The optical properties of Er3+ and Tm3+ in KCaF3 crystal

Journal of Luminescence 43 (1989) 185—194 North-Holland, Amsterdam

185

TIlE OPTICAL PROPERTIES OF Er3~AND Tm3~IN KCaF3 CRYSTAL C.Y. CHEN

a

W.A. SIBLEY, D.C. YEH and C.A. HUNT

Physics Department, Oklahoma State University, Stillwater, Oklahoma 74078, USA Received 23 July 1988 Revised 6 February 1989 Accepted 9 March 1989 3 ± and Tm3 ± in single crystal KCaF The optical properties of the rare earth ions Er 3 + is observed compared 3 to arethe investigated. many different We find sitesthat that rare can earth be accommodated ions enter theinlattice otherreadily crystals. and Effective only oneenergy defect transfer site for Er occurs between the Er3 + and Tm3 + ions and the low phonon frequencies improve the optical transition quantum efficiency. This material is promising as a device host.

1. Introduction Infrared-to-visible upconversion processes are important for device applications. Research interest has concentrated on various single crystal or glass hosts doped with rare-earth ions and/or transition metal ions. The upconversion process discussed inand this paper multiple photon absorption energyconsists transferof as reviewed by Wright [1] and Auzel [2]. This process has proved to be relatively efficient [3] and has been well described by Johnson [4], Van Uitert et al. [5], Pollack [6] and others [7—9]. Recently, Yeh et al. [10] reported that the efficiency of the two-photon process 3~and Tm3~ionsupconversion in BaF between Yb 2/ThF43 ±heavy conmetal fluoride depends on the centration and glass the temperature. TheyTmalso observed a 1.2% projected efficiency at room temperature for a concentration of 0.05 mol% TmF and 2. 3Since an incident infrared intensity of 1 W/cm the process is non-linear the incident intensity must be specified for a given efficiency. Jouart [11] and Van Uitert and Johnson [7] have pointed out that the relative intensities of a

. . . . Present address: Department of Radiological Sciences, University of Oklahoma, Oklahoma City Campus — Health Sciences Centers, Oklahoma City, OK 73190—3020, USA.

fluorescence are strongly influenced by energy transfer when both Er3~and Tm3~ions are present in the same crystal. One purpose of this paper is to investigate energy transfer between Er3 + and Tm3~in a perovskite KCaF 3 crystal host [12]. A second purpose is to determine the concentration of dopant ions that can be incorporated3~ions into the in lattice and how many sites for Er the crystal. A careful studyexist of the energy level splitting of the Er3 + ions allows this determination for KCaF 3. The upconversion fluorescence intensity can be estimated by using transition probabilities, fluorescence lifetimes and energy transfer rates [1]. 2. Experimental procedures The crystals used in this study were grown at the Crystal Growth Laboratory of Oklahoma University. The crystals were grown in an State inert atmosphere using the Bridgman technique with 2 at.% Er3~and 0.5 at.% Tm3~added to the melt. Optical absorption measurements were made with a Perkin—Elmer 330 Spectrophotometer and the integrated intensities were calculated by numerical integration. Low temperature measurements were performed in a CTI Model 215C Cryodyne Cryocooler with a resistance heater

0022-2313/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

186

C. Y Chen et al.

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3 + and Tm3 ± in KCaF~crystal

Optical properties of Er

which allows temperature control within ±1K in the range of 15—300 K. A 150-W xenon-arc lamp was used for emission and excitation studies. The sample was excited with light through a Spex Spectromate Model 1680B Double Monochromator. The fluorescence was focused into a 0.8-rn Spex Monochromator with a mirror to direct the light to the appropriate detectors. A cooled RCA C31034 Photomultiplier Tube (PMT) and an Optoelectronics OTC-22-53 PbS cell were used for detection. Signals from the detectors were preamplified and passed to a lock-in amplifier synchronized with a variable light chopper in the excited beam with the output displayed on an X—Y recorder or stored in a Hewlett—Packard HP-86B minicomputer. Lifetime measurements were performed by using a Biomation 610B Transient Recorder and a Nicolet 1070 Signal Averager. Lifetimes as short as 10 p.s could be measured. The excitation spectra were corrected for source, rnonochromator, detector response. The concentrations of and erbium and thulium were measured using the ICP technique by Dr. Yok Chen at Oak Ridge National Laboratory. The erbium and thulium concentrations in the crystals are 1.62 at.% and 0.35 at.%, respectively. The index of refraction for this crystal was measured by Dr. E. Arakawa also at ORNL. The index of refraction at various wavelengths are n1 1.409 (400 nm), n2 = 1.405 (500 nm), n3 1.398 (600 nm), n4 1.394 (800 nm), n5 1.391 (1400 nm), 1.389 (1800 nm), and n7 1.388 (2000 nm). =

