Photoluminescence of nanocrystalline YVO4:TmxDy1−x prepared by a modified Pechini method

This paper reports the luminescence effects of Tm³⁺ doped YVO₄:Dy nanocrystalline synthesized by a modified Pechini method. The structure and morphology were characterized by using X-ray diffraction (XRD) and transmission electron microscope (TEM). The relationship between the ratio of Tm³⁺/Dy³⁺ and the chromaticity is studied, i.e. Tm³⁺ ion doping effectively tunes the emission color of YVO₄:Tm_xDy_1−x phosphors. The best white light emission was observed with YVO₄:1%(Tm_0.6Dy_0.4). These results indicate that thulium doped YVO₄:Dy phosphors are promising white-emitting luminescence materials.     

Diode-pumped doubly Q-switched mode-locked YVO4/Nd:YVO4/KTP green laser with AO and GaAs saturable absorber

By using both acousto-optic modulator and GaAs saturable absorber, a diode-pumped Q-switched and mode-locked(QML) YVO₄/Nd:YVO₄/KTP green laser is presented. The experimental results show that the doubly QML YVO₄/Nd:YVO₄/KTP green laser has good stability and nearly 100% modulation depth. The average output power and the pulse width of the Q-switched envelope have been measured. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 137 MHz and its pulse width is estimated to be about 280 ps. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML green laser is given and the numerical solutions of the equations are in good agreement with the experimental results.     

Influence of Ce on the luminescence properties of Er/Yb:YVO4 crystals

YVO₄ single crystals doped with Ce³⁺, Er³⁺ and Yb³⁺ ions were grown by the Czochralsski technology. The luminescence properties of Er³⁺/Yb³⁺:YVO₄ single crystals with different concentration of Ce³⁺ were studied, and the energy transfer mechanism between Er³⁺, Yb³⁺ and Ce³⁺ was discussed based on their energy level properties. The branching ratios of the ⁴I_11/2 → ⁴I_13/2 transition in different samples were calculated. The results indicate that codopants of Ce³⁺ greatly enhance the population rate of the ⁴I_13/2 level due to the fast resonant energy transfer between Er³⁺ and Ce³⁺, i.e., ⁴I_11/2(Er³⁺) + ²F_7/2(Ce³⁺) → ⁴I_13/2(Er³⁺) + ²F_5/2(Ce³⁺).     

Enhancement of stability and peak power in a diode-pumped doubly QML YVO4/Nd:YVO4 laser with EO and Cr:YAG saturable absorber

A diode-pumped doubly Q-switched and mode-locked (QML) YVO₄/NdYVO₄ laser is realized with the electro-optic (EO) modulator and Cr⁴⁺:YAG saturable absorber, in which the repetition rate of the Q-switched envelope is controlled by the active EO modulation while the mode-locked pulses inside the Q-switched envelope depend on both the actively modulated loss and the passive saturable absorption. The experimental results show that the doubly QML laser can generate more stable and shorter pulses with higher peak power when compared with the singly passively QML laser with Cr⁴⁺:YAG. At the pump power of 20 W and the repetition rate 1 kHz, a 21 ns Q-switched pulse envelope with a average mode-locked peak power of 544 kW is obtained, which is the shortest Q-switched pulse envelope to my knowledge. In comparison to the singly passively QML laser with Cr⁴⁺:YAG, the doubly QML laser has compressed the Q-switched envelope pulse width 70% and improved the mode-locked pulsed peak power 27 times. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the numerical solutions of the equations are in good agreement with the experimental results.     

Annealing behavior of single mode planar waveguide in YVO4 produced by He ion implantation

We report, for the first time to our knowledge, the formation of single mode planar waveguide in z-cut YVO₄ by 400 keV, 500 keV He ion implantation in fluence of 3 × 10¹⁶ ions/cm² at room temperature or at liquid nitrogen temperature (77 K). We investigated annealing behavior of the guiding mode and near-field image in the waveguide by prism-coupling method and end-face coupling method respectively. We found that the effective refractive index of the TE₀ mode was different before and after annealing for the samples implanted at room temperature, while, annealing had nearly no influence on the effective refractive index of the TE₀ mode of the samples implanted at liquid nitrogen temperature (77 K). After annealing at 600 K for 1 h, no guiding mode was observed in the sample implanted by 400 keV He ion in fluence of 3 × 10¹⁶ ions/cm² at room temperature. The Rutherford backscattering/channeling technique was used to investigate the damage reduction after annealing treatments. The minimum yield of the implanted, annealed sample was 5.43%. We reconstructed the refractive index profiles in the waveguide under different condition by applying intensity calculation method.     

