Infrared to visible upconversion in Cs3Yb2Cl9:Tm3+

JOURNAL

OF

LUMINESCENCE ELSEVIER

Journal

of Luminescence

63 (1995) 327-337

Infrared to visible upconversion in Cs3Yb2C19 : Tm3 + Toni Riedener”, Hans U. Giidel”, *, George C. Valleyb, Ross A. McFarlaneb “Institutfiir Anorganische Chemie, Freiestrasse 3, 3000 Bern 9, Switzerland bHughes Research Laboratories, 3011 Malibu Canyon Road, Malibu. CA 90265, USA Received

5 July 1994; revised 28 September

1994; accepted

28 September

1994

Abstract Tm” doped Cs3Yb2C19 and Cs3Y2C19 was synthesized and single crystals were grown using the Bridgman technique. Absorption, upconversion luminescence and excitation spectra are presented as well as power- and concentrationdependent measurements. Under near-infrared (NIR) CW excitation of the *F,,* + 2F,,z transition of Yb3+, Tm3+ doped Cs3Yb2C19 exhibits intense NIR (3H4 -+ 3H,), red (3F3 + 3H6), blue (‘G4 + 3H6 and ‘D, -+ 3F,) and near-UV luminescence. It is a stepwise energy transfer with a very pronounced power and concentra(‘Dz --t 3H,) upconversion tion dependence. The unusual (compared to fluoride and oxide systems) red upconversion luminescence, assigned to a 3F3 -+ 3H, transition, is due to the low-energy phonons ( I 280 cm- ‘) in this lattice. The power dependence and the dependence of the excitation spectra upon the detection wavelength are reproduced by simple models.

1. Introduction

The current scientific interest in upconversion processes in lanthanide compounds is primarily motivated by the search for new materials for phosphors and solid state lasers. Near-infrared (NIR) to visible (MS) upconversion processes are of particular interest because suitable excitation sources are available both for research (Ti:sapphire) and application (semiconductor diode laser) purposes [l]. Pr3+, Nd3+, Er3+ and Tm3+ doped fluoride and oxide host materials have received most of the attention sofar [2-61 We are investigating chloride and bromide lattices in which the phonon energies are significantly smaller than in oxides and fluorides [7, S] . In Er3+ systems we found that this lowering of the phonon energies had a dramatic effect on the excited state dynamics and the steadystate populations under CW excitation [9]. *Corresponding

author.

In the present paper we choose a chloride lattice with a dimer structure in wljch the shortest metal-metal distance is only 3.6 A. The host lattice Cs3Yb2C19 acts as a sensitizer, and the active ion is Tm3+. The principle of Yb3+ -+Tm3+ energy transfer upconversion has been employed in various other systems [lo- 133. Due to the short Yb3+-Tm3+ separation we expect highly efficient nonradiative transfer processes. Tm3+ doped Cs3YzC19 was used to determine the absorption spectra of Tm3+.

2. Experimental 2.1. Synthesis Starting materials for the synthesis of Tm3+ doped Cs3YbzClg and Cs3Y2C19 were TmCI,, YbC13 and YC13, all prepared by the ammonium chloride route described in Ref. [14] using the

0022-2313/95/$09.50 0 1995 - Elsevier Science B.V. All rights reserved SSDI 0022-23 13(94)00065-4