=

=

=

=

=

3. Results and discussions absorption dataabsorption were takendata at 77at K77and 300Optical K. From the optical K, it was possible to construct 3~and Tm3~ions as energy shown level in fig.diagrams 1. Evifor Er dence was presented in a previous paper [13] that Er3~ions in RbMgF 2~ 3 crystals substitute for Mg2~ Mg ions in the two different symmetry sites for in this crystal. It is probable that the Er3~and Tm3~ions present in KCaF 2~ 3 substitute for Ca ions due to charge compensation and ionic radii considerations. A large number of site symmetries 3~in CaF have been reported for Er 2 [14]. In order

5’ 20

4F 2H 712 1112

7 6 5 C44.

15

F912

4

4’

~~9/2

3

3

2

C22

c24.

3H ,

4

5

1,

H4

11

113/2

W

2’

1

5

4 3~ Tm3~ 6 0 ~15/2 .Er 3 ± 3± Fig. 1. Energy level diagram of Er and Tm ions and upconversion mechanism. .

to determine whether the Er3~ions in KCaF 3 occupy many different sites, a low temperature, high resolution measurement was made. Figure 2 illustrates the excitation spectrum of the ~‘l5/2 ~ 4S 3/2 transition taken at 15 K and 77 K by detect4F ing the 9/2 emission. The resolution is ±5cm~. 3~ions. In a noncubic crystal4S field, there are only two excited levels for the 3/2 level of at the15Er The excitation spectrum in fig. 2(a) K clearly shows the energy splitting of 106 cm between the two 4S 3/2 levels and 3suggests, since only two + ions only occupy one lines are observed, the Er symmetry site in KCaF 3. 4S It is possible to observe only these two ~~15/2 to 3/2 transitions only the lowest level of the ~~15/2 multiplet since is populated at 15 K. As the temperature rises to 77 K, three more transitions occur in the spectrum at 18 447, 18340, and 18309 cm1 as shown in fig. 2(b). This is the result of the Boltzrnann population increase in the next two higher levels of the ~Ii5/2 multiplet. Emission spectra of 4S 3/2 to 115/2 are shown in fig. 3. Eight lines are observed at 15 K as portrayed

C. Y. Chen et al.

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3 + and Tm3 + in KCaF

Optical properties of Er

ic ~ z

545

106

312

540

KCaF3:Er, Tm

8 6

187

4s

WAVELENGTH (rim) 550

3 crystal

1

0 (a)

~

106cm—

15K

4

—

2

18377 cml

18232 cm1

I-

E~ ic

z

—

(b)

W I-

— —384 ————-——346 —

Z 0

6

77K

———280

><

w 18200 18400 18600 PHOTON ENERGY (cm—i)

—166 —138 15/2 — 81 — 44 — — — 0 Fig. 4. The energy level diagram illustrating the 4S 3/2 and

3~in KCaF Fig. 2. (a) Excitation spectra 4S at 15 K showing the energy splitting between the two 3/2 levels. (b) Spectra at 77 K.

in fig. 3(a). These lines are 4Sdue to transitions from the lower energy level of 3/2 to the various ‘15/2 multiplet levels. If only one substitution site exists, WAVELENGTH 10 KCaF3:Er, 560Tm

z

6

540

(a)

8 (I) i—

555

(nm)

