Radiative and non-radiative decays from the excited state of Ti3+ ions in oxide crystals

The fluorescence spectra of Ti³⁺ in Y₃Al₅O₁₂ (YAG), Al₂O₃ (sapphire), YAlO₃ (YAP) observed at 10 K are composed of zero-phonon lines accompanied by the broad vibronic sidebands. The temperature dependence of the fluorescence lifetime and of the total intensity of the broadband measured in YAG and Al₂O₃ indicate that the radiative decay times from the excited states are nearly constant in the range 10–300 K. This demonstrates that the broadband radiative emissions in Ti³⁺:YAG and Ti³⁺:Al₂O₃ are due to magnetic dipole transitions or to electric dipole transitions induced by static odd-parity distortion, respectively. The decrease of the fluorescence lifetime with increasing temperature in Ti³⁺:YAG and Ti³⁺:Al₂O₃ is due to non-radiative decay from the excited state which occurs through phonon-assisted tunnelling between the excited and ground states. The radiative decay of Ti³⁺:YAP is enhanced with increasing temperature, indicating that radiative decay rate contains a term associated with odd-parity phonons. Nevertheless, a non-radiative decay rate of 3.6 × 10⁴ s^–1 observed in the temperature range 10–300 K is due to excited state absorption, which depopulates the excited state and quenches the fluorescence at the laser wavelength.     

The optical properties and laser characteristics of Cr and Nd co-doped Y3Al5O12 ceramics

We have fabricated Cr³⁺ and Nd³⁺ co-doped YAG (Cr;Nd:YAG) ceramics, and investigated their optical properties and laser characteristics. The Cr;Nd:YAG has two broad absorption bands at around 440 nm (⁴A₂→⁴T₁) and 600 nm (⁴A₂→⁴T₂) respectively, caused by Cr³⁺ ions. In the case of pumping at 440 nm, the maximum effective lifetime of the Cr;Nd:YAG was 737 μs with a 0.1 at% Cr³⁺ and 1.0 at% Nd³⁺ co-doped YAG sample. Cr³⁺ ions take a role of an effective sensitizer to convert the UV light of flashlamp. For single-shot laser operation, a 10.4 J output energy at 1064 nm was obtained with 0.1 at% Cr³⁺ and 1.0 at% Nd³⁺ co-doped YAG ceramic rod with a laser efficiency of 4.9%. The laser efficiency was found to be more than twice that of a 1.0 at % Nd³⁺:YAG ceramic rod.     

Spectroscopic properties of alexandrite crystals II

Several aspects of the optical spectroscopic properties of alexandrite crystals not previously investigated are reported here. For Cr³⁺ ions occupying the Al³⁺ lattice sites with mirror symmetry in BeAl₂O₄, the positions of the zero-phonon lines for absorption transitions to the ²T_1g and ⁴T_2g levels are identified, and vibronic transition peaks in the fluorescence spectrum are compared to transitions appearing in the Raman spectrum and Stokes excitation spectrum. In addition, the effects of radiation trapping are shown to lengthen the fluorescence lifetime of the ²E_g - ⁴A_2g transition for ions in these sites at low temperatures. For Cr³⁺ ions occupying the Al³⁺ lattice with inversion symmetry, the ground state splitting of the ²E_g - ⁴A_2g transition is reported and the decrease of the fluorescence lifetime with temperature is shown to be due to the increase in vibronic emission probability as well as increased probability of direct radiationless decay.     

Enhancement of 1.35 μm fluorescence lifetime of Cr<Superscript>4+</Superscript>-doped YAG crystal by Ytterbium codoping

The 1.35 μm fluorescence lifetime from Cr⁴⁺-doped YAG crystal was strongly enhanced by doping Yb ions. We demonstrate that Yb ions act as efficient sensitizers of the ³ B ₂ (³ T ₂) → ³ B ₁(³ A ₂) emission of Cr⁴⁺ ions. The lifetime enhancement factor depending on Yb concentration was studied. Our results show that the crystal could be used in microchip laser and has a promising potential as an efficient source with the near infrared emission.     

