Excited state absorption of Ti3+:YAlO 3 ⋆

Spectroscopic properties and excited-state absorption (ESA) of Ti³⁺ doped YAlO₃ are investigated in detail. The fluorescence band is centered at 623 nm with a room-temperature lifetime of 11.4 s and a quantum efficiency nearby one. Till now laser action could not be detected because of strong excited-state absorption into a charge transfer band located 45 500 cm^–1 above the Ti³⁺ ground state. At the Ar-laser pump wavelength the ESA cross-section is determined to be _ESA=1×10^–18 cm². This value is a factor of 44 higher than the ground-state absorption cross-section. Consequently the pumping efficiency is decreased by one or two orders of magnitude. The Ti:YAlO₃ results are compared with the spectroscopic data of Ti:Al₂O₃.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Time-resolved spectra of excited-state absorption in Er3+ doped YAlO3

A pump- and probe-beam technique is used for measuring time-resolved excited-state absorption (ESA) and stimulated-emission (SE) spectra of Er³⁺ doped YAlO₃. The Er^{3+ 4} I _15/2 ⁴ F _7/2 transition of the sample is excited at 488 nm by an excimer laser pumped dye laser. The ESA and SE of broadband xenon flashlamp light is monitored between 300 and 860 nm by an optical multichannel analyzer (OMA). The analysis of the experimental results provides information on the effective cross sections _ESA and _SE originating from several levels and on the populations of these levels. To our knowledge this represents the first detailed investigation of time-resolved ESA and SE over a broad spectral range in rare-earth doped materials.     

Infrared excited-state absorption and stimulated-emission cross sections of Er3+-doped crystals

Detailed spectra of the excited-state absorption and the stimulated-emission cross-sections of Er³⁺ -doped Y₃Al₅O₁₂, YAlO₃, LiYF₄, and KYF₄ crystals are analyzed in the infrared wavelength regions from 700 to 2100 nm. The spectra were measured with a pump and probe technique employing a double modulation scheme. For Er (2%): Y₃Al₅O₁₂ also the stimulated emission at 3 µm and the reabsorption due to excited-state absorption from the lower laser level are investigated.     

Electronic and optical properties of Ti3+ doped α-Al2O3 crystals: First-principles calculations

Based on density functional theory, the geometric and electronic structures of the Ti doped α-Al₂O₃ system (Ti:α-Al₂O₃) are calculated by using a first-principles method. Due to the Ti-doping, both the lengths and angles of bonds in the local geometries are distorted, from which a trigonal–triclinic structural transition may be expected. With increasing Ti-doping, an insulator–semimetal transition is observed in the Ti:α-Al₂O₃ system, mainly due to a complete spin polarization of electrons at the Fermi level. The dielectric function and absorption edge are further calculated. It is found that the absorption edge is decreased, and is much lower than that of pure Al₂O₃ crystals, which is in good agreement with the experiment. The results indicate the potential applications of the Ti:α-Al₂O₃ optical system.     

Stimulated emission and laser action of Pr3+-doped YA1O3

Groundstate absorption, fluorescence, excited-state absorption, and stimulated emission of Pr:YA1O₃ were measured in dependence on the polarization. The results reflect the anisotropy of the host lattice. Especially the polarized spectra of the stimulated emission fit very well with the laser data of this material. Seven cw laser transitions in the visible spectral range obtained so far in Pr:YA1O₃ with argon-ion laser pumping were identified in the spectra of the stimulated emission. In addition, two infrared cw laser transitions were realized. The best results were achieved for the³ P ₀³ F ₄ laser transition at 746.9 nm with a maximum slope efficiency of 24.6%, a maximum output power of 49.6 mW, and a laser threshold of 25 mW.     

