Rare-earth Oxyorthosilicate Research Articles

Hyperbolic-to-hyperbolic transition at exceptional Reststrahlen point in rare-earth oxyorthosilicates

Anisotropic optical crystals can exhibit a hyperbolic response within the Reststrahlen band (RB) and support directional polaritonic propagations when interacting with light. Most of the reported low-symmetry optical crystals showcase the evolution from hyperbolic to elliptic dispersion topologies, largely owing to their adjacent RBs being either overlapped or separated. Here, we report an exceptional Reststrahlen point (ERP) in rare-earth oxyorthosilicate Y2SiO5, at which two neighboring RBs almost kiss each other. Consequently, we observe the direct hyperbolic-to-hyperbolic topological transition: the hyperbolic branches close and reopen along with the rotating transverse axis (TA). At such ERP, the TA merges to the direction orthogonal to its proximate phonon mode, mainly due to the interplay between these two non-orthogonal phonon modes. We also find that even with the existence of only one single RB, the TA can rotate in-plane. Our findings are prevalent in isostructural rare-earth oxyorthosilicates, such as Lu2SiO5. The universally underlying physics of ERP and its corresponding special class of rare-earth oxyorthosilicates may offer playgrounds for continuously tuning phonon polariton propagation direction, and broadband controlling light dispersion of polaritonic nanodevices.

Diode-pumped mode-locked Yb:Sc2SiO5 laser generating 38 fs pulses.

We report on sub-40 fs pulse generation from a Yb:Sc2SiO5 laser pumped by a spatially single-mode fiber-coupled laser diode at 976 nm. A maximum output power of 545 mW was obtained at 1062.6 nm in the continuous-wave regime, corresponding to a slope efficiency of 64% and a laser threshold of 143 mW. A continuous wavelength tuning across 80 nm (1030 -1110 nm) was also achieved. Implementing a SESAM for starting and stabilizing the mode-locked operation, the Yb:Sc2SiO5 laser delivered soliton pulses as short as 38 fs at 1069.5 nm with an average output power of 76 mW at a pulse repetition rate of ∼79.8 MHz. The maximum output power was scaled to 216 mW for slightly longer pulses of 42 fs, which corresponded to a peak power of 56.6 kW and an optical efficiency of 22.7%. To the best of our knowledge, these results represent the shortest pulses ever achieved with any Yb3+-doped rare-earth oxyorthosilicate crystal.

A Review on X-ray Excited Emission Decay Dynamics in Inorganic Scintillator Materials

Scintillator materials convert high-energy radiation into photons in the ultraviolet to visible light region for radiation detection. In this review, advances in X-ray emission dynamics of inorganic scintillators are presented, including inorganic halides (alkali-metal halides, alkaline-earth halides, rare-earth halides, oxy-halides, rare-earth oxyorthosilicates, halide perovskites), oxides (binary oxides, complex oxides, post-transition metal oxides), sulfides, rare-earth doped scintillators, and organic-inorganic hybrid scintillators. The origin of scintillation is strongly correlated to the host material and dopants. Current models are presented describing the scintillation decay lifetime of inorganic materials, with the emphasis on the short-lived scintillation decay component. The whole charge generation and the de-excitation process are analyzed in general, and an essential role of the decay kinetics is the de-excitation process. We highlighted three decay mechanisms in cross luminescence emission, exitonic emission, and dopant-activated emission, respectively. Factors regulating the origin of different luminescence centers controlling the decay process are discussed.

