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Emission from the high lying excited states of Ho3+ ions in YAP and YAG crystals
ARTICLE IN PRESS
Journal of Luminescence 106 (2004) 269–279
Emission from the high lying excited states of Ho3+ ions in YAP and YAG crystals M. Malinowskia,*, M. Kaczkana, A. Wnukb, M. Szuflin˜skab a
Institute of Microelectronics and Optoelectronics PW, ul. Koszykowa 75, 00-662 Warsaw, Poland b ! Institute of Electronic Materials Technology, ul. Wolczyn ˜ ska 133, 01-919 Warsaw, Poland
Received 13 September 2002; received in revised form 17 January 2003; accepted 28 October 2003
Abstract The near UV and visible emissions of Ho3+ in YAP and YAG are investigated after selective one photon pumping of the 3D3 and 5S2 manifolds. The fluorescence decays of Ho3+ ions after pulsed excitation are measured as a function of temperature and holmium concentration. The shortening and non-exponentiality of the decays, observed with increasing activator concentrations, are indications of cross-relaxation (CR) among the Ho3+ ions. The CR schemes are proposed for both emitting 3D3 and 5S2 manifolds and the values for the transfer rates are experimentally determined and compared to the Inokuti–Hirayama theory. Mechanisms responsible for the temperature quenching are studied. r 2003 Elsevier B.V. All rights reserved. PACS: 71.55; 78.40; 78.55 Keywords: Upconversion; UV emission; Visible emission; Ho3+
1. Introduction Trivalent holmium (Ho3+)-doped crystals and glass fibers have been the subject of extensive investigation for the development of mid-infrared (IR) lasers for remote sensing and medical applications [1–3]. Also recently, Ho3+: Sr5(PO4)3F (SFAP) crystal has been investigated as a potential saturable absorber for Q-switching of IR lasers [4]. Among various holmium activated *Corresponding author. Tel.: +22-6607783; fax: +226288740. E-mail address: [email protected] (M. Malinowski).
matrix, yttrium aluminum garnet Y3Al5O12 (YAG) and yttrium aluminum perovskite, YAlO3 (YAP), otherwise known as yttrium orthoaluminate, doped by trivalent holmium (Ho3+), are commonly employed as 2 and 3 mm laser materials [5–7]. The rapid growth of applications requiring visible lasers, such as full-color displays, optical data storage and biomedical instruments, have stimulated the development of lasers in the visible spectral range. Experiments are in progress to evaluate optical properties that impact short wavelength potential laser performance of various materials. In particular, upper level lifetimes, branching ratios, concentration quenching rates and emission cross-sections are investigated.
0022-2313/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2003.10.008
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M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
As can be seen from the energy level diagram of trivalent holmium [8], this ion has several high lying metastable levels that can give rise to transitions at various wavelengths in the visible and ultra-violet (UV) region. Crystals of CaF2 [9], LiYF4 [10] and YAlO3 [11] have been demonstrated to lase in the green part of the spectrum between transitions from the excited 5S2 state to the ground 5I8 state. The green emission at 550 nm can also be excited using the frequency conversion processes due to energy transfer or a stepwise absorption of multiple photons [12]. Especially, upconversion processes in Ho3+-doped BaYb2F8 [13], LaF3 [14], CaF2 [15] crystals and optical glasses have been the subject of extensive studies. Experiments are in progress to develop an upconversion holmium laser producing green laser light of a shorter wavelength than the pump light from red or near-IR semiconductor diodes. The first upconversion laser described by Johnson and Guggenheim in 1971 [16] was BaY2F8 crystal doped by Ho3++Yb3+ ions and was pumped by a filtered flash lamp. Allain et al. [17] demonstrated continuous wave (cw) green upconversion lasing in Ho3+-doped fluorozirconate glass fiber pumped by a red line of krypton laser. In 1994 Thrash et al. [18] reported cw upconversion green laser operation in Yb3+-codoped KYF4:Ho3+ crystals as the result of Yb3+-Ho3+ energy transfer. Very recently, the Ho3+:ZBLAN fiber upconversion laser has been examined in detail by Funk and Eden [19] and Funk et al. [20]. While most of these studies have focused on fluoride systems much less is known about the spectroscopy of Ho3+-doped oxides. To our knowledge only in two garnets; YAG [21] and YSGG [22] Ho3+ energy levels have been determined in the wide energy range. Only recently we reported the detailed structure of Ho3+ energy levels in YAP [23]. It is known that rare-earth doped yttrium orthoaluminate YAlO3 (YAP) offers advantages of longer lifetimes and higher, polarized crosssections with respect to most of other oxide matrix. The room temperature optical spectra and optical transition probabilities for Ho3+ in YAP have been reported by Weber et al [24], also Weber et al. [25] demonstrated laser action in the IR region from the Ho3+:YAP. As a result of
extensive studies of 3 mm laser materials, IR level lifetimes and emission cross-sections for Ho3+:YAP have been established in Ref. [5]. Emission properties of the above mentioned systems have been extensively studied in the near IR range but the UV and visible emissions require more detailed investigation. The purpose of this work is to study the short wavelength emission properties of Ho3+ in YAP and YAG crystals. We analyzed the near UV, blue and green emissions of Ho3+ after selective one photon pumping of the 3 D3 and 5S2 manifolds. In order to investigate the cross-relaxation (CR) mechanisms leading to concentration quenching of the observed emissions, especially green 2S2 one, we deal here with different concentrations of activator. These studies are also motivated by our recent observation of room-temperature avalanche upconversion in several Ho3+ activated materials [26,27] leading to strong 5S2 green emission after cw orange excitation. In this process efficient CR of the emitting level is one of the fundamental conditions for avalanche to happen.
2. Experimental methods Ho3+-doped YAG and YAP single crystals used in this work were grown by the Czochralski technique in the ITME laboratory in Warsaw. They were doped with 0.1, 0.3, 1, 2 and 5 at% of Ho3+ with respect to Y3+. After growth crystals were annealed in a reducing atmosphere. Yttrium aluminum perovskite, YAlO3 (abbreviated YAP), otherwise known as yttrium orthoaluminate, has the orthorombic structure with four molecules per unit cell and space group symmetry Pnma. The rare earth ions enter the lattice at yttrium sites having a low; Cs point symmetry. Yttrium aluminum garnet, Y3Al5O12 (abbreviated YAG), has the cubic structure with eight formula unit per unit cell and space group symmetry O10 h . The rare earth ions enter the lattice at dodecahedral yttrium sites of D2 point symmetry. Fluorescence and excitation spectra were obtained using a tunable dye laser, operating with various dyes, pumped by a frequency doubled or tripled Continuum Surelite II Nd:YAG laser (10 ns
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279 3+
3
emission
YAP:0.1% Ho T=300 K
D
5
3 5
intensity [a.u.]
pulse length, 10 Hz repetition rate and 180 mJ energy per pulse at 532 nm). For pulsed excitation in the UV region the fourth harmonic of Nd:YAG laser or output wavelength of a tunable dye laser, 4155 cm1 up-shifted by stimulated Raman scattering (SRS) in a gaseous H2 cell, were used. CW UV excitation was also obtained with a high power filtered xenon lamp. The spectra were recorded using a 1 m monochromator with dispersion of 8 A mm1 and detected by EMI 9789 or RCA C 31034-02 cooled AsGa photomultiplier. Data acquisition was obtained with a Stanford SR 4000 boxcar averager controlled with a PC computer. Fluorescence lifetime measurements were made using a Stanford SR430 multichannel analyzer. Sample cooling was provided by a closed cycle He optical cryostat which allowed the temperature to be varied between 15 and 300 K.
271
F
S2
3
absorption 3
3
G3+ L8 3
5
5
3
G2+ H5
5
G5
3
350
400
G6
5
5
F3+ F2
5
5
F4 + S2
3
+ M10+ D4 + H6
300
5
+ K8
450
500
550
600
λ [nm] Fig. 1. UV and visible emission spectrum, due to transitions from the 3D3, 5F3 and 5S2 excited states, registered after cw excitation at 288.2 nm and room temperature absorption spectrum of YAP:0.1% Ho3+.