II I

15

D

4 >F-

w F-

Q

8 6

(b) ~

(I)

cQ w

4~~j~j2

_________________________ 17700

18100

18500

PHOTON ENERGY (cm1) Fig. 3. Emission spectra of 4S 77K, respectively. 312 ~I15/2 at (a) 15 K and (b)

~h15/2

multiplets of Er

3.

group theory [15] predicts eight lines for any site symmetry less than cubic. At 77 K, as shown in fig. 3(b), two more emission lines appear on the higher energy side at 18332 and 18294 cm’ with another six lines possibly overlapping with those lines observed at 15 K. This set of emission lines is 4S due transitions higher energy level to (106 cm~)offrom 3/2 the to the ‘15/2excited ground state multiplet. It appears that KCaF3 crystals have an ideal lattice for incorporating rare earth ions since only one symmetry site is observed. In addition, the fact that about 80% of the rare earth dopants in the melt enter the crystal indicates this material can be a good host for a new laser system. From the data shown in fig. 3(a) the energy splitting of the ~‘15/2 ground state multiplet can be determined. Figure 4 depicts the energy split4S 34’ in KCaF ting of 3/2 and ‘15/2 multiplets of Er 3. The energies for KCaF3 : Er, Tm are congiven in table sistent with 3~are those for RbMgF3 : Er1.[13]. The energy levels Er Theofmeasured oscillator strength, f, of the absorption transitions can be calculated by the following equation:

f

mcn 2 2N~Ja(~) dv, ire

(1)

188

C. Y. Chen et aL

/

Table 1 3* and Tm3~ in KCaF Emission of Er 3:Er, Tm at 15 K. Measurements are accurate to ±5 cm — 1

_______________________________________________ (cm~) Energy

Transition 115/2

—•

453/3

113/2

(nm) Wavelength

15173 15157 15129 15097 15009 14886 14826 14784

659.1 659.8 661.0 662.4 666.3 671.8 674.5 676.4

the previous section. Analyses using the Judd—Ofelt theory [16,17] to determine the oscillator strengths of transitions is

11795 11806 11771 11714 11635 11616 11544

847.9 847.0 849.5 853.7 859.5 860.9 866.3

9756

1025

115/2

6464 6127

1547 1632

3H

3H

± ions, x is KCaF3, the termNfor refractive index of the 3host crystal is the effective field of at Er a well-localized center in a the concentration

medium of isotropic refractive index n. Xed n(n2 + 2)2/9 and Xmd are used for electric

115/2

6

where m and e are the mass and charge of the electron, c is the velocity of the light, n is the

548.5 549.8 550.9 552.7 5535 557.0 559.1 560.3

4 113/2

4 —~

3 crystal

18232 18188 18151 18094 18066 17952 17886 17848

~I11/2

3F

3 ± and Tm3 + in KCaF

Optical properties of Er

12485 12524 12405

801.0 798.5 806.2

12341 12287 12264 12182 12137 12055 11856

810.3 813.9 815.4 820.9 823.9 829.5 843.4

=

=

dipole and magnetic dipole transitions, respectively. The integrated absorption coefficient is fa(v)dv. The refractive index (n) at the corresponding wavelength is found by fitting a curve determined from the known n values provided in

now routine with the relevant equations and assumptions presented in many papers [18—23].We have used the same approach as previously [24—27] to determine the Q 3~in KCaF 1 values for Er 3: 2, f~ 2 and 0.57 X 1020 cm2. From these parameters, the 0.74 x cm 4 0.87 X 1020 cm calculated oscillator strengths can be computed. Table 2 tabulates the measured and calculated = =

=

oscillator strengths as well as their residuals. The root-mean-square deviation of the oscillator strength for this study is 4.9 X 10~ which is comparable to the rms deviation found by applying the Judd—Ofelt theory to Er3~in other hosts [18,22,24,25]. Table 3 provides the values of 3~in several various hosts. The smaller valueof ofEr~26 suggests that the Judd—Ofelt parameters most KCaF 3 has higher rigidity for the ions situated [28]. Also, the lower ~2 value indicates 3~ions are in the site which is not far that fromthe cubic. Er The total spontaneous-emission probability is given by: 64 ~

