Passive mode-locking Yb<Subscript>3+</Subscript>:Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript> laser at 1040 nm with SESAM

Passive mode locking of the new crystal Yb₃₊:Sc₂SiO₅ (Yb:SSO) laser at 1040 nm in a w-type cavity is reported for the first time to our knowledge. The laser operated at a repetition frequency of 94 MHz. A pulse duration of 4.2 ps was produced with an average power of 543 mW and a highest pulse energy of 5.7 nJ, corresponding it’s maximum peak power can reach to 1.4 kW.     

Continuous wave laser operation of Yb<Superscript>3+</Superscript>:YVO<Subscript>4</Subscript>

We have demonstrated the continuous wave laser operation of Yb³⁺:YVO₄. For Ti:Al₂O₃ laser pumping at 985 nm, a maximum slope efficiency of 41.1% and a threshold pump power of 76 mW were obtained. The maximum output power was 433 mW at a laser wavelength of 1037 nm.Using a cw diode laser around 974 nm as a pump source, a slope efficiency of 10.9% and a maximum output power of 152 mW were achieved at a laser wavelength of 1039 nm. The laser threshold pump power was 608 mW with respect to the absorbed pump power. The effective emission cross-sections for the ²F_5/2²F_7/2 transition were determined using the Füchtbauer–Ladenburg equation. The maxima of the effective absorption and emission cross-sections were found at 984.5 nm (6.74×10^-20 cm²) in -po larization and 985.5 nm (4.28×10^-20 cm²) also in -p olarization. The upper laser level lifetime was measured with suppression of radiation trapping and is around 318 s. PACS 42.55.Rz; 42.55.Xi; 42.70.Hj     

Wavelength tunable femtosecond laser based on short Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> codoped phosphate fiber

A wavelength tunable laser system mode-locked by nonlinear polarization evolution based on a 3 cm-long homemade Er³⁺/Yb³⁺ codoped phosphate fiber has been reported. By simply adjusting the polarization controllers, the central wavelength of the mode-locked spectrum can be tuned over 1537.1–1563.3 nm continuously. Moreover, 264-fs pulse with 3-dB spectral width of 39.6 nm and peak power of 7.8 kW at a 7.55 MHz repetition rate is generated directly from the all-fiber ring cavity.     

CW and tunable laser operation of Yb<Superscript>3+</Superscript> doped CaF<Subscript>2</Subscript>

We present the spectroscopic properties and room temperature of a cw tunable laser operation with Yb³⁺ doped CaF₂ single crystals grown in our laboratory. A laser slope efficiency of 50% with respect to the absorbed 920 nm pump power was obtained, and the laser wavelength could be tuned between 1000 and 1060 nm. PACS 42.55R; 42.70An erratum to this article can be found at     

LiNbO<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript> crystals for optical temperature sensors

Possibilities of using lithium niobate crystals, doped with ytterbium (LN:Yb³⁺) as materials for optical temperature sensor (OTS), are discussed. We consider both the radiative and absorptive characteristics of the crystals, assuming that their temperature dependences are caused by the Boltzmann factor of the initial population of Stark sublevels of the ion. It is shown that the crystals can be used as materials for OTS in the temperature range 150–400 K.     

Intense White Luminescence from Combustion Synthesized Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript>: Yb<Superscript>3+</Superscript>/Yb<Superscript>2+</Superscript> Single Phase Phosphor

The Ca₁₂Al₁₄O₃₃: Yb³⁺/Yb²⁺ single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb³⁺ and Yb²⁺ states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb³⁺ state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb²⁺ ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb²⁺ state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb²⁺ to Yb³⁺ has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb³⁺ and Yb²⁺ states have been examined and discussed. Quantum yield of emission has also been determined for different samples.     