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corresponding rare earth oxides Mz03 (99.999%, Johnson Matthey Alpha Products), ammonium chloride ( > 99.8%, Merck) and concentrated hydrochloric acid (37%, p.a., Merck), and cesium chloride ( > 99.5%, suprapur, Merck). Nonstoichiometric amounts (57 mol% CsCl, 43 mol% MC13, M = Y, Yb, Tm) were sealed in silica ampoules and the crystals were grown utilizing the Bridgman technique. Due to the sensitivity of the substances to moisture, all handling was done under dry conditions in a glove box. Good crystal quality was achieved with the Cs3Yb2C19 samples, but the Cs3Y2C19 crystals were of poor quality. 2.2. Spectroscopy All spectra were measured unpolarized with a random crystal orientation. The absorption spectra were recorded on a Cary 5e (Varian) spectrometer, cooling the sample (enclosed in a copper cell) with a closed-cycle cryostat (Air Products). For the upconversion luminescence spectra and power dependence measurements, the samples were sealed into silica tubes, filled with 400 mbar helium gas for heat dissipation, and cooled with a nitrogen or helium gas-flow technique. The crystals were excited with a multimode, standing wave Ti:sapphire laser (Schwartz Electra Optics), pumped by an argon-ion laser in all-lines mode (Spectra Physics 204515/4S). For excitation spectra, the wavelength control was achieved by an inchworm driven (Burleigh PZ-501) birefringent filter and a wavemeter (Burleigh WA2100). The sample luminescence was dispersed by a 0.85 m double monochromator (Spex 1402) with gratings blazed at 500 nm (1200 grooves/mm). For the slit widths tsed the instrumental resolution was typically 1 A. The signal was detected with an cooled photomultiplier tube (RCA 31034) and a photon counting system (Stanford Research SR400). Device control and data acquisition were done by a personal computer. To measure the power dependence, the beam was attenuated with a series of neutral density filters (Balzers). A part of the luminescence spectra and power dependence measurements were done using a one meter Czerney-Turner monochromator (Hilger) driven by a computer controlled stepper motor,

63 (1995) 327-337

a GaAs-photocathode detector (Hamamatsu R639) and a lock-in amplifier. The data were analyzed using Igor (Wave Metrics). The luminescence spectra were corrected for the wavelength dependence and sensitivity of the setup. The model calculations were done with ACSL (Advanced Continuous Simulation Language, Mitchell and Gauthier).

3. Results Fig. 1 shows an overview of an unpolarized absorption spectrum of Cs,Y,C&,: 10% Tm3+ at 15 K. The various parts of the figure were m$asured with different spectral band widths (l-10 A). The assignment of the energy levels is straightforward from a comparison with literature data [15, 163. The upconversion luminescence was measured systematically on a series of Cs3YbzClg crystals doped with l%, 5%, 10% and 15% Tm3+ at three temperatures (10 K, 80 K and room temperature) and different pump intensities (10, 25, 50, 100 and 200 mW). They were all excited around 10670 cm-’ into the ‘Fsj2 state of the Yb3+ ion. The crystals glow red and blue at any temperature, which can easily be seen with the naked eye. The blue luminescence gets more intense with increasing pump power and decreasing Tm3+ concentration. The corresponding spectra at 10 K of Cs3Yb2C19 : 5% Tm3+ are shown in Fig. 2 for pump powers of 10 and 200 mW, respectively. The traces are scaled to an equal height of the 3H4 + 3Hs transition by multiplying the 10 mW spectrum by a factor of approximately 25. The assignment of the transitions was done by comparison with the absorption and ‘G4 excited luminescence spectra of the Tm 3+ doped C~3YzCl9 crystal. The peak labeled with an asterisk (*) is the second order of the ‘D2 + 3H, band. The intensity distribution is clearly dependent on the excitation power, the high-energy transitions originating in ‘G4 and ID2 states becoming more dominant at high laser powers. The lower trace in Fig. 3 presents an absorption spectrum of the ‘F,,, + 2F5,2 transition of Yb3+ in Cs3Yb2Clg: 1% Tm3+ at 15 K. It shows the three pure crystal-field origins of 2F5,2 (two of them are

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‘F3

F4

‘H,

‘D*

‘I, >3P,

I

I

20000

25ooo

30

Wavenumbers [cm-‘] Fig. 1. Unpolarized absorption spectrum of Cs3Y2C19:10% Tm’+ at 15 K

cutoff due to stray light) at 10196 cm-’ (I, + I,), 10234 cm-’ (I,) and 10670 cm-’ (I,), as well as a pattern of vibronic side bands. The assignment of the levels was done by comparison with neat Cs3YbzBrg [17]. The highest energy vibrational side bands are situated about 280 cm- ’ from the corresponding origin. The structure between 10270 and 10550 cm-’ is the superposition of two vibrational side band patterns, one built on the IS + I6 origin and the other one on 14. The upper trace in Fig. 3 is the excitation spectrum of C+Yb& : 5% Tm3+ at 125 K, monitoring the ‘H4 + 3H, luminescence on the Tm3+ at 12410 cm-‘. It is very similar to the absorption spectrum, clearly demonstrating that the Yb3+ host lattice acts as a sensitizer. The lowest-energy crystal-field transition is split as a result of exchange splitting both in the initial and final state [18] . This splitting is instrumentally resolved in the Ti:sapphire laser excitation spectrum but not in the Cary absorption spectrum. The spectrum is not corrected for the wavelength dependence of the output power of the Ti:sapphire laser. The power dependence of the upconversion luminescence was measured for three concentrations (l%, 5% and 15%) at three different temper-