3H 4~

6

5636 5497 5445 5333 5262 5169 5088 4988

1774 1819 1837 1875 1900 1935 1965 2005

A(aJ; bJ’)=

3 3(2J+1)hc

(XedSed+XmdSmd),

which is related to the radiative lifetime excited state i by

(2) TR

of an

(3) TRI

where

~

Sed

and

Smd

are the electric dipole and

C. Y. Chen et at

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3 ± and Tm3 + in KCaF

Optical properties of Er

3 crystal

189

Table 2 3~ in KCaF Measured and calculated oscillator strength of Er 3 at 300 K. All transitions are from the ~~i5/2 indicated

level to the level

Level

Wavelength (nm)

Average frequency (cm)

Oscillator strength (10_8) Measured Calculated

Residual (b08)

~~11/2 19/2 4F

966 797 650 542 518 486 449 441 405 377

10352 12547 15385 18450 19305 20576 22272 22676 24691 26525

21.4 7.8 78.2 14.1 118.8 72.1 11.4 3.7 26.4 218.6

4.09 —3.20 —0.54 —2.1 —2.9 4.54 —7.8 —6.8 —1.1 1.58

4S 9/2 2H3/2 4F 11/2 7/2 2H 4G 9/2 11/2

magnetic dipole line strength between the J manifolds, the average frequency i in eq. (2) of the transition was taken to be the center of the gravity of the emission band and the summation in eq. (3) is over electric and magnetic dipole transitions to all terminal states. With the electric dipole transition comprising most of the spontaneous-emission probabilities of any level, the magnetic dipole emission probabilities of ~I53/2 ~I15/2, ~I11/2 ~ and ~I9/3 ~I11/2 were also calculated, The branching ratio was calculated from the equation —‘

—~

—~

A (i 3) ~ j~

=

~,

.

=

T~,A(i,

J).

(4)

The measured and calculated values for A, B, and T are shown in table 4 An ion in an excited state decays with a measured lifetime T given by l/’r

=

WR + WMP + WET +

...,

17.3 11.0 78.7 16.3 121.7 67.6 19.2 10.5 27.5 217.0

where WR is the radiative rate of the state, WMP the nonradiative rate due to multiphonon emission, and WET the nonradiative rate due to energy transfer. If WET is assumed to be small, the multiphonon emission rates can be determined directly by subtracting the calculated radiative rates from the inverses of the measured lifetimes. Several methods have been developed to calculate the temperature dependence of the multiphonon emission rate [24]. The theory developed by Huang and Rhys [29] is based on a single-configuration coordinate model in which nonradiative transitions between levels occur with a single-phonon energy h a. Their expression for the nonradiative rate is given by: WNR

=

N exp

(

—

s0 1 r

,~=o(s~ ~

—

r / ~

1

(5) /j!(p+j)!,

Table 3 3 + in various hosts Judd—Ofelt intensity parameters of Er KCaF 3 BZYTLE ~ ZBLA b) c) 20cm2) 0.74 2.44 2.54 4.59±0.25 £24 (10~° cm2) 0.87 1.55 1.39 1.21±0.21 Q2 (10 ~6 (1020 cm2) 0.57 1.18 0.97 0.48±0.33 a) Ref (25) b) Ref. (24). ~ Ref. (18).

\ P +1

S (6)

rwhere exp(N is h c~/kT),and a constant on p the E/h order ca is of the1013 number si, =

—

=

of phonons which must be emitted in order to conserve energy during the transition. The quantity E is the energy gap between the ermtting level and the closest lower electronic level. The Huang—Rhys factor is 5~ ~%E/2hca, where /.~E is a measure of the relative offset between these levels. The fundamental vibration frequencies can =

190

C. Y. Chen et a!.

/

3 + and Tm3

Optical properties of Er

+

in KCaF 3 crystal

3~in KCaF Table 4 Predicted spontaneous-emission probabilities of Er 3 Transition 4

