Z. Pan

Host‐dependent optical transitions of Er3+ ions in lead–germanate and lead‐tellurium‐germanate glasses

Differential scanning calorimetry, Raman scattering, optical absorption, and upconversion of infrared to green luminescence have been studied for Er3+‐doped lead‐germanate glass 55GeO2⋅20PbO⋅10BaO⋅10ZnO⋅5K2O (GPBZK) and lead‐tellurium‐germanate glass 30GeO2⋅30PbO⋅30TeO2⋅10CaO (GPTC). Judd–Ofelt intensity parameters of Er3+ in the two host glasses were determined and used to calculate radiative transition rates and lifetimes. Values of the radiative quantum yield of the 4S3/2→4I15/2 transition and the infrared (797 nm) to green (547 nm) upconversion efficiency of Er3+ were obtained. It has been found that the 4S3/2→4I15/2 radiative transition rate of Er3+ in GPTC glass is about twice that in the GPBZK glass and the upconversion efficiency in the GPTC glass is about four times larger than that in the GPBZK glass. These host‐dependent properties are mainly attributed to the enhanced local field and the reduced multiphonon rate in lead‐tellurium‐germanate glass compared to lead‐germanate glass.

INFRARED TO VISIBLE UPCONVERSION IN ER3+-DOPED-LEAD-GERMANATE GLASS : EFFECTS OF ER3+ ION CONCENTRATION

The upconversion mechanism of Er3+ ions has been studied for lead‐germanate glasses containing Er2O3 concentrations from 0.1 mol % to 2 mol %. Intense green emission was observed at room temperature due to 4S3/2→4I15/2 transition excited by a cw near‐infrared laser beam at 797 nm. This green emission shows a similar intensity for samples with different Er3+ ion concentrations. A weak blue emission of 410 nm originating from the 2H9/2→4I15/2 transition was also observed. This blue emission and a red emission from the 4F9/2 level increase with the increase in Er3+ ion concentration. The bright green emission is attributed to the excited level absorption while the blue emission is due to a third step excitation where energy transfer between excited ions owing to their Coulomb interaction plays a key role.

Enhanced photoresponse in ZnO nanowires decorated with CdTe quantum dot

The photoresponse of ZnO nanowires irradiated with photons having energies below the band gap of ZnO (3.4eV) was studied before and after deposition of CdTe quantum dots via a pulsed electron-beam technique. The small amount of deposited CdTe did not increase the dark current of the samples. However, a substantial increase in the steady state photocurrent was observed after CdTe deposition suggesting a clear photosensitization effect. Results revealed that CdTe influences the photoconductivity transients of ZnO by minimizing its interaction with oxygen in air as well as providing additional traps that serve to increase the photocurrent time constant.

Raman spectra and thermal analysis of a new lead–tellurium–germanate glass system

Abstract Differential scanning calorimetry (DSC) and Raman scattering studies of a new glass system, lead–tellurium–germanate glasses in the form of (90−x)GeO2·xTeO2·27PbO·10CaO with x=0, 10, 20, 30, and 40, are reported. The glass samples were fabricated using a conventional melt-quenching method. The Raman spectra and possible glass structures are discussed for different TeO2 contents. The results indicate that increasing TeO2 content up to 40 mol% in the glass system decreases the glass transition temperature and melting temperature, and suppresses the crystallization tendency in the fiber pulling temperature range. The lead–tellurium–germanate glass, GTPC, possesses a larger refractive index and a smaller maximum phonon energy than that of a lead–germanate glass, 63GeO2·27PbO·10CaO, and shows a better thermal stability compared to a tellurite glass, 75TeO2·20ZnO·5Na2O (TZN). These improved properties could be beneficial for fabricating rare-earth doped fiber devices.

Vibrational spectra of bismuth silicate glasses and hydrogen-induced reduction effects

Abstract Raman (10–1200 cm −1 ) and infrared reflectance (100–2000 cm −1 ) spectra are reported for bismuth silicate glasses of the formula x Bi 2 O 3 · (10− x )SiO 2 with x = 9, 8, 7, 6, 5, 4. The basic vibrational characteristics and the possible glass structure are discussed. The results provide evidence that bismuth oxide behaves as the network former while silicon oxide is present as isolated SiO 4 tetrahedra. Hydrogen treatment was performed at 300°C on a bismuth silicate glass with x = 5. The intensity of most Raman bands decreased with increasing exposure time. The infrared spectrum after reduction showed vibrational bands at 1124 and 1220 cm −1 which are characteristic of SiOSi stretching vibrational modes in a fused-silica network. The spectral changes observed indicate the formation of bismuth metallic colloids as well as the formation of the bridged SiOSi structure in the glass surface layer during reduction.

Raman scattering cross-section and non-linear optical response of lead borate glasses

Lead borate glasses in the system xPbO · (10 - x)B2O3(5 ⩽ x ⩽ 9) have been studied by Raman scattering and Z-scan non-linear optical measurements. It is found that both Raman scattering cross-section and non-linear refractive index, n2, of glasses increase significantly with increasing lead oxide content. The increase in Raman cross-section is dominated by an increase in the low-frequency Raman scattering. The results indicate that both electronic and nuclear responses contribute to the observed optical non-linearity which is further enhanced by two-photon resonance effects for glasses with x ⩾ 80 mol%. The nuclear response is attributed to low-frequency vibrational modes and the electronic response to highly polarizable lead cations in the glass matrix.

