Multiphonon Relaxation Rates Research Articles

Multiphonon relaxation in fluoride and ternary sulfide laser crystals with neodymium ions

It is shown that, in ternary sulfide crystal matrices, in contrast to fluoride matrices with similar phonon spectra, the multiphonon relaxation rate as a function of the number p eff of "effective phonons" (the energy gap ∆ E min between the J ' and J levels of the Nd 3+ ion) has a steep slope, which does not decrease with increasing p eff . This indicates that, compared to fluoride crystals, the multiphonon relaxation rate of mid-IR transitions (in the range 4-5 µ m) in ternary sulfide matrices sharply decreases, by three to four orders of mag- nitude.

Er 3+-doped GeGaSbS glasses for mid-IR fibre laser application: Synthesis and rare earth spectroscopy

With an infrared transparency extended to 10 μm, low multiphonon relaxation rates and suitable rare earth solubility, sulphide glasses in the Ge–Ga–Sb–S system allow radiative emission from rare earth ions in the mid-IR range. The Er 3+ ion, widely studied in glass fibres for optical amplification at 1.5 μm, presents an interesting transition for mid-IR applications around 4.5 μm ( 4I 9/2 → 4I 11/2). Thus, the aim of this work is to evaluate the Er 3+-doped Ge 20Ga 5Sb 10S 65 glass as a potential fibre laser source operating in the 3–5 μm mid-IR spectral region. For that purpose, absorption and emission spectra were recorded from visible to mid-IR and the radiative lifetimes of the involved excited levels ( 4I 9/2, 4I 11/2 and 4I 13/2) were determined. Experimental results were compared with those obtained from a Judd–Ofelt analysis based on the absorption cross-sections of all observable transitions. The 4I 9/2 radiative quantum efficiency was estimated to be 64% and the emission cross-section at 4.6 μm was found equal to 2.85 × 10 −21 cm 2. Core only and core/clad Er 3+-doped Ge 20Ga 5Sb 10S 65 glass fibres were successfully drawn with minimum optical losses of about 1.5 and 10 dB/m at 5.2 and 3.5 μm, respectively. Fluorescence signals in the mid-IR (at 2.7 and 4.6 μm) were clearly observed with both fibres.

Polarized spectral properties of Er3+ ions in NaGd(WO4)2 crystal

Polarized spectral properties of Er3+:NaGd(WO4)(2) single crystal are reported. The crystal was grown by the Czochralski method. The Judd-Ofelt theory was applied to analyze the polarized absorption spectra and then calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios. Fluorescence decay curves of the(4)I(13/2), I-4(11/2), and S-4(3/2) multiplets for the Er3+ ions were measured. Stimulated emission cross-sections of the I-4(13/2)-> I-4(15/2) transition obtained by the Fuchtbauer-Ladenberg formula and the reciprocity method were compared. Multi-phonon relaxation rates of the crystal were estimated. Green up-conversion fluorescence around 531 and 552 nm was observed, and the possible up-conversion mechanisms were proposed.

Optical properties of Er3+-doped alkali fluorophosphate glasses

A systematic study of the optical absorption and luminescence spectra of Er3+-doped alkali fluorophosphate glasses (RTFP) 50(NaPO3)–10TeO2–20AlF3–19RF–1Er2O3 (R=Li, Na and K) has been performed. The phenomenological Judd–Ofelt intensity parameters have been determined from the spectral intensities of the absorption bands in order to calculate the radiative transition probabilities, radiative lifetimes and branching ratios for various excited luminescent states. Using the visible and near infrared emission spectra, full width at half maximum (FWHM), emission cross-sections (σe) and figure of merit (FOM) were evaluated and compared with other hosts. Especially, the numerical values of these parameters indicate that the emission transition 4I13/2→4I15/2 transition at 1.534μm in Er3+-doped fluorophosphate glasses may be highly useful in optical communication. The decay characteristic of 4S3/2 excited level has also been recorded and analyzed. The calculated and experimental lifetimes were compared in terms of quantum efficiencies and multiphonon relaxation rates.