Radiation damage study of MeV ions-implanted Nd:YVO4 crystal

Damage formation mechanism of Nd:YVO₄ implanted with MeV ions is investigated. MeV Si⁺ ions were implanted into Nd:YVO₄ crystal, and the lattice damage was measured using Rutherford backscattering spectroscopy/channeling (RBS/C) method. The damage creation kinetic indicates a significant contribution from electronic energy loss to the surface damage. A detailed analysis allows us to deduce the different contributions from electronic and nuclear stopping powers to the lattice damage production. An obvious difference in extent of damage from 1 MeV and 3 MeV Si⁺ implantations also implies that there exists a threshold value of the electronic energy deposition for damage formation. The exact value of threshold is obtained by comparison with the experimental data obtained from 3 MeV O⁺, F⁺ and Si⁺ implantation results, which turns out to be (1.7 ± 0.1) keV/nm.     

Er,Yb:YVO4 waveguides deposited on amorphous SiO2 by PLD and UVPLD

In this work, erbium, and erbium and ytterbium co-doped YVO₄ waveguiding thin films were deposited on amorphous SiO₂ substrates by pulsed laser deposition (PLD) and ultraviolet-assisted pulsed laser deposition (UVPLD). The influence of the deposition technique on the structure, morphology, and optical properties of the films was investigated. At lower dopant concentrations the films prepared by UVPLD show better crystallinity and optical properties. All the samples show preferred orientation of the (001) zone axes parallel to the substrate surface. The polycrystalline samples show difference in the refractive indexes ?n (?n = n_TE − n_TM) for the TE and TM polarizations.     

Mild hydrothermal solid-phase synthesis of YVO4 nanocrystals

The tetragonal YVO₄ nanocrystals are facilely prepared via a hydrothermal solid-phase synthesis method directly utilizing bulk phase materials of V₂O₅ and Y₂O₃ as precursor. Whether additives (acid and EDTA) exist or not, the reaction can be performed in the mild temperature range from 130 to 200 °C. The products are characterized with XRD, FTIR, TEM and PL. The effect of acid and amounts of EDTA on the morphology of the product is investigated. The YVO₄ nanoparticles exhibit novel photoluminescence emission bands at 330 and 606 nm under 254 nm excitation. A growth mechanism of yttrium orthovanadate is proposed.     

EPR and vibrational studies of YVO4:Tm, Yb single crystal

A well oriented YVO₄ single crystal, with 5% Yb³⁺ and 2% Tm³⁺ nominal doping, was investigated using the Raman and EPR techniques.The EPR measurements suggest that Yb³⁺ ions occupy eight-coordinated Y³⁺ sites forming bisdisphenoids of the D_2d symmetry. An inhomogeneous distribution of rare-earth ions leads to a significant distortion of the local point symmetry (C₁). It seems that strong dipole–dipole interactions between Yb³⁺ ions are responsible for the distortion. As a result, two types of ytterbium magnetic centers appear. They correspond to paired magnetic centers and distorted isolated paramagnetic centers that are strongly sensitive to the magnetic field directions and some imperfections of the crystal. Pair centers can be recorded through the rotation around the c-crystal axis, whereas isolated centers can be measured when the crystal is rotated around the a-crystal axis. With the increasing temperature, the ytterbium signal disappeared at about 23 K and a group of narrow lines became visible. These lines, observed in the range of 240–550 mT, correspond to the Gd³⁺ (S = 7/2) ions, doped to the structure unintentionally from the basic materials.     

