Spectroscopy and dynamics of upconversion in Tm3+: YLiF4

Journal of Luminescence 48 & 49 (1991) 517—521 North-Holland

517

Spectroscopy and dynamics of upconversion in Tm3~:YLiF4 M. Dulick, G.E. Faulkner, N.J. Cockroft and D.C. Nguyen Los Alamos National Laboratory, MS J564, Los Alamos, NM 87545, USA

3~:YLiF Spectroscopic and dynamic properties of Tm 4as an upconversion material with blue emission are presented. Absorption, fluorescence and excitation spectra at low temperatures were used to determine the crystal field levels of all manifolds except ‘S0 of Tm~°. 3~ The in YLiF observed energy levels were fitted in a least-square analysis to determine crystal-field t in YLiF parameters of the S4 site of Tm 4 with a standard deviation of 16 cm~’between the calculated and measured energy levels. Judd—Ofelt calculations using measured integrated absorption yield the intensity 4. 3F parameters f1~40for Tm° From these parameters, the calculated stimulated 2. emission An estimate cross of section the dipole—dipole of the ‘D~-~interaction 4 transition which is 2.4 gives x 10_to rise to cm~, quenching in good agreement of the excited withstate the measured was derived value from of 3the x l0~’° fluorescence cm decay curves at different concentrations.

1. Introduction

We recently reported a new upconversion 3~:YLiF Tm 4laser that emits blue light at 77 K and room temperature when pumped sequentially by two lasers at 781 and 649 nm [1,2]. 3~:YLiF To optimize the upconversion process in Tm 4, a detailed knowledge of its energy levels and excited state dynamical processes is necessary. Jenssen et al. 3~:YLiF has reported the energy structures of 3~ Tm substitutional 4and assigned S4 symmetry the Tm ion site [3]. However, tosignificant discrepancies between their results and our data prompted us to remeasure the 4f crystal field levels. With the new energy levels, we have repeated the crystal field analysis to obtain a new set of crystal field parameters for Tm3~in YLiF 4. Additionally, Judd-Ofelt calculations have been performed to predictThe transition intensities and radiative lifetimes. temperature and concentration dependence of the fluorescence decay 3F curves of the 450 nm ‘D2 4 transition is studied to evaluate the concentration quenching effect.

tal field levels of all manifolds except ‘S,, of Tm3* ions in YLiF 4 at low temperatures. Absorption and fluorescence spectra were collected with a Spex Model 1704 1 m monochromator. Fluorescence and excitation spectra were recorded with either a dye laser or Nd:YAG third harmonic excitation and absorption spectra with Xe and tungsten lamps. All measured energies are quoted for vacuum. Fluorescence decay curves were recorded on a Tektronix Model digitizing scope. The samples were11402 mounted on a oscilloclosedcycle refrigerator with temperature control between 10 and 300 K.

3. Results and discussion 2

is

often written

free for f’ as The a sum ofion six Hamiltonian terms HFI = Hee + H,~

0+ H00 + H000 + H

+ H

(1)

—*

2. Experimental High resolution absorption, fluorescence and excitation spectra were used to determine the crys-

0022-2313/91/503.50

© 1991

-

corresponding to the repulsive Coulomb, spinorbit, spin—spin, spin—other—than orbit, electrostatically correlated spin—orbit, and configuration interactions [4,5]. In all, the free ion Hamiltonian gives rise to a total of thirteen adjustable 2,radial and parameters which E5, are and designated as E’, E E3 for He~e,E4, E6 for H~ 1,~ for H00,

Elsevier Science Publishers B.V. (North-Holland)

M. Dulick er a!. / Spectroscopt- and dvnamic.s ot upconversion in Tm

518

5(k

=

: YLiI—

4

0, 2, 4) for H..~+ H~ 2, 4, 6)

than those of the other transitions. Thirdly, the

The S4 crystal field Hamiltonian can be expanded in terms of spherical tensors to give

laser output at 450.2 and 452.7 nm was found to contain both polarizations, consistent with the predicted overlap of two crystal field transitions at

M for ~

H(,

0,and P~)k

=

B~C~+ B~C~+ B~C~ 4 ~)+ B~(C~+ C°~). )2) + B~(C~+ C The combined analysis of polarized absorption, fluorescence and excitation spectra yielded the assignment of 56 levels out of a total of 70 levels allowed by S 4 symmetry. Of the observed crystal field levels, 50 levels with the greatest experimental confidence were included in a least-squares fit of assigned levels to the eigenvalues of the energy matrix for the 1-lamiltonian H1 + H( ~. The final fit gives a standard deviation of 16 cm ‘and values of the free ion and crystal field Hamiltonian parameters (table I). A comparison between experimental and calculated energy levels is shown in table 2. A least-square fit of the integrated absorption data yield the following Judd—Ofelt parameters: 2 [14 = (1.08±0.12)x fl, = (2.43 0.10)fl~ x =10~° cm±0.04) x 10 21) cm2 [6]. 10_20 cm2, ±and (0.67 =