Color Change Upconversion Mechanism of Y<Subscript>6</Subscript>O<Subscript>5</Subscript>F<Subscript>8</Subscript>: Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> Microtubes by Using Time-Resolve Spectra

The mechanism of the upconversion processes in Y₆O₅F₈: 2%Er³⁺/X%Yb³⁺ (X = 3, 10, 20) microtubes has been explored. The luminescent properties of the as prepared sample is investigated by utilizing up- /downconversion, decay and time resolve spectra. The results indicate that the red and green emission are clearly competitive depending on the Yb³⁺ concentration. High Yb³⁺ concentration induces the enhancement of the energy-back-transfer (EBT), process, which leads to the quenching of green emission and enhances the red emission. So it is possible to utilize the temporal evolutions of emission bands to deeply understand the color change UC mechanisms.     

Photon avalanche upconversion and pump power studies in LaF<Subscript>3</Subscript>:Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> phosphor

Hexagonal LaF₃:Er³⁺/Yb³⁺ phosphor material has been synthesized by chemical precipitation method to obtain high near-infrared to green upconversion (UC) efficiency. Its thermal, structural and fluorescence properties have been studied. UC emission bands have been observed up to 315 nm in UV region. The effect of input pump power on the intensities of various emission bands has been studied in detail and photon avalanche UC mechanism has been identified. On increasing the excitation power, some bands have shown saturation in intensity. Also, at higher pump intensities two new UC bands were observed and their origin has been discussed. The phosphor has also been tested for possible UC-based fingerprint detection.     

A ceramic based Yb<Superscript>3+</Superscript>:YAG laser

We report the performance of a laser based on a 9.8% doped ceramic Yb³⁺:YAG operating in quasi-Continuous Wave (QCW) and in Continuous Wave (CW) at room temperature. The maximum output power of 9 W with a slope efficiency of 73% at 1030 nm was achieved in QCW. We investigated the problems originated from thermo-mechanical properties of the ceramic and we studied their effects on the laser per-formance finding that either the slope efficiency or the laser threshold are thermal-losses independent. Finally, we report a characterization of a low losses tunable cavity for several laser wavelengths.     

A high efficient antireflective down-conversion Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Bi<Superscript>3+</Superscript>, Yb<Superscript>3+</Superscript> thin films

The high efficient antireflective down-conversion Y₂O₃:Bi, Yb films have been prepared successfully on Si(100) substrates by pulsed laser deposition (PLD) method, Upon excitation of ultraviolet photon varying from 300 to 400 nm, near-infrared emission of Yb³⁺ was observed for the film, can be efficiently absorbed by silicon (Si) solar cell. Most interestingly, there is a very low average reflectivity 1.46% for the incident light from 300 to 1100 nm. To the best of our knowledge, this is the lowest reflectance for the down-conversion thin films prepared by cost efficient method. The surface topography of the high efficient antireflective films can be controllably tuned through the substrate template regulation by optimizing process parameters. Besides, the results showed that there is a close relationship between luminescent property and morphology of the film. With the change of the surface morphology, the intensity of Bi³⁺ and Yb³⁺ emission peaks increase first and then decrease. The obtained results demonstrate that this film can enhance the Si solar cell efficiency through light trapping and spectrum shifting.     

Enhanced Up-Conversion Emission in Al<Superscript>3+</Superscript> Co-Doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript>,Tm<Superscript>3+</Superscript> Powder Phosphors

Yb³⁺-Tm³⁺ co-doped up-conversion powder phosphors using Zn(Al_xGa_1-x)₂O₄ (ZAGO) as the host materials were synthesized via solid-state reaction successfully. In addition, the morphology, structural characterization and up-conversion luminescent properties were all investigated by scanning electron microscope (SEM), x-ray diffraction (XRD) and fluorescence spectrophotometer (F-7000), respectively. Under the excitation of a 980 nm laser, all as-prepared powders can carry out blue emission at about 477 nm (corresponding to ¹G₄ → ³H₆ transition of Tm³⁺ ions), and red emission at about 691 nm (attributed to ³F₃ → ³H₆ transition of Tm³⁺ ions). Also, the influence of doping Al³⁺ ions were investigated. In brief, the doping of Al³⁺ ions has no effect on the position of emission peak. Howbeit the up-conversion efficiency and intensity of ZAGO:Yb,Tm phosphors are stronger than ZGO:Yb,Tm and ZAO:Yb,Tm phosphors, while the crystallinity is the opposite. More particularly, all as-prepared powder phosphors emit strong luminescence, which is observable by the naked eye, demonstrating the potential applications in luminous paint, luminescent dye, etc.     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

