optical properties, fluorescence mechanisms and energy transfer in tm3+, ho3+ and tm3+ -ho3+ doped...

October 1995

ELSEVIER Optical Materials 4 ( 1995 ) 797-810

Optical properties, f luorescence mechanisms and energy transfer in Tm 3 +, Ho 3 + and Tm 3 +-Ho 3 + doped near-infrared laser glasses,

sensitized by Yb 3 +

Bo Peng *, Tetsuro Izumitani lzumitani Special Lab., R&D Center, Hoya Corporation 3-3-1 Musashino, Akishimashi, Tokyo 196, Japan

Received 8 April 1994; revised 22 August 1994; accepted 4 July 1995

Abstract

The optical properties of the rare elements T m 3+, H o 3+ and Yb 3+ were systematically investigated in various glasses. The Tm 3 + doped aluminozircofluoride glass shows higher quantum efficiency, longer lifetime and stronger fluorescence intensity t h a n T m 3 ÷ doped YSGG crystal and other T m 3 ÷ doped glasses for the 3H4 ~ 3H 6 transition. Similar quantum efficiency, longer lifetime and stronger fluorescence intensity were also found in HO 3 + doped aluminozircofluoride glass for the 5I 7 ~ 5[ 8 transition. The higher quantum efficiencies of T m 3 + and Ho 3 + in aluminozircofluoride glass are due to the longer lifetime and the lower phonon energy. The fluorescence mechanisms and energy transfer in the Y b 3 + - T m 3 ÷ system, Y b 3 +-HO 3 + system and Yb 3 +- T m 3 + - H o 3 + system were studied. The very strong fluorescence intensities in the y b 3 + - T m 3+ system for T m 3+ and the Y b 3+-

Tm3÷-Ho 3÷ system for Ho 3+ which are 1.68 times that of Tm 3÷ doped YSGG crystal and 2.25 times that of Tm3+-Ho 3÷ codoped YSGG crystal are attributed to the efficient Y b 3 + -* Tm 3 +, Y b 3 + ~ H o 3 + and Tm 3 ÷ ~ Ho 3 + energy transfer processes. The fluorescence processes are described by cross relaxations of 2F5/2 ~ 3H 5 ~ 3H 4 "-* 3H 6 ~ 2F7/2 and 2F5/2 ~ 3H 5 ( o r 2Fs/2 --~ 516 ~ 3H s ) ~ 3H 4 .---) 517 --~ 518 ~ 3H 6 ~ 2F7/2.

1. Introduction

Recently, near-infrared tunable solid state lasers have been investigated in solid materials [ 1-8] for various application, for example medical application, eye safe laser radar, atmosphere pollution monitoring,

etc. For obtaining efficient, compact and cheap sources of laser radiation in this region we examined the pos- sibility of glass. We systematically investigated the optical properties of Tm 3 +, Ho 3 + and Yb 3 + in various

glasses and studied the fluorescence mechanism, energy transfer and energy backtransfer in the Yb 3 +-

* Research and Development Center, Mitusi Matsushima Corp., 1-2-30, Sanno, Hakata-ku, Fukuoka, 812 Japan.

0925-3467/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0925-3467(95)00032- 1

Tm 3 + system, the Yb 3 +-HO 3 + system and the Yb 3 +-

Tm3+-Ho 3+ system by measuring the absorption

spectra, fluorescence spectra and lifetimes of these ions. We also researched the dynamics in Yb 3 + -Tm 3 + -Ho 3 +

system. It has been found that the Tm 3 + or Ho 3 + doped

aluminozircofluoride glasses have higher quantum efficiencies due to the longer lifetime and lower phonon energy.

2. Experiment procedure and theoretical background

2.1. Experiment procedure

The batch compositions of the glasses are listed in Table 1. High pure materials were used to eliminate

B. Peng, T. lzumitani / Optical Materials 4 (1995) 797~810 798

Table 1 The composition of base glasses in this study

Glass Composition (cat%)

Fluoride(AZF) Gallate Aluminate Fluorophosphate Germanate Silicate Borate Phosphate

28.6A1F3-14.5ZrF4-51.6 ( MgF2 + CaF2 + SrF 2 + BaF2 ) -6.4 ( NaF + NaCI ) 50GaOi.5-8KOo.5-14CaO- 19SrO-9BaO 48A10~ 5-36CaO-8MgO-8BaO 10PO2.5-33A1F3-4YF3-48.3 ( MgF2 + CaF2 + SrF2 + BaF2 )-5NaF 57.5GEO2-16.5BaO-26KOo 5 50SiOz-5AIOI.5-24LiOo 5-12NaOo s-9SrO 74.85BO15-16.96BAO-8.19LaOi.5 65PO25-9A1015-8 ( MgO + BaO)- 18KOo.5

contamination by impurities. Fluoride and fluorophosphate glasses were prepared by melting the batch materials in glassy carbon crucible in nonreactive gas atmosphere inside a silica tube. After the melting, the glass was rapidly cooled to the glass transition temperature and annealed. Aluminate and gallate glasses were melted in a platinum crucible under dry nitrogen atmosphere control and glass was cast in graphite molds and annealed. Silicate, germanate and phosphate glasses were melted in a platinum crucible in air. The melt was poured into graphite mould and annealed. All samples were fabricated to a size of 25 × 25 × 5 mm 3 and opti- cally polished.

The absorption spectra were measured by a Hita- chi330 spectrophotometer at room temperature. The emission spectra were obtained by exciting the samples with a diode laser operating around 790 nm, 980 nm or a xenon lamp. The light from the light source was chopped at 80 Hz and focused to a 5 × 25 mm 2 face of the sample. The position of 1 mm from the edge was excited to minimize the reabsorption of emission. The emission from the sample was focused to a monochro- mator and detected by Ge detector and InAs detector cooled at 77 K. The signal was intensified with a lock- in amplifier and processed by a computer. The fluorescence lifetimes were measured by exciting the samples with a Nd:YAG pulse laser and a xenon lamp and detecting by a S-1 photomultiplier tube and an InAs- detector. The fluorescence decay curves were recorded and averaged with a computer-controlled transient dig- itizer.

2.2. Theoretical background

The spontaneous-emission probability (Arad) and emission cross section of T m 3 + and H o 3 + were cal-

culated by Judd-Ofelt theory developed by Judd [9], Ofelt [ 10], Krupke [ 11 ], and Weber [ 12]. The measured oscillator strength of the absorption transition can be calculated from Eq. ( 1 ),

P= ( mcn2 / Tre2Nx) fK( v ) dr, ( 1 )

where m and e are the mass and charge of the electron, c is the velocity of light, N is the concentrations of absorption centers, fK(v) dv is the integrated absorption coefficient. X is a factor for the effective field at a well-localized center in a medium of isotropic refractive index, n. It is given by Xed = n ( n 2 q- 2) ~- / 9 for elec- trical dipole transitions and )(md=n 3 for magnetical dipole transitions.