3. Results 3.1. Visible emission spectra After cw UV excitation of YAP:Ho3+ by a xenon lamp, various emission lines were observed in a wide spectral range from UV to visible as seen in a low resolution, room temperature spectrum shown in Fig. 1. In Fig. 2 we present the high resolution emission spectrum of the 3D3 level in 0.3% Ho3+:YAP registered after selective laser excitation of the 3D3 Stark component located at 296 nm (33,733 cm1) [23] at 15 K. This fluorescence is dominated by two strong transitions, centered around 360 and 412 nm, originating from the 3D3 to the 5I7 and 5I6 multiplets, respectively. At low temperatures we have also observed weak emissions originating from the excited 5G4 and 5F3 levels. However, as seen from Fig. 1, the main visible emission channel of Ho3+ is in the green part of the spectrum and corresponds to the 5 S2-5I8 transition. It was also observed that as the concentration of the activator changes the relative intensities of the observed lines also change. For higher concentrations the intensity of lines attributed to the 3D3 emission transitions is growing with respect to the intensity of the 5S2 emission suggesting different
magnitude of the concentration quenching and reabsorption processes. For comparison the absorption spectrum of YAP:Ho3+ is shown in the lower part of Fig. 1. 3.2. Selective 5S2 excitation Strong, green (centered at 550 nm) emission of holmium compounds results from the 5S2-5I8 transition. In the upper part of Fig. 3 the fluorescence decay transients for the 5S2-5I8 transition of Ho3+ in YAP and YAG, respectively, are shown as a function of activator concentration at room temperature. In YAP:Ho3+ the pulsed excitation was into 5 I8-5S2 absorption at 18,488 cm1 [23]. The temperature dependence of the 5S2 decay shows a constant shortening as temperature increases. The room temperature decay measured in 5% Ho3+doped YAP sample is nearly exponential with lifetime of 12.5 ms. As can be seen, 5S2 decays are concentration dependent with a weak non-exponentiality observed only in the 5% sample. At low temperatures the fluorescence decays of the 1% and 5% YAP:Ho3+ samples are initially nonexponential with a late part of decays approaching the isolated ion lifetime. As shown in Ref. [23] at
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
272 100
10
YAP:Ho 3+ T=15 K
80
5 60
8
5
I7
I
6
40
4
20
2
0 358
359
360
362 410
361
411
412
413
414
10
5
I
6
0 415 458
459
460
5
461
462
705
710
3.5 3.0
8
5I
5 4
F5
5 S
2.5
2
6 2.0 4
1.5
2
1.0 0.5
0 500
505
510
515
565
570
575
580
680
685
690
695
700
WAVELENGTH [nm]
10
green emission intensity [a.u.]
green emission intensity [a.u.]
Fig. 2. Summary of the short wavelength emission transitions from the 3D3 level of holmium observed in YAP:0.1% Ho3+ at low temperature.
0
YAP:Ho3+ T=300K
0.1% 0.3% 1.0% 2.0% 5.0%
-1
10
-2
10
-3
10
0
50
100
150
200
250
10
0
3+
YAG:Ho T=300K
-1
10
-2
10
0.1% 0.3% 2% 5%
-3
10
0
10
10
0
YAP:Ho3+ -1
10
-2
10
0.1% 0.3% 1.0% 2.0% 5.0%
-3
10
-4
10
0
10
20
t [µs]
20
t [µs]
UV emission intensity [a.u.]
UV emission intensity [a.u.]
t [µs]
30
100 0.1% 0.3% 2% 5%
10-1
YAG:Ho3+
10-2
10-3 0
5
10
15
t [µs]
Fig. 3. Selectively excited 5S2 and 3D3 fluorescence decay curves for different concentrations of Ho3+ in YAP and YAG measured at room temperature.
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
15 K the calculated quenching rate is nearly quadratically dependent on the fractional holmium concentration N. In YAG:Ho3+ we have excited into the highest component of the 5S2 multiplet lying at 18,444 cm1 [21] and registered green 5S2 emission at 557 nm (17,953 cm1) [28]. Fluorescence decays have been measured as a function of holmium concentration and temperature. The temperature dependence of the 5S2 decay shows a peak in the lifetime value at about 30 K. As the temperature is further increased the lifetime decreases from its maximal value of 7.4 to 3.44 ms at 300 K in the 0.1% Ho3+ sample. In 5% sample at room temperature it decreases to 3.2 ms. Generally decays are exponential, but are much faster than in the case of Ho3+:YAP. Results for the temperature dependence of 5S2 fluorescence lifetime are given in the upper part of Fig. 4. In the case of departure from exponential decay an effective lifetimes teff were calculated as teff ¼
80
9
YAP:5% Ho
5
3+
S emission
τ [µs]
YAG:5% Ho
5 4 3
20
2
τ [us]
150
200
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4
250
3
300
D emission 3
3+
YAP:5% Ho
3+
YAG:5% Ho
0
50
100
150
3.3. Selective 3D3 excitation After selective one-photon (OP) pulsed, UV laser excitation we have registered fluorescence decays at about 360 nm which correspond to the 3 D3-5I7 transition of Ho3+ ion. The room temperature decays of the 3D3 state of Ho3+ in YAP for several activator concentrations are shown as a semi-log plot in the lower part of Fig. 3. Only at the highest concentration of 5% the decay is slightly non-exponential, similar behavior has been observed also at 15 K. The low temperature, low concentration lifetime of the 3D3 level in YAP was measured to be 7.9 ms. Crystals of YAG:Ho3+ represent faster and generally nonexponential decays even at room temperature (see Fig. 3). At 10 K and for the lowest Ho3+ doping level the 3D3 lifetime in YAG was measured to be 2.8 ms. Results for the temperature dependence of 3 D3 fluorescence lifetime are given in the lower part of Fig. 4.
200
250
4. Discussion
6
3+
40
100
0 IðtÞ dt=Ið0Þ; where IðtÞ is the fluorescence intensity emitted from the level.
8 7
50
RN
2
60
0
273
300
T [K] Fig. 4. Temperature dependence of the 5S2 and 3D3 fluorescence lifetimes in 5% holmium-doped YAP and YAG crystals.
In the investigated YAP and YAG systems it was observed that, as the concentration of Ho3+ is increased, the fluorescence lifetimes decrease and decays, for both 3D3 and 5S2 fluorescing levels, start being non-exponential. This concentration quenching is attributed to the non-radiative CR between ions. Fig. 5 shows an energy level diagram of the Ho3+ ions illustrating the possible CR processes. By close examination of the Stark energy level structure of Ho3+ in YAG and YAP [21,26], a number of resonant CR pathways may be found for the 5S2 level (see Tables 1 and 2), and also several quasi-resonant (with the energy mismatch of 1–5 cm1) processes for the 3D3 level (see Tables 3 and 4). The energy levels of YAG and YAP have been determined at cryogenic temperatures, so due to the concentration and temperature line broadening and line shift CR transfer with a mismatch of a few per cm could be considered as the resonant case. When the energy
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
274 3+
3+
Ho
Table 2 Resonant, 5S2-5I4=5I8-5I7 cross-relaxation pathways between two Ho3+ ions in YAG:Ho3+
Ho
E [1000xcm -1 ] 3D 3
30
3G 5 5G 6 3 K8 5S 2
20
5I
10
5
0
4
I5
5I
6
5I
7
5
I8
5 S2 E (cm1)
5
I4 E (cm1)
5
I8 E (cm1)
5
18,450 18,458 18,532 18,532 18,540 18,540 18,540 18,546 18,546 18,546 18,546
13,563 13,509 13,288 13,288 13,288 13,349 13,622 13,288 13,288 13,509 13,622
531 506 151 160 51 41 457 138 160 418 531
5418 5455 5395 5404 5303 5232 5375 5395 5418 5455 5455
I7 E (cm1)
Fig. 5. Partial energy level diagram of Ho3+ describing most probable CR pathways.