On the luminescence origin in Y2SiO5:Ce and Lu2SiO5:Ce single crystals

Photoluminescence characteristics of single crystals of Ce3+-doped rare-earth oxyorthosilicates RE2SiO5 (RE: Y, Lu) with different Ce3+ concentrations are studied in the 4.2–500 K temperature range by the steady-state and time-resolved spectroscopy methods. The concentrations of single and dimer Ce3+-related centers of different types in the investigated crystals are evaluated from their EPR spectra. On the basis of the obtained results and analysis of literature data, the origin of the Ce3+-related centers responsible for the luminescence of these crystals is clarified and a new interpretation of their luminescence spectra is proposed. The suggestion is made that the dominating higher-energy (violet) doublet emission band of Y2SiO5:Ce and Lu2SiO5:Ce (denoted earlier as the Ce1 emission) arises from the Ce3+ ions substituting for the host crystal lattice rare-earth RE3+ ions in both RE1 and RE2 lattice sites and, thus, can be considered as the superposition of the strong Ce1 and weak Ce2 emission bands. The weaker lower-energy (blue) broad emission band (denoted earlier as the Ce2 emission) is ascribed to the dimer {Ce3+ - Ce3+} centers. No centers with the local X1 structure, containing Ce3+ ions and neighboring O2− ions (the CeO9 clusters embedded into the crystal lattice of the X2 structure), reveal themselves in the EPR spectra.

Influence of the ratio of rare earth oxyorthosilicate R2SiO5 (R = La, Y) hosts on the structure and optical properties of co-doped Pr3+ /Dy3+ phosphors

Praseodymium (Pr3+) and dysprosium (Dy3+) co-doped mixed lanthanum/yttrium oxyorthosilicates (La2-xYxSiO5, x = 0.5, 1 and 2) phosphors were synthesized using urea-assisted combustion method. The X-ray diffraction (XRD) patterns showed the pure monoclinic phases of La2SiO5 (x = 0) and Y2SiO5 (x = 2) and mixed phases of La2-xYxSiO5. The diffuse reflectance spectra of the phosphors showed a broad absorption peak with a maximum around 376 nm assigned to O2−→ Pr4+ charge transfer band. The band gaps of the Pr3+/Dy3+ co-doped La2-xYxSiO5 increased from 2.7 to 5.1 eV as the value of x increased from 0 to 2. The luminescent properties were measured when the samples were excited using three different sources, namely 450 W Xenon lamp, 325 He-Cd laser and electron beam. The photoluminescence (PL) emission spectra recorded using monochromatized Xenon lamp showed tunable emission for different excitation wavelengths. For example, when the PL spectra were recorded under 294 nm excitation, using Xenon lamp, emissions from self-trapped excitons (STE) in SiO2, f→f transitions of Dy3+ and Pr3+ were observed. The emission monitored under 349 nm excitation showed the STE peak and the four emission lines of Dy3+. However, when the phosphors were excited using a 325 nm He-Cd laser, the STE emission and four Dy3+ lines were observed in all the samples, but the four manifolds emission from the 1D2→3H4 transition of Pr3+ were only observed for x = 1.5 and 2. Furthermore, the cathodoluminescence (CL) spectra showed the STE band, four emission lines of Dy3+ and four manifolds emission from the 1D2→3H4 transition of Pr3+. The tunable colours of the phosphors were shown in International Commission on Illumination (CIE) coordinate diagram.

Optical properties and chemical composition analyses of mixed rare earth oxyorthosilicate (R2SiO5, R=La, Gd and Y) doped Dy3+ phosphors prepared by urea-assisted solution combustion method

Dysprosium (Dy3+) doped lanthanum gadolinium oxyorthosilicate (LaGdSiO5), lanthanum yttrium oxyorthosilicate (LaYSiO5) and gadolinium yttrium oxyorthosilicate (GdYSiO5) phosphors (in powder form) were synthesized by urea-assisted combustion method. The X-ray diffractometer analysis confirmed that the LaGdSiO5, LaYSiO5 and GdYSiO5 crystalized in monoclinic phases. The chemical composition of the phosphors was analyzed by measuring the atomic and molecular ionic species using the time of flight secondary ion mass spectroscopy (ToF SIMS). In addition, ToF SIMS imaging technique was used to determine the distribution of the Dy3+ dopant ions on the surface on the phosphors. The average crystallite sizes and lattice strains of the phosphor were increased by Dy3+ doping. The field emission scanning electron microscope images showed that the powders were made up of an agglomeration of particles with no regular shape. The photoluminescence data showed narrow line emission peaks at the wavelengths of 485nm (minor emission) and 573nm (major emission) associated with the f→f transitions of Dy3+. The photoluminescence (PL) measurements showed that the emission peak of LaGdSiO5:Dy3+ was ~3× more intense than those of LaYSiO5:Dy3+ and GdYSiO5:Dy3+ when excited using monochromatic xenon lamp with a wavelength of 241nm. However, when the powders were excited using a 325nm He–Cd laser, the highest PL emission intensity was observed from GdYSiO5:Dy3+.