The oscillator strength of a transition from the level J to J ' is

P( aJ, bJ ' ) = [ 87rZmv/3h( 2J + 1 ) e 2]

X [Sed(aJ, bJ ' ) + Smd(aJ , bJ ' ) ] , (2)

where Sed and Smd are electric dipole and magnetic dipole line strength which are given by

Seo(aJ, bJ') = e 2 2 0 , ( ( f N J I U"' [fNJ')) 2, (3)

Stud(a J, bJ') = ( ~reh/mc)2( Q~NJlL + 2S l fSJ ' ) ) 2. (4)

The matrix elements in Eqs. (3), (4) are calculated using formulas given by Carnall et al. [ 13] and Weber [ 12]. In this work the matrix elements calculated by Carnall et al. [ 14] and Weber [15] have been used. Although the matrix elements are not exactly appro- priate for T m 3+ o r H o 3 + in glasses, the differences should be small for the present purposes. The intensity parameters Ot for various glasses were calculated by using the least-squares fitting approach method based

B. Peng. T. lzumitani / Optical Materials 4 (1995) 797~I0 799

on the measured oscillator strengths of the various observed transitions. The spontaneous emission prob- abilities of electric dipole radiation and magnetic dipole radiation were given by

Aed(aJ, bJ') =64"rr4ua/3h(2J+ l)c3*XedSed, ( 5 )

Amd(aJ, bJ') =647r4u3/3h(2J+ 1)c3*XmdSed • (6)

The spontaneous-emission probability (Araa) is

Arad(aJ, bJ') =Aea(a,], bJ') +Ama(aJ, bJ') . (7)

The emission cross section o- is given in the form of Ara d divided by the half-width of the fluorescence spectrum AA, which is

O" = ( A4/8"l'i'ch 3A A)Ara d. ( 8 )

The quantum efficiency r/is defined by

T] = Arad T . (9)

The spontaneous-emission probability (Arad) of Yb 3 + was calculated by Fuchtbauer-Ladenburg equation [ 16 ] :

Arad = ( 8 7 r n 2 / N o h~) [ ( 2 J + 1 ) / (2 J ' + 1)]

X~K(u) dr , (10)

where n is the refractive index, No is the concentration of Yb 3 + ions, and Ap is the peak wavelength of absorption. ~K(v) d u is the integrated absorption coefficient. The energy transfer efficiencies were calculated by the Reisfeld equation [ 17]:

r /= 1 - ~'/Zo, (11)

where zis donor and accepter codoped donor' s lifetime, r0 is the lifetime of donor only.

3. Results and discussion

3.1. Optical properties of Tm 3 +, Ho 3 + and Yb 3 + in various glasses

Table 2 shows the total spontaneous-emission probability Ara a of Tm 3 + in various glasses calculated by Eqs. (3) through (7). The number and types of radiative and nonradiative transitions are restricted by selection rules. In the Russell-Saunders limit, these selection rules include:Magnetic-dipole:

A S = A L = 0 , A J = 0 , __+ 1 ( 0 ~ 0 ) ,

Electric-dipole:

AS=0, AI= + 1, IALI, [AJ[ <21,

Electric-quadrupole:

AS=0, IALI, IAJI _<2 (0,--,0, 0,--, 1).

Therefore, the magnetic dipole radiations only occurred in 3H 5 ~ 3H6, 3H 5 ---> 3H 4, 3F 4 ---> 3F 3 and 3F 3 ~ 3F 2 transitions. The branch ratios of Tm 3 + in various glasses were listed in Table 3. For the 3H 4 ~ 3H 6 transition, the branch ratios of all glasses are 1. The total spontaneous-emission probability Araa and the branch ratios o f H o 3 + in various glasses were presented in Table 4 and Table 5 using same calculation method. The magnetic dipole radiations occurred in the 517 ~ 518,5I 6 --> 517, 5I 5 ~ 516, 5I 4 ---> 5I 5 and 5F 4 ---'> 5F 5 transitions. The branch ratios of 5I 7 ----> 518 transition in various glasses are 1. The optical properties of the 3H 4 "-> 3H 6 ( T m 3 + ) transition, 5I 7 ---> 5I 8 ( HO 3 + ) transition and 2F5/2 ~ 2F7/2 (Yb 3 + ) transition were shown in Table 6, Table 7, and Table 8. It can be seen that the spontaneous-emission probability Araa is Yb 3 ÷ > Tm 3+ > Ho 3+ in order and the lifetime is Ho 3+ > Tm 3+ > Yb 3÷ in order. As well known a sensitizer is necessary for three-level laser, this suggests that Yb 3 ÷ and Tm 3 ÷ will be sensitizers for the 3H 4 ----> 3H 6 (Tm 3 + ) and 5I 7 ----> 5I 8 ( H o 3 ÷ ) transitions in order to obtain an efficient, compact and cheap sources of laser radiation in infrared range. Table 9 and Table 10 present the intensity parameters J2, of Tm 3 + and HO 3 + for various glasses. It is observed that the oxide glasses have large g22 and small ~26 values, whereas the fluoride glass has comparable values for all three O,. According to the discussion by Izmitani [ 18 ], this suggests that the fluoride glass has lower ion polarization than oxide glasses. From Table 10 it also can be found that the ~26 o f H o 3 + in fluoride glass is larger than that of Ho 3 + in various glasses. Since the 5I 7 ----> 5I 3 ( H o 3 + ) t r a n s i t i o n

is mainly affected by Y26 and Ara 0 is proportional to ~6 [ 13], the fluoride glass should give higher spontaneous-emission probability Ara d for the 5I 7 ---> 5I 8 ( H o 3 + )

transition. The phonon energy in various glasses are determined by measuring the IR spectroscopy and shown in Table 11. It is borate > phosphate > silicate > germanate > aluminate > gallate > fluoride in order except fluorophosphate glass because it is hard

800 B. Peng, T. lzumitani / Optical Materials 4 (1995) 797-810

Table 2 Predicted spontaneous-emission probability (Aed + Amd) of Tm 3 ÷ in various glasses (s- ~ )

Glasses

Transition Fluoride Aluminate Gallate Germanate Silicate Phosphate Fluorophosphate

3H4 ~ 3H 6 115.01 297.02 344.79 200.46 160.69 201.64 153.65 3H 5 ~ 3H 6 178.33 285.68 327.93 203.19 181.23 220.83 201.06

3H 4 4.068 5.63 6.67 4.61 3.89 5.43 4.34 3F 4 ~ 3H 6 602.89 1294.84 1524.13 892.74 716.90 972.45 725.33

3H 4 46.57 104.21 123.09 76.11 58.71 81.93 58.08 3I"-I 5 15.34 25.71 28.61 15.08 14.61 18.46 20.44

3F 3 ~ 3H 6 1302.71 1518.58 1657.08 765.51 898.59 1204.56 1477.95 3H 4 35.27 34.62 39.15 16.33 20.94 30.38 35.26 3H 5 129.83 444.56 522.77 331.32 237.46 273.92 179.25 3F4 2.25 5.36 5.95 3.71 3.28 3.47 3.43