atures (10 K, 80 K and room temperature). The data of a 5% Tm 3+ doped Cs3Yb2 Cl 9 crystal at 80 K are presented in Fig. 4(a) for illustration. The measured intensity is plotted against the pump intensity in a doubly logarithmic plot. For a better comparison, all the intensities are arbitrarily scaled to one at the lowest pump power. The slopes of the four curves are significantly different, and also their levelling-off at high pump powers is different. The slope of the 3H4 (U) is the most linear of the four, whereas the slopes of the ‘Gq ( x) and ID2 (0) show a very pronounced flattening at high pump powers. We choose the slopes at the lowest pump power used in our experiments for the analysis in Section 4.2. They are given in Table 1. A series of luminescence excitation spectra of Cs3YbzClg : 5% Tm3+ at 10 K, detected at four different wavelengths is shown in Fig. 5(a). The traces are scaled to get an equal height of the band at 10670 cm-‘. With increasing energy of the observed luminescence, the intensity ratio of excitation origins and vibrational side bands shows a dramatic increase. At the highest luminescence energies (traces C and D), the vibrational side bands are no longer visible, only the three

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r

;I:L 3, + ‘H, 3F3+ 3H,

‘G,

4

x 25

10mW

‘H,

‘D, -r 3H, 200 mW

16000

18000

20000

22000

24000

26000

l-4 28000

Wavenumbers [cm*‘] Fig. 2. Upconversion luminescence spectra of CssYb#&:5% Tm3+ at 10 K with two different pump intensities at 10670 cm-‘: 10 mW (upper trace) and 200 mW (lower trace). The upper trace is multiplied by a factor of 25 to get an equal height of the 3H, + 3Hs transition. The peak labeled with an asterisk (*) is the second order of the ‘DZ --+3Hs luminescence.

Excitation

Absorptior I

u

I~“~I~~~‘I’~“I~~“,~~~‘I~~~~,~‘~~I”’~,~~~’~’~~~,~~“I’~~~,~~,,,~~~~,~~~~,~’

10200

10300

10400

10500

10600

10700

10800

10900

I10100

Wavenumbers [cm-‘] Fig. 3. Excitation spectrum of CssYb2Clg:5% Tmr+ at 125 K in the region of IF,,* + ‘F,,, transitions, monitoring the aH4 + 3H6 luminescence transition on the Tm3+ (12410 cm-‘, upper trace). Absorption spectrum of Cs,YbzC1,:l% Tm3+ at 15 K (lower trace). Electronic origins are labeled in Cjv point group notation.

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1000

‘;; .t: a d 100 2 x .z % 9 E 10

1

1

(f$+_zJ 2

100 Power watt/cm2]

1000

lK,

2

j:ji,,I

100 Power fWatt/cm’]

1ooa

Fig. 4. a) Measured power dependence of the upconversion luminescence in Cs,Yb2C19:5% Tm 3+ in double logarithmic representation: ID2 -+ 3F,(21706cm-‘, O), ‘G, +aH,(20755cm-‘, x),~F~ -t3H,(14384cm-‘,O)and 3H,, + 3H, (12378 cm-t, n) with a constant excitation wavelength of 10670cm-‘. b) Calculated power dependence using the model of Antipenko [26] and considering the processes and rate constants in Table 2. All the intensities are arbitrarily scaled to 1 for the lowest power.