Spectrum and energy levels of Cr3+ ions and exchange-coupled Cr3+ pairs in lanthanum aluminate

From the optical spectra the energy levels of the Cr³⁺ ion in LaAlO₃ were determined. In the cubic field approximation we obtained with the parametersDq=1750cm^-1,B=661 cm^-1, andC=2908 cm^-1 a good agreement between the calculated and measured energies. For higher Cr³⁺ concentrations the fluorescence spectrum of the exchange-coupled first nearest (1N) Cr³⁺ pairs was separated from the vibronic fluorescence spectrum of the single ions by selective excitation. From the experimental data the energy level scheme of the 1N pairs is constructed for the states ¦⁴ A ₂·⁴ A ₂〉 and ¦⁴ A ₂·² E〉. In the ground state the pairs are ordered antiferromagnetically due to an isotropic exchange interaction with an exchange integralJ=(?68 ± 1) cm^-1. From the fluorescence decay the intrinsic lifetimes of the fluorescent states of the single ions, the 1N and the 2N pairs were determined to be equal to (86± 4) msec, (1.75± 0.25) msec, and (31 ± 5) msec respectively. The fluorescence decay shows an energy transfer between the single ions and the pairs.     

Luminescence of impurity 3d and 4f metal ions in different crystalline forms of Al2O3

The ² E-⁴ A ₂ luminescence spectra of Cr³⁺ ions in Al₂O₃ are investigated in the course of transitions between the structural forms γ-δ-θ-α. The spectral lines observed are assigned to Cr³⁺ ions in these structural forms, which are identified by an X-ray powder diffraction analysis. The lifetimes of the Cr³⁺ excited states in transient forms of Al₂O₃ are measured. Investigations of the luminescence spectra of Al₂O₃: Eu³⁺ demonstrate that the Eu³⁺ ions can form regular centers only in α-Al₂O₃ and, unlike the Cr³⁺ ions, give no rise to similar centers in moderately ordered θ-Al₂O₃.     

调谐激光晶体Cr3+:ZnWO4光致发光特性的研究

研究了调谐激光晶体Ｃｒ³⁺：ＺｎＷＯ₄的光致发光特性。报道了它的吸收光谱、激发光谱、发射光谱及其随温度的变化、零声子跃迁和发射寿命等实验结果，并讨论了激发特性、电子－声子耦合作用、ＺｎＷＯ₄中Ｃｒ³⁺的发射寿命曲线等相关问题。 关键词：     

Nanocrystalline BaTiO3, doped with Eu and Cr: Fluorescence spectroscopy and spontaneous ordering effects

The results of optical studies of Cr³⁺ and Eu³⁺ ions doped nanocrystalline ferroelectric BaTiO₃ produced by the sol-gel process (particle size 20-40 nm) are reported. The Cr³⁺ impurity ions ²E-⁴A₂ (R-lines) fluorescence spectra in BaTiO₃ revealed significant differences from that reported in literature for the bulk material. At least three types of Cr³⁺ centers were found in the spectra. The temperature dependence of optical second harmonic generation in nanocrystalline BaTiO₃: Eu³⁺ shows a strong hysteresis in C_4v-O_h ferroelectric phase transition region, which was explained by the ordering effects in the system of electric dipole moments of dipole nanocrystals. The temperature dependencies of radiative lifetime of Eu³⁺⁵D₀ excited level reveal some hysteresis too. The possibility of influence of the ordering in the system of BaTiO₃:Eu³⁺ nanocrystals on the effective index of refraction of the medium and thus on the Eu³⁺⁵D₀ radiative lifetime, due to the local field effects is discussed.     

X α studies of laser crystals

This paper reports the spin-polarized self-consistent-charge discrete variational (SCC-DV-X ) calculations for the garnets GGG(Ca, Mg, Zr), GGG, YGG and YAG. The calculated values of 10Dq for these garnets are reported for the first time, and are in good agreement with the experimental values. The relation between the calculated values and the distances of the central ion and the ligand ions is discussed. The energy values of the doublet state² E _g were also obtained, and the reason why YAG:Cr³⁺ only emits sharp fluorescence whereas GGG(Ca, Mg, Zr):Cr³⁺ can also emit strong broad fluorescence are explained.     

Light-induced charge transfer and kinetics of the NIR absorption of Nb4+ polarons in SBN crystals at low temperatures