Identification of trace impurities in pure and doped YAlO3 and Y3Al5O12 crystals by their fluorescence and by the EMA method

Trace uncontrolled impurities as Ce³⁺, Cr³⁺, Eu³⁺ and Nd³⁺ were detected in YAlO₃ and YAlO₃: Cr crystals from their fluorescence by high sensitive photon counting detection. The concentrations of Ce, Cr, Eu, Nd and some other impurities (Tm, Ho, Mn, Mg, Fe) were also determined by electron beam excited X-ray microanalysis (abbr. EMA) in YAlO₃ and Y₃Al₅O₁₂ crystals (pure or doped). Both the methods (fluorescence detection and EMA) are compared, fluorescence properties of some trace impurities were determined and their influence on local distribution and transfer processes (Ce³⁺Nd³⁺, Cr³⁺ Nd³⁺) in these crystals are discussed.The authors are grateful to J. Kvapil, Jos. Kvapil and K. Blaek from Preciosa a.s., Division Monokrystaly, Turnov, Czech Republic, for supplying us with YAlO₃ and Y₃Al₅O₁₂ single crystals.     

Upconversion in YAlO3:Er pumped at 800 nm

Fluorescence at 3 m and 545 nm in a YAlO₃:Er (30 at.%) crystal has been measured as a function of wavelength, intensity and polarization for the excitation in the 800 nm wavelength region. Emission at 545 nm is an indicator of losses for 3 m emission due to excited-state absorption and upconversion. The experimental results allow to determine the excitation mechanisms, the upconversion coefficient W ₂ and the optimum pump wavelengths to generate 3 m radiation.     

Coherent population trapping with a train of pulses via a continuum and its application to the suppression of excited-state absorption

We discuss the coherent population trapping (CPT) effect induced in a three-level Λ-type system by a train of short pulses. Specifically, we consider a modified Λ system with the upper level replaced by a continuum of states. We show that CPT with a train of pulses can be applied to the suppression of excited-state absorption of pump radiation in crystalline laser materials. The utility of the technique for pumping a promising yellow-orange solid-state medium Ti:YAlO₃ and overcoming excited-state pump absorption is analyzed.     

High Pressure Spectroscopy of Ti Doped Al2O3 and YAIO3 Host Crystals

We report high-pressure luminescence spectroscopy studies of Ti^3? in Al₂O₃ and YAlO₃. High-pressure luminescence spectra were measured for Al₂O₃:Ti^3? up to 90kbar and for YAlO₃:Ti^3? up to 181 kbar. In both cases, a blue shift of the luminescence peak with pressure was observed and is attributed to an increase in the octahedral crystal-field strength (10 Dq) with pressure. In the case of YAlO₃;Ti^3?, an additional luminescence peak was observed at 181 kbar and below ～ 200 K, and is attributed to a metastable state. The metastability of the ²E excited state was induced by pressure that removes the equivalency of three energy minima of the ²E state coupled to the ? mode.     

Radiation and thermally induced effects in YAlO3:Mn crystals

We have studied effects induced by γ-radiation and temperature in Mn-doped YAlO₃ crystals. The studies have been performed by means of optical spectroscopy that include measuring of optical absorption changes induced by γ-radiation and elevated temperature as well as thermally stimulated luminescence (TSL). It has been shown that under γ-irradiation of YAlO₃:Mn crystals, along with the ionization of Mn_Al⁴⁺ ions (Mn_Al⁴⁺→Mn_Al⁵⁺+e⁻), some additional coloration processes take place. This additional coloration is characterized by a wide intense band centered at 26,000- that is ascribed to color centers intrinsic to YAlO₃ lattice. This coloration is removed by the way of crystal warming at , while the coloration caused by Mn_Al⁵⁺ ions is removed at higher temperature . The observed TSL glow of irradiated crystals reveals three peaks near 360, 400 and that correspond to three types of traps. Parameters of the traps have been determined. The TSL emission corresponds to intra-center luminescence of Mn_Al⁴⁺ and Mn_Y²⁺ ions. The possible ionization and trapping mechanisms in YAlO₃:Mn crystals are discussed.     

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.     