Photoluminescence spectra of MnIn2S4

MnIn₂S₄ single crystals grown by the directional crystallization method were investigated by using the temperature and excitation power dependencies of photoluminescence (PL) spectra. PL spectra consist of one broad band resulting from donor-acceptor pair recombination. The analysis of the temperature quenching of the PL intensity yields one defect donor level with a thermal ionization energy of about 0.17 eV. The broad band of PL spectra indicates that radiative recombination is related to multiphonon optical processes. The energy of the involved phonon was found to be around 0.025 eV and the energy of the acceptor level is about 0.86 eV.     

Diode-pumped efficient laser operation and spectroscopy of Tm,Ho:YVO4

Spectroscopic characterization of co-doped Tm,Ho:YVO₄ crystal grown by the Czochralski method has been performed including absorption spectrum, emitting spectrum and luminescence decay lifetime. The polarization emitting spectrum around 2 μm is accomplished by exciting a singly Ho³⁺ doped YVO₄ crystal to exclude the influence of Tm³⁺³F₄-³H₆ transition and the emission cross section is deduced from both Fuchtbauer-Ladenburg (F-L) equation and reciprocity method (RM). In addition, we report up to 10.4 W continuous wave (CW) output with a conversion efficiency of 40% and 10.3 W Q-Switch output with 12.5 kHz pulse repetition rate of diode-pumped cryogenic Tm,Ho:YVO₄ laser. For Q-Switch operation, the minimum pulse width of 28.2 ns is obtained, all of which demonstrate that the Tm,Ho:YVO₄ is excellent laser material for 2 μm radiation.     

Photoluminescence properties of Y2O3:Tb and YBO3:Tb green phosphors synthesized by hydrothermal method

In this work, two Tb³⁺ activated green phosphors: Y₂O₃:Tb³⁺ and YBO₃:Tb³⁺ were prepared by hydrothermal method. Photoluminescence properties of both phosphors were studied in details. Both phosphors exhibit similar luminescent characteristics symbolized by the dominant green emission at 545 nm. Concentration quenching occurs at the Tb³⁺ concentration of 1.60 atomic% and 2.57 atomic% for Y₂O₃:Tb³⁺ and YBO₃:Tb³⁺, respectively. Luminescence decay properties were characterized to better understand the mechanism of concentration quenching. Based on the calculation, the concentration quenching in both phosphors was caused by the dipole–dipole interaction between Tb³⁺ ions.     

Effect of Calcium on Nd:GdVO4 single crystal

The Ca_0.08Gd_0.92VO₄ crystal and Nd-doped Ca_0.08Gd_0.92VO₄ crystals were grown by the Czochralski method; the lattice parameters of the crystals were determined by XRD. The results showed that substituting Nd³⁺ caused an increase in lattice parameter; the segregation coefficient of Nd³⁺ ions in the crystal is lower than that of Nd:GdVO₄ crystal; the absorption spectra, fluorescence spectra, absorption cross-section and the emission cross-section of the Nd:Ca_0.08Gd_0.92VO₄ crystal were investigated at room temperature; furthermore, the Raman performance of Ca_0.08Gd_0.92VO₄ crystal was improved.     

Thermal properties and infrared spectra analysis of monoclinic RbGd(WO4)2 single crystals

Monoclinic rubidium gadolinium bis(tungstate) single crystals, RbGd(WO₄)₂ (RGW), have been grown by the spontaneous nucleation from high-temperature solutions. The thermal properties were firstly studied by measuring DSC, TG and specific heat. The melting point was determined to be 1089 °C. The measured specific heat ranges from 0.141 J g^− 1 K^− 1 to 0.564 J g^− 1 K^− 1 in the temperature range from 60 °C to 700 °C, a value that is slightly smaller than that of KGd(WO₄)₂. An infrared spectrum of the crystal was recorded in the frequency range of 50 to 1000 cm^− 1 and all vibration frequency peaks were assigned.     

Spectroscopic properties of red Bi4Ge3O12 by vertical Bridgman method

Red Bi₄Ge₃O₁₂ (BGO) single crystals had been grown by vertical Bridgman (VB) method. The structure of this crystal was determined by XRD. The absorption and emission spectra of the red BGO in visible and near infrared region (NIR) were measured at room temperature. The emission intensity of the red BGO is weaker than that of ordinary BGO at about 500 nm. Interestingly, the red BGO shows a significant emission band centered at about 1495 nm. The red BGO faded and its properties recovered after ultraviolet (UV) irradiation or annealing.     