Table 1 Resonant, 5S2-5I4=5I8-5I7 cross-relaxation pathways between two Ho3+ ions in YAP:Ho3+
Table 3 Nearly resonant, with the energy difference of DE, 3 D3-5G6=5I8-5I5 cross-relaxation pathways between two Ho3+ ions in YAP:Ho3+
5
S2 E (cm1)
5
I4 E (cm1)
5
I8 E (cm1)
5
I7 E (cm1)
3
D3 E (cm1)
5
G6 E (cm1)
5
I8 E (cm1)
5
I5 E (cm1)
DE E (cm1)
18,453 18,463 18,476 18,476 18,476 18,476 18,482 18,482 18,488
13,296 13,474 13,277 13,296 13,350 13,474 13,277 13,277 13,394
66 278 147 147 133 185 111 133 185
5223 5267 5346 5327 5259 5187 5316 5338 5279
33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019
22,005 22,005 22,005 22,005 22,005 22,020 22,020 22,020 22,033 22,084 22,095 22,095 22,124 22,146 22,165 22,165 22,195 22,218 22,218 22,225 22,225 22,273
0 185 210 278 318 210 318 410 278 278 278 410 318 465 410 465 494 410 494 410 465 465
11,207 11,207 11,207 11,298 11,339 11,207 11,313 11,408 11,259 11,207 11,207 11,339 11,207 11,339 11,259 11,319 11,321 11,207 11,298 11,207 11,259 11,207
193 8 17 6 7 2 4 1 5 6 5 5 6 1 5 6 1 4 3 3 0 4
mismatch between optically excited ‘‘donor’’ ion and an unexcited ‘‘acceptor’’ is larger, the energy transfer can occur accompanied by the emission or absorption of phonons. As, for several rare-earth the non-resonant transition assisted with the emission of one phonon could be of comparable intensity to the resonant ones [29,30] the energy level schemes of Ho:YAG and Ho:YAP were also analyzed for this type of processes. For the 5S2 level non-resonant transitions of the type 5S2-5I5=5I8-5I7+DE and 5S2-5I6= 5 I8-5I6+DE were considered and found to be out of resonance by about 2300 and 1400 cm1, respectively, which is much higher then the cut-off
frequency of the phonon spectrum in YAG and YAP. These highly non-resonant processes, necessitating the emission of several optical phonons, are much less probable than the resonant ones.
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
Analysis of the 3D3 level is less definitive than that of 5S2 because, except identified quasiresonant process of the type 3D3-5G6=5I8-5I5 (see Tables 3 and 4), there are also other processes with the emission of low energy phonons, namely 3 D3-3G5=5I8-5I6+DE, 3D3-5F2=5I8-5I5+DE, 3 D3-5F4=5I8-5I4+DE, and in YAP only 3 D3-3K8=5I8-5I5+DE. We compared these processes by using the Dexter formalism [31]. According to Dexter the electric dipole–dipole energy transfer process probability is proportional to the product of the oscillator strengths of the emission fem and absorption fab transitions of participating ions. Oscillator strengths for purely electronic dipole transitions involved in the CR process were obtained from the Judd–Ofelt calculations [32,33] using the available data on holmium activated YAG and YAP crystals [24,28]. In Table 5 possible pathways for the 3D3 level, their
Table 4 Nearly resonant, with the energy difference of DE, 3 D3-5G6=5I8-5I5 cross-relaxation pathways between two Ho3+ ions in YAG:Ho3+ 3 D3 E (cm1)
5
G6 E (cm1)
5
I8 E (cm1)
5 I5 E (cm1)
DE E (cm1)
33,072 33,072 33,072 33,072 33,072 33,072 33,072 33,072
22,014 22,056 22,072 22,265 22,265 22,265 22,284 22,284
418 457 418 498 506 520 520 531
11,477 11,477 11,422 11,301 11,311 11,328 11,311 11,322
1 3 4 4 2 1 3 3
275
energy mismatches and the products of f-values ( 1016) for corresponding donor and acceptor transitions are shown. Analysis of the data presented in Table 5 suggests that indeed, in addition to the resonant process, two other CR pathways accompanied with the emission of low energy phonon could be active. The next step in our investigation was to determine the CR rates and their concentration and temperature dependence. When the fluorescence decay is exponential CR transfer rates could be defined as X ¼ 1=t 1=t0 ; where t is the fluorescence decay time and t0 is the isolated ion lifetime measured in the low concentration sample. For non-exponential decay X could be defined as: Ið0Þ 1 X ¼ RN : t IðtÞ dt 0 0
ð1Þ
The concentration and temperature quenching of the Ho3+ fluorescence in YAP and YAG was directly observed by examining UV (360 nm) and green (550 nm) decays after selective 3D3 and 5S2 manifold excitation. The quenching rates X ; calculated according to Eq. (1) are plotted as a function of holmium concentration in Fig. 6 in double-logarithmic representation. In these calculations t0 lifetimes of levels 3D3 and 5S2 were taken to be 7.6 and 58 ms in YAP and 2.3 and 4.5 ms, respectively, in YAG. At room temperature, and over the range of investigated concentrations, close to linear dependence of X is clearly visible for the 3D3 and 5S2 luminescence. Values of calculated CR rates for the 5S2 level in YAP:Ho3+
Table 5 Possible cross-relaxation processes for the 3D3 level of Ho3+ ion in YAP and YAG Process
YAP
YAG 1
D3-3G5=5I8-5I6+DE D3-5G6=5I8-5I5 3 D3-3K8=5I8-5I5+DE 3 D3-5F2=5I8-5I5+DE 3 D3-5F4=5I8-5I4+DE 3 3
16
DE (YAP) (cm )
fab fem (10
81pDEp489 Resonant 87pDEp304 430pDEp450 500pDEp1310
8870 77 2597 6 23
)
DE (YAG) (cm1)
fab fem (1016)
21pDEp453 Resonant 6pDEp457 375pDEp689 706pDEp1198
3049 94 3079 7 18
DE is the energy mismatch for these transitions and the product of corresponding oscillator strength values is fab fem :
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
276 10
7
-1
X [s ]
3+
10
6
10
5
10
4
10
3
0.1
YAG:Ho 3+ YAP:Ho T=300 K
S=1
1
}
3
}
5
D3 S2
10
N Ho [at.%] Fig. 6. CR rate for the 5S2 and 3D3 states at room temperature versus holmium ions concentration.
are in a reasonable agreement with the data presented by Osiac [34]. Experimental data were used to evaluate the values of the nearest neighbor trapping rates X01 using the relation: X R01 6 X01 ¼ X =CA ni ; ð2Þ R0i i¼1 where ni is the coordination number of ion pair i and CA is a fractional concentration of acceptors. Taking into account the crystallographic structure of YAP and YAG the lattice sum calculations ( radius sphere of were performed in the 12.5 A influence which includes 168 and 120 ion sites in YAP and YAG, respectively. Calculated X01 values are comparable in magnitude to the low concentration decay rate t1 0 which, together with the generally exponential character of decays, except for the highest concentration of 5% Ho3+, explain the linear concentration dependence of CR. According to the generalized Inokuti and Hirayama [35] model developed in Ref. [36], 1 1 for times shorter than X01 that is when t0 pX01 and in the absence of donor–donor transfer, this model guarantees an exponential decay rate, which is linearly dependent on N: Similar, linear dependence of the green emission CR quenching rate on the concentration has been
observed in LiYF4:Ho3+ by Gomes et al. [37]. The authors, assuming a random distribution of ions in the matrix and the electric-multipole character of interaction, defined the function describing the average value of the CR rate and, by the numerical calculation, showed its linear dependence with the activator fractional concentration up to 12%. For the 3D3 emission, purely exponential decays are no longer observed suggesting stronger donor– acceptor interaction. Indeed, as seen from Fig. 6, CR rate values are higher than for the 5S2 emission but also follow linear dependence on the concentration. This is an indication that the weak quenching approximation is still valid, and is consistent with the model presented in Ref. [36]. The next step in our investigation was to determine the strength and character of the donor–acceptor interaction in Ho3+:YAP and YAG. Assuming, that the donor–donor transfer is negligible at low temperatures, the time dependence of the 3D3 emission could be described by the Inokuti and Hirayama [35] equation of the form: ln I=I0 þ t=t0 ¼
4pnA R301 ðX01 tÞ3=s Gð1 3=sÞ; ð3Þ 3
where t0 is the lifetime of the 3D3 state at low concentration, nA is the number of acceptors per unit volume, and R01 and X01 are the nearest neighbor separation and donor–acceptor interaction rate, respectively, s identifies the nature of interaction and G is the gamma function. From Fig. 7 it is seen that, for the appropriate choice of s; a plot of ln I=I0 þ t=t0 against t3/s gives a straight line. In the case of Ho3+ activated YAP and YAG the plots can be best fitted for s ¼ 6; indicating dipole–dipole nature of interaction. These fits yield the nearest-neighbor donor–acceptor transfer rates X01 to be 5.1 105 and 4.8 106 s1 for 5% Ho3+-doped YAP and YAG samples, respectively. The fluorescence lifetimes t were also observed to be temperature dependent between 10 and 300 K. Low temperature maximum in the 5S2 lifetime of Ho3+:YAG has been also observed in several Ho3+ activated solids like ZBLAN glass [38], LiNbO3 [39] and KGd(WO4)2 [40] crystals. This behavior is related to the thermalization of
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
3+
3
YAG:5% Ho
cross rel. rate X [1/s]
Ln(I/I0)+t/t0
3
-1.0
10
3+
D emission 3
-1.5
3+
-1
YAG: 5% Ho
em. D3 at 27415 cm exp. -1 E=495+2 cm
∇
YAP:5% Ho
-0.5
6
-1
5
5
10
em. S2 at 17953 cm exp -1 E=410+4 cm
∇
0.0
277
-2.0
-2.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.00
4.0
0.02
0.04
0.08
0.10
1/T [K ]
t
ð4Þ where DE is the separation between the 5S2 and 5F4 states, which is 36 and 19 cm1, respectively in YAG and in YAP, gi is the degeneracy of the multiplet; g1 ¼ 9 and g2 ¼ 5: As shown in Figs. 8 and 9 the 5S2 and 3D3 CR rates in 5% Ho3+-doped crystals are rapidly increasing with temperature which is an indication that the non-resonat CR processes dominate. For the non-resonant energy transfer its temperature dependence could result either from its multiphonon character or from the thermal activation of the participating levels. According to Miyakawa and Dexter [42] the temperature dependence of the energy transfer rate is expressed by ð5Þ
where N is the number of phonons participating in the energy transfer process, expressed as N ¼ DE=_o; where DE is the difference between the energies of the donor and acceptor levels.
