Ultraviolet excited luminescence of Ti: YAlO3

Solid State Communications, Vol. 80, No. 3, pp. 231-233, 1991. Printed in Great Britain.

0038-1098/91 $3.00 + .00 Pergamon Press plc

ULTRAVIOLET EXCITED LUMINESCENCE OF Ti: YAIO3 I. Vergara and J. Garcia So16 Departamento de Fisica Aplicada C-IV, Universidad Aut6noma de Madrid, Cantoblanco, 28049 Madrid, Spain and B. Henderson Department of Physics and Applied Physics, University of Strathclyde, Glasgow G4 0NG, Scotland, U.K.

(Received 10 April 1991 by D. Van Dyck) The lifetime of the red emission of Ti 3+ : YA103 under UV excitation has been measured as a function of temperature and emission wavelength. The slightly different values of the luminescence decay time and bandwidth when measured under UV and visible excitations constitute a new probe of the red emission of Ti 3+ : YAIO3 taking place on two different centers: one single Ti 3+ ions and the other pairs T? +/Ti 4+ . The emission intensity has been measured as a function of excitation power for laser radiation at 266 nm and 532 nm laser lines and shown to follow a linear behavior for UV exicitation. For visible excitation there is a slight deviation from linearity at high excitation intensity, due to an excited state absorption process. I. INTRODUCTION

2. EXPERIMENTAL DETAILS

THE RECENT use of Ti-sapphire (A1203) crystals as the gain medium in broadly tunable solid state lasers for the near-IR region has stimulated a new interest in the optical properties of Ti 3+ in other host materials [1, 2]. Thus, the spectroscopy of Ti 3+ has been recently studied in a number of crystals [3-5] and glasses [6, 7] as possible alternative matrices for new titanium activated tunable solid state lasers. One of these crystals is Ti3+ :YA103, the emission band of which extends from 540 to 800 nm. Thus, the spectral range is shifted into the yellow region. The longer room temperature lifetime of 11.4 #s, compared to A1203, makes Ti3+ :YA103 more suitable for flashlamp pumped laser action. Thus far, there is only one report of laser action in Ti:YAIO3 [8]. The difficulty in obtaining laser action in Ti3+:YAIO3 has been attributed to excited state absorption [9, 10] at the pump wavelength into a charge transfer band produced by Ti 3÷/Ti 4+ pairs centers [11]. These sites present a UV absorption band peaking at 280 nm, which also excites the broad red emission band. In this paper we report the luminescence behavior of titanium in YA103 crystals under variable excitation intensity in the UV region. Such experiments give a more detailed understanding of the excited state and explore alternative mechanisms for pumping the laser emission. The results are compared with those obtained for direct excitation of the crystal field transition.

The host crystal yttrium ortholuminate (YAIO3) has the perovskite crystal structure with space group Pbnm. AI ions occupy octahedral sites distorted to triclinic site symmetry Ci. Ti ions are incorporated in these Al-sites. YAIO3:Ti crystals used in this work were grown from high purity oxides using the Czochralski technique. To carry out the emission experiments, the 266 nm quadrupled and 532 doubled lines of a high power pulsed Nd :YAG laser (Spectra Physics model DCR 2/2A3378) were used to excite in the charge transfer absorption edge and in the crystal field transition band. Fluorescence pulses were signal averaged by an oscilloscope. (Tektronix 2440) and the lifetimes were calculated using least square fits. At the same time continuous fluorescence measurements were made using a conventional spectrofluorimeter, Jovin Ivon model JY3CS. 3. EXPERIMENTAL RESULTS AND DISCUSSION Figure 1 shows the fluorescence (emission and excitation) spectrum of the Ti:YAIO3 crystal. The emission band (dashed line) corresponds to the well known red emission which has been tested to be used for laser applications [8-10]. The excitation spectrum of this band is also included in Fig. 1. Two excitation bands appear. The visible band at 480 nm is associated

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U L T R A V I O L E T E X C I T E D L U M I N E S C E N C E O F Ti:YAIO3