Synthesis and luminescent properties of LaPO4:Eu microspheres

LaPO₄:Eu³⁺ microspheres were synthesized, using LaCl₃, EuCl₃ and (NH₄)₂HPO₄ as starting materials. The morphology, formation mechanism, and luminescent property of samples were systemically studied. X-ray diffraction (XRD) and infrared spectroscopy (IR) show that LaPO₄:Eu³⁺ microspheres have a pure monoclinic phase. Cetyltrimethyl ammonium bromide (CTAB) usually forms spherical micelles above a critical micelle concentration, which plays an important role in the formation of LaPO₄:Eu³⁺ microspheres. The excitation spectrum of LaPO₄:Eu³⁺ microspheres consists of several sharp lines due to the direct excitation of the Eu³⁺ cations from the ground state to higher levels of the 4f-manifold. The emission intensity of microspheres is higher than irregular particles because of the lowlier surface area. The lifetimes of Eu³⁺ ions in the LaPO₄:Eu³⁺ microspheres are determined to be 2.41 ms.     

Photoluminescence properties of ion beam synthesized β-FeSi2

Effects of structural changes near the interface, the implantation dose and the precipitate morphology on photoluminescence (PL) properties of ion-beam synthesized (IBS) β-FeSi₂ have been investigated. The intrinsic PL spectrum attributed to the interband transition of β-FeSi₂ was found by examining the dependence of the PL peak energy on temperature and the excitation power.     

Syntheses of Mn3O4 and LiMn2O4 nanoparticles by a simple sonochemical method

Mn₃O₄ and LiMn₂O₄ nanoparticles were prepared by a simple sonochemical method which is environmentally benign. First, Mn₃O₄ nanoparticles were prepared by reacting MnCl₂ and NaOH in water at room temperature through a sonochemical method, operated at 20 kHz and 220 W for 20 min. Second, LiOH was coated onto the resulting Mn₃O₄ under the same sonochemical conditions as above. The thickness of coated LiOH on Mn₃O₄ obtained from the reaction ratio of 3:1 between LiOH and Mn₃O₄ was about 4.5–5.5 nm range. Then, by heating those LiOH-coated Mn₃O₄ particles at the relatively low temperature of 300–500 °C for 1 h, they were transformed into phase-pure LiMn₂O₄ nanoparticles of about 50 to 70 nm size in diameter.     

Radiative recombination in Cu2ZnSnSe4 monograins studied by photoluminescence spectroscopy

In this study we investigated the optical properties of Cu₂ZnSnSe₄ monograin powders that were synthesized from binary compounds in the liquid phase of flux material (KI) in evacuated quartz ampoules. The monograin powder had p-type conductivity. Radiative recombination processes in Cu₂ZnSnSe₄ monograins were studied using photoluminescence spectroscopy. The detected low-temperature (T = 10 K) photoluminescence band at 0.946 eV results from band-to-impurity recombination in Cu₂ZnSnSe₄. The ionization energy of the corresponding acceptor defect was found to be 69 ± 4 meV. Additional photoluminescence bands detected at 0.765 eV, 0.810 eV and 0.860 eV are proposed to result from Cu₂SnSe₃ phase whose presence in the as-grown monograins was detected by Raman spectroscopy and SEM analysis. Considering photoluminescence results, it is proposed that the optical bandgap energy of Cu₂ZnSnSe₄ is around 1.02 eV at 10 K.     

Magnetic properties of β-FeSi2 single crystals

We investigated temperature and magnetic field dependences of the magnetization of β-FeSi₂ single crystals in the temperature range of 5-300 K in magnetic fields up to 15 kOe. The temperature dependence of the magnetic susceptibility of the Cr- and Ni-doped sample can be explained by temperature-dependent contributions due to paramagnetic centres and due to the carriers excited thermally in the extrinsic conductivity region. The values of the paramagnetic Curie temperature as well as activation energy of the donor and acceptor levels are estimated.