6 3
cross rel. rate X [1/s]
5 1 5 1 g1 t1 0 ð F4 Þ exp ðDE=kB TÞ þ g2 t0 ð S2 Þ ¼ ; t0 ðTÞ g1 exp ðDE=kB TÞ þ g2
10
-1
em. D3 at 27565 cm exp. -1 E=280+5 cm
YAP: 5% Ho
3+
10
5
10
4
5
-1
em. S2 at 18550 cm exp -1 E=150+18+2 cm ∇
the 5F4 state lying only several tens of wave numbers above 5S2. Thus, in the expression for the radiative decay in Eq. (1) thermalization between the 5S2 and 5F4 states must be considered [41]
Fig. 8. Plot of CR rate between Ho3+ ions in YAG as a function of 1/T for 10oTo300 K. The squares are the experimental data and the straight lines indicate thermally activated processes calculated according to Eq. (7).
∇
Fig. 7. Plot of ln ðI=I0 Þ þ t=t0 against t1/2 for 5% Ho3+-doped YAP and YAG, the straight line fit indicates a near-neighbor transfer rates X01 of 5.1 105 and 4.8 106 s1, respectively.
X ðTÞ ¼ X ð0Þð1 exp ð_o=kTÞÞN ;
0.06 -1
1/2
3
10 0.00
0.02
0.04
0.06
0.08
0.10
-1
1/T [K ] Fig. 9. Plot of CR rate between Ho3+ ions in YAP as a function of 1/T for 10oTo300 K. The squares are the experimental data and the straight lines indicate thermally activated processes calculated according to Eq. (7).
As discussed here CR pathways are only slightly non-resonant, with the maximal energy mismatch of the order of few hundreds wave numbers, that is well below the cutoff energy of the phonon spectrum, which is about 700 and 780 cm1 in YAP and YAG, respectively. In this case the temperature dependence of CR is rather due to the
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278
changing of the thermal populations of the crystalfield levels of donor and acceptor transitions. The temperature dependence of the energy transfer rate can thus be described by a Boltzmann-type expression of the form: X ðTÞ ¼ X ð0Þexp ðDE=kT Þ;
ð6Þ
where DE is an activation energy. From the data presented in Tables 1–4 it could be seen that for both 5S2 and 3D3 levels resonant CR transitions initiate not from the lowest but from high-lying Stark sublevels of the ground 5I8 multiplet which are coupled by the Boltzmann statistics. In this case, the transfer rate X is defined by Eq. (6). However, it is also seen that the plots of CR rates as a function of 1/T could not be approximated by a single straight line, indicating presence of at least two different, thermally activated processes. We fitted the experimental results by taking the sum of two thermally activated processes, only in the case of 5S2 CR rate in YAP we needed to consider three processes, using the relation: X X¼ X0i exp ðDEi =kTÞ: ð7Þ i
From Figs. 8 and 9 it is seen that the slopes of the plots for higher temperatures yield activation energies of a few hundreds of wave numbers. This, together with the results of our earlier analysis and an examination of the energy values listed in Tables 1–4 shows that CR processes of the type: 5 S2(18476)-5I4(13277)=5I8(147)-5I7(5345) and 5 S2(18546)-5I4(13509)=5I8(418)-5I7(5455) could be considered as responsible for the quenching of the 5S2 fluorescence in YAP and YAG, respectively. For the 3D3 holmium emission in YAG the temperature dependence of the quenching rates together with the data in Table 5 indicates the process of the type 3D3(33072)-5G6(22265)= 5 I8(498)-5I6(11301) with the mismatch of 4 cm1. In the case of YAP:Ho3+ the indication of the single responsible process is more difficult because the first three processes listed in Table 5 could be active. However, the 3D3 CR rate temperature dependence in YAP shown in Fig. 9 could be understand in terms of a thermal activation of the 278 cm1 Stark level in the 5I8 multiplet [21].
Finally, the energies DE of a few wave numbers, obtained from the fitting in Figs 8 and 9, are comparable to the values of the splitting of the lowest 5I8 holmium Stark energy levels which are 4 and 5 cm1 in YAG and YAP, respectively [21,23], and result from the thermal activation of these levels in the low temperature limit.
5. Conclusions The short wavelength emission properties of Ho3+ ion in YAP and YAG crystals were studied and analyzed. The temporal behavior of the 5S2 and 3D3 fluorescence after pulsed excitation was analyzed as a function of temperature and holmium ions concentration. The most efficient cross-relaxation (CR) schemes were proposed for both emitting 3D3 and 5S2 manifolds and the values for the transfer rates were experimentally determined and compared to the results of Inokuti–Hirayama theory. The CR rates were found to be temperature dependent, which results from the thermal activation of the Stark energy levels in the 5I8 manifold.
Acknowledgements This work was supported by the Polish State Committee of Science (KBN)—Project no. 8 T11B 08319.