Defect Structure and Up-conversion Luminescence Properties of ZrO<Subscript>2</Subscript>:Yb<Superscript>3+</Superscript>,Er<Superscript>3+</Superscript> Nanomaterials

The up-converting ZrO₂:Yb³⁺,Er³⁺ nanomaterials were prepared with the combustion and sol–gel methods. FT-IR spectroscopy was used for analyzing the impurities. The crystal structures were characterized with X-ray powder diffraction and the mean crystallite sizes were estimated with the Scherrer formula. Up-conversion luminescence measurements were made at room temperature with IR-laser excitation at 977 nm. The IR spectra revealed the conventional and OH⁻ impurities for the combustion synthesis products. The structure of the ZrO₂:Yb³⁺, Er³⁺ nanomaterials was cubic except for the minor monoclinic and tetragonal impurities obtained with the sol–gel method. The materials showed red (650–700 nm) and green (520–560 nm) up-conversion luminescence due to the ⁴F_9/2→⁴I_15/2 and (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺, respectively. The products obtained with the combustion synthesis exhibited the most intense luminescence intensity and showed considerable afterglow.     

Photodynamic processes in CaF<Subscript>2</Subscript> crystals activated by Ce<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> ions

The photochemical properties of CaF₂ crystals activated by Ce³⁺ and Yb³⁺ ions are studied. A model of the photodynamic processes induced by pumping UV or VUV radiation in active media is suggested and experimentally verified. This model explains both the presence of color centers of electronic and hole nature in crystals activated by cerium and the mechanism of suppressing of solarization processes after additional activation of the samples by Yb³⁺ ions. The cross sections of the processes of free-carrier capture by various ytterbium impurity centers are estimated. These impurity centers are established to be effective centers of recombination of free carriers of both signs.     

The upconversion luminescence and magnetism in Yb<Superscript>3+</Superscript>/Ho<Superscript>3+</Superscript> co-doped LaF<Subscript>3</Subscript> nanocrystals for potential bimodal imaging

Biocompatible upconversion nanoparticles with multifunctional properties can serve as potential nanoprobes for multimodal imaging. Herein, we report an upconversion nanocrystal based on lanthanum fluoride which is developed to address the imaging modalities, upconversion luminescence imaging and magnetic resonance imaging (MRI). Lanthanide ions (Yb³⁺ and Ho³⁺) doped LaF₃ nanocrystals (LaF₃ Yb³⁺/Ho³⁺) are fabricated through a rapid microwave-assisted synthesis. The hexagonal phase LaF₃ nanocrystals exhibit nearly spherical morphology with average diameter of 9.8 nm. The inductively coupled plasma mass spectrometry (ICP-MS) analysis estimated the doping concentration of Yb³⁺ and Ho³⁺ as 3.99 and 0.41%, respectively. The nanocrystals show upconversion luminescence when irradiated with near-infrared (NIR) photons of wavelength 980 nm. The emission spectrum consists of bands centred at 542, 645 and 658 nm. The stronger green emission at 542 nm and the weak red emissions at 645 and 658 nm are assigned to ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively. The pump power dependence of luminescence intensity confirmed the two-photon upconversion process. The nanocrystals exhibit paramagnetism due to the presence of lanthanide ion dopant Ho³⁺ and the magnetization is 19.81 emu/g at room temperature. The nanocrystals exhibit a longitudinal relaxivity (r ₁) of 0.12 s^?1 mM^?1 and transverse relaxivity (r ₂) of 28.18 s^?1 mM^?1, which makes the system suitable for developing T2 MRI contrast agents based on holmium. The LaF₃ Yb³⁺/Ho³⁺ nanocrystals are surface modified by PEGylation to improve biocompatibility and enhance further functionalisation. The PEGylated nanocrystals are found to be non-toxic up to 50 μg/mL for 48 h of incubation, which is confirmed by the MTT assay as well as morphological studies in HeLa cells. The upconversion luminescence and magnetism together with biocompatibility enables the adaptability of the present system as a nanoprobe for potential bimodal imaging.     

