Journal of Luminescence 102–103 (2003) 72–76
Energy transfer processes of Nd3+ in Y2O3 ceramic A. Lupeia,*, V. Lupeia, T. Tairab, Y. Satob, A. Ikesuec, C. Gheorghea a
Institute of Atomic Physics, Magurele Str. Atomistilor 111 C.P., Bucharest MG 76900, Romania b Laser Research Center, Institute of Molec. Science, Okazaki 444-8585, Japan c Japan Fine Ceramics Center, 2-4-1 Mutsuno, Nagoya 456-8587, Japan
Abstract The paper presents the results on the energy transfer processes of Nd3+ doped Y2O3 transparent polycrystalline ceramics. The 4F3/2 decays are highly non-exponential at concentrations larger than 0.1 at%. For concentrations up to 3 at% the emission kinetics can be described in terms of a static cross-relaxation (calculated by using the discrete structure and two types of interactions—short range and dipole–dipole) plus a migration term. Only C2 sites are considered in the quenching. The transfer parameters that describe coherently the decays up to 3 at% are determined. The high concentration effects and the concentration dependence of the quantum efficiency are discussed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser materials; Transparent ceramic; Rare earth; Energy transfer processes; Nd3+; Y2O3
1. Introduction The sesquioxides crystals are subjects of study in the last years as laser materials due to their very good thermal properties and the facility of doping with rare earths (RE3+) ions. The high melting point (over 24001C) makes the crystal growth by pulling from melt very difficult. Different attempts to overcome the crystal growth difficulties were recently reported, such as laser heated pedestal technique [1] or Nacken–Kyropulos technique in a RF-heated rhenium crucible [2]. An alternative method to obtain laser quality sesquioxides is the transparent ceramics technique [3]. The Y2O3–yttrium sesquioxide forms at room temperature a cubic C-type structure, belonging to the IA3% space group. The unit cell contains 16 *Corresponding author. Tel.: +457-44-72; fax: +457-42-43. E-mail address:
[email protected]fim.ro (A. Lupei).
formula units with 32 cations that form 24 sites of C2 symmetry oriented parallel to h1 0 0i crystal axes and 8 sites of C3i symmetry, with threefold axes along h1 1 1i: The charge compensation is achieved by oxygen vacancies: the C2 site is an eight-fold cubic structure with two oxygen vacancies on a face diagonal, while C3i correspond to a cube with two vacancies on a body diagonal. The ( and the cationic density lattice constant is 10.6 A (2.687 1022/cm3) is rather high compared to other laser crystals. The RE3+ dopants are assumed to occupy randomly both sites and no correlated clustering has been reported up to now. The Nd3+ optical spectra in Y2O3 single crystals [4] contain mainly the contribution from C2 sites; no lines assigned the transitions in C3i have been reported. Though a satellite structure has been reported in Nd:Y2O3 optical spectra, no analysis or interpretation has been given. The observed concentration quenching of 4F3/2 emission has
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 5 1 2 - 4
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not been analyzed in terms of energy transfer processes. The purpose of this paper is the study of energy transfer processes and their effects on quantum efficiency of 4F3/2 emission in Nd:Y2O3 transparent ceramics, in connection with high-resolution spectral data.
2. Experiment The Y2O3 transparent ceramics doped with Nd3+ from 0.1 to 10 at% were prepared at Japan Fine Ceramics Center, Nagoya, Japan, by a technique described elsewhere [3]. For the optical spectra a high-resolution monochromator (0.3 cm1 resolution) and a photon counting technique and a multichanel analyzer TurboMCS for detection were used. The decays were measured by non-selective excitation with the second harmonic of a YAG:Nd Quanta Ray laser (B10 ns pulse width) at low intensity to avoid high pumping up-conversion effects.
