emission observed in the thermally stimulated luminescence of the crystals with residual concentration of chromium is associated with luminescent centers different from typical isolated Cr 3+ ions. The spectral properties of these centers are studied.

The crystal structures of the lanthanum and neodymium gallates and yttrium aluminates, doped with neodymium and erbium were investigated by means of X-ray structure analysis (full profile Rietveld refinement, powder diffractometers HZG-4 and DRON-4, Cu and Co Kα-radiation). A comparative crystallochemical analysis of the studied structures was performed. All examined compounds belong to the orthorhombically distorted GdFeO3-type of structure, space group Pbnm, Z=4. It is shown that the structure deformation parameters such as the rare earth cation shift, the bond length distortion of the polyhedra, and the tilt angles of octahedra increase in the sequence: LaGaO3–NdGaO3–YAlO3:Nd–Y0.5Er0.5AlO3.

The LaGaO 3 crystal structure was investigated by means of X-ray structure analysis (powder diffractometer HZG-4, CuK α ): structure type GdFeO 3 , space group Pbnm, a =5.52299(8) A; b =5.49138(9) A; c =7.7725(1), A, Z =4, R I =0.098. The shortest interatomic distances (A) are: Ga–O, 1.967(3); La–O, 2.46(3) ; O–O, 2.77(4); Ga–Ga, 3.8862(1); La–Ga, 3.287(1); La–La, 3.866(5). Atoms have the following coordination numbers: La, 9 or 10; Ga, 6. Compounds formed in the LaGaO 3 –GdGaO 3 quasibinary system have been studied. An anomalous behaviour of the La 1− x Gd x GaO 3 solid solution lattice parameters has been observed.

The color centers, which are generated in yttrium–aluminium perovskite YAP:Nd(1 at.%) and YAP:Er(50 at.%) crystals under the influence of ultraviolet and γ-irradiation, have been studied by absorption spectroscopy. Both stable and transient at room temperature color centers are generated. It is shown that the transient color centers are mainly responsible for the decrease of laser generation efficiency of Nd:YAP and YAP:Er irradiated crystals, although physical mechanisms leading to efficiency decrease are different in these materials.

Y 3 Al 5 O 12 single crystals doped with praseodymium and magnesium ions have been prepared. The reversible color change of these crystals are observed when annealing in oxidizing or reducing atmospheres. The change is ascribed to the formation of Pr 4+ in the as-grown crystal, caused by the second dopant, Mg 2+ . The absorption spectra of YAG:Pr,Mg in the range 200–1100 nm, as grown and annealed in air and H 2 /N 2 atmosphere, are presented and discussed. Additional broad absorption bands are observed for the as-grown crystals and those annealed in oxidizing atmosphere. Crystallographic investigations of the original crystal and after annealing in a reducing atmosphere as described above, show no distinct structural differences. A redox mechanism is proposed to explain the color change during annealing.

Recently, ZnO attracts a wide interest as a promising material for the application in optoelectronic devices working in the blue and ultraviolet region and (when doped with magnetic impurities) in spintronic devices. Unfortunately, the technology of good, large (e.g.: 2.5–5 cm in diameter) single crystals is very difficult, even as compared with other II–VI compounds. We report on the successful growth of the ZnO crystals with a chemical vapour transport (CVT) method and on the characterisation of them. The source material is synthesised at 650 °C from oxygen and zinc vapours and subsequently, baked (as a powder) to achieve stoichiometry. The crystals grow (with the rate 1–2 mm per day) in the graphite-covered quartz ampoules containing pure (6N) hydrogen or nitrogen and a small amount of water vapour. The crystals, both as-grown and annealed in pure oxygen, are characterised by the measurements of photoluminescence spectra, transmission spectra, far infrared transmission, X-ray diffraction and electrical transport. The surface region is analysed by the secondary ion mass spectroscopy (SIMS).

The energy transfer between Yb3+ and Ho3+ ions and the energy migration on Yb3+ ions as a function of temperature were studied in gadolinium gallium garnet crystals doped with Yb3+ (5%) and Ho3+ (3%). It was found that the commonly accepted energy transfer theories do not provide a good description of the system studied. The application of several theories gives contradictory results.

The influence of rare earth and 3d impurities on the process of ionizing recharge of genetic defects under gamma-irradiation in Gd 3 Ga 5 O 12 and Y 3 Al 5 O 12 laser crystals has been studied by absorption spectroscopy. Impurities with stable trivalent states (Nd 3+ , Er 3+ , Sm 3+ , etc) do not change the character of absorption spectra of the colour centers formed during gamma-irradiation. Impurities (Cr, Fe, Ce) which can easily change valency during irradiation, compete with growth defects in trapping of the charge carriers generated by irradiation.