Surface-Enhanced Raman Spectroscopy Using Silver-Coated Porous Glass-Ceramic Substrates

Surface-enhanced Raman scattering (SERS) has been studied using a silver-coated porous glass-ceramic material as a new type of substrate. The porous glass-ceramic is in the CaO–TiO2–P2O5 system prepared by controlled crystallization and subsequent chemical leaching of the dense glass-ceramic, leaving a solid skeleton with pores ranging in size from 50 nm to submicrometer. Silver was coated on the surface of the porous glass-ceramic by radio frequency (RF) sputtering or e-beam evaporation in vacuum. SERS spectra of excellent quality were obtained from several dyes and carboxylic acid molecules, including rhodamine 6G, crystal violet, isonicotinic acid, and benzoic acid, using this new substrate. This new substrate showed a good compatibility with these molecules. The porous glass-ceramic with a nanometer-structured surface accommodated both test molecules and silver film. The absorbed molecules were therefore better interfaced with silver for surface-enhanced Raman scattering.

Formation of Ag nanoparticles and enhancement of Tb3+ luminescence in Tb and Ag co-doped lithium-lanthanum-aluminosilicate glass

Tb3+ and Ag co-doped glass nano-composites were synthesized in a glass matrix Li2O-LaF3-Al2O3-SiO2 (LLAS) by a melt-quench technique. The growth of Ag nanoparticles (NPs) was controlled by a thermal annealing process. A broad absorption band peaking at about 420 nm was observed due to surface plasmon resonance (SPR) of Ag NPs. The intensity of this band grows with increasing annealing time. The transmission electron microscopic image (TEM) reveals the formation of Ag NPs in glass matrix. Photoluminescence (PL) emission and excitation spectra were measured for glass samples with different Ag concentrations and different annealing times. Plasmon enhanced Tb3+ luminescence was observed at certain excitation wavelength regions. Luminescence quenching was also observed for samples with high Ag concentration and longer annealing time. Our luminescence results suggest that there are two competitive effects, enhancement and quenching, acting on Tb3+ luminescence in the presence of Ag NPs. The enhancement of Tb3+ luminescence is mainly attributed to local field effects due to SPR. The quenching of luminescence suggests an energy transfer from Tb3+ ions to Ag NPs.

Er-doped ZnO films grown by pulsed e-beam deposition

Erbium (Er)-doped ZnO thin films were grown on fused silica (SiO2) substrates by pulsed electron-beam deposition (PED) and analysed by Rutherford backscattering spectrometry (RBS), ultraviolet–visible absorption, and photoluminescence (PL). Subsequent annealing at 700 °C produces remarkable effects on the optical properties of Er-doped films. Under 325 nm excitation, a dramatic increase of deep-level emission from 450 to 680 nm was observed from annealed Er-doped ZnO films. Under 488 nm excitation, the PL spectrum of annealed Er-doped ZnO films revealed sharp and well-resolved Stark-splitting peaks in both the green emission of transition and the red emission of transition of Er3+ ions, which suggests that the Er ions have been incorporated inside the crystalline ZnO grains after thermal annealing.

Luminescence of Er3+ in Oxyfluoride Transparent Glass-Ceramics

Abstract Erbium doped silicate, germanate, and tellurium-germanate oxyfluoride glasses were prepared in a bulk form. Through appropriate heat treatment of the as-prepared glasses, transparent glass-ceramics (TGCs) were obtained with the formation of β-PbF 2 : Er 3+ nanocrystals in the glass matrix were confirmed by X-ray diffraction. Well-defined diffraction peaks were observed in the samples after heat-treatment. The average crystal diameter of these precipitated crystals from full-width at half-maximum (FWHM) of the diffraction peak was estimated to be between 8 and 13 nm. Optical absorption, photoluminescence, and upconversion luminescence were measured on as-prepared glass and glass-ceramics. Luminescence spectra in the TGC samples revealed well-resolved, sharp stark-splitting peaks, which indicates that a majority of Er 3+ ions has been incorporated into the crystalline phase of the nanocrystals. The intensity of the visible and near infrared luminescence mostly increases in TSG compared to that in the as-prepared glass. In 1.53 μm absorption and emission bands, the maximum absorption peak is blue-shifted from 1531 to 1507 nm, whereas the maximum emission peak is red-shifted from 1535 to 1543 nm in TGC, as compared with that in glass. The bandwidth at half-maximum (BWHM) of the emission band is significantly broader in TGC than in glass, which is beneficial to the erbium-doped fiber amplifier (ED-FA). Upconversion luminescence was measured using 800 nm near-infrared light excitation. Drastically increased upconversion luminescence was observed from the TGC as compared to that from their corresponding as-prepared glasses. In addition to a strong green emission centered at 545 nm because of 4 S 3/2 → 4 I 15/2 transition and a weaker red emission centered at 662 nm because of 4 F 9/2 → 4 I 15/2 transition, generally seen from the Er 3+ doped glasses, two violet emissions centered at 410 nm because of b H 9/2 → 4 I 15/2 transition and centered at 379 nm because of 4 G 11/2 → 4 I 15/2 transition were also observed from the TGC. The increased luminescence was attributed to the decreased effective phonon energy and the increased energy transfer between the excited ions when Er 3+ ions were incorporated into the precipitated β-PbF 2 nanocrystals. The results indicated two attractive spectroscopic properties of the Er 3+ doped TGC samples, compared to glass samples, namely a reduced multiphonon decay rate and a reduced inhomogeneous broadening. In addition, these oxyfluoride TGC materials were robust, easy and flexibile to process, and possible to be fabricated in the fiber form for device applications.