3P 2 "~ 3H 6 401.56 362.36 409.16 155.01 198.71 320.87 411.05 3H 4 228.63 818.28 984.85 662.99 448.35 614.88 312.32 3H~ 149.47 179.25 200.29 92.92 105.64 141.97 172.59 3F 4 5.82 17.64 20.91 13.79 9.80 12.88 7.84 3F 3 0.03 0.03 0.04 0.04 0.03 0.04 0.03

~G4 ~ 3H6 483.39 1238.99 1432.09 860.93 669.52 885.32 646.18 3H 4 91.33 108.06 120.86 57.36 63.53 89.91 101.34 3H 5 401.56 599.79 691.68 362.19 336.82 475.84 437.39 3F 4 118.45 253.58 296.31 175.50 139.09 193.76 137.69 3F 3 36.86 42.14 47.47 22.24 24.65 36.21 40.47 3F 2 10.80 17.23 19.27 10.28 9.76 13.17 13.35

D2 ~ 3H6 5464.45 8055.51 8767.17 4568.36 4667.01 5622.91 6937.64 3H 4 5380.58 20046.9 23603.7 15822.3 10770.54 14674.3 7380.62 3H 5 69.86 68.01 73.85 30.33 39.98 58.23 69.01 3F 4 791.48 1768.37 2043.50 1271.51 964.15 1329.07 878.30 3F 3 405.69 1370.33 1582.14 1044.87 732.05 1006.62 552.32 3F 2 531.89 1099.03 1210.2 720.80 609.02 800.19 703.17 1G4 73.54 222.61 249.08 166.40 119.81 158.99 99.34

to es t imate the exact ly phonon energy. F luor ide glass

has a lower phonon energy than other glasses. Longer

f luorescence l i fe t imes were also observed in f luoride

glasses containing both T m 3 + and n o 3 + as shown in

Tables 6 and 7. Fig. 1 and Fig. 2 il lustrate the depend-

ence o f the T m 3+ and Ho 3+ f luorescence intensities on

quan tum eff iciency. It was found that the f luorescence

intensit ies o f 3H4--~aH 6 ( T m 3+) transi t ion and

5I 7 ~ 5I 8 ( H o 3 + ) transit ions are mainly control led by

the quan tum eff iciencies dr iven pr imari ly by the pho-

non energy. F luor ide glass and gallate glass give the

h igher quan tum eff iciencies due to the lower phonon

energy and longer l i fe t ime al though the fluoride glass

has nei ther a large cross sect ion for spontaneous-emis-

sion nor large spontaneous-emiss ion probabil i ty. Con-

sequently, the fluoride glass and gallate are considered

as suitable base glass. The chemica l propert ies o f gal-

late glass are not good, the T m 3+, Ho 3+ doped alu-

minozi rcof luor ide glass (hereaf ter cal led fluoride glass

in this paper) which has good chemica l propert ies for

applicat ion [ 19] should be better material for devel-

oping the near- infrared solid state lasers.

3.2. T h e a n a l y s e s o f f l u o r e s c e n c e m e c h a n i s m s a n d

e n e r g y t r a n s f e r p r o c e s s e s in the Yb 3 + - T m 3 + s y s t e m ,

Yb 3 + - H o 3 + s y s t e m a n d Yb 3 + - T m 3 + - H o 3 + s y s t e m

As discussed before a sensi t izer is necessary for a

three- level laser and fluoride glass is a good base mate-

rial for T m 3+ and Ho 3+ laser transit ion in the

near- infrared region. W e studied the f luorescence mechan i sms and energy transfer processes in Yb 3 +- T m 3 + system, Yb 3 + - H o 3 + system and Yb 3 + - T m 3 +-

Ho 3 + system in fluoride glass.

B. Peng, T. Izumitani / Optical Materials 4 (1995) 797-810

Table 3 Calculated branching ratio for emitting states of 3H 4, 3H 5, 3F 4, 3F 3, 3F2, IG 4 and ~D2 in Tm 3 + dopde various glasses

801

Glasses


3H,~ ~ 3H 6 1 1 1 1 I 1 1 3H4 ~ 3H 6 0.978 0.980 0.980 0.978 0.979 0.976 0.979

3H 4 0.022 0.020 0.020 0.022 0.021 0.024 0.021 3F 4 ~ 3H 6 0.907 0.909 0.909 0.907 0.907 0.906 0.902

3Hz 0.070 0.073 0.073 0.077 0.074 0.076 0.072 3Hs 0.023 0.018 0.018 0.026 0.019 0.028 0.026

3F 3 ~ 3 H 6 0.886 0.758 0.745 0.685 0.774 0.797 0.871 3H 4 0.024 0,017 0.018 0.015 0.018 0.020 0.021 3H 5 0.088 0.222 0.234 0.297 0.205 0.181 0.107 3F 4 0.002 0.003 0.003 0.003 0.003 0.0(12 0.001

3F 2 ~ 3H6 0.511 0,264 0.253 0.168 0.261 0.294 0.455 3H 4 0.291 0,594 0.610 0.717 0.588 0.564 0.346 3H 5 0.190 0,130 0.124 0.100 0.139 0.130 0.191 3F~ 0.008 0,013 0.013 0.015 0.013 0.012 0.008 3F 3 0 0 0 0 0 0 0

~G4 ~ 3H6 0,423 0,548 0.549 0.576 0.539 0.523 0.469 3H~ 0.080 0.048 0.046 0.039 0.051 0.053 0.074 3Hs 0.352 0.265 0.265 0.245 0.270 0.281 0.318 3F 4 0.104 0.112 0.114 0.118 0.112 0.114 0.100 3F3 0.032 0.017 0.018 0.015 0.020 0.021 0.029 3F z 0.009 0.008 0.007 0.007 0.008 0.008 0.010

1D 2 ~ 3H 6 0.430 0.244 0.234 0.193 0.261 0.239 0.418 3H 4 0.423 0.608 0.629 0.670 0.602 0.617 0.444 3H~ 0.005 0.002 0.002 0.001 0.002 0.003 0.004 3F 4 0.062 0.054 0.054 0.054 0.053 0.057 0.053 3F3 0.032 0.042 0.042 0.044 0.041 0.043 0.033 3F, 0.042 0.033 0.032 0.031 0.034 0.034 0.042 IG 4 0.006 0.007 0.007 0.007 0.007 0.007 0.006

Yb3+-Tm ~+ system and Yb3+-H03+ system The fluorescence mechanisms of the Yb 3"-Tm 3 +

system and Yb 3+-HO 3+ system can be described as follows. The energy absorbed by Yb 3 ÷ is transfered to T m 3 + ( 3H 5) o r H o 3 + (516). In the Yb 3 + - T m 3 + system,

the T m 3 + 3H 5 state relaxes nonradiatively by multiphonon emission to the 3H 4 level, then emits fluorescence at 1.8 Ixm. In the y b 3 + - H o 3 + system, the H o 3+