Table. 1 Initial slopes of the measured (from Fig. 4a) and calculated (from eq. 2) power dependence in CssYb+&:5% Tm3+ Transition

Initial slopes Experimental Theoretical

‘Hz, -+ 3Hs 3F3 -+ 3Hh ‘G, - ‘He ‘DZ -t ‘F.,

1.4 2.0 2.6 3.4

2.0 2.0 3.0 4.0

crystal-field origins of 2F5,2 (Yb3’) remain. The origin peak at 10234 cm- ’ (r,) has a stronger decrease in intensity with increasing monitoring energy than the one at 10196 cm-’ (I-, + r,). The influence of the Tm3+ concentration on the upconversion luminescence, already observed with the bare eye, was measured at different temper-

atures and different pump intensities. Fig. 6 shows as an example the upconversion luminescence spectra of Cs3Yb2Clg crystals doped with l%, 5%, 10% and 15% Tm3+ at 105 K. The spectra are scaled to an equal height of the 3H4 + ‘He luminescence for an easier comparison. The higher energy part ( > 20000 cm-‘) is blown up by a factor of 10 in all spectra. The intensity distribution is obviously concentration dependent. The higher-energy transitions originating in ‘Gq, ‘D, and 3F3 states are relatively more prominent in the dilute systems.

4. Discussion

Cs3YbzClg, Cs3Y2C19and Cs3Tm2Clg crystallize in the trigonal space group R3c [19] with Z = 6, in the so-called Cs3TlzC19 structure [20]. This

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63 (1995) 327-337

a>

A h

10200

10400

10600

10800

11000

10200

Wavenumbers [cm-‘]

10400

10600

10800

11000

Wavenumbers [cm-‘]

Fig. 5. a) Measured excitation spectra of Yb 3’ in CsJYb,Cl,:5% Tm3+ at 10 K, monitoring the following luminescences on Tm3+: 3H., -P 3H6 (12410 cm-‘), A; 3F3 + 3Hs (14294 cm-‘), B; ‘Gg -P 3H,r (20790 cm-‘), C; ‘Dz -P 3Hs (271OOcm-r), D. The spectra in b) were calculated using Eq. (3).

‘G4+ 3Hs ‘D, + 3F4

3Fs+ 3H, P-.

i

‘D, + 3H,

15% Tm3+

x 10

10% Tm3+

x 10

b

P’

5% Tm3+

h

_jiJ 1% Tm3+ I”“~‘“‘I’“‘I’“‘I”“~““,‘““““,““I””I’”’I””,””l””,‘““““I”“” 12000 14000 16000 18000 20000 22000 Wavenumbers [cm-‘]

x10,

x10_

I

24000

26000

28000

Fig. 6. Upconversion luminescence spectra of Cs3Yb2C19,doped with different Tm3+ concentrations(l%, 5%, 10% and 15%)at 105 K. The higher energy part ( > 20000 cm-r) is multiplied by 10. All the spectra are scaled to an equal height for the 3H4 + 3Hs luminescence at 12300 cm-‘.

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of Luminescence 63 (I 995) 327-337

333

structure is built up of M&l;- dimers, which contain two face-sharing MClz- octahedra. The M3+ single ions thus occupy sites of C3” symmetry. The M3+-M3” distance within the dimers is approximately 3.6 A, i.e. the smallest possible separation in a chloride lattice. An important consequence of the short intradimer distance in Cs3Yb2C19 is a strong exchange interaction leading to relatively large splittings of the electronic levels. The ground-state splitting was found to be 3.25 cm-’ by inelastic neutron scattering [18]. Since proximity is also a very important factor in nonradiative energy transfer processes, we expect particularly high transfer rates for cross-relaxation and upconversion processes within a given dimer. The closest M3+_M3 + separation between neighbouring dimers in the Cs3MzC19 lattice is approximately 5.8 A, short enough for a relatively efficient energy transfer by electric multipole mechanisms [9]. We thus expect thef-j-excitations to have an excitonic character in Cs3Yb2Cl,, so that intentional and unintentional traps can be fed by energy migration with high efficiency. In our system Tm3+ ions are introduced as intentional traps for the Yb3+ excitation. The host lattice thus acts as a sensitizer.

upconversion phenomena described for Tm3+ and Yb3+ codoped fluoride lattices [lo-131 with one significant exception: Tm3+ doped Cs3Yb2Clg crystals exhibit a very distinct red glow besides the blue luminescence. This red transition lies at the edge of the sensitivity of the human eye, and using a spectrometer it is found at about 14200 cm-’ as a dominant upconversion luminescence, especially at low pump powers. We assign it to a 3F3 -+ 3H, transition. The fact that it is not observed in fluoride lattices can be explained by the energy gap law [24]. Referring to Fig. 7 we see that the 3F3-3H4 energy gap is about 1600 cm- ‘. In our crystal the highest phonon energy is about 280 cm- ‘, as derived from Fig. 3, so that nearly six vibrational quanta are needed to bridge the electronic energy gap in a 3F3--+3H4 multiphonon relaxation process. In fluoride and oxide lattices the highest phonon energies are 35&400 cm- ’ and typically > 550 cm-‘, respectively [25]. The 1600 cm- ’ energy gap can thus be bridged by 445 and 3 vibrational quanta, respectively. The rate constant k,, for a multiphonon relaxation process across an electronic energy gap can be formulated as