113/2 11/2 9/2

4

115/2

—• ~*

__

115/2 4 113/2 45a15/2 4 113/2 4

111/2 ~115/2

4F 9/2

4

113/2

4 111/2

Average frequency 1) (cm 6430 10140 3710 12296 5866 2156 15100 8670 4960

Aed (s 1) 31.7 37.2 5.2 41.9 12.9

Amd (~_1)

TR

(ms) Calculated

Measured (15 K)

26.4

17.2 20.0

9.2 13.5

Calculated branching ratio 1 0.74 0.26 0.75 0.23 0.02 0.92 0.04 0.03

7.7 17.8 1.4

454.6 21.9 15.8

2.0

0.54

2.0

0.81

4

19/2 155/5 4 113/2 4 111/2 4 19/2

3/2

2H

111/2

2804 18059 11629 7919 5763 18895 20131 21775 22186 24141 17711 14001

0.3 337.4 135.5 10.3 17.2 923.8 875.5 390.4 333.7 195.9 383.1 99.3

19/2

11845

3.4

9041 25924

8.6 3164.3

—~

11/2

115/2

4F7/2 ‘~‘~ ~115/2 4F5/2 a ~‘i5/2 4F3/2 _ ~ 115/2 9/2

115/2 4

0 0.67 0.27 0.02 0.03 1 1 1 1 0.28 0.56 0.14

1.1 1.1 2.6 3.0 1.4

4

4F

9/2 —•

115/2

0.01 0.01 1

0.3

16

1000 800

(a)

(a)

600 400 .123 1 phonons -i3 22004 7 x 410 cm 4 S C I3,~—I ‘15/2I I Lii

° C,

I

is

I

(b)

i~ 800 w IL

4000i35

10 9 x =410 0.217 cm1 0phonons 84 4 1fl/~~ 115/2

.~

W

6

~ ~

10

I I

I

I

)b) 000

0

0

8 6

200 ~ x 410cm 15/2

100

113/2

200

300

TEMPERATURE (K) Fig. 5. Temperature dependence of the (a) 453/3 and (b) 4F emission lifetimes, 9/2

2

~“

‘15/2

100

200

300

TEMPERATURE (K) 41i1/2 and (b) ~I,3/3 emission lifetimes. Fig. 6. Temperature dependence of the (a)

C.Y. Chen et at

/

3 ± and Tm3

Optical properties of Er

be determined from infrared [30,31] and Raman spectra. Infrared spectra for KMnF3 [30] reveal three broad peaks centered near 125, 224 and 420 cm ~ The initial values of these parameters were substituted into eq. (6) and the values of S0 and were adjusted to give the best fit to the ob-

+

in KCaF 3 crystal

900

3H

191

WAVELENGTH (nm) 800 700

625

4 EMISSION

—

4

I

served multiphonon emission rates. With the use

$

tive rates in eq. (5), the lifetimes were obtained. These of the calculated calculated multiphonon lifetimes form emission the solid andlines radiain figs. 5 and 6. The best fit values of S~,p, and hw

(I)

I

>-

are also4F shown in the for 4S figure. The values of1.hciiThis ~Iii/2, 9/2 and with 3/2the arehighest-energy all 410 cm phonon value is consistent obtained from the infrared reflectivity data. This suggests that the decay involves the emission of high-energy phonons and occurs in approximately the lowest order permitted by energy conservation. Previous studies in energy transfer [7—11,32—34] have indicated that energy absorbed by one rareearth ion can be transferred to other rare-earth ions. Evidence for this effect occurs in the excitation produces spectra since exciting of type. rare-earth ion emission fromone thetype other The excitation spectrum of the Er3 + ~‘13/2 emission for KCaF 3, containing 1.62 mol% ErF3 and 3F 0.35 mol% 3F TmF3, is illustrated in fig. 7. The 4 and 3

1000 5 I ~~13/2

900

WAVELENGTH (nm) 800 700

I

I

625 I

EMISSION 3F

J~9/2

ce $3 >C/I

z2

w ~

12 14 16 WAVENUMBER (i03 cm1) Fig. 7. The corrected excitation spectrum for Er3 + (~I11/2and 4F 3~(3F 3F 9/2) and Tm 3 and 4) transitions (detected by ~13/2 emission). 10

z

w

2

I— F-

z —

Il

[\~j~\~/2

1

t

c 10

12

14

16

WAVENUMBER (10~cm1) Fig. 8. The corrected excitation spectrum for Er3 ± (4 Fg/ 3 ± (3 F 3F 3H 2) and Tm 3 and 4) transitions (detected by 4 emission).