Magnetic Resonance and Optical Spectroscopy of Yb<Superscript>3+</Superscript> in CsCaF<Subscript>3</Subscript> Single Crystal: an Analysis of Distortions of the Crystal Lattice near Yb<Superscript>3+</Superscript>

A trigonal Yb³⁺ paramagnetic center in the CsCaF₃ single crystal was studied by magnetic resonance and optical spectroscopy methods. The structural model of the complex and the empirical energy level scheme were established. The transferred hyperfine interaction parameters and the crystal field ones were determined. The crystal field parameters were used to analyze the lattice distortions in the vicinity of Yb³⁺ using the superposition model.     

Green,blue, red,near-infrared up-conversion luminescence of Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript>/Tm<Superscript>3+</Superscript> co-doped YVO<Subscript>4</Subscript> nanoparticles

YVO₄:Yb³⁺,Er³⁺; YVO₄:Yb³⁺,Tm³⁺; and YVO₄:Yb³⁺,Er³⁺,Tm³⁺ were all synthesized via sol-gel method with a subsequent thermal treatment. Specifically, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ phosphors were prepared with different annealing temperatures to study the influence of temperature. The transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffractometer (XRD), and photoluminescent (PL) spectrofluorometer were used to investigate the morphology, crystal structure, and up-conversion luminescent properties of all samples. In summary, all samples were granular-like nanoparticles and well crystallized with the same tetragonal phase as YVO₄. Under the irradiation at 980 nm, YVO₄:Yb³⁺,Er³⁺ phosphors can generate green emission at 525 and 553 nm and red emission at 657 nm, while YVO₄:Yb³⁺,Tm³⁺ phosphors can generate blue emission at 476 nm, red emission at 648 nm, and near-infrared emission at 800 nm. Notably, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ samples can exhibit green emission, blue emission, red emission, and near-infrared emission at the same time, which might endow the as-prepared samples with potential applications in many fields, such as luminous paint, infrared detection, and biological label.     

Room-temperature diode-pumped Yb<Superscript>3+</Superscript>-doped LiYF<Subscript>4</Subscript> and KYF<Subscript>4</Subscript> lasers

We present a comparative analysis on the growth, the spectroscopic features, and the cw laser action of room-temperature Yb(5%):LiYF₄ (YLF) and Yb(10%):KYF₄ (KYF) crystals. Optical slope efficiencies of 33% and 52% have been demonstrated for Yb:YLF and Yb:KYF crystals, respectively. A remarkable wide wavelength tunability from 1.01 to 1.07 μm has been obtained for both laser crystals.     

Intense Upconversion Luminescence of Yb<Superscript>3+</Superscript>-Er<Superscript>3+</Superscript> in Li<Subscript>2</Subscript>O Content Tungsten-tellurite Glasses

The Er³⁺ -Yb³⁺ codoped in Li₂O content tungsten -tellurite (TWL) transparent glasses are synthesized and measured the absorption, Raman and upconversion luminescence (UPL) spectra. At room temperature intense green emission peak at 560 nm ( ⁴S_3/2→⁴I_15/2) and red emission peak at 670 nm ( ⁴F_9/2→⁴I_15/2) of Er³⁺ observed even at minimum 86 mW pumping power of infrared 980 nm excitation. For structure of the TWL glass, Raman spectrum result revealed that an important role of WO₃ in the formation of glass network linkage with Li₂O. Under this influence estimated lifetime of the ⁴I_11/2 of Er³⁺ was 1.89 μs and due to lower phonon energy of the glass produce strong upconversion signal. The effect of Er₂O₃ concentration on emission intensity result indicated that green emission intensity initially increase in compare to red emission. Under the 980 nm pump power variation measured the relatively increases the red emission to the green emission intensity and analyze the possible upconversion mechanism and process.