Excited state absorption in thulium doped YVO4 crystals

Excited state absorption (ESA) of Tm³⁺ ions in YVO₄ crystal was measured using a pump and probe technique. The measurements have been performed in the wide spectral range in both the near infrared and visible region between 6000 cm^-1 (NIR) and 24000 cm^-1 (VIS). ESA absorption cross section spectra have been calculated for transitions from the ³ F ₄ and ³ H ₄ states of Tm³⁺ and compared to those evaluated experimentally. The excited state absorption in YVO₄:Tm³⁺ should not influence the laser operation related to the ³F₄→³H₆ transition around 1900 nm, but it will be a significant loss factor for a potential laser action associated with the ³H₄→³F₄ transition around 1.48 μm. PACS 42.55.Xi; 42.62.Fi     

Excited state absorption in Mn2+ doped phosphate glass

The possibility of obtaining laser action from Mn²⁺ doped glass has been investigated. The excited state absorption at the expected laser wavelength was measured and explains the unsuccessful attempts to obtain laser action.     

Spectroscopic study of thulium doped transparent glass ceramics

A systematic investigation on fluorescence spectroscopy of trivalent thulium doped in oxyfluoride glass ceramics containing LaF3 nanocrystals has been carried out in a spectral range from 400 to 900 nm under the direct excitation of 1D2 level at a low temperature. Specific optical transitions related to the fluorescence emissions are studied based on experimental measurements in frequency and time domain. Fluorescence emissions from the ions in crystal phase are distinguished from what in glass phase and their spectroscopic properties are explored. The dynamical process shows that the temporal decay of fluorescence emission consists of two parts： a rapid decay from the ions in glass phase and a slower decay from the ions in crystal phase.     

Excited state absorption in CaMoO4

We report the first measurement of excited state absorption (ESA) in a calcium molybdate (CaMoO₄) crystal, and compare the ESA spectrum with a stimulated emission spectrum calculated in terms of the McCumber theory of phonon terminated lasers. It appears that the magnitude of ESA would preclude laser action in this crystal. The temporal ESA dependence has been measured and discussed in terms of the most likely assignment of energy levels involved in ESA.     

Excited state absorption of Ag--centres in RbBr

The induced absorption of Ag^--centres in RbBr pumped by a strong excimer laser pulse is measured over a wide spectral range (250 to 950 nm) using a weak cw probe beam (Xe-lamp). A strong transient absorption around 510 nm is observed and is attributed to the intraionic transition from the relaxed excited 5 ³P₁ state to upper S- or D-levels.     

Extended X-ray absorption fine structure studies of thulium doped GaN epilayers

The local structure of Tm³⁺ ions incorporated into GaN epilayers was studied by means of Extended X-ray Absorption Fine Structure. The samples were doped either in situ during growth by Molecular Beam Epitaxy or by ion implantation of layers grown by Metal Organic Chemical Vapour Deposition. The implantation was done at ion energy of 300 keV and different nominal fluences of 3×10¹⁵, 4×10¹⁵ cm⁻² and 5×10¹⁵ cm⁻². The concentration of Tm in the samples studied was measured by Wavelength Dispersive X-ray analysis. For the in situ doped sample with concentration of 0.5%, and for all of the implanted samples, Tm was found on the Ga site in GaN. The ion implanted sample and an in situ doped sample with a similar concentration of Tm showed the same local structure, which suggests that the lattice site occupied by Tm does not depend on the doping method. When the average Tm concentration for in situ doped samples is increased to 1.2% and 2.0%, Tm is found to occupy the Ga substitutional site and the presence of a substantial number of Tm ions in the second coordination sphere indicates dopant clustering in the films. The formation of pure TmN clusters was found in an in situ doped sample with a dopant concentration of 3.4%.     

Excited state absorption measurements and laser potential of Cr4+ doped Ca2GeO4

4+ doped Ca₂GeO₄ in the 450–2000 nm spectral region at room temperature are presented. Weak ESA bands due to the transition are observed between 1200 and 2000 nm and stronger ESA bands due to peak below 1200 nm. The overlap of the ESA band with the emission which extends from 1100 to about 1600 nm will significantly affect the laser performance of Cr⁴⁺ doped Ca₂GeO₄. The effective peak stimulated emission cross section at 1350 nm in E∥b polarization is only about 3×10^-20 cm². However, our results indicate that tunable laser action between 1300 and 1500 nm should, in principle, be possible. Received: 28 October 1996     

Excited state intramolecular proton transfer of 1,2-dihydroxyanthraquinone by femtosecond transient absorption spectroscopy