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and satellite lines. Thus in the 4I9/2 (1)-4F3/2 (1), 4 I9/2 (1)-4F5/2, 4I9/2 (1)-2P1/2 transitions (where 1 denotes the first Stark component of the manifold) at least two satellites (M1 and M2) with intensities ratio of 2:1 are observed. This is illustrated in Fig. 1 for two concentrations (1 and 3 at% Nd ) in 4I9/2 (1)-4F5/2 (1) transition. The shifts of the satellite lines from main lines N are transition dependent and less than 10 cm1. The concentration dependence of relative intensities of satellites to main lines or between them suggests that they correspond to Nd3+ ion pairs. The intensity ratios and structural data allow us to assign these lines to two types of pairs: pairs of identical Nd3+ centers at nearest distances, most probably Nd3+ similar C2–C2 pairs (8 sites at ( and pairs of dissimilar distances smaller than 4 A) 3+ Nd centers; i.e. C2–C3i pairs (4 sites at distances ( smaller than 4 A). For higher concentrations additional satellites are observed. The large inhomogeneous line widths of main lines N, at high concentrations, are due to unresolved
3. Absorption measurements The low temperature absorption spectra for Nd3+:Y2O3 ceramic samples were investigated in different optical spectral regions and contain besides the Nd3+ main lines (N), several satellites close to the N lines and vibronic side bands. At concentrations up to B1 at%, the position of the main lines correspond to that previously reported for Nd3+ in Y2O3 single crystals [4], which have been assigned to Nd3+ in C2-sites. Our spectra show some lines that could be assigned to Nd3+ in C3i sites. Thus, in the 4I9/2-4F7/2 transition (that is magnetic dipole allowed too) an additional line at 18744 cm1 is observed. In the same transition an absorption line for Nd3+ in octahedral sites of YAG has been previously reported [5]. The Nd concentration effects on low temperature absorption spectra are: red shifts (up to 5 cm1 over the range 1–10 at% Nd) of main lines N and satellites, a non-linear (almost quadratic) increasing of the relative intensities of satellite lines to main lines and significant broadening of main
Fig. 1. The absorption spectra corresponding to 4I9/2 (1)-4F5/2 (1) transition of Nd3+ in Y2O3 ceramic for 1 and 3 at%.
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satellites corresponding to pairs or larger Nd3+ associations, in connection to high cationic density of Y2O3 and Nd3+–Nd3+interactions.
4. Energy transfer processes The emission decays of 4F3/2 level of Nd3+ in Y2O3 were measured with a resolution of 50 ns. At low concentration (0.1 at%) the emission decay of the C2 sites in the range from 10 to 300 K is almost exponential with a lifetime t0 of B310 ms. Attention has been paid to avoid reabsorption effects and this value is close to that determined for powder samples in refraction index matching conditions [6]. This is likely to be the radiative lifetime, since the non-radiative contribution, for a gap of B4700 cm1 in a system with phonons no larger than 600 cm1, could be negligible. At higher concentrations the decay is strongly non-exponential, suggesting the existence of energy transfer inside the system of Nd3+ ions that quenches the emission. At low pump intensities, the quenching is induced by a two-ion crossrelaxation down-conversion on intermediate levels. The cross-relaxation could be: a resonant process (4F3/2-4I15/2):(4I9/2-4I15/2) or one phonon-assisted processes (4F3/2-4I15/2):(4I9/24 I13/2) and (4F3/2-4I13/2):(4I9/2-4I15/2). Since the transitions in cross-relaxation do not contain magnetic dipole contribution, it is very unlikely that the C3i sites are involved, unless a strong lowering of C3i symmetry takes place. A C2–C3i direct transfer is unlikely from the same reasons. The experimental transfer functions PðtÞ ¼ lnðI=I0 Þ þ t=t0 for several Y2O3 samples with Nd3+ from 1 to 10 at% Nd3+ are given in Fig. 2 in a t1/2 scale. For a 1 at% sample a very small fast drop followed by a quasilinear part that evolves after B10 ms into a t1/2 dependence is observed. This t1/2 dependence indicates the presence of a static (direct) energy transfer by a dipole–dipole interaction between donors (excited Nd3+ C2 ions) and acceptors (non-excited Nd3+ C2 ions). At higher concentrations (up to B3 at%) the decay is more complex and evolves to an exponential behavior at large times as shown in Fig. 3 for the 3 at% sample; suggesting the presence of
Fig. 2. The experimental concentration dependence of energy transfer functions P(t) of 4F3/2 Nd3+ emission in Y2O3 at 300 K, under excitation with 532 nm, in a t1/2 scale.
Fig. 3. Experimental and calculated (grey line) transfer function for 4F3/2 emission at 300 K, for a sample with 3 at% Nd3+ in Y2O3.
