Optical gain by upconversion in Tm–Yb oxyfluoride glass ceramic

Evidence of positive optical gain is observed in Tm³⁺–Yb³⁺-codoped oxyfluoride glass ceramic in an upconversion pump and probe experiment. The ¹G₄ level of the Tm³⁺ ions is populated by an upconversion mechanism under excitation of the Yb³⁺ ions at 975 nm with a high-power pulsed laser and give rise to an intense emission from the ¹G₄ to the ³F₄ levels. The ¹G₄→³F₄ electronic transition is stimulated with a low signal at 650 nm as a probe.     

Judd–Ofelt analysis and upconversion emission of Er3+–Yb3+ co-doped oxyfluoride glass ceramics containing LaF3 nanocrystals

Er³⁺–Yb³⁺ co-doped glasses and glass ceramics containing LaF₃ nanocrystals were prepared and their absorption spectra obtained. The Judd–Ofelt parameters Ω _t (t?=?2,?4,?6) for f–f transition of Er³⁺, as well as spontaneous emission probabilities, branching ratios and radiative lifetimes for stimulated emission of each band were determined. Addition of Er³⁺ and Yb³⁺ ions into low-phonon energy LaF₃ nanocrystals makes the upconversion emission of Er³⁺–Yb³⁺ co-doped glass ceramic much stronger than that of Er³⁺–Yb³⁺ co-doped glass.     

Effect of PbCl2 addition on structure, OH− content, and upconversion luminescence in Yb3+/Er3+-codoped germanate glasses

Yb³⁺/Er³⁺-codoped oxychloride germanate glasses have been synthesized by a conventional melting and quenching method. Structural properties were obtained based on Raman-spectra investigation, indicating that PbCl₂ plays an important role in the formation of the glass network and has an important influence on the phonon density and the maximum phonon energy. The Judd–Ofelt intensity parameters and quantum efficiencies were calculated based on the Judd–Ofelt theory and lifetime measurements. The enhanced upconversion luminescence intensity of Er³⁺ with increasing PbCl₂ content could not be explained only by the maximum phonon-energy change of the host glasses. For the first time, the effect of PbCl₂ addition on phonon density, OH^– content, and upconversion luminescence in oxychloride glasses has been discussed and evaluated. The results show that the effect of phonon density and OH^– content on upconversion luminescence in oxychloride glasses is much stronger than that of the decrease of the maximum phonon energy. The possible upconversion luminescence mechanisms have also been estimated and are discussed.     

Quasi-one dimensional Er3+–Yb3+ codoped single-crystal MoO3 ribbons: Synthesis,characterization and up-conversion luminescence

The quasi-one dimensional (Q1D) Er³⁺–Yb³⁺ codoped single-crystal MoO₃ ribbons with width range from 1 to 5 μm, and maximum length about 30 μm have been synthesized by the vapor transport method. The samples were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscope, and luminescence spectra. By a 975 nm laser diode (LD) as excitation source, the blue, green and red emission bands centered at about 408, 532, 553 and 657 nm were detected, which attributed to the ²H_9/2 → ⁴I_15/2, ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively. The three-, and two-photon process was responsible for the blue, green and red up-conversion emissions mechanism for the Q1D Er³⁺–Yb³⁺ codoped single-crystal MoO₃ ribbons, respectively. The results suggested that the Q1D Er³⁺–Yb³⁺ codoped single-crystal MoO₃ ribbons will have potential applications in remote bio-imaging and surface enhanced Raman scattering.     

Optical properties and upconversion in Yb3+—Tm3+ co-doped oxyfluoride glasses and glass ceramics

A theoretical study has explored the changes brought upon the electron density of the open (C_2v) and cyclic (D_3h) forms of ozone by protonation. Although protonation results in a strong deformation of the electron density of ozone with notably different charge distributions for the stable forms of O₃H⁺, the electric dipole polarizability is almost stable for the open protonated species. The polarizability of the cyclic form is less affected by protonation than that of the open one. The anisotropy of the dipole polarizability discriminates very clearly between open and closed protonated ozone. We expect the present findings to advance our understanding of the chemical reactivity of O₃H⁺.     

Energy transfer and frequency upconversion in Yb3+–Er3+-doped PbO-GeO2 glass containing silver nanoparticles

We report the infrared-to-visible frequency upconversion in Er³⁺–Yb³⁺-codoped PbO-GeO₂ glass containing silver nanoparticles (NPs). The optical excitation is made with a laser at 980 nm in resonance with the ²F_5/2→²F_7/2 transition of Yb³⁺ ions. Intense emission bands centered at 525, 550, and 662 nm were observed corresponding to Er³⁺ transitions. The simultaneous influence of the Yb³⁺→Er³⁺ energy transfer and the contribution of the intensified local field effect due to the silver NPs give origin to the enhancement of the whole frequency upconversion spectra.     