5I 6 state radiatively transfers by emission at 2.84 ixm to 5I 7 manifold, then emits fluorescence at 2.04 Ixm. The fluorescence mechanisms are cross relaxations of 2F5/2 ~ 3H 5 "9 3H 4 ~ 3H 6 ~ 2F7/2 for the Y b 3 + - T m 3 +

system and 2F5/2 ---4' 516 ~ 517 ~ 5I 8 ---* 2F7/2 for the

y b 3 + - H o 3+ system as shown in Fig. 3. Fig. 4 and Fig. 5 show the dependence of the 2Fs/2(yb3+ ) level lifetimes on Tm 3 + and Ho 3 ÷ concentrations in fluoride glasses. When the concentrations of Tm 3 + increase,

the lifetime decrease quickly. The dependence of 3H 4 ~ 3H 6 ( T m 3 + ) and 5I 6 ~ 5I 7 ( H o 3 + ) f luorescence

decays on the concentration of Yb 3 ÷ in fluoride glasses is illustrated in Fig. 6 and Fig. 7. It can be seen that the fluorescence lifetime do not change remarkably. We calculated the energy transfer and energy backtransfer efficiencies by Eq. ( 11 ). The results are also shown in Figs. 4-7 . It shows that the energy transfer from Yb 3 + to T m 3 ÷ and H o 3 + is very ef f ic ient and the energy

backtransfer f rom T m 3 + o r H o 3 + to Yb 3 + is low. F ig . 8

shows the fluorescence spectra of Yb 3 ÷-Tm 3 + doped fluoride glass and Tm 3 + doped YSGG crystal. It can be seen that the Yb 3 ÷-Tm 3 ÷ doped fluoride glass are 1.68 times of Tm 3 + doped YSGG crystal. This strong emission is due to the efficient energy transfer from Yb 3÷ to Tm 3÷ and low energy backtransfer from Tm 3+ to Yb 3+. Fig. 9 gives the measured lifetime,

802 B. Peng, 7". lzumitani / Optical Materials 4 (1995) 797-810

Table 4 Predicted spontaneous-emission probability (Aed + Amd) of Ho 3 + in various glasses ( s - ~ )

Glasses


5I 7 ~ 5I 8 58.07 70.22 69.53 39.7 61.65 49.25 90.42 5I 6 ~ 5t 8 99.25 95.12 80.16 50.91 72.26 65.74 183.13

5I 7 19.58 24.92 28.59 7.33 15.01 19.21 34.11 5I 5 ~ 5I 8 31.43 37.06 32.41 11.19 27.97 26.33 67.97

517 49.35 45.19 32.95 12.44 29.85 30.13 93.52 5I 6 7.4 12.93 13.72 8.72 9.4 8.62 10.13

5I 4 "~ 518 0.23 0.18 2.51 1.08 3.32 2.98 10.18 517 25.6 22.02 11.93 5.6 14.08 14.9 47.58 5I 6 20.58 19.64 7.42 5.19 12.68 13 37.97 5I 5 4.79 5.5 1.81 3.23 3.26 4.17 6.82

5F 5 ~ 5I 8 1064.2 1618.04 2011.68 628.4 1196.33 1315.46 1995.98 5I 7 254.38 400.28 506.3 1 64.92 268.7 318.78 461.28 5I 6 54.46 68.69 218.6 24.39 49.2 49.42 99.75 5I 5 4.28 5.01 5.17 1.83 3.58 3.53 7.55 5I 4 0.03 0.06 0.18 0.02 0.06 0.05 0.07

5S 2 ~ 5I 8 723.25 603.63 185.98 145.28 458.15 401.27 1340.38 5I 7 487.2 403.38 256.89 97.35 287.7 271.98 899.04 sI 6 85.59 91.04 80.68 27.48 67.08 67.6 157.33 5I 5 20.63 18.01 13.13 4.64 13.55 13.05 37.11 5I 4 20.48 20.44 25.02 6.1 17.01 15.37 37.06 5F 5 0.20 0.34 0.48 0.15 0.28 0.35 0.35

5F 4 ~ 518 1948.67 2433.94 2697.67 868.09 1820 1894.03 608.47 517 224.23 426.28 639.07 194.5 306 385.63 416.25 5I 6 152.33 265.24 353.86 110.3 193.18 220.13 286.62 5I 5 76.13 88.35 96.21 31.06 66.11 48.46 142.65 3I 4 12.19 11.73 14.56 3.6 9.91 8.3 23.97 5F 5 5.68 14.49 19.88 10.72 9.6 9.14 7.43

5F 3 ~ 5I 8 1124.9 930.82 600.97 225.64 716.23 601.9 2106.68 5F z ~ 5I 8 948.94 784.96 509.66 539.48 648.01 513.43 1742.5 3K 8 ~ 5I 8 355.1 559.04 660.59 257.65 403.87 377.33 615.18 5G 6 ~ 5I 8 7766.6 26237.6 37027.4 15937.2 16211.8 15038.9 10466.3 5G 5 ~ 518 1887.9 3209.7 4364.68 1343.5 2391.39 2760.93 3417.23 5G 4 ~ 518 387.5 626.47 785 245.1 462.4 309.97 729.83 3K 7 ~ 5I 8 150.27 247.24 280.94 114.68 166.88 154.76 262 3H6 ~ 518 1326.6 1569.6 1846.7 550.98 1173.65 1274.44 2163.69 3H 5 ~ 518 2409.2 8160.68 11417.4 5023.4 5072.8 4809.94 3283.98 3L 9 ~ 518 623.1 849.67 851.41 3611.89 578 512.24 1096.04 3F 4 ~ 518 1495.4 2989.3 4335.68 1320.3 3206.58 2644.95 2460.3

3MIo ~ 518 911.55 . . . . . 1011.32 - - 1637.61 5D 4 ~ 5I 8 5882.7 . . . . . 8505.3 - - - 10779.3 3H 4 ~ 5I 8 5139.7 . . . . . 7771.74 - - 9301.5

ca l cu l a t ed s p o n t a n e o u s e m i s s i o n c r o s s sec t ion , and

e m i s s i o n and a b s o r p t i o n spec t r a o f Y b 3÷ 16 ca t%,

T m 3 + 2 ca t% d o p e d f luor ide g lass . In the Y b 3 ÷ - H o 3 +

s y s t e m , the e m i s s i o n o f 517 ~ 518 ( H o 3 + ) is no t ve ry

s t r o n g b e c a u s e it is e a s y fo r 516 ~ 552 u p c o n v e r s i o n

[ 2 0 ] .