Blue, green and red upconversions in Ho2O3-doped fluorophosphate glasses

Near infrared (NIR) to visible upconversions of a fluorophosphate glass of composition (mol%) 7Ba(PO3)2–32AlF3–30CaF2–18SrF2–13MgF2 doped with various concentrations (0.1, 0.3 and 1.0mol%) of Ho2O3 have been investigated by exciting at 892nm at room temperature. Three upconverted bands originated from the 5F3→5I8, (5S2, 5F4)→5I8 and 5F5→5I8 transitions have been found to center at 491nm (blue), 543nm (green) and 658nm (red), respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been interpreted by the excited state absorption (ESA), energy transfer (ET) and cross relaxation (CR) mechanisms involving population of the metastable (storage) energy levels by multiphonon deexcitation effect. It is evident from the infrared reflection spectral (IRRS) analysis that the upconversion phenomena are expedited by the low multiphonon relaxation rate in fluorophophate glasses owing to their high intense low phonon energy of ∼600cm−1 which is very close to that of fluoride glasses (500–600cm−1).

Theoretical studies of nonradiative 4f–4f multiphonon transitions in dielectric crystals containing rare earth ions

Detailed calculations have been performed of multiphonon relaxation rates of optical excitations in Nd-doped LiYF 4 and Pr-doped CsCdBr 3 crystals in the frameworks of the exchange charge model of the crystal fields and rigid ion harmonic models of lattice dynamics. It is shown that the empirical energy gap low emerges from the exponential diminishing of spectral densities of n-phonon correlation functions with the increase of an order n. Calculated transition probabilities for 2-, 3-phonon processes agree with experimental data. However, for energy gaps exceeding the maximum phonon energy more than twice, the existing theory which neglects the lattice anharmonicity brings about underestimated relaxation rates.

Nonoxide Glass-Forming Systems-Glass Formation and Structure, and Optical Properties of Rare-Earth Ions in Glasses

Nonoxide glasses composed of halides and/or chalcogenides (sulfides, selenides, and tellurides) as their main components have been extensively investigated due to their characteristic structures and properties. Among them, the present review focuses on glass formation based on nonfluoride-halide (halides other than fluoride) systems and the structure of the glasses as well as the transition properties of rare-earth ions in the nonoxide glasses. Systematic examination of glass formation in nonfluoride-halide systems based on AXn-MX-M′X2 with a single halide ion revealed that the systems of A=Zn, Cd, Ag, Cu and Li are vitrified by a conventional melting-cooling method, where X=Cl, Br, or I, M=alkali metal (K, Rb, Cs), and M′=alkaline earth metal (Ba). The coordination structure around the cations, An+, was investigated by means of X-ray absorption spectroscopy and other methods. The structural model for these halide glasses is also discussed. The studies about the transition properties of rare-earth ions in the nonoxide glasses showed that the multiphonon relaxation rates are strongly suppressed. Detailed analyses on the fluorescent decay curves for Er3+-doped Ga2S3-based glasses are presented.

Relaxation properties of rare-earth ions in sulfide glasses: Experiment and theory

Both radiative and nonradiative relaxation rates for a series of rare-earth ions doped $20\mathrm{Ge}\text{\ensuremath{-}}5\mathrm{Ga}\text{\ensuremath{-}}10\mathrm{Sb}\text{\ensuremath{-}}65\mathrm{S}$ $(\mathrm{GeGaSbS})$ sulfide (chalcogenide) glasses have been determined. Temperature-dependent lifetimes were carried out for various excited levels of the sample. Radiative decay rates were derived by using the Judd-Ofelt approach. Nonradiative decay rates are evaluated by comparing the inversion of measured lifetimes with the calculated radiative decay rates. We have found that the multiphonon relaxation rates should be a predominant decay mechanism among the excited states if the energy gap to the next lower level is smaller than $2500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and the decay mechanism can be determined using the semiempirical ``energy-gap law.'' For an energy gap larger than $2500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, additional nonradiative decay processes become dominant over the multiphonon decay. Additional nonradiative decay processes have been quantitatively identified with the diffusion-limited relaxation calculations.