Core-shell LaF<Subscript>3</Subscript>:Er,Yb nanocrystal doped sol–gel materials as waveguide amplifiers

The oleic acid (OA)-modified LaF₃:Er,Yb and LaF₃:Er,Yb–LaF₃ core-shell nanocrystals are synthesized. The lifetime values could be further improved by incorporating core-shell nanocrystals. A kind of sol–gel derived organic–inorganic hybrid material (SGHM) allows for 50 wt.% or even more of both the two nanocrystals in the matrix, and we give the explanation from scattering analysis. It’s precisely because we use the erbium nanocrystals rather than erbium organic complexes, and avoid the undesirable luminescence quenching by Er–Er clustering with a high Er³⁺ concentration. LaF₃:Er,Yb–LaF₃/SGHM transparent films and optical waveguides are also fabricated. The nanocomposite films show strong 1550 nm luminescence intensity under the excitation of 980 nm after heat treatment below 150 °C and the full-width-at-half-maximum is about 51 nm. The loss and optical gain of the waveguide are measured. A relative gain of about 3.5 dB is measured at 1550 nm in a 1.7 cm long waveguide.     

Extrinsic surface scattering and intrinsic absorption loss of vinyl based hybrid organic–inorganic materials for optical waveguides applications

Hybrid photosensitive materials were prepared by using vinyltriethoxysilane (VTES), tetraethoxysilane (TEOS) and tetrabutoxytitanate (TTBu) precursors through sol–gel technique. The materials are intended for optical telecommunication applications. Thus, high optical transmissions at the second and third optical telecommunication windows are essential. Extrinsic surface scattering due to surface roughness and intrinsic absorption due to aliphatic CH and OH groups are always ascribable to the optical power attenuations at aforementioned optical telecommunication windows. Optical waveguide based on hybrid sol–gel materials were fabricated, characterized and analyzed in order to investigate the extrinsic and intrinsic sources of attenuation and their contributions. The fabricated samples were characterized for propagation loss, surface condition, and Fourier transform IR (FTIR) absorption spectra. Propagation loss were measured by means of cut-back method as 1.6 and 6.9 dB/cm at 1310 and 1550 nm wavelengths, respectively. Surface scattering loss was modeled based on measured rms roughness of 0.724 nm and turned out to contribute less than 0.01% of the total propagation loss. FTIR absorption spectra show the persistent existence of aliphatic CH and OH groups within the final hybrid sol–gel materials.     

Synthesis of hybrid organic–inorganic nanocomposite materials based on CdS nanocrystals for energy conversion applications

Efficient solar energy conversion is strongly related to the development of new materials with enhanced functional properties. In this context, a wide variety of inorganic, organic, or hybrid nanostructured materials have been investigated. In particular, in hybrid organic–inorganic nanocomposites are combined the convenient properties of organic polymers, such as easy manipulation and mechanical flexibility, and the unique size-dependent properties of nanocrystals (NCs). However, applications of hybrid nanocomposites in photovoltaic devices require a homogeneous and highly dense dispersion of NCs in polymer in order to guarantee not only an efficient charge separation, but also an efficient transport of the carriers to the electrodes without recombination. In previous works, we demonstrated that cadmium thiolate complexes are suitable precursors for the in situ synthesis of nanocrystalline CdS. Here, we show that the soluble [Cd(SBz)₂]₂·(1-methyl imidazole) complex can be efficiently annealed in a conjugated polymer obtaining a nanocomposite with a regular and compact network of NCs. The proposed synthetic strategies require annealing temperatures well below 200 °C and short time for the thermal treatment, i.e., less than 30 min. We also show that the same complex can be used to synthesize CdS NCs in mesoporous TiO₂. The adsorption of cadmium thiolate molecule in TiO₂ matrix can be obtained by using chemical bath deposition technique and subsequent thermal annealing. The use of NCs, quantum dots, as sensitizers of TiO₂ matrices represents a very promising alternative to common dye-sensitized solar cells and an interesting solution for heterogeneous photocatalysis.     