Y b 3 + - T m 3 + - H o 3 + s y s t e m

A s d i s c u s s e d p r e v i o u s l y , the Y b 3 + - H o 3 + s y s t e m is

no t g o o d fo r H o 3 + 2 Izm la se r p e r f o r m a n c e . W e s tud ied

the Y b 3 + - T m 3 + - H o 3 + s y s t e m fo r the o p t i m i z a t i o n o f

H o 3+ 2 Ixm lase r p e r f o r m a n c e . Fig . 10 s h o w s that the

T m 3+ (3H4----~3H6) f l u o r e s c e n c e j u s t o v e r l a p s the

B. Peng, T. Izumitani / Optical Materials 4 (1995) 797-810

Table 5 Calculated branching ratio for emitting states of 517, 516, 515 ,5I 4 ,SF 5 ,552 and 5F 4 in Ho 3 + doped various glasses

803

Glasses


5I 7 ~ 5I 8 1 1 1 1 1 1 1 5I 6 ~ -Sl s 0.835 0.792 0.737 0.874 0.828 0.774 0.843

5I 7 o. 165 0.208 0.263 o. 126 o. 172 0.226 o. 157 515 ~ 5I 8 0.356 0.389 0.410 0.346 0.416 0.405 0.396

5I 7 0.560 0.475 0.417 0.384 0.444 0.463 0.545 516 0.084 0.136 0.173 0.270 0.140 0.132 0.059

5I 4 ~ 518 0.004 0.004 0.106 0.071 0.100 0.085 0.099 5I 7 0.500 0.465 0.504 0.371 0.422 0.425 0.464 516 0.402 0.415 0.313 0.344 0.380 0.371 0.370 515 0.094 0.116 0.077 0.214 0.098 0.119 0.067

5F 5 --* 518 0.772 0.773 0.733 0.767 0.788 0.780 0.778 517 0.185 0.191 0.185 0.201 0.177 0.189 0.180 5I 6 0.040 0.033 0.080 0.030 0.032 0.029 0.039 515 0.003 0.003 0.002 0.002 0.003 0.002 0.003 514 0 0 0 0 0 0 0

5S 2 ~ 5I 8 0.542 0.531 0.331 0.517 0.543 0.521 0.542 5I v 0.364 0.355 0.456 0.346 0.341 0.363 0.364 5I 6 0.064 0.080 0.144 0.098 0.080 0.088 0.064 51~ 0.015 0.016 0.023 0.016 0.016 0.017 0.015 51a 0.015 0.018 0.045 0.022 0.020 0.020 0.015 5F 5 0 0 0.001 0.001 0 0.001 0

5F4 ~ 5I 8 0.805 0.751 0.706 0.712 0.757 0.733 0.410 5I 7 0.093 0.132 0.167 0.160 0.127 0.149 0.280 5I 6 0.063 0.082 0.093 0.091 0.080 0.085 0.193 5I 5 0.032 0.027 0.026 0.025 0.028 0.026 0.096 5I 4 0.005 0.004 0.004 0.003 0.004 0.003 0.016

5F 5 0.002 0.004 0.005 0.009 0.004 0.004 0,005

H o 3 + ( 517 --) 518) a b s o r p t i o n b a n d . It is e x p e c t e d to b e

e f f i c i e n t f o r e n e r g y t r a n s f e r f r o m T m 3 + to H o 3 +. T h e

d e p e n d e n c e o f T m 3 ÷ ( 3H 4 ----) 3H6) f l u o r e s c e n c e d e c a y s

o n H o 3 + c o n c e n t r a t i o n s a n d energytransfer efficiencies f r o m T m 3+ (3H4) to H o 3+ (517) a r e c a l c u l a t e d b y Eq .

( 1 1 ) in t he Y b 3 + - T m 3 ÷ - H o 3 + s y s t e m a n d a re s h o w n

Table 6

in Fig. 11. The dependence of H o 3+ (5I~518) fluorescence decays on Tm 3 + concentrations and energy b a c k t r a n s f e r e f f i c i e n c i e s f r o m H o 3+ (517) to T m 3+

(3H4) a re calculated by Eq. (11) in the yb3+-Tm 3÷- Ho 3 + system and are shown in Fig. 12. From Figs. 11 and 12, it is indicated that the energy transfer from

Comparison of laser spectroscopic properties of Yb 3+ 2F5/~ level in various glasses

Up(Cm ~) N(ca t%) AR(S 1) ZM(~S) A h ( n m ) t r (10-Z°cm3)

Fluoride 10267 8 582 2280 67 0.60 Aluminate 10225 2 1309 1030 64 1.07 Fluorophosphate 10267 2 667 2120 67 0.64 Germanate 10267 2 750 1190 64 0.58 Silicate 10256 2 629 1040 63 0.56 Phosphate 10256 2 913 1560 66 0.79

Note: Up is the average frequency, N is the dopant concentration, AR is the spontaneous-emission probability, z M is the measured lifetime, A A is the halfwidth of emission spectrum, and cr is the emission cross section.

804 B. Pen g, T. lzumi tani / Optical Materials 4 (1995) 797-810

Table 7 Comparison of laser spectroscopic properties of Tm 3 ÷ 3H 4 level in various glasses

b'p (cm -1 ) N (cat%) AR (S -1 ) zM (ms) AA(nm) tr( 10-z° cm 3)

Fluoride 1820 8 115.01 6.38 240 0.32 Gallate 1860 2 344.79 2.44 278 0.63 Aluminate 1860 2 297.02 1.52 280 0.59 Fluorophosphate 1820 4 153.65 1.12 245 0.43 Germanate 1840 1 200.46 0.60 163 0.68 Silicate 1835 1 160.69 0.25 159 0.61 Phosphate 1835 1 201.64 - - 152 0.84

Note: Vp is the average frequency, N is the dopant concentration, A R is the spontaneous-emission probability, ~'r~ is the measured lifetime, AA is the halfwidth of emission spectrum, and tr is the emission cross section.

Table 8 • ' f H ~+ Comparison of laser spectroscopic properties 0 o- 5I 7 level in various glasses

Vp (cm -I ) N (cat%) Ar~ (s - l ) rM (ms) AA (nm) ~r ( 10-2° cm 3)

Fluoride 2035 2 58.07 26.7 118 0.53 Gallate 2055 2 69.53 8.2 141 0.38 Aluminate 2055 2 70.22 7.2 144 0.41 Fluorophosphate 2035 2 90.42 5.6 123 0.79 Germanate 2045 2 39.70 0.36 84 0.40 Silicate 2040 2 61.65 0.32 82 0.70 Phosphate 2040 2 49.29 - - - 78 0.62

Note: Up is the average frequency. N is the dopant concentration, A R is the spontaneous-emission probability, 7M is the measured lifetime, A A is the halfwidth of emission spectrum, and o" is the emission cross section.