Oxysulfide optical ceramics doped by Nd 3+ for one micron lasing

The spectroscopic characteristics of translucent oxysulfide optical ceramics doped by Nd 3+, which can be perspective for 1-μm lasing with high quantum efficiency is considered. Analysis of multiphonon relaxation (MR) rate of the low 4I 11/2 laser level in the ceramics is provided using nonlinear theory of MR for multifrequency model of lattice vibration. MR rate dependence for the transitions with equal number of phonons p=4 on the extent of the phonon spectrum and Nd 3+ to the nearest ligands distance R 0 is established experimentally and analyzed. For the initial 4F 3/2 laser level in the Gd 2O 2S:Nd 3+ and La 2O 2S:Nd 3+ ceramic samples an analysis of the energy transfer kinetics is provided. Different stages of energy transfer—direct energy transfer and a stationary migration-controlled stage are observed and analyzed. Dipole–dipole mechanism of energy transfer is established and microparameters of energy transfer and migration are determined.

Mid-IR transitions of trivalent neodymium in low phonon laser crystals

Mid-IR Nd 3+ transitions perspective for laser oscillation were analyzed in the CaGa 2S 4, PbGa 2S 4, and PbCl 2 crystals and compared with low phonon fluoride crystals. Fluorescence kinetics decay of the high-lying 4G 7/2 and 4G 5/2; 2G 7/2 and low-lying 4I J levels and its temperature dependence in the range of 13–295 K were measured. For 5 μm mid-IR transitions of Nd 3+ the radiative relaxation rates are found to be several times higher and multiphonon relaxation (MR) rates are several times lower in lead and calcium thiogallate crystals compared to low phonon fluoride crystals. MR rate dependence on the Nd 3+ to the nearest ligand distance R 0 is established experimentally and analyzed in the frame of point-charge model of nonlinear theory of multiphonon relaxation. Mid-IR Nd 3+ 4–5.5 μm fluorescence spectra of 4I i – 4I j transitions perspective for laser oscillation were measured for the first time in the studied crystals.

Optical properties of Dy3+ ions in alkali tellurofluorophosphate glasses for laser materials

Optical absorption and emission spectra were performed for Dy3+ ions doped alkali tellurofluorophosphate glasses. The intensity parameters from the measured oscillator strengths were evaluated using the Judd–Ofelt (J–O) theory. The observed and calculated oscillator strengths (f) are found to be in reasonable agreement with one another. The evaluated J–O parameters are used to predict radiative transition probabilities (AR), total radiative transition probabilities (AT), radiative lifetimes (τR) and branching ratios (βR) for various excited levels of Dy3+ doped alkali tellurofluorophosphate glasses. The predicted τR and βR values of 4F9/2 transition are compared with the experimentally measured values. The lifetime of the 4F9/2 level was measured with a quantum efficiency of 80% for Dy3+: RTFP (R = Li, Na, K) glasses. Stimulated peak emission cross-sections (σe) for different emission transitions were estimated from emission spectra. From the magnitudes of the stimulated emission cross-sections, branching ratios, multiphonon relaxation rates (WMP), the most potential laser transitions are identified and the utility of these glasses as laser active materials is discussed. A thermalization process between the 4F9/2 and 4I15/2 levels is also reported.

Infrared to visible upconversion of Nd3+ions in KPb2Br5low phonon crystal

In this work we present a detailed analysis of the infrared to visible upconversion in Nd(3+)-doped KPb(2)Br(5) low phonon crystal by using both steady-state and time-resolved luminescence spectroscopy. Efficient blue, green, orange, and red emissions have been observed under excitation into the (4)F(5/2) and (4)F(3/2) states. The low phonon energy of this crystal leads to a significant reduction of the multiphonon relaxation rates which allows most of the excited states to relax radiatively. To investigate the nature of the upconversion processes, emission spectroscopy and lifetime measurements for the visible fluorescence, performed by using one photon excitation, have been compared to the upconverted emissions and lifetimes obtained under infrared pulsed excitation. The analysis of the experimental results indicates that the upconverted emissions occur via energy-transfer upconversion.