Synthesis, Rietveld Analysis, and Solid State Nuclear Magnetic Resonance of X2‐Sc2SiO5

Compounds containing rare earths are of increasing technological interest especially because of their unique mechanical, magnetic, electrical, and optical properties. Among them, rare earth oxyorthosilicates are attractive scintillators for γ‐ and X‐ray spectroscopy and detection. However, there are many structural aspects of those compounds that are not clear. In this research, the structure parameters for Sc2Si2O5, X2‐polymorph, have been refined from powder X‐ray diffraction (XRD) data and the29Si MAS NMR spectrum is reported for the first time. X2‐Sc2SiO5polymorph was synthesized by the sol–gel method and characterized by XRD and29Si MAS NMR. The XRD pattern was indexed in a monoclinic unit cell with space groupI2/c; the resulting unit cell parameters werea=9.9674(2) Å,b=6.4264(9) Å,c=12.0636(2) Å, and β=103.938(1)°. The29Si MAS NMR spectrum showed a unique signal at −79.5 ppm, compatible with the unique Si crystallographic site in the unit cell. Finally, the band valence method has been applied to the calculation of a “shift parameter,” which is correlated with the NMR chemical shift.

Cerium oxidation state in LSO:Ce scintillators

Trivalent cerium ions form the luminescence centers in several important families of scintillation materials including the rare earth oxyorthosilicates, pyrosilicates, and aluminates. When comparing the experimentally determined scintillation properties of cerium-doped scintillators to theoretical models of scintillation mechanisms, there is often speculation regarding the fraction of the total cerium that exists in the radiative trivalent charge state (Ce/sup 3+/) rather than the nonradiative tetravalent state (Ce/sup 4+/). Until now, however, no technique has been developed to quantitatively measure both Ce/sup 3+/ and Ce/sup 4+/ in single crystal scintillators . We report here for the first time direct measurements of Ce/sup 3+/ and Ce/sup 4+/ in Lu/sub 2/SiO/sub 5/:Ce single crystals. Synchrotron radiation was used to measure the X-ray absorption on the M/sub 4/ and M/sub 5/ edges of Ce, and the results were compared to model samples of Ce/sup 3+/ (Ce/sub 2/O/sub 3/) and Ce/sup 4+/ (CeO/sub 2/) which provided clear signatures of the two charge states. The spectra were obtained with a high-resolution superconducting tunnel junction spectrometer on beamline 4.0.2 at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The results clearly show 100% Ce/sup 3+/, independent of light yield and sample coloration. Therefore, energy migration to the luminescence centers appears to be the determining factor in the scintillation efficiency of these samples, rather than variations in the Ce/sup 3+//Ce/sup 4+/ ratio.

Czochralski growth and characterisation of large Ce 3+:Lu 2SiO 5 single crystals co-doped with Mg 2+ or Ca 2+ or Tb 3+ for scintillators

The growth of large-diameter (up to 80 mm) rare-earth oxyorthosilicate crystals is discussed. The scintillating performance of well-known Ce:Lu 2SiO 5 (LSO) crystals, which are the key contenders for positron emission tomography, and the Ce 3+:Mg 2+:Lu 2SiO 5− x , Ce 3+:Ca 2+:Lu 2SiO 5− x and Ce 3+:Tb 3+:Lu 2SiO 5 crystals developed in GPI of RAS are compared.