Role of Ti in the luminescence process of Al2O3:Si,Ti

Al₂O₃:Si,Ti, prepared under oxidizing condition at high temperature, gives PL emission around 430 nm when excited with 240 nm. The Al₂O₃:C, TL/OSL phosphor, also shows emission around 430 nm, which corresponds to characteristic emission of F-center. Thus, to identify the exact nature of luminescent center in Al₂O₃:Si,Ti, fluorescence lifetime measurement studies were carried out along with the PL,TL and OSL studies. The PL and TL in Al₂O₃:Si,Ti show emission around 430 nm and the time-resolved fluorescence studies show lifetime of about 43 μs for the 430 nm emission, which is much smaller than the reported lifetime of ∼35 ms for the 430 nm emission (F-center emission) in Al₂O₃:C phosphor. Therefore, the emission observed in Al₂O₃:Si,Ti phosphor was assigned to Ti⁴⁺ charge transfer transition. Fluorescence studies of Al₂O₃:Si,Ti do not show any traces of F and F⁺ centers. Also, Ti⁴⁺ does not show any change in the charge state after gamma-irradiation. On the basis of the above studies, a mechanism for TSL/OSL process in Al₂O₃:Si,Ti is proposed.     

Polarization of emission spectra from Ti3+-doped oxide crystals

The Ti³⁺ ion in YAlO₃ (YAP), Y₃Al₅O₁₂ (YAG), and Al₂O₃ crystals occupies distorted octahedral sites relative to the nearest neighbour ligand ions. Such distortions are of even-parity in YAG where the zero-phonon lines in emission occur via magnetic dipole transitions. In contrast, the zero-phonon transitions occur by electric dipole processes in Ti³⁺:YAP and Ti³⁺:Al₂O₃ where there are odd-parity distortions from octahedral symmetry. This paper reports measurements of the zero-phonon lines of Ti³⁺ ions in YAP, YAG, and Al₂O₃ at 10 K. The zero-phonon lines of Ti³⁺:YAP are strongly polarized perpendicular to the tetragonal axis and those of Ti³⁺:Al₂O₃ parallel to the trigonal axis. The experimental results are shown to be in accord with a molecular orbital model of the radiative transition according to which the transition intensities derive from odd-parity ligand wavefunctions induced into even-parity ground and excited Ti³⁺ wavefunctions by odd-parity crystal distortions.     

Generation of continuous-wave coherent radiation tunable down to 190.8nm in β-BaB2O4

Continuous-wave coherent radiation tunable in the wavelength range 190.8–196.1 nm has been generated by sum-frequency generation in -BaB₂O₄. The fundamental beams were supplied by a Ti:Al₂O₃ laser and a frequency-doubled argon-ion laser.     

Excited state absorption and stimulated emission of Nd3+ in crystals. Part I: Y3Al5O12, YAlO3, and Y2O3

4 F_3/2 excited state of the Nd³⁺ ion in Y₃Al₅O₁₂, YAlO₃, and Y₂O₃ were measured in a continuous wave pump- and probe experiment in a wide spectral range from 850 nm (780 nm for Y₃Al₅O₁₂) to 1500 nm. The cross sections were determined from a comparison with the emission spectra and the simultaneously measured ground state absorption bleaching. The strongest excited state absorption transitions were found in the 1220–1400 nm spectral region due to transitions to the ²G_9/2 and ⁴G_7/2 levels. The spectral positions of the measured transitions are in good agreement with the theoretically expected transitions calculated from the known Stark-level splittings. Received: 4 December 1997/Revised version: 8 May 1998     

On the growth and lasing characteristics of thick Nd:GGG waveguiding films fabricated by pulsed laser deposition

Pulsed laser deposition of epitaxial, single-crystal Nd:Gd₃Ga₅O₁₂ (Nd:GGG) films on Y₃Al₅O₁₂ substrates with thicknesses up to 135 m and propagation losses as low as 0.1 dB/cm is reported. Rutherford backscattering spectrometry has shown constant stoichiometry for the films throughout their depth. Fluorescence properties were similar to that of the bulk Nd:GGG crystal used as a target material for the deposition and lasing action has been observed at 1059.0 and 1060.6 nm after pumping by a Ti:sapphire laser operating at 808 nm. A laser threshold of 18 mW has been obtained and a slope efficiency of 17.5% has been observed using an output coupler of 4.5%. The low losses in combination with the high numerical aperture (0.75) and the thickness of the structures make them suitable for high-power diode pumping. PACS 42.70.Hj; 42.55.-f; 42.82.Et; 42.79.Gn; 81.15.Fg     