Table 9 The intensity parameter ~ ( 10 -20 cm z) of Tm 3+ in various glasses

Glass O,2 /24 ~6

Fluoride (AZF) 1.92 1.68 1.13 Gallate 4.98 1.54 0.60 Aluminate 4.91 1.65 0.63 Fluorophosphate 2.75 2.28 1.18 Germanate 4.05 1.01 0.28 Silicate 3.31 1.21 0.48 Phosphate 4.86 1.68 0.77

2F5/2--* 3H 5 transfer occurs. From Fig. 13 we can see that the difference of energy state between HO 3+ (516)

a n d T m 3+ ( 3 H s ) is very small and the respective transitions of Ho 3+ (sI6) and Tm 3+ (3H5) are close to

being resonant. This means that the excitation energy can be transfered efficiently from Ho 3+ (516) to Tm 3+ (3H5). We measured the lifetime of n o 3+ (516) as a function of the Tm 3+ concentrations in the Yb 3+- Tm 3 + -Ho 3 + system and calculated the energy transfer efficiency based on the lifetime data. The results are

Tm 3+ (3H4) to Ho 3+ (517) is very efficient, when the H o 3 + concentration is over 2 cat% the energy transfer efficiency is higher than 90%, and the energy back- t r a n s f e r f r o m H o 3 + (517) to T m 3 + ( 3H 4) is l ow . T h e r e -

fore, it was suggested that the fluorescence mechanism o f H o 3 + (517 ~ 5I 8) t r a n s i t i o n in t h e Y b 3 + - T m 3 + -

H o 3+ system can be considered by 2F5/z 3H 5 ---) 3H 4 --~ 5I 7 ~ 5I 8 ~ 3H 6 ---) 2F7/2 c r o s s relaxations as shown in Fig. 13. But the 2F 5/2 (yb3 + ) also transfers the energy to 5I 6 (Ho 3+) at the same time as the

Table 10 The intensity parameter g2 ( 10 20 cm 2) of Ho 3 + in various glasses

Glass ~ O4 g26

Fluoride (AZF) 1.86 1.90 1.32 Gallate 5.70 3.10 0.37 Aluminate 4.90 2.50 0.68 Fluorophosphate 2.1 3.5 2.5 Germanate 3.30 1.14 O. 17 Silicate 3.60 2.30 0.65 Phosphate 3.33 3.01 0.61

B, Peng, T. Izumitani / Optical Materials 4 (1995) 797-810 805

Table 11 The phonon energy in various glasses ~o (cm ~)

Fluoride 540 Gallate 660 Aluminate 800 Germanate 820 Vanadate 850 Fluorophosphate 1050 Silicate 1050 Phosphate 1300 Borate 1350

• 400

2

2OO

m

Et

0 0

G a l l a t e O / / / O ~ A 1 u mlaFtle u°ride

0 Fluorophosphate

~ G e r m a n a t e Sil , icate, , , ,

20 40 60 80 I00 Quantum efficiency(~)

Fig. 1. The dependence of Tm 3 + (3H 4 -"* 3H6) fluorescence intensity on quantum efficiency.

~2

400

200

~ O"

C) /OGal l a t e

0 Aluminate

Fiuorophosphate

~)Si l , icate ~ Germanate , , ,

20 40 60 80 I00 Quantum eff iciency(~)

Fig. 2. The dependence of Ho 3 + (sit ---, 512) fluorescence intensity on quantum efficiency.

p r e s e n t e d in Fig . 14. It c a n b e s e e n t h a t t he e n e r g y

t r a n s f e r f r o m H o 3+ (516) to T m 3+ (3H5) is v e r y eff i -

c i en t . W h e n t h e T m 3 + c o n c e n t r a t i o n is o v e r 4 c a t % ,

t he e n e r g y t r a n s f e r e f f i c i e n c y a p p r o a c h e s n e a r 1 0 0 % .

W e c o u l d not o b s e r v e t he 5I 6 ~ 517 ( H o 3 + ) t r a n s i t i o n

a t 2 . 84 I~m in th i s case . C o n s i d e r i n g the d i s c u s s i o n

a b o v e , t h e f l u o r e s c e n c e m e c h a n i s m o f t he H o 3+

( 517 ~ 518) t r a n s i t i o n in Y b 3 + - T m 3 + -Ho 3 + system c a n

3H5

3H+

3H~

\

sI6

sI?

I 2F?/2 sIB

Tm ~ + yb 3 + Ho 3 +

Fig. 3. The fluorescence mechanisms for Yb3+-Tm TM and Yb 3+- no 3+ systems.

0.5

~ 0 . 4

. - 0 . 3

~ 0 . 2

0.1

O

o f ~ I /

O ~ O

/ []

I I 1 2

Tm ~+ (cat%)

100

9O

8O

70

Fig. 4. The lifetimes of Yb 3+ (2F5/2---->2F7/2) v e r s u s concentrition of Tm 3+ in Yb 3+ 16 cat% doped fluoride glasses and the energy transfer efficiency from Yb 3 ÷ to Tm ~ +.

0.5

0 . 4

~0.3

.~0.2

0.I

'o

[?

D /

I 1

llo ~* (cat%)

lOO

L]--

9O

8O

©

[ 70 2

Fig. 5. The lifetimes of Yb 3 + (2F5/2 ~ 2F7/2) versus concentrition of Ho 3+ in Yb 3+ 16 cat% doped fluoride glasses and the energy transfer efficiency from Yb 3+ to Ho 3+ .

806 B. Peng, 7". lzumitani / Optical Materials 4 (1995) 797-810

20 20

15

--0--0 0 0

~ 0 D

0 m

I [ I I I

7 I0 13 16 19

10 @

Fig. 6. The lifetimes of Ho 3 ÷ (5I 6 ~ 5 I 7) versus concentrition of Yb 3 + in Ho 3 ÷ 0.1 cat% doped fluoride glasses and the energy transfer efficiency from Ho 3 ÷ to Yb 3 +.

10 10

e. 468

g 3.12'

< 1.5G

---i I " o:2.,o,,.,/ \

1500 1700 1900

.--?

d

g

Fig. 9. The emission and absorption spectra of Yb-Tm codoped fluoride glass• Yb 16cat%, Tm 2cat%.

.~ 5

L~

--0-- O- O-- O-- O-- 0--0-_....0...

/ _ D_D--D--D--D--D ~D

I I I I I

4 8 12 16 20

2O@

Fig. 7. The lifetimes of Tm 3+ (3H4---*3H6) versus concentrition of Yb 3 + in Tm 3 ÷ 0.3 cat% doped fluoride glasses and the energy transfer efficiency from Tm 3 + to Yb 3 +.

--7.

g

-- t . _ ~

I

1660 1820 1980 2140

i

Fig. 10. The emission spectrum of Tm 3÷ (3H4-'~ 3H6) in Tm doped fluoride glass and the absorption spectrum of Ho 3 + ( 517 ---* 512) in Ho doped fluoride glass.

g

o =

1500

a

, \ ~k

4.