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ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279 [8] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Energy level structure and transition probabilities of the trivalent lanthanides in LaF3, Argonne National Laboratory, Argonne, IL, 1975. [9] Yu.K. Voronko, A.A. Kaminskii, V.V. Osiko, A.M. Prokhorov, JETP Lett. 1 (1965) 3. [10] I.G. Podkolzina, A.M. Tkaczuk, V.A. Fedorov, P.P. Feofilov, Opt. Spektrosk. 40 (1976) 196. [11] A.A. Kaminskii, V.M. Garmosh, G.A. Ermakov, V.A. Akkerman, A.A. Filmonov, K. Kurbarov, Izv. Akad. Nauk SSSR Ser. Neorg. Mater. 22 (1986) 1576. [12] F. Auzel, C. R. Acad. Sci. Paris 262 (1966) 1016. [13] G.D. Gilliand, R.C. Powell, L. Esterowitz, Phys. Rev. B 38 (1988) 9958. [14] B.R. Reddy, S. Nash-Stevenso, P. Vankateswarlu, J. Opt. Soc. Am. B11 (1994) 923. [15] S.R. Bullock, B.R. Reddy, P. Vankateswarlu, S.K. Nash-Stevenson, J. Opt. Soc. Am. B14 (1997) 553. [16] L.F. Johnson, H. Guggenheim, Appl. Phys. Lett. 19 (1971) 44. [17] J.Y. Allain, M. Monerie, H. Poignant, Electron. Lett. 26 (1990) 261. [18] R.J. Thrash, R.H. Jarman, B.H.T. Chai, A. Pham, Topical Meeting on Compact Blue-Green Lasers, CFA5-1/73, Salt Lake City, February 10–14, 1994. [19] D.S. Funk, J.G. Eden, IEEE J. Sel. Top. Quantum Electron. 1 (1995) 784. [20] D.S. Funk, S.B. Stevens, S.S. Wu, J.G. Eden, IEEE J. Quantum Electron. 32 (1996) 638. [21] J.B. Gruber, M.J. Hills, M.D. Seltzer, S.B. Stevens, C.A. Morrison, G.A. Turner, M.R. Kokta, J. Appl. Phys. 69 (1991) 8183. [22] J.B. Gruber, M.J. Hills, M.D. Seltzer, S.B. Stevens, C.A. Morrison, J. Appl. Phys. 72 (1992) 5253. [23] M. Malinowski, R. Piramidowicz, Z. Frukacz, G. Chadeyron, R. Mahiou, M.F. Joubert, Opt. Mater. 12 (1999) 409.
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Journal of Luminescence 106 (2004) 269–279
Emission from the high lying excited states of Ho3+ ions in YAP and YAG crystals M. Malinowskia,*, M. Kaczkana, A. Wnukb, M. Szuflin˜skab a
Institute of Microelectronics and Optoelectronics PW, ul. Koszykowa 75, 00-662 Warsaw, Poland b ! Institute of Electronic Materials Technology, ul. Wolczyn ˜ ska 133, 01-919 Warsaw, Poland
Received 13 September 2002; received in revised form 17 January 2003; accepted 28 October 2003
Abstract The near UV and visible emissions of Ho3+ in YAP and YAG are investigated after selective one photon pumping of the 3D3 and 5S2 manifolds. The fluorescence decays of Ho3+ ions after pulsed excitation are measured as a function of temperature and holmium concentration. The shortening and non-exponentiality of the decays, observed with increasing activator concentrations, are indications of cross-relaxation (CR) among the Ho3+ ions. The CR schemes are proposed for both emitting 3D3 and 5S2 manifolds and the values for the transfer rates are experimentally determined and compared to the Inokuti–Hirayama theory. Mechanisms responsible for the temperature quenching are studied. r 2003 Elsevier B.V. All rights reserved. PACS: 71.55; 78.40; 78.55 Keywords: Upconversion; UV emission; Visible emission; Ho3+
1. Introduction Trivalent holmium (Ho3+)-doped crystals and glass fibers have been the subject of extensive investigation for the development of mid-infrared (IR) lasers for remote sensing and medical applications [1–3]. Also recently, Ho3+: Sr5(PO4)3F (SFAP) crystal has been investigated as a potential saturable absorber for Q-switching of IR lasers [4]. Among various holmium activated *Corresponding author. Tel.: +22-6607783; fax: +226288740. E-mail address: [email protected] (M. Malinowski).
matrix, yttrium aluminum garnet Y3Al5O12 (YAG) and yttrium aluminum perovskite, YAlO3 (YAP), otherwise known as yttrium orthoaluminate, doped by trivalent holmium (Ho3+), are commonly employed as 2 and 3 mm laser materials [5–7]. The rapid growth of applications requiring visible lasers, such as full-color displays, optical data storage and biomedical instruments, have stimulated the development of lasers in the visible spectral range. Experiments are in progress to evaluate optical properties that impact short wavelength potential laser performance of various materials. In particular, upper level lifetimes, branching ratios, concentration quenching rates and emission cross-sections are investigated.
0022-2313/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2003.10.008
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As can be seen from the energy level diagram of trivalent holmium [8], this ion has several high lying metastable levels that can give rise to transitions at various wavelengths in the visible and ultra-violet (UV) region. Crystals of CaF2 [9], LiYF4 [10] and YAlO3 [11] have been demonstrated to lase in the green part of the spectrum between transitions from the excited 5S2 state to the ground 5I8 state. The green emission at 550 nm can also be excited using the frequency conversion processes due to energy transfer or a stepwise absorption of multiple photons [12]. Especially, upconversion processes in Ho3+-doped BaYb2F8 [13], LaF3 [14], CaF2 [15] crystals and optical glasses have been the subject of extensive studies. Experiments are in progress to develop an upconversion holmium laser producing green laser light of a shorter wavelength than the pump light from red or near-IR semiconductor diodes. The first upconversion laser described by Johnson and Guggenheim in 1971 [16] was BaY2F8 crystal doped by Ho3++Yb3+ ions and was pumped by a filtered flash lamp. Allain et al. [17] demonstrated continuous wave (cw) green upconversion lasing in Ho3+-doped fluorozirconate glass fiber pumped by a red line of krypton laser. In 1994 Thrash et al. [18] reported cw upconversion green laser operation in Yb3+-codoped KYF4:Ho3+ crystals as the result of Yb3+-Ho3+ energy transfer. Very recently, the Ho3+:ZBLAN fiber upconversion laser has been examined in detail by Funk and Eden [19] and Funk et al. [20]. While most of these studies have focused on fluoride systems much less is known about the spectroscopy of Ho3+-doped oxides. To our knowledge only in two garnets; YAG [21] and YSGG [22] Ho3+ energy levels have been determined in the wide energy range. Only recently we reported the detailed structure of Ho3+ energy levels in YAP [23]. It is known that rare-earth doped yttrium orthoaluminate YAlO3 (YAP) offers advantages of longer lifetimes and higher, polarized crosssections with respect to most of other oxide matrix. The room temperature optical spectra and optical transition probabilities for Ho3+ in YAP have been reported by Weber et al [24], also Weber et al. [25] demonstrated laser action in the IR region from the Ho3+:YAP. As a result of
extensive studies of 3 mm laser materials, IR level lifetimes and emission cross-sections for Ho3+:YAP have been established in Ref. [5]. Emission properties of the above mentioned systems have been extensively studied in the near IR range but the UV and visible emissions require more detailed investigation. The purpose of this work is to study the short wavelength emission properties of Ho3+ in YAP and YAG crystals. We analyzed the near UV, blue and green emissions of Ho3+ after selective one photon pumping of the 3 D3 and 5S2 manifolds. In order to investigate the cross-relaxation (CR) mechanisms leading to concentration quenching of the observed emissions, especially green 2S2 one, we deal here with different concentrations of activator. These studies are also motivated by our recent observation of room-temperature avalanche upconversion in several Ho3+ activated materials [26,27] leading to strong 5S2 green emission after cw orange excitation. In this process efficient CR of the emitting level is one of the fundamental conditions for avalanche to happen.
2. Experimental methods Ho3+-doped YAG and YAP single crystals used in this work were grown by the Czochralski technique in the ITME laboratory in Warsaw. They were doped with 0.1, 0.3, 1, 2 and 5 at% of Ho3+ with respect to Y3+. After growth crystals were annealed in a reducing atmosphere. Yttrium aluminum perovskite, YAlO3 (abbreviated YAP), otherwise known as yttrium orthoaluminate, has the orthorombic structure with four molecules per unit cell and space group symmetry Pnma. The rare earth ions enter the lattice at yttrium sites having a low; Cs point symmetry. Yttrium aluminum garnet, Y3Al5O12 (abbreviated YAG), has the cubic structure with eight formula unit per unit cell and space group symmetry O10 h . The rare earth ions enter the lattice at dodecahedral yttrium sites of D2 point symmetry. Fluorescence and excitation spectra were obtained using a tunable dye laser, operating with various dyes, pumped by a frequency doubled or tripled Continuum Surelite II Nd:YAG laser (10 ns
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279 3+
3
emission
YAP:0.1% Ho T=300 K
D
5
3 5
intensity [a.u.]
pulse length, 10 Hz repetition rate and 180 mJ energy per pulse at 532 nm). For pulsed excitation in the UV region the fourth harmonic of Nd:YAG laser or output wavelength of a tunable dye laser, 4155 cm1 up-shifted by stimulated Raman scattering (SRS) in a gaseous H2 cell, were used. CW UV excitation was also obtained with a high power filtered xenon lamp. The spectra were recorded using a 1 m monochromator with dispersion of 8 A mm1 and detected by EMI 9789 or RCA C 31034-02 cooled AsGa photomultiplier. Data acquisition was obtained with a Stanford SR 4000 boxcar averager controlled with a PC computer. Fluorescence lifetime measurements were made using a Stanford SR430 multichannel analyzer. Sample cooling was provided by a closed cycle He optical cryostat which allowed the temperature to be varied between 15 and 300 K.