3~are shown in this figure, which bands of energy Tm transfer from Tm3 ± to Er3 Furindicates thermore, the Er3 + 4F 9/2 band also34’ appears 3H in the excitation spectrum for the Tm 4 —* ~ H6 emission3 +(fig. 8). This suggests energy transfer to Tm3 from TheErupconversion mechanism for a crystal contaming Er3 ± and Tm3 + is depicted in fig. 1. Up~.

~.

converted fluorescence can be generated from the 4S 4F 3/2 and the 9/2 manifold when a populated manifold is excited. Two 1-p.m photons are 4F required to cause population of the 7/2 level. 4F level and a second to One to populate the ~I1i/2 excite 4F this electron to the 7/2 level. Excitation 2H of the 7/2 level decays rapidly via the 11/2 level 4S to the emitting 3/2 level. In addition, the upconversion en’ussion of the Tm3~3F 4 level An maymibe observed when the ~I11/2 level is excited. dent infrared photon with energy corresponding 3~~Ii1/2 absorption band provides excitation to the 411i/2 level. The excited Er3~ion to the Er non-resonantly transfer energy to a Tm3 ± ion, exciting it to the 3H 3H 5 level. 5 excited state 3H The decays rapidly to the 4 level by multiphonon relaxation. Finally,a either same or a nearby 3~ion absorbs secondtheinfrared photon and Er transfers the energy to the same Tm3~ion, exciting the Tm3 ± from 3H 3F 3F 4 to 2 level. The 3F2 state relaxes by multiphonon relaxation to the 4 state which emits fluorescence at 805 nm. .

192

C. Y. Chen et at

/

3 ± and Tm3 ± in KCaF

Optical properties of Er

The upconversion efficiency is defined as

(7) where ~ is the emitted light intensity and ‘abs is the absorbed incident intensity. The incident absorbed light intensity was measured to be 0.63 mW/cm2 at 15 K and 0.31 mW/cm2 at 300 K. The peak output of the exciting light is 962 nm and the bandwidth is 8 nm. Details of the techmque used to determine ~ have been described previously [25]. The room temperature upconversion efficiencies of the Er3 + 4S 3/2 (green) and 4F 9/2 (red) emission for this intensity of incident light were 6.1 x 10_6 and 4.6 x 106, respectively. 3 + 3F No Tm 4 upconversion emission was detected at either 15 or 300 K which 3 + ~ could be due to the lowSince concentration of Tm energy transfer plays an important role in laser and upconversion processes, it is helpful to attempt to predict the impurity types and concentrations that maximize the desired process. A simple rate equation model can be used to 1oui/1’ais’

=

describe 3~and upconversion andFigure energy1 illustrates transfer beTm3~ions. the tween Er upconversion process in a KCaF 3 crystal on doped with Er3~and Tm3~ions. The notation the

3 crystal

3H affecting the populations of the other processes ~i ii/2 and 5 levels. The pumping intensity is sufficiently weak that the ground states are not appreciably depopulated. The W~terms represent the transition rates between levels i and i. The C,, terms are transfer rates between levels i and j. The lifetime of level I in the absence of ion—ion transfer is denoted by ‘r,. a.~is the absorption cross section of Er3~ions and ~2 is the incident pumping flux. NE and NT are the concentrations 3 ± and Tm3 ± ions in the crystal. of Er In order to solve rate equations such as (8)—(11) it is necessary to have experimental data. For example, since the emission intensity, I., is related to population for the n the by I,electron n,h w/r,, it is possible to emitting determinelevel, the n, values in the rate equations by measuring the various relative emission intensities. In fig. 1, I~ would represent the emission intensity from the ~113/2 level and I~’would be the emission intensity from the 3H 4 level. For this case the relevant experimental parameters for KCaF3 15 Ka~ are 11/12 0.0397, I1’/12 0.02, I~’/I~at0.35, 2 s~, T~t 5.0 1021 58 x s~, T 1 cm,50~2 S1, 1.23 T 1x lOi?cm_ 100 51, W 2 1’ 2~1~ iO~ 3 and ~ W~ 10 s~, NE 1.94 x 1020 cm NT 4.20 X 1019 cm3. We find n 1’/n1 0.24, n 1/n2 0.05, n1’/n2 0.02 from the expression ~,