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quenching assisted by migration on donors. For larger concentrations the transfer function P(t) becomes again almost linear in t1/2, as evident for the 10 at% sample, suggesting a static energy transfer process. However, for this sample the slope of the t1/2 dependence is B17 times larger than that for 1 at% sample. The data up to 3 at% Nd could be described by the theoretical transfer function for a random distribution of Nd3+ ions [7,8]: X % PðtÞ ¼ ln½1 CA þ CA expðWi tÞ þ Wt; i
ð1Þ where the first term describes the static transfer and the second the migration assisted process. The summation in Eq. (1) is performed only on C2 sites and CA is Nd3+ concentration. The static term for low Nd3+ concentrations can be approximated by simple functions of time at small and large times. For a dipole– dipole interaction, with a transfer rate written as Wi ¼ CDA =R6i ; the transfer function at large times leads to a t1/2 dependence (Forster– Dexter continuous distribution) that allows the estimation of CDA microparameter, in our case CDA=3.7 1039 cm6 s1. At short times, the static contribution in Eq. (1) can be approximated by a linear temporal dependence and its duration t1 ER6i =CDA [7] depends on the rate to the first acceptor site. In our case if one take the nearest C2 positions and estimated CDA, t1 would be B0.5– 1 ms; however the experimental value for 1 at% is an order of magnitude larger. This disagreement is consistent with the existence of a fast drop in emission. Both facts can be interpreted as the effect of a strong short-range interaction (most likely superexchange) acting inside the first nearest ( and four eight C2–C2 pairs (four sites at 3.54 A ( The emission of these pairs end in sites at 4 A). B1–2 ms and the quasilinear part duration is determined by dipole–dipole interaction with the ( i.e. t1B third acceptors sphere [9,10] at 5.3 A, 6 ms, as observed experimentally. The shortrange transfer rates that fit the data are Wi >5 106 s1. The measured migration rate % ¼ depends quadratically on concentration, W 2 W0 CNd ; and the estimated migration parameter
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is W0 B450 s1(at%)2 if CNd is in at%. The fitting for 3 at% Nd3+ sample with these parameters is illustrated in Fig. 3. As mentioned above, Eq. (1) cannot describe the experimental data for 6% or 10% with the same microparameters. The transfer function for 10 at%, with a t1/2 dependence up to long times, suggests additional quenching mechanisms. The very large transfer microparameters (especially CDA) together with the close packing of Nd3+ in Y2O3 induce a strong Nd3+ dependence of emission quantum efficiency (B0.4 for 1 at%) that could increase the laser threshold for continuous wave emission. If one takes as a ‘‘material only’’ figure of merit the product ZCNd for cw laser potential, it shows a maximum around 1 at% Nd, unlike for YAG:Nd where it has a maximum in the range of B3 at% [10]. This restricts the range of Nd3+ concentrations in Y2O3 useful for laser emission.
5. Discussion and conclusions The paper presents spectroscopic and energy transfer results from the investigation of Nd3+:Y2O3 transparent ceramics, as a new variant of a laser material. The positions of main Nd3+ lines (N) in ceramic Nd3+:Y2O3, at low concentrations, are identical to those reported for single crystals. The absence of any other spectral satellites than those corresponding to Nd3+ pairs (M) as well as the relative intensity of M lines, show that no structural defects are present in the crystalline lattice of ceramic grains and that the distribution of Nd3+ ions at the available lattice sites is random. Thus, from spectroscopic and microstructural point of view, the ceramic Nd3+:Y2O3 materials are similar to single crystals. At least two satellite lines were observed in high resolution absorption spectra and assigned to pairs of identical Nd3+ centers at nearest distances, C2– ( and C2 pairs (8 sites at distances smaller than 4 A) dissimilar Nd3+ centers , C2–C3i pairs (4 sites at ( Pairs of Eu3+ in Y2O3 distances smaller than 4 A). observed as satellites near C3i lines have been analyzed previously [11–13] and the existence of a short-range interaction within the pairs, most
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probably superexchange, has been reported for Eu3+ in Y2O3 [12]. The global emission decays of 4F3/2 Nd3+ level at 300 K show strong concentration dependence. The decays for concentrations up to B3 at% present a complex non-exponential behavior, that is interpreted in terms of a cross-relaxation process involving at least two types of interactions: a short range—probably superexchange—connected with interaction within near neighbor pairs and dipole– dipole and a migration term. Though the system contains two types of Nd3+ sites, no energy transfer or cross-relaxation to C3i sites is considered for low concentrations since no magnetic dipole allowed transitions are involved in these processes. The efficient energy transfer for low concentrations (dipole–dipole microparameter CDA B3.7 1039 cm6 s1 is B20 times larger than that for Nd3+ in YAG CDA 40 6 1 B1.8 10 cm s [9,10]) and the additional transfer mechanisms at higher concentrations determines a strong drop of emission quantum efficiency with concentration and restricts the range of Nd3+ concentration in Y2O3 useful for laser emission to the range of B1 at%. At higher concentrations other quenching mechanisms have to be considered. Besides the
quenching by impurities other possible quenching mechanism could be the involvement of C3i sites. Due to Nd3+–Y3+ ionic radii mismatch the local symmetry could be slightly lowered and electric– dipole transitions involved in energy transfer become allowed. The possibility of three-ion cross-relaxation could be also considered.
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