4.1. Upconversion in Tm3’ doped Cs,Yb2C19

where C and a are host dependent constants and p = AElhw is the number of high-energy phonons of energy hw required to bridge the energy gap AE [24]. The exponential decrease of k,, with p explains the observed behaviour. For p values up to 45 the nonradiative 3F3 + 3H4 relaxation dominates all other processes. For p = 6 the radiative 3F3 + 3H6 transition becomes competitive and shows up in the chloride spectrum. A 3F3 + 3H6 luminescence was also found in ‘G, excited luminescence spectra not shown here. But part of the 3F3 excitation still relaxes nonradiatively to 3H& as evidenced by the intense 3H4 + 3H6 luminescence in Figs. 2 and 6. Due to the very small energy gap of % 580 cm-’ between 3F2 and 3F3, no luminescence from the 3F2 state was found. The population of the ‘D2 level with a fourth energy transfer step would require that the lattice absorbs a surplus energy of z 3300 cm- ‘. Since this corresponds to about twelve vibrational quanta in our chloride, this mechanism is not very probable. Another mechanism for the population

There are a number of examples in the literature, in which a system is excited by the ZF,,2 -+ 2F,,2 transition of Yb3+ around 10000 cm-’ and this excitation is then transferred to Tm3+ [lo-131 or other rare earth ions incorporated into the Yb3+ lattice [17, 21, 221. In mixed Yb3+-Tm3+ systems this transfer can be very efficient, and Tm3+ can be excited by successive energy transfer steps to excited states in the visible and UV range. These upconversion processes, which were first described by Auzel [23] are schematically shown in Fig. 7. The thickness in Fig. 7 of the arrows gives an impression of the relative intensity of the luminescence transitions. The luminescence spectra shown in Fig. 2 provide clear evidence that upconversion of the type described above does take place in our Tm3 ’ doped Cs3Yb2Clg crystals. Upon excitation at 10670 cm-’ we observe luminescence transitions up to the near UV. The behaviour is similar to the

k,, = C. e-“P,

(1)

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Ttl?+

63 (1995) 327-337

Yb3+

Fig. 7. Schematic diagram of possible upconversion processes and observed VISjUV luminescences in Cs,Yb$& : Tm3+. The broken arrows show the sequential Yb3+ + Tm3+ energy transfer steps. The dotted arrows indicate an upconversion process within the Tm3 + system. The thickness of the arrows indicates the intensities of the luminescences.

of ‘D2 (iG4, 3H, + ‘Dz, 3F4 within the Tm3+ system) was used in a model by Antipenko et al. for the behaviour of [26] to account BaYbzFs : Tm3+. This model is described and used for a discussion of the power dependence data in the next section. The 3H4 + ‘He transition is relatively most intense in the samples with the highest Tm3+ concentration, i.e. we observe a “concentration quenching” of the higher excited ID1 and ‘G, states. In the concentrated samples there are obviously nonradiative pathways to populate 3H4 such as crossrelaxation processes between Tm3+ ions with sufficiently small separations. The following processes are energetically plausible: 3Hs, ‘G4 + 3H4, 3HS or 3H6, ‘Dz + ?H4, 3F 3/3F,. They all lead to the observed decrease of the blue (lDz + 3F, and ‘G, -+ 3H6) and the corresponding increase of the red and near-infrared (3F3 + 3Hs and 3H4 -P 3H6) luminescence. This concentration quenching is also reflected in the decrease of the lifetimes of the lD2