1660 1820 1980 2140

Wavel ength(nm)

Fig. 8. The fluorescence spectra of Tm 3+ (3H4"~ 3H6 ) transition in (a) Yb 3+ (28 . 8x 1020 ions/cm3), Tm 3+ (3 .6× 1020 ions /cm 3)

doped fluoride glass pumped by LD980 (50 mW); (b) Tm 3÷ ( 15.2× 1020 ions /cm 3) doped fluoride glass; (c) Tm 3+ (8 .0× 1020 ions /cm 3) doped YSGG crystal. (b) and (c) pumped by LD790 (50 mW).

6.5

s.2

.•3.9 "~ 2.6

1.3

"-1 /

/ ~ O n

/\o o

I 1 1

Ito 3÷ 0 1 2 3 4

Tm 3÷ 4 4 4 4 4

100

80

6o

4O

2O

Fig. 11. The lifetime of Tm 3+ (3H4~31-I6) and energy transfer efficiencies from Tm 3+ (3H4) to Ho 3+ (sI7) in Yb-Tm-Ho doped fluoride glasses.

B. Peng, T. lzumitani / Optical Materials 4 (1995) 797-810 807

30

24

18

12

~ 0 ~ 0

~ 0

100

8O

60

~ 0 El ; 40 ~"

~ E ~ r n ~ 20

I I I Tm a÷ 0 1 2 3 4

Ho 3+ 2 2 2 2 2

Yb 3÷ I0 i0 I0 i0 1O

Fig. 12. The lifetimes of Ho 3 + ( 5I 7 -o 512 ) and energy transfer efficiencies from Ho 3+ (517) to Tm 3+ (3Ha) in yb3+-Tm3+-Ho 3+ doped fluoride glasses.

2 F s / 2 ~ - ~ . . . . . ~ ~

zFT/2

\ \

3H5 t 3J-l~

__Sl 7

,~I .

yb 3. Tm3+ Ho3+

Fig. 13. The fluorescence mechanisms for Yb3+-Tm3+-Ho 3+ system.

b e d e s c r i b e d by 2F5/2 ---) 3H5, 5I 6 --) 3H 5 --) 3H 4 ---)

517 ---) 5I 8 ---) 3H 6 ~ 2F7/2 c r o s s r e l a x a t i o n s as s h o w n in

Fig. 13. This means that the energy was transfered from Y b 3+ (2F5/2) to T m 3+ (3H5) a n d H o 3+ ( 5 [ 6 ) O c c u r s

at the same time, and the energy on 516 (Ho 3 + ) state was transfered to the 3H 5 (Tm 3 + ) state efficiently, then relaxes nonradiatively by multiphonon relaxation from 3H 5 (Tm 3+) to 3H 4 ( T m 3 + ) ; a f t e r th is p r o c e s s , t he

energy on the 3H 4 (Tin 3+ ) state was transfered to the 517 (Ho 3 + ) state efficiently, and finally relaxes radiatively to the 518(H03 + ) state. Fig. 15 presents the emission spectra of the Ho 3+ (517~518) transitions in T m 3 + - H o 3 + doped fluoride glass, Y b 3 + -T in 3 +-HO 3 +

doped fluoride glass and Tm 3+-Ho 3+ doped YSGG

crys ta l . It c an b e s e e n tha t t he e m i s s i o n i n t ens i t y o f the

H o 3+ (5I 7 ---)518) t r ans i t i ons in y b 3 + - T m 3 + - H o 3+

d o p e d f luo r ide g l a s s is 2 .25 t i m e s h i g h e r t h a n in T m 3 - -

H o 3 + d o p e d Y S G G crys ta l . W e a l so c o m p a r e d the

e m i s s i o n c r o s s s e c t i o n (0-) and p h o n o n e n e r g y

b e t w e e n Y b 3 + - T m 3 + - H o 3 + doped fluoride glass a n d

T m 3 + - H o 3 + d o p e d Y S G G crys ta l . T h e p h o n o n e n e r g y

are 540 c m - ' f o r g l a s s a n d 640 c m - t fo r Y S G G . T h e

e m i s s i o n c r o s s s e c t i o n ( ( r ) a re 4 . 6 × 10 - ~ c m 2 fo r

3

2

j ~

D / / [] (-o O

I I I

Tm 3÷ 0 1 2 3 4

11o 3÷ 2 2 2 2 2

Yb 3÷ I0 i0 I0 lfl 1O

100

80

4O

20

Fig. 14. The lifetimes of Ho 3 + (5I 6 --~ 517) and energy transfer efficiencies from Ho 3+ (5I 6) to Tm 3+ (3H5) in yb3+-Tm3+-Ho 3+ doped fluoride glasses.

&

o=

1500

a

~_, d I

1660 1820 1980 2140

Wavelength(ran)

r 2300

Fig. 15. The fluorescence spectra of Ho 3 + (5I 7 ~ 518) transition in (a) Yb 3÷ (22.3× 1020 ions/cm3), Tm 3+ (7.2× 1020 ions/cm 3) Ho 3+ (4.4>( 102°ions/cm 3) doped fluoride glass pumpedby LD980 (50 mW); (b) Tm 3+ (15.0× 1020 ions/cm3), Ho 3+ (3.1 × 1020 ions/cm 3) doped fluoride glass; (c) Tm 3÷ (8.0>( 1020 ions/cm3), Ho 3+ (0.5>( 1020 ions/cm 3) doped YSGG crystal. (b) and (c) pumped by LD790 (50 mW).

808 B. Peng, T. lzumitani / Optical Materials 4 (1995) 797-810

glass and 5 × 10 -2 ' cm 2 for YSGG. There are no big differences between glass and crystal. The glass has higher emission intensity because optimum concentrations of the sensitizer and dopant were used and higher concentrations of the sensitizer and dopant can be doped in glass. From this results the Y b 3 + - T i n 3 ÷ - H o 3 +

doped fluoride glass is a promising candidate for tunable glass laser application operating around 2 Ixm. Since the intergral spectra between Yb 3+ (2F5/2--9 2F7/2) emission and Tm 3 + (3H 5 ~ 3H6) absorption or H o 3+ (516-'--~ 518) absorption are very small as shown in Fig. 16 and Fig. 17, the sensitizer concentrations of Yb 3 + should be doped in the system as high as possible. Fig. 1 8 shows the relationship between emission intensity and Tm 3+, Ho 3+ concentrations in Yb 3÷ 12.5 cat% doped fluoride glasses. When Tm 3 + is at 4 cat% and H o 3 + is at 2.5 cat%, the emission intensity shows a maximum. Since the concentration of H o 3 + should be as low as possible for a three-level laser, the Yb 3 ÷ 12.5 cat%, T m 3 + 5.5 cat%, H o 3 + 1 cat% doped fluoride glass was selected as candidate for application of tunable glass laser operating around 2 Ixm. Fig. 19 presents the measured lifetime, calculated emission cross section, emission and absorption spectra of the Yb 3 ÷ 1 2.5 cat%, Tm 3 ÷ 5.5 cat%, H o 3+ 1 cat% doped fluoride glass.