1,2-Dihydroxyanthraquinone (alizarin) shows dual emission bands with a large Stokes shift from a “locally-excited (LE)” and “proton-transferred (PT)” tautomers in the excited state. Excited state intramolecular proton transfer (ESIPT) reaction of alizarin is tunable by changing concentration, solvent polarity, excitation wavelength, and etc. ESIPT reaction of alizarin in the excited state was investigated by steady-state absorption/emission spectroscopy and femtosecond transient absorption spectroscopy. In ethanol solution, the lifetime of PT tautomer of alizarin was measured as 87 ps, in addition to 0.35 and 8.3 ps vibrational cooling dynamics for the LE and PT tautomers of alizarin, respectively. In binary mixtures of ethanol and water, the excited state dynamics became more complicated; the LE and PT tautomers appeared to decay with 8.9 and 30.8 ps lifetimes, which is much shorter compared to the lifetime of the PT tautomer in ethanol. A long-lived nonradiative state in the excited states of alizarin was found as well, which was proposed as a “trapped” state with tightly hydrogen-bonded water molecules. The ESIPT reaction of alizarin was blocked in a 1:1 mixture of ethanol-water due to strong hydrogen bonding between water molecules and alizarin, which was further confirmed by the efficient coupling of alizarin to TiO₂ nanoparticles in the 1:1 binary mixture of ethanol-water.     

Excited state absorption and energy-transfer mechanisms of up-conversion luminescence in Er-doped oxyfluoride glass ceramics at different temperatures

Oxyfluoride silicate glass SiO₂-Al₂O₃-Na₂CO₃-NaF-LaF₃-ErF₃ was synthesized. The glass transition and crystallization temperatures were determined by differential thermal analysis. Glass ceramics containing LaF₃:Er³⁺ crystallites of size ∼20 nm were formed in the glass matrix after the heat treatment of the precursor glass in the vicinity of the crystallization temperature. Up-conversion luminescence, excitation spectra as well as time-resolved up-conversion luminescence of the glass and glass ceramics were studied at different temperatures. The up-conversion transients showed that at room temperature the dominant mechanism of the up-conversion luminescence in the glass ceramics is excited state absorption while at lower temperatures energy-transfer mechanism prevails. The origins of these differences are discussed in terms of the transitions between Stark manifolds of ⁴I_15/2, ⁴I_11/2 and ⁴F_7/2 states in Er³⁺ ions.     

Luminescent thulium centers in ZnSe crystals doped with thulium and lithium

Luminescence, absorption and Zeeman effects characteristic of Tm³⁺ ions in ZnSe crystals doped with Tm and Li are studied. The parallel and perpendicular g-values in the ground state of a center, responsible for the strongest luminescence, are 13·74 and zero, respectively. A model for the center is proposed; the center consists of an interstitial Tm impurity surrounded by four nearest neighboring Se²⁻ ions and by two Zn²⁺ ions and four coplanar Li⁺ ions in the next nearest neighbor sites. The local symmetry of the center is tetragonal, D_2d.     

Excited state dynamics in silver nanoparticles embedded in phosphate glass

Excited state dynamics in silver nanoparticles embedded in aluminophosphate glass was studied by ultrafast optical pump–probe technique. The absorption process of pump radiation and the electron–phonon relaxation on the 10^?13–10^?11 s scale were analyzed in the framework of two-temperature model. The time evolution of the light-induced transient diffraction grating shows an uncommon relaxation on the nanosecond time scale. This relaxation is assigned to phonon–phonon scattering process as well as to the energy transfer from photoexcited electronic states in glass matrix to silver nanoparticles.     

Mid-infrared luminescence and energy transfer of Dy3+/Tm3+ doped fluorophosphate glass

This work reports the observation of emissions at 2.9 μm, 1.8 μm and 1.47 μm from Dy³⁺/Tm³⁺ codoped fluorophosphate glass upon excitation of a conventional 800 nm laser diode. Judd–Ofelt intensity parameters and radiative properties of Dy³⁺ ions in present glasses were calculated using the Judd–Ofelt theory. The mechanism and microparameters of energy transfer processes were investigated based on photoluminescence performance and lifetime measurements. The Dy³⁺/Tm³⁺ codoped fluorophosphate glass possessing advantageous spectroscopic characteristics as well as excellent thermal stability is a promising candidate for an efficient 2.9 μm laser.     

Excited state absorption spectroscopy for thulium-doped zirconium fluoride fiber

The excited state absorption (ESA) transitions at 1050 and 1420 nm play a fundamental role in thulium-doped fiber amplifiers (TDFA). We present a novel setup to measure the spectral cross-sections of these transitions in amplifier fibers and the results of this measurement in case of thulium-doped fluorozirconate (ZBLAN) fibers. Besides a standard system for fiber attenuation measurements and a long-pass filter, we use only components from the fiber amplifier setup, including the active fiber. No special parts are needed. We show that this fiber optic method delivers reliable results for different lengths of the doped fibers. The oscillator strengths of the measured transitions are calculated and compared to values published in the literature.     