From these values, we calculated the3F relevant spectroscopic parameters of the ‘D7-~ 4 upconversion laser transition (table 3). Four important observations relate to this table. First, the excited state absorption required for the upconversion laser is relatively strong. 3H The excited state absorption cross section ( 4—~‘D2) is calculated, from a measured line width of 2.5 cmi to be 2.9x 3Fbranching ratio for 2. Secondly, the 57% the 1020lasing cm transition (‘D,—s 4) is much higher Table I Final fit free ion and crystal field Hamiltonian parameters for Tm~:YLiF 4. E’2 E E4 E3 E5 E’~

6756.86±40.68 35.59± 0.04

P~ P4

991.92±176.65 6.23± 68.46

286.81 ±42.23 B~ 321.86± 19.70 677.78 ± 0.61 Pt~ 902.45 ±255.88 —16.33± 1.67 B~ —638.51 ± 39.31 33.38 ± 1.27 B~ —150.33 ± 47.00 ~ 2617.08 ± 3.28 Real(B~) 888.33 ± 31.31 M° 4.87 Im( B~) 0 2 274 Real(B~) 612.99± 28.09 M M4 1.86 Im(B~) 0 ____________________________________________________________

each wavelength. Finally, the total stimulated emission cross section of experimental 2.4x 10 ‘4 cmvalue at 450.2 nm compares well to the of 3 x 10 ‘°cm2[1,2]. Since the upconversion laser of refs. [1,2] relies on a single ion process, cross relaxation involving coupled Tm ions introduces loss and thus reduces the laser efficiency. These cross relaxation mechanisms are reflected in reduced fluorescence lifetimes and non-exponential decay curves. It is found that the fluorescence decay of the D2 level at relatively high Tm concentrations shows dependence on both temperature and concentration. In fig. I, the inverse of fluorescence lifetimes of the ‘D2 level at 1.5% concentration is plotted versus inverse temperature, with a curve fitted to the following empirical expression 1/T~,hoers0d= 1/T0+ R0 exp( — ~E/kT). (3) The best fit gave an intrinsic lifetime r 0 of 77.5 ~i.s, R of 7.1 x 10~s’ and an activation energy ~E of 32cm ‘. YLiF~has an effective phonon energy of 490cm’ [7] whereas the energy gap between ‘D2 and ‘G4 is 6400 cm~ (table 2). The 13 phonons required to bridge this gap make lifetime reduction —______

__________

~ ~

j

~ — 10

-~

11114

11(16

002

010

1112

Irremperature (Inverse °K( Fig. I. Decay rate of ‘D 4F cence lifetime) as a function of inverse (inverse temperature. The 2 -. 4 fluorescence of fluoresasymptotic limit at T = 0 K is 0.0129 js or an intrinsic lifetime of 77.5 ~s.

M. Dulick et a!.

/

3~:YLiF

Spectroscopy and dynamics of upconversion in Tm

Table 2

519

3~:YLiF

Observed and calculated energy levels of Tm SLJ 3H 6

4

4in cm~.

1’

Observed

Calculated

2 3,4

300

270

56 270 305

52 269 296

—19 —5

2 2

Error

SLJ 3F,

30

1 2 3,4

Observed 15094 15203

4

2

—

1

1

15275

Calculated 15105 15200 15232 15269

1 3,4 2

20973 21186 21 272

20988 21194 21 267

Error —11 3 —

6

9 iG

—15 —8

319

338

3,4

334 372

339 394

3,4

407

395

12

1

21 300

2

419

423

—4

2

—

3,4

21 554

21 310 21467 21 545

1

21 562

21 546

2

27961

27963

—2

2 3,4

27991 28053 28075

28997 28051 28067

—6 2 8

— 34729*

34582 34563 34609 34778 34880 34995 35014 35024

—

—

23,4 1 1 3,4 2 1 3,4

—15

2

— —

35 194 35197

—

2

—22

5 —10 —

9

3F 4

3H~

1 1 3,4

5599 5756 5757

5599 5766 5756

2 2

5820 5942

5815 5938

1 3,4 2 3,4 1 3,4 1

5968 5972 8284 8300 8319 8501 8519

5973 5972 8285 8300 8319 8497 8528 8549 8551

2

—

3,4

—

1

8 535

8 550

0 —10 1 5

‘D2

4 —5 0 —1 0 0 4 —9

1 iI

—

—

34778 34769* —

34999* 34998*

16

147

—

0 —lii —

—15 —26 —

3H 4

3P

2

12599

12606

—7

1

12624

12613

11

0

1

35538

35538

0

3,4 11 3,4 2

12643 12745 12804* 12835 12891

12634 12741 12799 12832 12924

9 45 3 —33

3Pi

1 3,4

36470 36566

36471 36565

—1 1

3P,

2

—

37871

3,4

38049

38071

—

—22

3F 3

3,4

14520

14517

3

2

—

2

14549

14545

4

1

38241

38166 38220

3,4

14594

5

2

—

14589 14594

—

—

74728

1

14597

14610

—13

15

—

21 —

*Excluded from the final fit.