Influence of structural evolution on fluorescence properties of transparent glass ceramics containing LaF 3 nanocrystals

The structural evolution and the fluorescence properties of Er 3+-doped oxyfluoride glass ceramics were investigated for samples at different stages of LaF 3 crystallization. TEM observation revealed the homogeneous precipitation of spherical LaF 3 crystals sized in the range of 10–15 nm among the glassy matrix for a sample heated to 690 °C. The crystallization induced the splitting of Er 3+ emission peaks and the appearance of upconversion in the visible wave-band, implying the incorporation of Er 3+ into a low-phonon-energy crystalline fluoride environment. In the isothermal experiment at 650 °C, with the increasing of crystal size that resulted from prolonged heating time, the relative intensity of upconversion emission at 660 nm to that at 540 nm decreased significantly, probably due to the reducing of multiphonon relaxation rate between 4S 3/2 and 4F 9/2, which was believed to be caused by weaker interactions of the Er 3+ electronic states in nanocrystals with the vibrational excitations of the surrounding glass medium and with the mixed vibrational modes of nanocrystallites and glass for the system with larger particle size. The remarkable difference in spectrum shape of the visible emissions under 378 nm excitation and 980 nm excitation implied that a certain amount of Er 3+ ions still remained in the glass matrix after crystallization.

IRRS, UV-Vis-NIR absorption and photoluminescence upconversion in Ho 3+-doped oxyfluorophosphate glasses

Infrared reflection spectroscopic (IRRS), ultraviolet-visible-near infrared (UV-Vis-NIR) absorption and photoluminescence upconversion properties with special emphasis on the spectrochemistry of the oxyfluorophosphate (oxide incorporated fluorophosphates) glasses of the Ba(PO 3) 2–AlF 3–CaF 2–SrF 2–MgF 2–Ho 2O 3 system have been studied with different concentrations (0.1, 0.3 and 1.0 mol%) of Ho 2O 3. IRRS spectral band position and intensity of Ho 3+ ion doped oxyfluorophosphate glasses have been discussed in terms of reduced mass and force constant. UV-Vis-NIR absorption band position has been justified with quantitative calculation of nephelauxetic parameter and covalent bonding characteristics of the host. NIR to visible upconversion has been investigated by exciting at 892 nm at room temperature. Three upconverted bands originated from the 5F 3→ 5I 8, ( 5S 2, 5F 4)→ 5I 8 and 5F 5→ 5I 8 transitions have found to be centered at 491 nm (blue, medium), 543 nm (green, very strong) and 658 nm (red, weak), respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been explained by the excited state absorption (ESA), energy transfer (ET) and cross relaxation (CR) mechanisms involving population of the metastable (storage) energy levels by multiphonon deexcitation effect. It is evident from the IRRS study that the upconversion phenomena are expedited by the low multiphonon relaxation rate in oxyfluorophosphate glasses owing to their high intense low phonon energy (∼600 cm −1) which is very close to that of fluoride glasses (500–600 cm −1).

Controlling fluorescence lifetime of rare-earth element in amorphous inorganic solids via very small compositional adjustments

Fluorescence lifetime of hypersensitive 4f-4f transitions of rare-earth elements embedded in amorphous inorganic solids can be dramatically modified by compositional adjustment of the hosts tantamount to not more than 1 mol % without any elaborated thermal treatments. It is possible to modify a spontaneous emission rate of Dy3+:(F11∕26,H9∕26)→H15∕26 transition in chalcogenide Ge–As–S glasses through selective addition of low levels of Ga and CsBr. Along with the change of the spontaneous emission rate, multiphonon relaxation rate involved in the (F11∕26,H9∕26) state also significantly varies upon the minute compositional adjustment. The combination of these effects results in the measured lifetime of the fluorescing (F11∕26,H9∕26) level being greatly enhanced. Such behaviors are attributed to the hypersensitive nature associated with the H15∕26↔F11∕26 transition and preferential coordination of bromine in the nearest-neighboring shell of the Dy3+ ions, which is formed spontaneously during the vitrification process of the host materials. These experimental observations show the most extreme dependence of the fluorescence lifetime on small compositional changes reported compared to any other noncrystalline solid-state dielectric. As such coutilization of many hypersensitive transitions of rare-earth elements and those host materials used in this study may present a unique opportunity to control absorption and emission properties, especially fluorescence lifetimes, through a minute compositional adjustment.