271
F
S2
3
absorption 3
3
G3+ L8 3
5
5
3
G2+ H5
5
G5
3
350
400
G6
5
5
F3+ F2
5
5
F4 + S2
3
+ M10+ D4 + H6
300
5
+ K8
450
500
550
600
λ [nm] Fig. 1. UV and visible emission spectrum, due to transitions from the 3D3, 5F3 and 5S2 excited states, registered after cw excitation at 288.2 nm and room temperature absorption spectrum of YAP:0.1% Ho3+.
3. Results 3.1. Visible emission spectra After cw UV excitation of YAP:Ho3+ by a xenon lamp, various emission lines were observed in a wide spectral range from UV to visible as seen in a low resolution, room temperature spectrum shown in Fig. 1. In Fig. 2 we present the high resolution emission spectrum of the 3D3 level in 0.3% Ho3+:YAP registered after selective laser excitation of the 3D3 Stark component located at 296 nm (33,733 cm1) [23] at 15 K. This fluorescence is dominated by two strong transitions, centered around 360 and 412 nm, originating from the 3D3 to the 5I7 and 5I6 multiplets, respectively. At low temperatures we have also observed weak emissions originating from the excited 5G4 and 5F3 levels. However, as seen from Fig. 1, the main visible emission channel of Ho3+ is in the green part of the spectrum and corresponds to the 5 S2-5I8 transition. It was also observed that as the concentration of the activator changes the relative intensities of the observed lines also change. For higher concentrations the intensity of lines attributed to the 3D3 emission transitions is growing with respect to the intensity of the 5S2 emission suggesting different
magnitude of the concentration quenching and reabsorption processes. For comparison the absorption spectrum of YAP:Ho3+ is shown in the lower part of Fig. 1. 3.2. Selective 5S2 excitation Strong, green (centered at 550 nm) emission of holmium compounds results from the 5S2-5I8 transition. In the upper part of Fig. 3 the fluorescence decay transients for the 5S2-5I8 transition of Ho3+ in YAP and YAG, respectively, are shown as a function of activator concentration at room temperature. In YAP:Ho3+ the pulsed excitation was into 5 I8-5S2 absorption at 18,488 cm1 [23]. The temperature dependence of the 5S2 decay shows a constant shortening as temperature increases. The room temperature decay measured in 5% Ho3+doped YAP sample is nearly exponential with lifetime of 12.5 ms. As can be seen, 5S2 decays are concentration dependent with a weak non-exponentiality observed only in the 5% sample. At low temperatures the fluorescence decays of the 1% and 5% YAP:Ho3+ samples are initially nonexponential with a late part of decays approaching the isolated ion lifetime. As shown in Ref. [23] at
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
272 100
10
YAP:Ho 3+ T=15 K
80
5 60
8
5
I7
I
6
40
4
20
2
0 358
359
360
362 410
361
411
412
413
414
10
5
I
6
0 415 458
459
460
5
461
462
705
710
3.5 3.0
8
5I
5 4
F5
5 S
2.5
2
6 2.0 4
1.5
2
1.0 0.5
0 500
505
510
515
565
570
575
580
680
685
690
695
700
WAVELENGTH [nm]
10
green emission intensity [a.u.]
green emission intensity [a.u.]
Fig. 2. Summary of the short wavelength emission transitions from the 3D3 level of holmium observed in YAP:0.1% Ho3+ at low temperature.
0
YAP:Ho3+ T=300K
0.1% 0.3% 1.0% 2.0% 5.0%
-1
10
-2
10
-3
10
0
50
100
150
200
250
10
0
3+
YAG:Ho T=300K
-1
10
-2
10
0.1% 0.3% 2% 5%
-3
10
0
10
10
0
YAP:Ho3+ -1
10
-2
10
0.1% 0.3% 1.0% 2.0% 5.0%
-3
10
-4
10
0
10
20
t [µs]
20
t [µs]
UV emission intensity [a.u.]
UV emission intensity [a.u.]
t [µs]
30
100 0.1% 0.3% 2% 5%
10-1
YAG:Ho3+
10-2
10-3 0
5
10
15
t [µs]
Fig. 3. Selectively excited 5S2 and 3D3 fluorescence decay curves for different concentrations of Ho3+ in YAP and YAG measured at room temperature.
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
15 K the calculated quenching rate is nearly quadratically dependent on the fractional holmium concentration N. In YAG:Ho3+ we have excited into the highest component of the 5S2 multiplet lying at 18,444 cm1 [21] and registered green 5S2 emission at 557 nm (17,953 cm1) [28]. Fluorescence decays have been measured as a function of holmium concentration and temperature. The temperature dependence of the 5S2 decay shows a peak in the lifetime value at about 30 K. As the temperature is further increased the lifetime decreases from its maximal value of 7.4 to 3.44 ms at 300 K in the 0.1% Ho3+ sample. In 5% sample at room temperature it decreases to 3.2 ms. Generally decays are exponential, but are much faster than in the case of Ho3+:YAP. Results for the temperature dependence of 5S2 fluorescence lifetime are given in the upper part of Fig. 4. In the case of departure from exponential decay an effective lifetimes teff were calculated as teff ¼
80
9
YAP:5% Ho
5
3+
S emission
τ [µs]
YAG:5% Ho
5 4 3
20
2
τ [us]
150
200
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4
250
3
300
D emission 3
3+
YAP:5% Ho
3+
YAG:5% Ho
0
50
100
150
3.3. Selective 3D3 excitation After selective one-photon (OP) pulsed, UV laser excitation we have registered fluorescence decays at about 360 nm which correspond to the 3 D3-5I7 transition of Ho3+ ion. The room temperature decays of the 3D3 state of Ho3+ in YAP for several activator concentrations are shown as a semi-log plot in the lower part of Fig. 3. Only at the highest concentration of 5% the decay is slightly non-exponential, similar behavior has been observed also at 15 K. The low temperature, low concentration lifetime of the 3D3 level in YAP was measured to be 7.9 ms. Crystals of YAG:Ho3+ represent faster and generally nonexponential decays even at room temperature (see Fig. 3). At 10 K and for the lowest Ho3+ doping level the 3D3 lifetime in YAG was measured to be 2.8 ms. Results for the temperature dependence of 3 D3 fluorescence lifetime are given in the lower part of Fig. 4.
200
250
4. Discussion
6
3+
40
100
0 IðtÞ dt=Ið0Þ; where IðtÞ is the fluorescence intensity emitted from the level.
8 7
50
RN
2
60
0
273
300
T [K] Fig. 4. Temperature dependence of the 5S2 and 3D3 fluorescence lifetimes in 5% holmium-doped YAP and YAG crystals.
In the investigated YAP and YAG systems it was observed that, as the concentration of Ho3+ is increased, the fluorescence lifetimes decrease and decays, for both 3D3 and 5S2 fluorescing levels, start being non-exponential. This concentration quenching is attributed to the non-radiative CR between ions. Fig. 5 shows an energy level diagram of the Ho3+ ions illustrating the possible CR processes. By close examination of the Stark energy level structure of Ho3+ in YAG and YAP [21,26], a number of resonant CR pathways may be found for the 5S2 level (see Tables 1 and 2), and also several quasi-resonant (with the energy mismatch of 1–5 cm1) processes for the 3D3 level (see Tables 3 and 4). The energy levels of YAG and YAP have been determined at cryogenic temperatures, so due to the concentration and temperature line broadening and line shift CR transfer with a mismatch of a few per cm could be considered as the resonant case. When the energy
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274 3+
3+
Ho
Table 2 Resonant, 5S2-5I4=5I8-5I7 cross-relaxation pathways between two Ho3+ ions in YAG:Ho3+
Ho
E [1000xcm -1 ] 3D 3
30
3G 5 5G 6 3 K8 5S 2
20
5I
10
5
0
4
I5
5I
6
5I
7
5
I8
5 S2 E (cm1)
5
I4 E (cm1)
5
I8 E (cm1)
5
18,450 18,458 18,532 18,532 18,540 18,540 18,540 18,546 18,546 18,546 18,546
13,563 13,509 13,288 13,288 13,288 13,349 13,622 13,288 13,288 13,509 13,622
531 506 151 160 51 41 457 138 160 418 531
5418 5455 5395 5404 5303 5232 5375 5395 5418 5455 5455
I7 E (cm1)
Fig. 5. Partial energy level diagram of Ho3+ describing most probable CR pathways.