Sr_1-xBa_xNb₂O₆ (SBN) crystals with open tungsten-bronze structure show enhanced photorefractive properties with doping of impurities such as Ce, Cr, Rh etc. Under illumination with Kr⁺ laser (647 nm) or Ar⁺ laser light (488 nm or 514 nm) or UV light at low temperature, pure and doped SBN crystals show a broad polaron absorption band around 0.7 eV (6000 cm^-1). The first step of a theoretical model involves the excitation of electrons by illumination from Cr³⁺/Ce³⁺ to higher excited states or the conduction band. The excited electrons can then be trapped by Nb⁵⁺ to form Nb⁴⁺ polarons and further on can directly tunnel through or hop over the potential barrier (with a value Δ≈0.15±0.02 eV) to recombine with Cr⁴⁺/Ce⁴⁺ ions. The experimental intensity dependence, temperature dependence, and decay process of the light-induced Nb⁴⁺ polarons can be fitted with the help of this model. Small, but systematic, differences lead to the additional assumption of different recombination rates of polarons at distinct distances from the Cr⁴⁺/Ce⁴⁺ recombination centers and therefore many parallel decay channels are active where each decay channel obeys a monoexponential decay law. A stretched exponential decay function is employed to fit in this case the decay process of the Nb⁴⁺ polarons at different temperatures and under illumination with different intensities. Due to the high dielectric constant value (ε₃₃ and ε₁₁ have values in the 10²-10³ range) at low temperature, the long range Coulomb attraction (to Ce³⁺ _Sr/Ba) or repulsion (from Cr³⁺ _Nb) of the electronic polaron is suppressed. The leading role in the attraction and the following trapping of the electronic Jahn–Teller polaron, both on Cr³⁺ _Nb and Ce³⁺ _Sr/Ba centers, is played by the indirect dipole–dipole interaction via the soft TO-mode. Received: 27 November 1998 / Revised version: 22 January 1999 / Published online: 7 April 1999     

Electron paramagnetic resonance and luminescence of chromium in calcium germanate crystals

The luminescence of Ca₂GeO₄: Cr⁴⁺ single crystals at wavelengths in the range of 1.3 μm upon excitation with a 1-μ m semiconductor laser is investigated in the temperature range up to 573 K. At T<110 K, the Ca₂GeO₄: Cr⁴⁺ crystals are characterized by the electron paramagnetic resonance, which is attributed to the Cr⁴⁺ ions substituted for Ge⁴⁺ ions. The components of the g tensor and its principal axes are determined. It is revealed that the Cr⁴⁺ impurity centers in calcium germanate affect the crystal symmetry to a lesser degree compared to Cr⁴⁺ ions in forsterite. The observed deviation of the temperature dependence of the electron paramagnetic resonance from the Curie law is explained by the transition to the excited state with a low activation energy, as is the case in impurity 3d ions in diamond-like semiconductors. The inference is made that the giant effective degeneracy multiplicity of the excited state is associated with the initiation of soft phonon modes in the crystal upon excitation of the defect.     

Cr3+−Cr3+ excited state Raman scattering from molecular vibrations in binuclear Cs3Cr2Cl9 induced by exchange striction

On increasing the temperature, several new bands appear at higher energy from their parent vibrational modes in the Raman spectrum of the molecular complex Cr₂Cl^3?₉. The parent and new bands have intensities that follow the thermal population factors of various ⁴A_2g⁴A_2g Cr³⁺ pair states. This behaviour is attributed to exchange striction which produces a change in vibrational energy with excited state for those normal modes with large net axial Cr³⁺?Cr³⁺ displacements.     

Laser and absorption saturation measurements of Cr crystals, pumped by broadband pulsed 940 nm radiation

The characteristics of a 940 nm pumped Cr⁴⁺:forsterite laser have been investigated using a pulsed colour centre laser as the excitation source. Laser threshold, slope efficiency and temporal behaviour have been determined for high and low Cr⁴⁺ doped material. 940 nm pumping is found to be more efficient than 532 and 730–770 nm excitation. Discrepancies with 1064 nm pumping are attributed to excited state absorption. Absorption saturation measurements have been used to assess the potential applications of Cr⁴⁺:forsterite, Cr⁴⁺:YAG and Cr⁴⁺: Lu₃A1₅O₁₂ (LuAG) as passive Q-switches for Nd lasers in the 940 nm region.     

Luminescence properties and energy transfer processes in YAG:Yb,Er single crystalline films

The paper is dedicated to the study of the optical properties of YAG:Yb,Er single-crystalline films (SCF) grown by liquid phase epitaxy. The absorption, cathodoluminescence and time-resolved photoluminescence spectra and photoluminescence decay curves were measured for the SCFs with different doping levels of Er³⁺ (from 0.6 to 4.2 at.%) and Yb³⁺ (from 0.1 to 0.6 at.%). The spectra, excited by synchrotron radiation in the fundamental absorption range of the YAG and in the intraionic absorption bands of both dopants, reveal energy transfer from the YAG host to the Er³⁺ and Yb³⁺ ions and between these ions.     