Energy response of the TL detectors based on YAlO3:Mn crystals

This study demonstrates the energy response of thermoluminescent (TL) detectors based on YAlO₃:Mn crystals. Experimental results of the relative sensitivity of YAlO₃:Mn²⁺ detectors to various kinds of photon radiation (from ⁶⁰Со, ¹³¹Cs, ¹⁹²Ir, and ¹³⁷Cs isotopes; X-ray from 220 kVp; and photon radiation from a linear accelerator (LINAC) at 5, 10, and 15 MV) agree with the theoretical energy response from Monte Carlo simulation. In addition to YAlO₃ (YAP), energy response was calculated for other yttrium-containing oxides such as Y₂O₃, Y₃Al₅O₁₂ (YAG), and Y₄Al₂O₉ (YAM). A possibility of filtering (modification) of the energy response of high atomic number (Z) materials by the metallic filters was shown.     

Luminescence of F + and F centers in YAlO3

In YAlO₃ crystals grown in vacuum or reduced by annealing at low oxygen pressure, the luminescence of F centers in the band at 420 nm, with τ=30 ms at 9 K, excited in the bands at 212 and 242 nm, as well as the luminescence of F ⁺ centers in the band at 355 nm, with τ=2.7 ns, excited in the main band at 220 nm and weaker bands at 190 and 288 nm, was detected. On the basis of the results obtained and data in the literature, the behavior of the emission of F ⁺ and F centers in oxides of the Al₂O₃-Y₂O₃ system is analyzed on the example of the compounds α-Al₂O₃, YAlO₃, and Y₃Al₅O₁₂. The role of antisite defects in the stabilization of F-like luminescence and absorption centers in multisublattice oxides is discussed.     

Ultraviolet luminescence of single crystals and single-crystal films of YAlO3

The nature of intrinsic emission bands of yttrium orthoaluminate in the UV spectral region at _max=220 nm (5.63 eV) and 330 nm (4.13 eV) is studied on the basis of the luminescence of single crystals and single-crystal films of YAlO₃ and Ce: YAlO₃ excited by synchrotron radiation sources with an energy of 3–25 eV at 9 and 300 K. The single crystals and single-crystal films were obtained, respectively, from solution and solution-melt by liquid-phase epitaxy and are characterized by considerably different concentrations of substitutional and vacancy defects. It is found that only the luminescence band at 300 nm, which has the decay time τ=4.1 ns and is excited in a band shifted from the range of interband transitions by 0.25 eV, has exciton-like character. The luminescence band at 220 nm with τ=0.1 µs at 9K, which is observed only for YAlO₃ single crystals and is absent in the luminescence of single-crystal films, is associated with antisite defects of the Y _Al ³⁺ type, which are a specific type of cationic isoelectronic impurities. It is shown that the phosphors based on single-crystal films of YAlO₃ have a simpler scintillation decay kinetics than their bulk analogues due to the absence of channels of excitation energy dissipation associated with the antisite defects of Y _Al ³⁺ type and vacancy defects.     

Laser emission at 1077 nm in Nd3+-doped calcium aluminosilicate glass

Laser emission at 1077 nm (⁴F_3/2⁴I_11/2) was demonstrated in Nd³⁺-doped low silica content calcium aluminosilicate (LSCA) glass under 810-nm pumping by a Ti:sapphire laser. The slope efficiency of the laser emission was found to be 34% with a 20-mW threshold power. The excited-state absorption (ESA) and stimulated emission were measured between 850 and 1520 nm. The results showed two ESA bands from 931 to 1070 nm (⁴F_3/2²D_3/2,²G_9/2,⁴G_11/2,²K_15/2) and from 1160 to 1414 nm (⁴F_3/2,⁴G_9/2⁴G_7/2,²K_13/2). The stimulated emission for the laser transition was found to be 45-nm broad with a peak value of 1.8×10^-20 cm². Thermal and mechanical properties of the LSCA glass were compared to the ED-2 and LHG-8 glasses. While the mechanical properties of the LSCA glass are superior to those of the commercial glasses, its temperature coefficient of optical path length is the highest among the three hosts. PACS 42.60.Pk; 42.70.Ce; 78.66.Jg