Characterization of structural transitions and lattice dynamics of hybrid organic–inorganic perovskite CH_3NH_3PbI_3

By combining temperature-dependent x-ray diffraction(XRD) with temperature-dependent Raman scattering, we have characterized the structural transitions and lattice dynamics of the hybrid organic–inorganic perovskite CH_3NH_3PbI_3.The XRD measurements cover distinct phases between 15 K and 370 K and demonstrate a general positive thermal expansion.Clear anomalies are found around the transition temperatures.The temperature evolution of the lattice constants reveals that the transition at 160 K/330 K is of the first-/second-order type.Raman measurements uncover three strong lowfrequency modes, which can be ascribed to the vibration of the Pb/I atoms.The temperature evolution of the modes clearly catches these transitions at 160 K and 330 K, and confirms the transition types, which are exactly consistent with the XRD results.The present study may set an experimental basis to understand the high conversion efficiency in methylammonium lead iodide.     

Infrared luminescence, thermoluminescence and defect centres in Er and Yb co-doped ZnAl2O4 phosphor

Er and Yb co-doped ZnAl₂O₄ phosphors were prepared by solution combustion synthesis and the identification of Er and Yb were done by energy-dispersive X-ray analysis (EDX) studies. A luminescence at 1.5 μm, due to the ⁴I_13/2 →⁴I_15/2 transition, has been studied in the NIR region in Er and Yb co-doped ZnAl₂O₄ phosphors upon 980 nm CW pumping. Er-doped ZnAl₂O₄ exhibits two thermally stimulated luminescence (TSL) peaks around 174°C and 483°C, while Yb co-doped ZnAl₂O₄ exhibits TSL peaks around 170°C and 423°C. Electron spin resonance (ESR) studies were carried out to identify defect centres responsible for TSL peaks observed in the phosphors. Room temperature ESR spectrum appears to be a superposition of two distinct centres. These centres are assigned to an O⁻ ion and F⁺ centre. O⁻ ion appears to correlate with the 174°C TSL peak and F⁺ centre appears to relate with the high temperature TSL peak at 483°C in ZnAl₂O₄:Er phosphor.     

Investigation of the mechanism of upconversion luminescence in Er3＋/Yb3＋ co-doped Bi2Ti2O7 inverse opal

Bismuth (Bi)-doped materials have attracted a great deal of attention because of their broadband nearinfrared (near-IR) emission around the wavelength utilized in telecommunications. In this study, broad near-IR emission band from 1 100 to 1 650 nm is generated in the Bi-doped 90GeS2-10Ga2S3 glass and glass-ceramics under 820 nm of light excitation. Based on the analysis of the absorption and emission spectra, the origin of this broadband emission is ascribed to the Bi2-2 dimers. The precipitation of β-GeS2 nanocrystals drastically enhances the emission intensity and lifetime of Bi-doped chalcogenide glass.     

Microwave sol–gel derived NaCaGd(MoO4)3:Er3+/Yb3+ phosphors and their upconversion photoluminescence properties

Ternary molybdate NaCaGd_1−x(MoO₄)₃:Er³⁺/Yb³⁺ phosphors with the proper doping concentrations of Er³⁺ and Yb³⁺ (x = Er³⁺ + Yb³⁺, Er³⁺ = 0, 0.05, 0.1, 0.2 and Yb³⁺ = 0, 0.2, 0.45) were successfully synthesized by microwave sol–gel method for the first time. Well-crystallized particles formed after heat-treatment at 900 °C for 16 h showed a fine and homogeneous morphology with particle sizes of 3–5 μm. The optical properties were examined comparatively using photoluminescence emission and Raman spectroscopy. Under excitation at 980 nm, the doped particles exhibited a strong 525-nm emission band, a weak 550-nm emission band in the green region, which correspond to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, and a very weak 655-nm emission band in the red region, which corresponds to the ⁴F_9/2 → ⁴I_15/2 transition. The optimal Yb³⁺:Er³⁺ ratio was obtained to be 9:1, as indicated by the composition-dependent quenching effect of Er³⁺ ions. The pump power dependence of upconversion emission intensity and Commission Internationale de L’Eclairage chromaticity coordinates of the phosphors were evaluated in detail.     

Silicon nanoparticle size-dependent open circuit voltage in an organic–inorganic hybrid solar cell

We have incorporated silicon nanoparticles (Si-nps) into organic–inorganic hybrid solar cells in place of the chalcogenide nanocrystals that are commonly employed in such devices. Poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) (PEDOT:PSS) and phenyl-C₆₁-butyric acid methyl ester (PCBM) were employed as hole and electron transport layers, respectively. We used transmission electron microscopy, Raman spectroscopy, and ultraviolet–visible spectroscopy to fully characterize the Si-nps and relate their characteristics to the performance of the hybrid solar cells. We show that the open circuit voltage (V_OC) was largely dependent on the size and amorphous volume fraction of Si-nps. Our findings imply that the amorphous phase and small size of Si-nps produce band gap widening that increases the V_OC when coupled with PCBM as acceptor. The maximum V_OC was up to 0.634 V in a hybrid device with 5.7 nm Si-nps.     