Some detailed investigations of optically detected Ē(2 E) →4 A 2 cross relaxation in ruby

Cross relaxation betweenē(² E) and⁴ A ₂ states of Cr³⁺ in ruby at an applied external magnetic field ofH=5336 Oe was measured by monitoring the ¦? 1/2〉_ex→ ¦? 3/2〉_g optical transition in the temperature region of 1.6 to 4.2 °K. The chromium concentration varied from 2.9· 10^?4 to 4 · 10^?6 Cr³⁺/Al³⁺. With a concentration greater than 2 · 10^?5, the light intensity of the observed transition increases when cross relaxation takes place, while below this value it decreases. By measurement of the fluorescent intensity of one transition and simultaneously inducing EPR ground state transitions, we monitored the effect of trapping. Taking the value for trapping from fluorescence decay time measurements, we have used rate equations for calculating the actual change of excited state population when cross relaxation occurs. With this phenomenological model we are able to explain our experimental data. Finally some calculations for the effective spin temperature in theē(² E) state as a function of Cr³⁺ concentration as well as for various applied magnetic fields have been done.     

Spectroscopy of multisites Cr3+ and Nd3+ in yttrium aluminium garnet

Cr³⁺ and Nd³⁺ non-equivalent crystal field sites were observed. By use of a high-resolution dye laser,²E and⁴A₂ splittings, zero-phonon⁴A₂ →²E,²T₁,⁴T₂ transitions and vibronic levels of²E doublets of Cr³⁺ non-equivalent centres were recorded. Energy transfers from Cr³⁺ multisites to Nd³⁺ were studied by site-selective spectroscopy and fluorescence decays.     

Radiative transitions and spectral-hole burning in MgO: Cr<Superscript>3+</Superscript> nanocrystals

Luminescence spectra of Cr³⁺ ions in nanocrystalline MgO obtained by sol-gel technology were studied. The radiative lifetime of excited ²E states of cubic and tetragonal Cr³⁺ centers was found to be considerably longer than that of bulk single crystals. This effect is caused by a modified effective refractive index of the inhomogeneous medium. Burning of long-lived spectral holes in the ⁴A₂-²E transition profile of cubic centers was observed, which sets nanocrystalline samples strongly apart from bulk crystals, where no hole burning was found.     

Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers

The objective of this study was to identify a material suitable to absorb radiation at the wavelength of neodymium-doped Yttrium Aluminum Garnet (Y₃Al₅O₁₂:YAG), 1064 nm. M-(M= Sm³⁺, Co²⁺, Co³⁺, Cr³⁺, and Cr⁴⁺) doped highly transparent YAG ceramics were fabricated, and their absorption spectra were measured. Unlike Co²⁺ and Cr³⁺-doped ceramic samples, Co³⁺ and Cr⁴⁺ and Sm³⁺-doped:YAG ceramics were found to have significant absorption at 1064 nm. However, the Sm³⁺-doped YAG clearly emerged as the best candidate because it is also transparent at 808 nm, the pumping wavelength laser diode (LD), and also at most absorption bands used for flash-lamp pumping.     

Dominant Cr3+ centers in LiNbO3 under hydrostatic pressure

Low-temperature luminescence spectra of stoichiometric Cr: LiNbO₃ and of congruent Cr, Mg: LiNbO₃ were studied. Cr³⁺ impurity ions preferentially occupy Li⁺ sites (Cr_Li) in the LiNbO₃ crystal lattice, while Cr³⁺ ions substituting for Nb⁵⁺ ions (Cr_Nb) occur in addition to Cr_Li centers in codoped Cr, Mg: LiNbO₃ crystals. Application of a high hydrostatic pressure leads to a transformation of (dominant in concentration) Cr³⁺ centers from low-to high-crystal-field centers. Due to a strong pressure-induced blue shift of the ⁴ T ₂ state resulting in crossing with the ² E state, the replacement of the broad band ⁴ T ₂ → ⁴ A ₂ emission by a narrow R-line emission ² E → ⁴ A ₂ occurs in the luminescence spectra of the samples. This effect of level crossing was observed for the dominant Cr _Li ³⁺ and Cr _Nb ³⁺ centers at pressures which correlated well with estimations based on the ⁴ T ₂-² E energy gap (230 and 1160 cm^?1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^?1/kbar, respectively).     

掺Nd3+激光玻璃的激光窄线选择激发

本文利用可调谐脉冲染料激光器的窄线输出,在低温和室温下,激发掺钕碲、锗、硅、磷酸盐玻璃和氟铍玻璃的⁴I_9/2→⁴G_5/2(²G_7/2)不均匀增宽吸收带。观测了荧光衰减对激发波长的关系,从而研究在不同玻璃基质中,处于不同格位的Nd³⁺离子的衰减特性以及他们之间的相互作用情况。 关键词：