Thermoluminescence and scintillation properties of rare earth oxyorthosilicate scintillators

In recent years the scintillation properties of several cerium-doped rare earth oxyorthosilicate scintillators, Ln/sub 2/SiO/sub 5/:Ce where Ln = Y, La - Lu, have been reported and, in some cases, extensively studied. In addition, binary and ternary compounds such as (Lu,Y)/sub 2/SiO/sub 5/:Ce, (Lu,Gd)/sub 2/SiO/sub 5/ and (Lu,Y,Gd)/sub 2/SiO/sub 5/:Ce have been reported. All of these crystals have either monoclinic P or C structures with characteristic SiO/sub 4/ tetrahedra and trivalent cations occupying two unique crystallographic positions. The trivalent cerium activator ions are assumed to occupy the cation lattice sites and possibly interstitial positions as well. The excited 5d state of Ce/sup 3+/ is split into 3 observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state (/spl sim/3 eV) with a Stokes shift of /spl sim/ 0.5 eV. The band gap is about 6 eV, and the index of refraction is close to 1.8, with some variation according to crystallographic axes. Despite these similarities, important differences remain among the crystals including scintillation efficiency, decay time, rise time, and afterglow. In this paper, we report thermoluminescence measurements between 10K and 350K that allow the determination of trapping levels that may influence scintillation properties. The thermoluminescence data shows that the various scintillators compositions have surprisingly dissimilar sets of traps that may at least partially explain some of the differences in their scintillation properties.

Czochralski growth of rare earth oxyorthosilicate single crystals

Single crystals of three Ce-doped rare earth oxyorthosilicates, Ln 2 (SiO 4 )O where Ln=Y, Gd, or Lu, were grown by the Czochralski technique. The formation of defects was strongly influenced by the crystal growth conditions for Ln= Gd but high quality boules for Ln=Y and Lu were grown under a variety of conditions. The luminescence properties of these materials reflect the interaction of the Ce 3+ activator with the crystal field of the host lattice

UV and gamma-ray excited luminescence of cerium-doped rare-earth oxyorthosilicates

Gamma-ray and UV-excitation of cerium-doped rare-earth oxyorthosilicates (RE = Y, Gd, and Lu) has been used to investigate the emission mechanism for this family of scintillators. The data clearly indicate the presence of two different luminescence centers, which are attributed to cerium substitution in the two different rare-earth crystallographic sites. While this model explains much of the present and previous UV data, the gamma-ray induced emission from GSO requires a better understanding of the energy transfer between Gd and Ce.

Site selectively excited luminescence of Eu 3+ in gadolinium, yttrium and lutetium oxyorthosilicates

In the present study the optical properties of the Eu 3+-activated rare earth oxyorthosilicate, RE 2(SiO 4)O:Eu 3+ (RE ≡ Gd, Y, Lu) powder samples were investigated. The luminescence spectra were recorded at 77 K using site selective excitation by a dye laser in addition to conventional UV excitation. Strong luminescence was observed only from the 5D 0 level. Site selective excitation gave two sites for the Eu 3+ ion in all oxyorthosilicates studied. In lutetium and yttrium compounds the sites are deduced to be very similar since only small differences exist in the 5D 0 → 7F 0 transition energies, and because the 7F j level schemes resemble each other. The absence of degeneracy in the 7F j levels indicates low symmetry for both sites, which are rather different. In gadolinium oxyorthosilicate the two cation sites differ significantly from each other. The emission from one of the sites showed doubled peaks probably due to local distortions.

Ce 3+- and Tb 3+-activated rare earth oxyorthosilicates

The luminescence properties of Ce 3+- and Tb 3+-activated rare earth oxyorthosilicates, RE 2(SiO 4)O (RE ≡ La, Gd, Y), have been studied. The excitation spectrum of Tb 3+ contains a strong 4f-5d band at about 240 nm. The position of the band shifts from 230 nm in the La compound to 245 nm in the Y compound. The emission spectrum of Tb 3+ reveals typical strong lines in the green region, originating from the 5D 4 level. With small activator concentrations the blue emission from the 5D 3 level dominates. The excitation spectrum of Ce 3+ in RE 2(SiO 4)O consists of two main bands at 300 and 360 nm. The emission of cerium occurs at the near UV-and blue region (395 nm; halfwidth 65 nm). In a Ce 3+, Tb 3+ codoped sample energy transfer from cerium to terbium is obtained. A very bright green emission is obtained when the codoped samples are excited with 360 nm.