=

=

=

=

= =

=

=

=

=

right side of each manifold represents the level. hand The labels used in the rate equations below 3~ are shown on the left-hand side. When the Er level is pumped, the appropriate steady-state rate equations are =

W~n2— Cli’nINT + Cl’inl’NE

=

a~42N~ — C22’n2NT

—

r~’n, o, (8) =

=

=

=

=

=

II

=

=

It is also possible to excite the

‘13/2

level

propriateand directly steady-state obtain valuable rate equations information. for excitation The apin this level are 1~i 0, (12) a~41N~ Cil’nINT + Cl’lnl’NE T1 =

=

—

‘r~n2— W21n2

=

0, (9)

=

—

—

C~’n

1fl

1N~—

Cl’lnl’NE

+

W2’1’n2’

—

‘r~’fl1’

0, (10) C W2’1’n2’have 0. been made in these (11) Several22’n2NT approximations equations. Stimulated emission terms have been ignored, transitions from the 4S 3/2 level 3F 3H to the level and from the 4 level to the 5 level are not taken into account to eliminate recycling which could then occur, back transfer from the 3H 5 levels to the ‘11/2 level 4F is neglected, and the 3F transfer to and from the 7/2 level or the 2 level is taken as insignificant compared with the = =

—

=

=

CIl’niNT

—

Cl’lnl’NE

—

2, 1’4, 0.1.70 x (13) lO~

T1’

=

=

In this case, s x 10—2i cm 2 s~ and I~’/i~ 0.23. Since only the lowest cm excited states are involved only I~and I~’can be measured. The other parameters are the same for =

=

excitation in the ~‘11/2 level and n 1’/n 0.16. The solution of these equations leads to the result: Cii’NT 16 51, (14) =

=

Ci’iNE C22~NT

0 s_i, 0 5~.

(15) (16)

C. Y. Chen et at

/

3 ± and Tm3

Optical properties of Er

It is noted that the energy transfer rate of C22’NT is smaller than for Cil’NT. ThisTm3 indicates that energy transfer between Er3 + and + ions occurs primarily at the lowest excited levels. This observation is helpful in understanding the work of Van der Ziel et a]. [35]. They used 1.5 p.m infrared excitation of Y~ Er~Tm~ F3 crystals and found that the green ~3/2 emission was quenched samples containing betweenthe 0.1Tm3 and+ 2% of Tm3in ± ions. When they increased content to more than 3% a nearly complete quenching of all visible Er3 + emission occurred. In all cases they found no Tm3 + emission. Our results suggest that when a sample is pumped with 1.5 p.m infrared radiation, most of the Er3 + energy transfer should occur from the Er3 ± ~I13/2 level to the Tm34’ 3H 4 level due to 3the high energy transfer + emission would be rate. Thus Moreover, the visiblealthough Er quenched. no Tm3 + emission was observed by Van der Ziel et al. [35], we believe that in most samples strong Tm3~infrared emission corresponding to the transition of 3H 4 3H 6 should be observed. Just as the research of Van der Ziel et al. [35] can be better understood through the combination of rate equations and experimental data, it is possible to obtain a quantative understanding of upconversion or energy transfer for other systems. In the future, a 2.8 p.m laser may be needed for quantative atmospheric measurements or for opti3.4’ 4 4 cal commumcation. The Er 111/2 ‘13/2 transition provides such a laser, but the long lifetime of the ~I13/2 level could present a problem. When 34’ to Tm3~4’ is the energy transfer rate from Er known and the levels at which transfer occurs can be identified, it may be posstble to choose the proper Er3 + and Tm3 + concentrations to maximize the efficiency.