Table 1 Resonant, 5S2-5I4=5I8-5I7 cross-relaxation pathways between two Ho3+ ions in YAP:Ho3+
Table 3 Nearly resonant, with the energy difference of DE, 3 D3-5G6=5I8-5I5 cross-relaxation pathways between two Ho3+ ions in YAP:Ho3+
5
S2 E (cm1)
5
I4 E (cm1)
5
I8 E (cm1)
5
I7 E (cm1)
3
D3 E (cm1)
5
G6 E (cm1)
5
I8 E (cm1)
5
I5 E (cm1)
DE E (cm1)
18,453 18,463 18,476 18,476 18,476 18,476 18,482 18,482 18,488
13,296 13,474 13,277 13,296 13,350 13,474 13,277 13,277 13,394
66 278 147 147 133 185 111 133 185
5223 5267 5346 5327 5259 5187 5316 5338 5279
33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019 33,019
22,005 22,005 22,005 22,005 22,005 22,020 22,020 22,020 22,033 22,084 22,095 22,095 22,124 22,146 22,165 22,165 22,195 22,218 22,218 22,225 22,225 22,273
0 185 210 278 318 210 318 410 278 278 278 410 318 465 410 465 494 410 494 410 465 465
11,207 11,207 11,207 11,298 11,339 11,207 11,313 11,408 11,259 11,207 11,207 11,339 11,207 11,339 11,259 11,319 11,321 11,207 11,298 11,207 11,259 11,207
193 8 17 6 7 2 4 1 5 6 5 5 6 1 5 6 1 4 3 3 0 4
mismatch between optically excited ‘‘donor’’ ion and an unexcited ‘‘acceptor’’ is larger, the energy transfer can occur accompanied by the emission or absorption of phonons. As, for several rare-earth the non-resonant transition assisted with the emission of one phonon could be of comparable intensity to the resonant ones [29,30] the energy level schemes of Ho:YAG and Ho:YAP were also analyzed for this type of processes. For the 5S2 level non-resonant transitions of the type 5S2-5I5=5I8-5I7+DE and 5S2-5I6= 5 I8-5I6+DE were considered and found to be out of resonance by about 2300 and 1400 cm1, respectively, which is much higher then the cut-off
frequency of the phonon spectrum in YAG and YAP. These highly non-resonant processes, necessitating the emission of several optical phonons, are much less probable than the resonant ones.
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Analysis of the 3D3 level is less definitive than that of 5S2 because, except identified quasiresonant process of the type 3D3-5G6=5I8-5I5 (see Tables 3 and 4), there are also other processes with the emission of low energy phonons, namely 3 D3-3G5=5I8-5I6+DE, 3D3-5F2=5I8-5I5+DE, 3 D3-5F4=5I8-5I4+DE, and in YAP only 3 D3-3K8=5I8-5I5+DE. We compared these processes by using the Dexter formalism [31]. According to Dexter the electric dipole–dipole energy transfer process probability is proportional to the product of the oscillator strengths of the emission fem and absorption fab transitions of participating ions. Oscillator strengths for purely electronic dipole transitions involved in the CR process were obtained from the Judd–Ofelt calculations [32,33] using the available data on holmium activated YAG and YAP crystals [24,28]. In Table 5 possible pathways for the 3D3 level, their
Table 4 Nearly resonant, with the energy difference of DE, 3 D3-5G6=5I8-5I5 cross-relaxation pathways between two Ho3+ ions in YAG:Ho3+ 3 D3 E (cm1)
5
G6 E (cm1)
5
I8 E (cm1)
5 I5 E (cm1)
DE E (cm1)
33,072 33,072 33,072 33,072 33,072 33,072 33,072 33,072
22,014 22,056 22,072 22,265 22,265 22,265 22,284 22,284
418 457 418 498 506 520 520 531
11,477 11,477 11,422 11,301 11,311 11,328 11,311 11,322
1 3 4 4 2 1 3 3
275
energy mismatches and the products of f-values ( 1016) for corresponding donor and acceptor transitions are shown. Analysis of the data presented in Table 5 suggests that indeed, in addition to the resonant process, two other CR pathways accompanied with the emission of low energy phonon could be active. The next step in our investigation was to determine the CR rates and their concentration and temperature dependence. When the fluorescence decay is exponential CR transfer rates could be defined as X ¼ 1=t 1=t0 ; where t is the fluorescence decay time and t0 is the isolated ion lifetime measured in the low concentration sample. For non-exponential decay X could be defined as: Ið0Þ 1 X ¼ RN : t IðtÞ dt 0 0
ð1Þ
The concentration and temperature quenching of the Ho3+ fluorescence in YAP and YAG was directly observed by examining UV (360 nm) and green (550 nm) decays after selective 3D3 and 5S2 manifold excitation. The quenching rates X ; calculated according to Eq. (1) are plotted as a function of holmium concentration in Fig. 6 in double-logarithmic representation. In these calculations t0 lifetimes of levels 3D3 and 5S2 were taken to be 7.6 and 58 ms in YAP and 2.3 and 4.5 ms, respectively, in YAG. At room temperature, and over the range of investigated concentrations, close to linear dependence of X is clearly visible for the 3D3 and 5S2 luminescence. Values of calculated CR rates for the 5S2 level in YAP:Ho3+
Table 5 Possible cross-relaxation processes for the 3D3 level of Ho3+ ion in YAP and YAG Process
YAP
YAG 1
D3-3G5=5I8-5I6+DE D3-5G6=5I8-5I5 3 D3-3K8=5I8-5I5+DE 3 D3-5F2=5I8-5I5+DE 3 D3-5F4=5I8-5I4+DE 3 3
16
DE (YAP) (cm )
fab fem (10
81pDEp489 Resonant 87pDEp304 430pDEp450 500pDEp1310
8870 77 2597 6 23
)
DE (YAG) (cm1)
fab fem (1016)
21pDEp453 Resonant 6pDEp457 375pDEp689 706pDEp1198
3049 94 3079 7 18
DE is the energy mismatch for these transitions and the product of corresponding oscillator strength values is fab fem :
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276 10
7
-1
X [s ]
3+
10
6
10
5
10
4
10
3
0.1
YAG:Ho 3+ YAP:Ho T=300 K
S=1
1
}
3
}
5
D3 S2
10
N Ho [at.%] Fig. 6. CR rate for the 5S2 and 3D3 states at room temperature versus holmium ions concentration.