Magnetocaloric effect in the layered organic–inorganic hybrid(CH_3NH_3)_2CuCl_4

We present a study of magnetocaloric effect of the quasi-two-dimensional(2D) ferromagnet(CH_3NH_3)_2CuCl_4 in ab plane(easy-plane). From the measurements of magnetic field dependence of magnetization at various temperatures,we have discovered a large magnetic entropy change associated with the ferromagnetic–paramagnetic transition. The heat capacity measurements reveal an abnormal adiabatic change below the Curie temperature T_c~8.9 K, which is caused by the nature of quasi-2D layered crystal structure. These results suggest that perovskite organic–inorganic hybrids with a layered structure are suitable candidates as working substances in magnetic refrigeration technology.     

Critical behavior in the layered organic–inorganic hybrid(CH_3NH_3)_2CuCl_4

The critical properties and the nature of the ferromagnetic–paramagnetic phase transition in the 2D organic-inorganic hybrid(CH_3NH_3)_2 CuCl_4 single crystal have been investigated by dc magnetization in the vicinity of the magnetic transition. Different techniques were used to estimate the critical exponents near the ferromagnetic–paramagnetic phase transition such as modified Arrott plots, the Kouvel–Fisher method, and the scaling hypothesis. Values of β = 0.22, γ = 0.82, and δ = 4.4 were obtained. These critical exponents are in line with their corresponding values confirmed through the scaling hypothesis as well as the Widom scaling relation, supporting their reliability. It is concluded that this 2D hybrid compound possesses strong ferromagnetic intra-layer exchange interaction as well as weak interlayer ferromagnetic coupling that causes a crossover from 2D to 3D long-range interaction.     

LaF<Subscript>3</Subscript>:Er,Yb doped sol–gel polymeric optical waveguide amplifiers

Oleic acid modified LaF₃:Er,Yb nanocrystals have been synthesized with a molar ratio of La:Yb:Er=75:20:5 and dispersed in toluene with 40 wt. % sol–gel derived organic–inorganic hybrid materials for films. The nanocomposite films showed strong 1540-nm luminescence intensity under the excitation of 980 nm after heat treatment below 150 °C. The full width at half maximum bandwidth was 51-nm wide around 1540 nm. We also fabricated reverse-mesa ridge waveguides using LaF₃:Er,Yb nanocrystals containing sol–gel materials. With an input pump of 150 mW, a signal enhancement of 2.3 dB at the wavelength of 1550 nm was measured in a 1.3-cm-long waveguide. PACS 42.00.00; 42.70.-a; 42.82.Et     

Blue upconversion luminescence in 12 CaO·7 Al2O3:Tm3 +/Yb3 + polycrystals

The effect of Yb^3 + concentration on the fluorescence of 12 CaO·7 Al₂O₃:Tm^3 +/Yb^3 + polycrystals is investigated. Under the excitation of 980 nm laser, the strong blue (477 nm) emission band is observed and attributed to ¹G₄ → ³H₆ of Tm^3 +. The ratio of blue to red emission increases with the increasing of Yb^3 + and remains constant at 10 mol% Yb^3 +. The pump dependence and upconversion mechanisms show that the two-photon cooperative upconversion process is responsible for the enhancement of the blue upconversion emission. The Commission Internationale de l'eclairage chromaticity coordinates (x, y) illustrate that the 12 CaO·7 Al₂O₃:1 mol% Tm^3 +/10 mol% Yb^3 + can emit high-purity blue light.     

Up conversion luminescence of Yb3+–Er3+ codoped CeO2 nanocrystals with imaging applications

The effects of Yb³⁺ doping on up conversion in Yb³⁺–Er³⁺ co-doped cerium oxide nanocrystals are reported. Green emission around 545 and 560 nm attributed to the ²H_11/2, ⁴S_3/2→⁴I_15/2 transitions and red emission around 660 and 680 nm due to ⁴F_9/2→ ⁴I_15/2 transitions under 975 nm excitation were studied at room temperature. Both green and red emission intensities increase as the Yb³⁺ concentration increases from 0%. Emission strength starts to decrease after the Yb³⁺ concentration exceeds a critical amount. The green emission strength peaks around 1% Yb³⁺ concentration while the red emission strength peaks around 4%. An explanation of competition between different decay mechanisms is presented to account for the luminescence dependence on Yb³⁺ concentration. Also, the application of up converting nanoparticles in biomedical imaging is demonstrated.