_~

— .~

±

in KCaE 3 crystal

193

(2) Apparently earth 2~ions inthe thisrare lattice andions onlysubstitute one type for of Ca defect site is observed as compared to numerous sites for CaF 2 [14]. (3) The low energy phonon modes reduce the energy lost to multiphonon transitions and thus the quantum efficiency is relatively high even at 300 K. 3+ (4) and Effective occurs Tm3 ~.energy Ratetransfer equations arebetween useful Er (and sufficiently accurate) to describe the process and to provide predictions for dopant concentrations appropriate for potential devices.

Acknowledgements The authors are indebted to Dr. Yok Chen for concentration determination and to Dr. E. Arakawa for measurement of the index of refraction. This work was supported by NSF Grant No. DMR-84-0676.

—‘

“~

References [1] J.C.

Wright, Top. AppI. Phys. 15 (1976) 239. [2] F.E. Auzel, Proc. IEEE 61(1973) 758. 13] L. Esterowitz, J. Noonan and J. Bahler, Appl. Phys. Lett. 26 (1967) 126. [4] L.F. Johnson, H.J. Guggenheim, T.C. Rich and F.W. Ostermayer, J. AppI. Phys. 43 (1972) 1125. 151 L.G. Van Uitert, S. Singh, Hi. Levinstein, L.F. Johnson, W.H. and J.E. Geusic, AppI. Phys. Lett. 15 (1969)Grodkiewicz 53. [6] S.A. Pollack, D.B. Chang and N.L. Moise, Appl. Phys. Lett. 49 (1986) 1578; J. Appl. Phys. 60 (1986) 4077. [7] L.G. Van Uitert and L.F. Johnson, J. Chem. Phys. 44 (1966) 3514. [8] J.D. Kingsley, J. AppI. Phys. 41(1970)175. [9] R.A. Hewes and J.F. Sarver, Phys. Rev. 182 (1969) 427. [10] D.C. Yeh, W.A. Sibley and M.J. Suscavage, J. AppI. Phys. 63 (1988) 4644.

4 Conclusions

[11] J.P. Jouart, J. Lumin. 21 (1980) 153. [12] R.W.G. Wyckhoff, Crystal Structures, 2nd ed. (Wiley, New York, 1963).

In conclusion, several important points can be made (1) The segregation coefficient for rare earth ions in KCaF 3 is excellent. High dopant levels can be achieved.

[13] M.D. Shinn, iC. Windscheif, D.K. Sardar and W.A. Sibley, Phys. Rev. B 26 (1982) 2371. [14] D.S. Moore and iC. Wright, J. Chem. Phys. 74 (1981) 1626. [15] iL. Prather, Atomic Energy Levels in Crystals, NBS Monograph 19 (US Dept. of Commerce, 1961).

194

C. Y. Chen et al.

/

3 ± and Tm3

Optical properties of Er

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in KCaF 3 crystal

[281 C.K. Jorgensen and R. Reisfeld, J. Less-Comn~onMet. 93 (1983) 107. [29] K. Huang and A. Rhys, Proc. R. Soc. (London) Ser. A 204 (1950) 406. [30] C.H. Perry and E.F. Young, J. Appi. Phys. 38 (1967) 4616. [31] I. Nakagawa, A. Tsuchida and T. Shimanouchi, J. Chem. Phys. 47 (1967) 982. [32] L.F. Johnson, J.E. Geusic and L.G. Van Uitert, Appl. Phys. Lett. 7 (1965) 127. [33] L.F. Johnson, L.G. Van Uitert, J.J. Rubin and R.A. Thomas, Phys. Rev. 133 (1964) A 494. [34] N.V. Danil’chuk, S.G. Lunter, Y.P. Nlkolaev, G.T. Petrovskii, Y.K. Fedorov and V.N. Shapovalov, Soy. Phys. Dokl. 27 (1982) 853. [35] J.P. Van der Ziel, L.G. Van Uitert, W.H. Grodkiewicz and R.M. Mikulyak, J. App!. Phys. 60 (1986) 4262.