are in a reasonable agreement with the data presented by Osiac [34]. Experimental data were used to evaluate the values of the nearest neighbor trapping rates X01 using the relation: X R01 6 X01 ¼ X =CA ni ; ð2Þ R0i i¼1 where ni is the coordination number of ion pair i and CA is a fractional concentration of acceptors. Taking into account the crystallographic structure of YAP and YAG the lattice sum calculations ( radius sphere of were performed in the 12.5 A influence which includes 168 and 120 ion sites in YAP and YAG, respectively. Calculated X01 values are comparable in magnitude to the low concentration decay rate t1 0 which, together with the generally exponential character of decays, except for the highest concentration of 5% Ho3+, explain the linear concentration dependence of CR. According to the generalized Inokuti and Hirayama [35] model developed in Ref. [36], 1 1 for times shorter than X01 that is when t0 pX01 and in the absence of donor–donor transfer, this model guarantees an exponential decay rate, which is linearly dependent on N: Similar, linear dependence of the green emission CR quenching rate on the concentration has been
observed in LiYF4:Ho3+ by Gomes et al. [37]. The authors, assuming a random distribution of ions in the matrix and the electric-multipole character of interaction, defined the function describing the average value of the CR rate and, by the numerical calculation, showed its linear dependence with the activator fractional concentration up to 12%. For the 3D3 emission, purely exponential decays are no longer observed suggesting stronger donor– acceptor interaction. Indeed, as seen from Fig. 6, CR rate values are higher than for the 5S2 emission but also follow linear dependence on the concentration. This is an indication that the weak quenching approximation is still valid, and is consistent with the model presented in Ref. [36]. The next step in our investigation was to determine the strength and character of the donor–acceptor interaction in Ho3+:YAP and YAG. Assuming, that the donor–donor transfer is negligible at low temperatures, the time dependence of the 3D3 emission could be described by the Inokuti and Hirayama [35] equation of the form: ln I=I0 þ t=t0 ¼
4pnA R301 ðX01 tÞ3=s Gð1 3=sÞ; ð3Þ 3
where t0 is the lifetime of the 3D3 state at low concentration, nA is the number of acceptors per unit volume, and R01 and X01 are the nearest neighbor separation and donor–acceptor interaction rate, respectively, s identifies the nature of interaction and G is the gamma function. From Fig. 7 it is seen that, for the appropriate choice of s; a plot of ln I=I0 þ t=t0 against t3/s gives a straight line. In the case of Ho3+ activated YAP and YAG the plots can be best fitted for s ¼ 6; indicating dipole–dipole nature of interaction. These fits yield the nearest-neighbor donor–acceptor transfer rates X01 to be 5.1 105 and 4.8 106 s1 for 5% Ho3+-doped YAP and YAG samples, respectively. The fluorescence lifetimes t were also observed to be temperature dependent between 10 and 300 K. Low temperature maximum in the 5S2 lifetime of Ho3+:YAG has been also observed in several Ho3+ activated solids like ZBLAN glass [38], LiNbO3 [39] and KGd(WO4)2 [40] crystals. This behavior is related to the thermalization of
ARTICLE IN PRESS M. Malinowski et al. / Journal of Luminescence 106 (2004) 269–279
3+
3
YAG:5% Ho
cross rel. rate X [1/s]
Ln(I/I0)+t/t0
3
-1.0
10
3+
D emission 3
-1.5
3+
-1
YAG: 5% Ho
em. D3 at 27415 cm exp. -1 E=495+2 cm
∇
YAP:5% Ho
-0.5
6
-1
5
5
10
em. S2 at 17953 cm exp -1 E=410+4 cm
∇
0.0
277
-2.0
-2.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.00
4.0
0.02
0.04
0.08
0.10
1/T [K ]
t
ð4Þ where DE is the separation between the 5S2 and 5F4 states, which is 36 and 19 cm1, respectively in YAG and in YAP, gi is the degeneracy of the multiplet; g1 ¼ 9 and g2 ¼ 5: As shown in Figs. 8 and 9 the 5S2 and 3D3 CR rates in 5% Ho3+-doped crystals are rapidly increasing with temperature which is an indication that the non-resonat CR processes dominate. For the non-resonant energy transfer its temperature dependence could result either from its multiphonon character or from the thermal activation of the participating levels. According to Miyakawa and Dexter [42] the temperature dependence of the energy transfer rate is expressed by ð5Þ
where N is the number of phonons participating in the energy transfer process, expressed as N ¼ DE=_o; where DE is the difference between the energies of the donor and acceptor levels.
6 3
cross rel. rate X [1/s]
5 1 5 1 g1 t1 0 ð F4 Þ exp ðDE=kB TÞ þ g2 t0 ð S2 Þ ¼ ; t0 ðTÞ g1 exp ðDE=kB TÞ þ g2
10
-1
em. D3 at 27565 cm exp. -1 E=280+5 cm
YAP: 5% Ho
3+
10
5
10
4
5
-1
em. S2 at 18550 cm exp -1 E=150+18+2 cm ∇
the 5F4 state lying only several tens of wave numbers above 5S2. Thus, in the expression for the radiative decay in Eq. (1) thermalization between the 5S2 and 5F4 states must be considered [41]
Fig. 8. Plot of CR rate between Ho3+ ions in YAG as a function of 1/T for 10oTo300 K. The squares are the experimental data and the straight lines indicate thermally activated processes calculated according to Eq. (7).
∇
Fig. 7. Plot of ln ðI=I0 Þ þ t=t0 against t1/2 for 5% Ho3+-doped YAP and YAG, the straight line fit indicates a near-neighbor transfer rates X01 of 5.1 105 and 4.8 106 s1, respectively.
X ðTÞ ¼ X ð0Þð1 exp ð_o=kTÞÞN ;
0.06 -1
1/2
3
10 0.00
0.02
0.04
0.06
0.08
0.10
-1
1/T [K ] Fig. 9. Plot of CR rate between Ho3+ ions in YAP as a function of 1/T for 10oTo300 K. The squares are the experimental data and the straight lines indicate thermally activated processes calculated according to Eq. (7).
As discussed here CR pathways are only slightly non-resonant, with the maximal energy mismatch of the order of few hundreds wave numbers, that is well below the cutoff energy of the phonon spectrum, which is about 700 and 780 cm1 in YAP and YAG, respectively. In this case the temperature dependence of CR is rather due to the
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278
changing of the thermal populations of the crystalfield levels of donor and acceptor transitions. The temperature dependence of the energy transfer rate can thus be described by a Boltzmann-type expression of the form: X ðTÞ ¼ X ð0Þexp ðDE=kT Þ;
ð6Þ
where DE is an activation energy. From the data presented in Tables 1–4 it could be seen that for both 5S2 and 3D3 levels resonant CR transitions initiate not from the lowest but from high-lying Stark sublevels of the ground 5I8 multiplet which are coupled by the Boltzmann statistics. In this case, the transfer rate X is defined by Eq. (6). However, it is also seen that the plots of CR rates as a function of 1/T could not be approximated by a single straight line, indicating presence of at least two different, thermally activated processes. We fitted the experimental results by taking the sum of two thermally activated processes, only in the case of 5S2 CR rate in YAP we needed to consider three processes, using the relation: X X¼ X0i exp ðDEi =kTÞ: ð7Þ i
From Figs. 8 and 9 it is seen that the slopes of the plots for higher temperatures yield activation energies of a few hundreds of wave numbers. This, together with the results of our earlier analysis and an examination of the energy values listed in Tables 1–4 shows that CR processes of the type: 5 S2(18476)-5I4(13277)=5I8(147)-5I7(5345) and 5 S2(18546)-5I4(13509)=5I8(418)-5I7(5455) could be considered as responsible for the quenching of the 5S2 fluorescence in YAP and YAG, respectively. For the 3D3 holmium emission in YAG the temperature dependence of the quenching rates together with the data in Table 5 indicates the process of the type 3D3(33072)-5G6(22265)= 5 I8(498)-5I6(11301) with the mismatch of 4 cm1. In the case of YAP:Ho3+ the indication of the single responsible process is more difficult because the first three processes listed in Table 5 could be active. However, the 3D3 CR rate temperature dependence in YAP shown in Fig. 9 could be understand in terms of a thermal activation of the 278 cm1 Stark level in the 5I8 multiplet [21].
Finally, the energies DE of a few wave numbers, obtained from the fitting in Figs 8 and 9, are comparable to the values of the splitting of the lowest 5I8 holmium Stark energy levels which are 4 and 5 cm1 in YAG and YAP, respectively [21,23], and result from the thermal activation of these levels in the low temperature limit.
5. Conclusions The short wavelength emission properties of Ho3+ ion in YAP and YAG crystals were studied and analyzed. The temporal behavior of the 5S2 and 3D3 fluorescence after pulsed excitation was analyzed as a function of temperature and holmium ions concentration. The most efficient cross-relaxation (CR) schemes were proposed for both emitting 3D3 and 5S2 manifolds and the values for the transfer rates were experimentally determined and compared to the results of Inokuti–Hirayama theory. The CR rates were found to be temperature dependent, which results from the thermal activation of the Stark energy levels in the 5I8 manifold.
Acknowledgements This work was supported by the Polish State Committee of Science (KBN)—Project no. 8 T11B 08319.
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