Vol. 80, No. 3

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Fig. 2. Temperature dependence of the emission band under (a) visible excitation and (b) ultraviolet with the T2g --+ Eg vibronic transition inside the d ~ excitation. configuration (in the octahedral field approximation). The UV excitation band at 280 nm is correlated with by fitting the temperature dependence of fluorescence direct population of the previously reported excited decay data to a suitable model. The measurements of the temperature dependence state [l l] followed by a rapid non-radiative relaxation to the Eg level of Ti 3+ ion. The results show that the of the fluorescence lifetime under both, visible and UV UV excitation produces a red emission 3.7 times more excitation, are shown in Fig. 2. For VIS excitation the efficient than the emission produced by VIS excitation. results are similar to those previously published [10, 12] The simplified diagram in the inset of Fig. 1 indicates with a room temperature lifetime of 11.4 #s and a low the mechanism of population of the Eg level of Ti 3+ via temperature lifetime of 17.5#s. However, the fluorUV excitation. In this diagram UV pumping directly escence lifetimes under UV excitation slightly differs populates the X level. Then the excited electrons relax from the VIS excitation. The room temperature non-radiatively very quickly to the Eg level, from lifetime is about 8.5ps and the low temperature which the broad emission takes place. At present, the lifetime is about 17 #s. The temperature dependence of the radiative rates identification of the level X is unknown, although it is clearly one level on the Ti 3+/Ti 4+ pair complex formed data can be fitted by the equation in the sample t1 l]. These centers are also responsible 1 + 1 coth (h2__~T) (1) for the Eg --+ X excited state absorption experimen- 1 ~" "~0 "~d tally observed. These experiments demonstrate that there are at where the first term, l/z0, accounts for magnetic least two types of absorbing centers in Ti:YAlO3; and electric dipole static contributions to the decay Ti3+/Ti 4+ pairs and isolated single Ti 3÷ ions. The rate [13]. This term also includes the temperature latter results only in the T2g -+ Eg absorption and, independent non-radiatively depopulation of the Eg presumably, the luminescence of these isolated centers level, such as excited state absorption of visible differs from that of the pairs. pumping radiation. The second term of the equation Thus, what we have studied is the red emission of indicates that the decay rate is enhanced with increasing Ti +3 ions under UV excitation in the charge transfer temperature due to phonon assisted processes using (C.T.) band (T2g --+ X) of Ti 3+/Ti 4+ centers for com- phonons of average energy ho~. parison with the emission resulting from excitation in The fits shown in Fig. 2(a) and (b) yield z0 = 24 #s, the crystal field band. The differences are also shown Zd = 68/~S and hco = 150cm -~ for visible excitation in Fig. l which illustrates the normalized emission andz0 = 23/~s, za = 68#sandho9 = 105cm-I f o r U V spectra of the Ti 3+ : YA103 sample under excitation in excitation. We can observe a difference in the effective both two excitation bands. There is a difference in the phonon energy in both processes. The phonon energy bandwidth of the emission and a slight shifting in observed for visible excitation is the same which has the peak position of the band. The difference in the been reported in the replica of the Eg --* T2g broad bandwidth is correlated with the energies of the emission [13]. The reduction of the phonon energy phonons which homogeneously broaden both pro- observed for UV excitation qualitatively explains the cesses discussed above, and which may be determined additional broadening of the E~ --, T~. This condition

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ULTRAVIOLET EXCITED LUMINESCENCE OF Ti : YAIO3

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yields an emission band intensity which varies linearly at low excitation power, but which deviates from linearity at high excitation power. This result is a probe that for high excitation power, the Eg ~ X excited state absorption of the Ti3+/Ti 4+ pair takes place, making the red emission less efficient. tx)

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REFERENCES 100.

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Fig. 3. Emission intensity as a function of the excitation power of (a) 532 nm and (b) 266 nm laser lines. In (a) the dashed line indicates the deviation from linearity at high power excitation.

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It should be noted that visible light should excite both, Ti 3+ and Ti 3+/Ti a+ pair centers. Therefore, the results obtained from visible excitation should correspond to the same averaging which should depend on the ratio between pairs and single Ti ions. This is corroborated by the fluorescence decay curves, not displayed here for brevity, in which UV excitation leads to an exponential decay whereas the visible excited fluorescence decay is not exponential. This fact indicates that both centers, Ti 3+ and Ti3+/Ti 4+ pairs, are present in the visible absorption band Eg -~ T2g. Finally, the emission intensity has been measured as a function of the excitation power using the 266 nm and 532nm laser lines. The results are shown in Fig. 3(a) and (b). The red emission band shows linear behaviour when exicted in the UV region (266 nm), in contrast to the results reported for Ti:A1203 [14] and Ti:MgAl204 [15], in which the UV excited Eg~T2g emission occurs via two photon excitation processes. Furthermore, our Ti : YAIO3 crystal does not show the blue emission band associated with isolated Ti4+ centers. Direct excitation in the visible region (532 nm)

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9. 10. 11. 12.

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