tD of 2 by multiphonon decay very unlikely cornpared to cross relaxation quenching. The 32 cm~ activation energy may arise from cross relaxation mechanisms involving an ion 3Hin the ‘D2 manifold and another in the ground 6 manifold, both of

30cm_i. Specific mechanisms with negligible energy mismatches will be reported elsewhere. The iD2 fluorescence decay curves for Tm concentrations of 1.5%, 5% and 7% exhibit increasing degrees of nonexponentiality at short time (fig. ~),

which have a lowest crystal field splitting of

consistent with cross relaxation quenching. Super-

52(1

M. Dulick e a!. / Spectroci-ops- and dynamics o/ upcontervion in Tm

YLiI-

4

fable 3 Parameters determined by Judd—Ofelt theor~ reles ant to

1),

‘F4 upconsersion.

(11cm 1D Abs. 780.8 nm (‘H6l,4 —~ ‘H4[,4)n polarized Abs. 648.8 nm (‘H4F,4—’ 21,4)rr polarized Em. 450.2 nm (iD,l.~_. ‘F4F,4) u polarized Em. 450.2 nm (‘D212 ‘F411)ir polarized Em. 452.7 nm I ‘D2F,4—’ ‘F41’1 ( o~polarized Em. 452.8 nm ) ‘D,[,4—o ‘F41,4)-rr polarized Branching ratios II)

1.1 x 10 2.9 x 10 1.5 x 10 0.9 x 10 ((.9 x 10 1.6 x 10

=

‘G4 transition

((.0067

—‘

‘F, transition

((.0345

‘F2 transition ‘H4 transition ‘H, transition ‘F4 transition ‘H0 transition Calculated radiative lifetime of ‘D2 levet

mental decay curves to the following expression 2~I

‘‘ ‘‘ 19 ‘‘

5 (meas. 2.5 (meas.) 4 (meas. 4 (meas.) 4 (estd. I 4 (estd.(

to describediffusion a dipole—dipole transfer process neglecting [8]. All energy three simulated curves

(4)

2

20

where r,, is the intrinsic radiative lifetime, N is Tm ion density, and cs is the dipole—dipole coupling constant. This expression has previously been used

curves calculated from a fit of all three experi2Nai —/r~—~.n-’

20

(.0390 11.0563 ((.0035 ((.5664 (.2927 84 ~

imposed on these decay curves are the predicted

In ~

~i’(crn

-1.0

1.5 7. -2.0

\

~

——

-3.0~

..-~ I,, —

77

-40

-%.0

,-

~-v-

—6.0 0.0

5.0

T

10.0

15.0

.-.--

I

20.0

2

25.0

30.0

35.0

40.0

45.0

Time 1asec Fig. 2. Fluorescence decay curves of 1.5, 5 and 7% Tm’~:YLiF4crystals superimposed on the simulated decay curves using eq. (41 (See text).

M. Dulick eta!.

/

34.. YLiF

Spectroscopy and dynamics of upconversion in Tm

were generated using the 2/s. same T0 of 72.4 Although r p.s and a values of 6.5 x i0~ cc 0 in this model should be the radiative lifetime, its value does not agree with the Judd—Ofelt calculation (84 p.s) and temperature dependence study (77.5 p.s). Further experiments are proposed to

4

521

calculation based on the Judd-Ofelt model provides values for the ground and excited state absorption cross section as well as emission cross sections. The parameters obtained are consistent with experimental values observed for the 450 nm upconversion laser.

resolve this discrepancy by lifetime measurement

in dilute samples. References 4. Conclusion

[1] D.C. Nguyen, G.E. Faulkner and M. Dulick, AppI. Opt.

30:YLiF The crystal_field levels of Tm 4 have been fitted in a least-square analysis that yields 16 cm_i standard deviation between the observed and calculated values. The iD2 fluorescence decay 3~:YLiF curves of Tm 4 exhibit nonexponential behaviors at short time due to a cross relaxation quenching mechanism. Discrepancy still exists between the calculated radiative lifetime and that estimated from the temperature and concentration dependence of fluorescence decay curves based on a dipole—dipole energy transfer model. Intensity

28 (1989) 3553. [2] D.C. Nguyen, G.E. Faulkner, ME. Weber and M. Dulick, SPIE Proceeding V.1223 (1990) 54. [3] H.P. Jenssen, A Linz, R.P. Leavitt, C.A. Morrison and D.E. Wortman, Phys. Rev. B 11(1975) 92. [4] B.R. Judd, Phys. Rev. 141 (1966) 4. [5] BR. Judd, H.M. Crosswhite and H. Crosswhite, Phys. Rev.

169 (1968) 130. [6] Details of the fit and data used will be presented elsewhere. [7] E.B. Sveshnikova, A.A. Stroganov and N.T. Timofeev, Opt.

Spektrosk. 64 (1988( 73.

[8] J.C. Wright, Topics in Applied Physics, Vol 15: Radiationless Processes in Molecules and Condensed Phases, ed.

F.K. Fong (Springer, Berlin, Heidelberg, 1976).