Tm-Ho and Tm-Tb energy transfer in tellurite glass

In Tm³⁺-Ho³⁺- and Tm³⁺-Tb³⁺-doped tellurite glasses, the IR fluorescence spectra and the lifetimes of the upper ³H₄ and lower ³F₄ lasing levels for 1.47 μm of Tm³⁺ were measured. The non-exponential decay is fitted using the Inokuti-Hirayama equation. The energy-transfer parameter C_DA, critical ion distance R₀, lifetime and non-radiative energy-transfer efficiency η between donor and acceptor ions are compared. The quenching mechanism has been explained. Both Ho³⁺ and Tb³⁺ ions reduce the lifetimes of the upper and lower lasing levels, with Tb³⁺ ions proving more efficient than the effects observed for Ho³⁺ ions. The optimised Tb₂O₃ concentration is about 1.0-1.5 wt%, which produced effective inversion and could be used for laser and amplifier applications.     

Electronic deactivation and energy transfer in doped oligophenylenevinylene nanoparticles

Suspensions of oligophenylenevinylene (nPV) nanoparticles withn = 2 vinylene units are doped with nPVs of longer chainlengths,n = 3–5. Absorption and fluorescence spectroscopy and steady-state and time-resolved fluorescence anisotropy measurements are used to determine the photo-physical properties of the suspensions. Undoped nanoparticles form highly oriented H-aggregates with low fluorescence quantum yields (Φ_F ≈ 0.1). Introduction of bulky substituents into the particle constituting molecules perturbs the intermolecular orientation. Upon doping, efficient energy transfer to the dopants is found, changing the color and leading to enhancement of the fluorescence quantum yields up to Φ_F = 0.6. The intermolecular orientation is not changed upon doping.     

Excited state intramolecular proton transfer mechanism of o-hydroxynaphthyl phenanthroimidazole

By utilizing the density functional theory(DFT) and the time-dependent density functional theory(TDDFT), the excited state intramolecular proton transfer(ESIPT) mechanism of o-hydroxynaphthyl phenanthroimidazole(HNPI) is studied in detail. Upon photo is excited, the intramolecular hydrogen bond is obviously enhanced in the S_1 state, which thus promotes the ESIPT process. Hydrogen bond is shown to be strengthened via comparing the molecular structures and the infrared vibration spectra of the S_0 and S_1 states. Through analyzing the frontier molecular orbitals, we can conclude that the excitation is a type of the intramolecular charge transfer excitation, which also indicates the trend of proton transfer in S_1 state. The vertical excitation based on TDDFT calculation can effectively repeat the absorption and fluorescence spectra of the experiment. However, the fluorescence spectrum of normal structure, which is similar to the spectrum of isomer structure is not detected in the experiment. It can be concluded that the fluorescence measured in the experiment is attributed to both structures. In addition, by analyzing the potential energy curves(PECs) calculated by the B3 LYP functional method, it can be derived that since the molecule to cross the potential barrier in the S_1 state is smaller than in the S_0 state and the reverse proton transfer process in the S_1 state is more difficult than in the S_0 state, the ESIPT occurs in the S_1 state.     

Excited state dynamics and energy transfer rates in Sr3Tb0.90Eu0.10(PO4)3

The emission spectrum of neat Sr₃Tb(PO₄)₃ upon excitation at 337 nm in the levels above ⁵D₃ is dominated by ⁵D₄ emission and no significant emission from ⁵D₃ is observed due to efficient cross relaxation involving the Tb³⁺ levels. On the other hand, the emission spectrum of the same host containing 10 mol% Eu³⁺ upon excitation at the same wavelength (in the Tb³⁺ levels) is dominated by strong emission bands from the ⁵D₀ level of Eu³⁺. This clearly indicates that Tb³⁺→Eu³⁺ energy transfer is present. The excitation spectrum of the Eu^{3+ 5}D₀ emission is dominated by Tb³⁺ bands extending in the UV region.The presence of 10 mol% Eu³⁺ in Sr₃Tb(PO₄)₃ very strongly shortens the ⁵D₄ decay time. The decay curve is not far from exponential, indicating that the energy transfer to Eu³⁺ is accompanied by fast energy migration. The transfer regimes are identified and the donor–donor and donor–acceptor transfer microparameters are quantified under the assumption of electric dipole–electric dipole interactions.     

Cooperative luminescence and absorption in ytterbium doped aluminosilicate glass optical fibres and preforms

The cooperative luminescence and absorption properties of Yb³⁺ doped aluminosilicate glass optical fibres and preforms are investigated in detail. In accordance with previous investigations, both the visible cooperative luminescence and the infrared luminescence decay measurements have been resolved into a single exponential decay component. We show that for a glass with similar Yb³⁺ dopant concentration but more Al³⁺, the glass emits less visible luminescence. Absorption loss measurements completed on fibre samples revealed a broad absorption in the 350-500 nm range, which we propose is due to a combination of Yb²⁺ absorption and cooperative absorption from Yb³⁺ ion pairs.