and ‘G4 with increasing Tm3+ concentration as observed in the fluoride crystals YF, [ll] and YLiFl [27]. An implication of the Auzel upconversion scheme in Tm3+-Yb3+ systems is a nonradiative 3HS + 3F4 relaxation after the first and before the second excitation step. We made no experiments in the infrared, but using Fig. 7 and applying the above arguments (BE z 2300 cm- ‘, 8 vibrational quanta) we expect a low efficiency of this nonradiative process in our chloride lattice. Since the 3F4 population is essential in the upconversion scheme in Fig. 7, we suspect that, similar to the 3H4 state discussed above, there are other pathways to populate 3F4 than the straightforward 3H5 + 3F4 relaxation. The following cross-relaxation processes within the Tm3+ system are possible candidates: 3H6, 3H4-+3F4, 3F4 or 3H4, 1G4+3F4, ‘D2 or 3H6, ‘Dz -+ 3F,, iGq. The ‘I, -+ 3H, transition was observed as a very weak band around 25200 cm- ‘. It is too weak to be

T. Riedener et al. J Journal of Luminescence

seen in Fig. 2. According to Fig. 7 the population of l I6 requires 5 excitation steps. Since we have found upconversion and cross-relaxation processes within the Tm3+ system to be important down to the lowest Tm3+ concentration of lo’, we cannot exclude the possibility of an upconversion process such as 3H4, ‘G, + rig, 3H, or ‘Dz, 3H4 + ‘Ig, 3F, or ‘Dz, 3F3 + ‘I,, 3H5 for the ‘I6 population. 4.2. Power dependence If the processes depicted in Fig. 7 were the only nonradiative energy transfer processes, the population N of a Tm3+ level after n successive energy transfer steps from Yb3 + would be given by N CCz”P”

(2)

for very low laser powers. In Eq. (2) a is the absorption coefficient of the *F,,* state of Yb3+ at the excitation wavelength and P is the laser power. For a fixed excitation wavelength a is constant and the exponent n could thus be derived from the slope of a double logarithmic plot of N versus P. We would expect slopes of 2,3 and 4 for the 3F3/3H,, ‘G4 and ’ D2 upconversion luminescence transitions, respectively. As shown in Fig. 4(a) and Table 1 the experimental behaviour follows this expected trend. The experimental 3F3 + 3H, power dependence has an initial slope of 2.0 f 0.1, in exact agreement with the model. For all the other curves in Fig. 4(a) the initial slopes are smaller than expected. In addition, all the curves in Fig. 4(a) level off to various degrees at higher power levels. All this clearly indicates that Eq. (2) is valid for power levels approaching zero but ceases to provide a good representation of the power dependence at higher powers. This is not surprising, because Eq. (2) does not take into account the multitude of secondary upconversion and cross-relaxation processes which can take place between close-lying Tm3+ ions. Excited state dynamics and steady-state populations in our systems are largely determined by such processes. They are important down to the lowest Tm3+ concentrations, and they are, together with a saturation of the 3F, level, responsible for the observed deviation of Fig. 4(a) from the straight lines with the slopes 2,3 and 4 expected from Fig. 7.

335

63 (I 995) 327-337

Similar deviations have been observed in other Yb3’-Tm3+ systems. In BaYo.sYb,,,99Tmo.oolF5 the following initial slopes were found: 1.7, 2.6/2.9 and 3.5 [12] . A flattening was also observed, even at low pump intensities of less than 50 mW/cm*. This was explained by a saturation of the ‘F, level, and a similar explanation was used for a similar observation in YF3:Yb3+ (lO-100%) Tm3+ (0.03%) [11] . Antipenko et al. have developed a model based on rate equations to calculate steady state populations and the power dependence of upconversion luminescence transitions in Tm3+ doped BaYb,F, [26]. We used this model with the parameter values chosen by Antipenko for a calculation and a comparison with our experimental results. The processes considered in this model and the rate constants used are collected in Table 2. It assumes that both the 3H5 and the 3F3/3F2 levels are relaxing by fast nonradiative processes to 3F4 and 3H4, Table. 2 Processes and rate constants used to calculate Fig. 4(b) with the model of Ref. [26] Upconversion or relaxation process

Rate constant

Spontaneous decay [s ‘1 ‘F, -+ 3H6 3H4 + ‘H, 3H4 + ‘F4 ‘G, -+ 3Hb ‘G,- 3Fz, + 3H, ‘GA + ‘H, + 3F3 + ‘F2 ‘DZ + 3Hs ‘DZ + 3F4 + ‘Hs 1DZ+3H4+3F3+3F2 ‘Fs,z -+ ‘F,,z