3.3. Dynamics of the 516 and 517 states in Yb 3+- Tm 3 +-Ho ~ + doped fluoride glass

If we let 5I 7 s t a t e (level 1) and 516 s t a t e (level 2) and our system is pumped to the 5I 7 state, the rate equation should be

/~-- II

//

950 1050 1150

Wave Iength (rim)

1250

Fig. 16. The emission spectrum of Yb 3 + (2F5/2 ~ 2F7/2 ) in Yb doped fluoride glass and the absorption spectrum of Trn 3 + (3H 5 ~ 3H6) in Tm doped fluoride glass.

" [ 7 - ~

j~

,!

! i I I t L

I ' - - - I

950 1050 1150 1250

Wavelength(nm)

.-?

Fig. 17. The emission spectrum of Yb 3 + (2F 5/2 ~ 2F7/2) in Yb doped fluoride glass and the absorption spectrum of Ho 3 + (516 ~ 512) in

Tm doped fluoride glass.

9

~ . 8

7

5

4

2

- 0

of - '/b 3÷ 19 cat%

I I I

~ 3 ÷eat~ 0 0.3 0.6 0.9 1.2

Fig. 18. The dependence of Ho 3 + ( 5I 7 ~ 512) emission intensities on Tm 3 +, Ho 3 + concentrations in fluoride glasses sensitized Yb 3 +.

dN/dt= -N/.c. (12)

The solution is

N(t) = N ( 0 ) exp( - t /z) , (13)

where ~-is the associated lifetime of 5I 7 level. When the excitation is produced in 5I 6 state, the rate equations are

d N 2 / d t - - -N2/'r'2, with N2(0 ) =No, (14)

d N 1 / d t = -N1/71 +A21N2, with Nl(0) =0 , (15)

whereAz, is the radiative and nonradiative spontaneous transition probability from level 1 to level 2. The solution for N1 (t) is given by [ 21 ]

B. Peng, T. lzumitani / Optical Materials 4 (1995) 797-810 809

b > ,

'y,

4.65 o = ~3.10

< 1.55

I I i

: 4 . 65 "10 - 2 ' cm ~

r :28.1ms

f ~

1700 1900 2100

Wavelength(run)

\

o =

Fig. 19. The emission and absorption spectra of Yb-Tm-Ho doped fluoride glass. Yb 12.5cat%, Tm 5.5cat%, Ho lcat%.

4.65

3.10 o

1.55

cr=4"65'10-*' era2 / /

711 x~x

1700 1900 2100

Wavelength(rim)

"-7.

t ~

"T.

=o

I I

4 8 Time(ms)

Fig. 20. The dependence of rio 3+ (516---:, 517) fluorescence intensity on time in ( a ) Yb 3 + 10cat%, Ho 3 + 1 cat%; (b) Yb 3 + 10cat%, Ho 3 + lcat%, Tm 3+ 0.5cat%; (c) Yb 3+ 10cat%, n o 3+ lcat%, Tm 3+ 1 cat%.

This equation means that the fluorescence coming from the level 1 rises at short times with a time constant equal to r2, and then decays exponentially with the time constant Z 1 provided T 1 > ,7" 2. When ~-, >> "/'2, Eq. (16) becomes

N 1 ~- T 2 N o W 2 1 exp(--t/7"1). (17)

This means that the fluorescence still decays exponentially with no rise time behavior, even though the excitation is produced in level 2.

Fig. 20 and Fig. 21 presents the fluorescence decay curves of H o 3+ (517) level and Ho 3+ (516) level with and without Tm 3 + ions in fluoride glasses sensitized by Yb 3+. When Yb 3+ (ZFs/2) was pumped by a laser pulse and the system has no Tm 3÷ ions, the energy transfers from Yb 3 + (2F 5/2)to n o 3 + (516) rapidly, the fluorescence coming from HO 3 + (516) decays exponentially without the rise time behavior shown in Fig. 20a. This decay behavior can be described by Eq. (13). In this case, the decay of the H o 3+ (517) level rises at short times with a time constant equal to the 5I 6 lifetime (5.8 ms), then relaxes radiatively with its own lifetime (28.2 ms) as shown in Fig. 21a. This process can be described by Eq. (16). When Tm 3÷ ions are added, the lifetime of HO 3 + (516) decrease quickly due to the energy transfer shown in Figs. 20b,c and the decay of the Ho 3+ (517) level does not exhibit the rise time behavior shown in Figs. 21b,c. In this case, the decay of rio 3+ (517) level can be described by Eq. (17).

i

I I I I I

10 20 30 40 50

T i m e ( ~ )

"q 'r2NoA 21 N , ( t ) - - -

.c I - .c 2 [exp( - t~ r l ) - exp( - t / z 2 ) ] .

Fig. 21. The dependence of Ho 3 + (5I 7 ~ 5I 2) fluorescence intensity on time in (a) Yb 3 ÷ 10cat%, Ho 3+ lcat%; (b) Yb 3÷ 10cat%, Ho 3 ÷ lcat%, Tm 3+ 0.5cat%; (c) Yb 3+ 10cat%, Ho 3+ lcat%, Tm 3+ 1 cat%.

810 B. Pen g, T. lzumitani / Optical Materials 4 (1995) 797~10

4. Conclusion

The experimental results indicate that the fluorescence intensities of the SH4 ~ 3H 6 (Tm 3 + ) transition and the 517 ~ s I s ( H o 3+) transitions are mainly controlled by the quantum efficiencies, determined by the phonon energy. Fluoride glass gives higher quantum efficiencies due to the lower phonon energy and longer lifetime, although the fluoride glass has smaller emission cross section and spontaneous-emission probability as compared to other glasses. Consequently, the fluoride glass are considered as base glass for devel- oping the near-infrared solid state lasers.

The very strong fluorescence intensities in the Yb 3 ÷ T m 3 + system for T m 3 + and the Y b 3 + - T m 3 +-HO 3 +

sys t em for Ho 3 + are 1.68 t imes o f T m 3 + doped Y S G G

crys ta l and 2.25 t imes T m 3 + - H o 3 + c o d o p e d Y S G G

crystal , and these b e h a v i o r can be a t t r ibu ted to the effi- c ien t Yb s + ~ T m 3 +, Y b 3 + ---) Ho 3 + and T m 3 +

Ho 3 + ene rgy t r ans fe r processes . T he f luorescence pro-

cess is desc r ibed by cross r e l axa t ions o f 2F5/2~

3H 5 ----) 3H 4 ~ 3H 6 ~ 2F7/2 and 2F5/2 ~ 3H 5, 316 ~ 3H 5

3H 4 ~ 5I 7 ~ 5I 8 ~ 3H 6 ~ 2F7/2. T he results presented in

this work are patent pending.

References

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optical properties, fluorescence mechanisms and energy transfer in tm3+, ho3+ and tm3+ -ho3+ doped...

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