Influence of PrCl3/PrF3 on the optical and spectroscopic properties of fluorogallate and fluoro-gallo-indate glasses

Thermal, optical and spectroscopic properties of fluorogallate PZG (PbF2–ZnF2–GaF3) and fluoro-gallo-indate PIG (PbF2–InF3–GaF3) glasses doped with PrCl3 and PrF3 have been investigated. Absorption and luminescence properties of these glasses have been studied in the visible and in the second telecom window around 1.3 μm. Fluorescence lifetimes of the 1G4 level have been compared between praseodymium chloride and fluoride doped glasses. Longer decay constants have been obtained for PrCl3 doping at low concentration (0.05 mol%). This effect has been related to a reduction of both the phonon energy in the surrounding of the rare earth ion and the electron–phonon coupling, leading to a decrease of the non-radiative multiphonon relaxation rate.

FT-IRRS, UV–Vis–NIR absorption and green upconversion in Er3+ doped lead silicate glass

Fourier transform infrared reflection spectroscopic (FT-IRRS), ultraviolet–visible–near infrared (UV–Vis–NIR) absorption and green upconversion fluorescence properties of an Er3+ ion doped high PbO (72.5 wt.%) content lead silicate glass have been studied at room temperature. FT-IRRS and UV–Vis–NIR absorption spectral band positions have been discussed quantitatively with theoretical calculation and covalent bonding characteristics of the glass respectively. NIR radiation upconversion into green fluorescence has been investigated by exciting at 979 nm. Two green upconverted bands with doublet characteristics originating from 4S3/2→4I15/2 and 2H11/2→4I15/2 transitions have been found to be centered at 550 (strong) and 530 (weak) nm respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been explained by the excited state absorption (ESA) and energy transfer (ET) mechanisms involving population of energy levels by multiphonon deexcitation and thermal effects. It is evident from IRRS studies that the upconversion phenomenon is expedited by the low multiphonon relaxation rate in lead silicate glass owing to its low phonon energy (990 cm−1) compared to that in silica glass (1122 cm−1).

Quantitative Identification of Phonon Modes Controlling the Multiphonon Relaxation in Heavy‐Metal Oxide Glasses

The phonon mode(s) controlling the multiphonon relaxation (MPR) in PbO–Bi2O3–Ga2O3 glass was analyzed, and the effect of GeO2 addition on the MPR process was investigated. MPR rates were obtained from the lifetimes of the Tm3+:3H4 level in glasses over the temperature range 20–280 K. In PbO–Bi2O3–Ga2O3 glass, phonons from the bending vibration between GaO4 tetrahedra (∼550 cm−1) controlled the MPR process. On the addition of GeO2, the phonon mode at ∼770 cm−1 due to the stretching vibration of GeO4 tetrahedra started to affect the MPR process. Phonon modes controlling the MPR process in PbO–Bi2O3–Ga2O3–GeO2 glass were both 550 cm−1 and 770 cm−1.

Er3+ doped boro-tellurite glasses for 1.5 μm broadband amplification

B2O3 has been introduced in Er3+-doped tellurite glasses in order to increase their phonon energy and the multiphonon relaxation rate of the Er3+:4I11/2 → 4I13/2 transition, which will allow efficient 980 nm pumping. The inclusion of additional glass components such as GeO2 and Al2O3 has been shown to enhance the thermal stability of the boro-tellurite glasses. No crystallization peak (Tc) was observed in glass samples containing GeO2 and Al2O3. The suitability of these boro-tellurite glasses as host for broadband erbium-doped fiber amplifiers and their advantages over the other tellurite glasses are discussed.

147-µm emission and multiphonon relaxation of Tm^3+ ions in potassium bismuth gallate glasses

We propose and demonstrate potassium bismuth gallate (KBG) glasses as suitable hosts for the rare-earth ion Tm3+ (thulium) operating at the 1.47-μm wavelength. The optical properties of Tm3+-doped KBG glasses were investigated. The bandwidth of the 3H4→3F4 transition is ∼120 nm and is higher than that of Tm3+-doped, tellurite-based glasses. The measured fluorescence lifetime is 0.15 ms at room temperature, and the quantum efficiency is 88%. Multiphonon relaxation rates for various transitions were evaluated and compared with other glass hosts. The energy transfer in Tm3+–Ho3+-codoped KBG glasses was also investigated, and these results indicate that Tm3+-doped KBG glasses are promising candidates for efficient broadband optical amplifiers operating in the 1.47-μm wavelength range.