117 794 40 309 539 + 365 14 + 30 + 6.3 3621 1891 + 62 1142 + 250 + 312 296

Upconversion steps [cm’/s- ‘1 ZF5;*>‘H6 --f 2F:/2, 3Hq-+ ‘Fs,z. ‘Fd - ZF,,2, 3F2-+ 2FSi2, 3H4 + ‘F,,,. ‘G, rGq. ‘H, + ‘DZ, 3F,

1.34x lo-” 3 x lo-l4 5x lomth 3 x lo-‘5

Cross relaxation [cm3/sm ‘1 3H‘t, 3H, + 3Fq, 3F4-+ ‘G& 3Hs + 3Hq, 3H,-+ ‘D,, 3H6 + 3H4. 3F,-+ Back transfer [cm3/s- ‘1 3H4, ‘F,,z -+ ‘H6, *Fs,z ‘G4, ‘F,,z -+ 3Hs, ‘Fg,z-+

4.1 X lo-” 2.6 x lo-”

I x lo-l6 6.6 x lo-l9 2.2 x lo- l9

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respectively. The ‘Dz level is populated by an upconversion process within the Tm3+ system: iGq, 3H4 + ‘Dz, 3F4. The rate constant for this process is not given by Antipenko, a value of 3 x 10-‘5cm3/s is assumed here. The result of our calculation is shown in Fig. 4(b) in an analogous representation to Fig. 4(a). It reproduces very well the principal trends of the observed power dependences. The slopes are significantly smaller than 2, 3 and 4 expected from Fig. 7 for the 3Hq, ‘D4 and ‘Dz populations, respectively. Also the levelling-off at high pump powers is qualitatively reproduced, it is least pronounced for the 3H4 level and most pronounced for ‘Gq as observed in Fig. 4(a). There is of course no quantitative agreement between Figs. 4(a) and (b), and we did not attempt to adapt the model rate constants to our chloride dimer system and we did not introduce the 3F3 and 3F4 levels to get a better agreement. We conclude from this analysis that the presence of upconversion and cross relaxation processes within the Tm3+ system and the saturation of the 3F4 level reduce the power dependence below the values expected from Fig. 7 alone and also lead to the levelling-off of the power dependences. Scanning over the range of Yb3+ ZF,,2 -+ 2F5,2 excitation around lOOOOcm-‘, we expect on the basis of Eq. (2) to measure different excitation spectra for the different transitions. This is indeed the case as shown in Fig. 5(a). With increasing number of Yb3+ -+ Tm3+ energy transfer steps the pure electronic origins become more dominant and the vibrational side bands disappear. According to Eq. (2) we expect the spectra C and D to be powers of 312 and 412, respectively, of the spectrum A. As shown in the previous paragraph, other processes are also important, and it is more realistic to use the experimental power dependences from Fig. 4(a) to calculate the excitation spectra : spectrum (state i) = (spectrum 3H4)sib,

(3)

where a and b are the initial power dependence slopes from Fig. 4(a) of the state i (i = 3H4, 3F3, lG& ‘Dz) and 3H4, respectively. The result is shown in Fig. 5(b). The observed trend of Fig. 5(a) is well reproduced. There are quantitative differences however, particularly for ‘G4 and ‘Dz, where the vibrational side bands are more intense in the cal-

63 (1995) 327-337

culated spectrum than in the experiment. These discrepancies might be at least partially due to the saturation of the most prominent line in the 3H4 excitation spectrum. Another factor is the Ti : sapphire laser output which is actually not constant over the range of the spectra in Fig. 5(a). But we do not have a reasonable explanation for the different power behaviour of the origins Is/F6 and I4 in Fig. 5(a). In summary, we find that the observed power dependence of the upconversion luminescences in Tm3+ doped Cs3 Ybz Cl 9 can be reasonably explained by using the most simple theoretical models.

Acknowledgements Part of this work was carried out while one of us (H.U.G.) was a visitor at the Hughes Research Laboratories, Malibu CA. We thank K. Kramer and N. Furer for their help with the synthesis and crystal growth. We are indebted to M. Hehlen and A. Hauser for valuable discussions and support with the measurements and to R.A. Cronkite for technical support. Financial support by the Swiss National Science Foundation is gratefully acknowledged.

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