Luminescence properties of the Tm3+ doped silicates Y2SiO5, CaY4(SiO4)3O and SrY4(SiO4)3O

JOURNAL OF

LUMINESCENCE

--

ELSEVIER

Journal of Luminescence 62 (1994) 157 171

Luminescence properties of the Tm3 + doped silicates Y 2SiO5, CaY4(Si04)30 and SrY4(Si04)30 C. Lia, A. Lagriffoula, R. Moncorgea. ~, J.C. Souriau”, C. Borel”, Ch. Wyonb b

Université de Lyon I, UA 442 CNRS, 69622 Villeurbanne, France LETI(CEA-Technologies Avancées) DOPT-SMDO CENG 85X, 38041 Grenoble Cedex, France Received 8 October 1993; revised 3 January 1994; accepted 10

March

1994

Abstract 3~doped silicates of formula Y The optical properties of powders and crystals of Tm 2SiO5, CaY4(Si04)O and SrY4(Si04)O are presented. These materials emit very interesting infrared fluorescences around 2 ~smwhich make them very efficient room temperature laser systems when pumped by diode lasers around 795 nm. The spectral and dynamical behaviour of these fluorescences in the three compounds is first analysed as a function of the temperature 3~doped and Y of the dopant concentration. Then a more detail study of the infrared and visible fluorescences in Tm 2SiO5 is developed, including a Judd Ofelt analysis of the absorption spectra and an analysis of the main energy transfer and multiphonon relaxation processes.

1. Introduction 3~ion plays an It is well-known that thesolid-state Tm important role in laser-type materials. It can be used directly as active center, in the infrared spectral region around 2 ~tm, in which case it is associated with a cross-relaxation (CR) energy transfer process, or in the visible region, in which case it is associated with an up-conversion (UP) process. The interest of the solid state lasers in the eyesafe spectral range around 2 p.m has been well documented in the recent years [1 4] 3F and room temperature CW lasers working on the 4 with ~H6 3~ion have been operated transition of the 3Tm doped host crystals [5 8]. Howa number of Tm ever, the need for compact and highly efficient laser —+

+

*

Corresponding author.

sources still motivates the search and the studies of new materials which can be diodes pumped by commerthe high power semiconductor laser now cially available. One possible way to increase the efficiency of these laser systems is to find materials in which the dopant can substitute for cations occupying low symmetry sites, possibly noninversion sites, in order to increase the oscillator strengths of the transitions, thus to get larger emission cross sections. From this point of view, the yttrium orthosilicate Y 2SiO5 and the silicate oxyapatites CaY4(Si04)30 and SrY4(Si04)30 doped with rare-earth seem toofbethe quite interesting.Y The laserions properties Nd3~doped 2SiO5 and CaY 4(Si04)30 crystals around 1.075 and 4F 1.067 ( 312 Y transition) and of the 3~Jtmdoped Ho 2SiO5, CaY4(Si04)3O, SrY4 (Si04)3O crystals around 2.085, 2.06, and 2 p.m

0022-2313/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDIOO22.2313(94)00021-4

—~ ~‘11/2

158

C’. Li et at.

Journal ot Lumineicence 62 (1994) 1~7 17/

(~I7—+ ~ transition) were reported a few years ago [9 13]. More recently, demonstration was made that Nd3 + doped Y2SiO5 could be very attractive for 4F room temperature laser action around 911 nm ( 32 19/2 transition) [14] and even more recently, we demonstrated room-temperature CW 3 + doped Y 3F 3H laser action of Tm 2SiO5( 4 doped 6) 3 ~ Er3 ~) around 2.05 p.m [15] and of (Yb Y 2SiO5 [16] and SrY4(Si04) 30 [17] around 1.57 3~ and 1.54 p.m, respectively ~ ‘152 Er transition). Because of a very (4J~3 important absorption band around S00 nm (six times more intense than in Tm3 doped Y 3A15O1 2) in the it-polarization 3~doped.CaY (E~c), the Tm 4(Si04)30 system also appeared to be an excellent candidate for improved diode pumped laser action around 2 p.m [18]. In this paper, we report on the spectroscopy 3 + doped and silithe fluorescence properties of the Tm cates Y 2SiO5, CaY4(Si04)30 and SrY4(Si04)30 hereafter called YSO, CYS and SYS. After a presentation of the experimental techniques in Section 2, and a brief description of the various systems in Section 3, we present first their infrared luminescence properties in Section 4, then the 3results of ~ doped a more extensive study concerning the Tm Y2SiO5 laser system (Judd Ofelt analysis, energy level diagram, multiphonon relaxation, up-conversion and cross-relaxation energy transfer processes). We conclude in Section 6. —~

2. Experimental The emission spectra were obtained by exciting the samples either with a CW Ti: sapphire laser or with the 3pulsed radiation of dye a frequency-doubled + laser-pumped laser (Quantel YAG:Nd model Datachrom) associated with a hydrogen Raman cell. This Raman cell allowed us to excite directly the 1G 3 + ion 4 excited level of the Tm located around 475 nm by using the Raman antiStokes 1 of the laser radiation obtained with the Rhodamine R6l0 dye 3~ions (exciton denomination). 1D was excited by The the 2 level of the Tm 355 nm frequency-tripled radiation of the same YAG:Nd3 + laser. The visible fluorescences were analyzed with the aid of a 1 m Jobin Yvon model HRS 1 monochromator, with a 1200 groove/mm

grating blazed at 500 nm, and detected by a Hamamatsu photomultiplier model Rl477. Then the signals are fed either into an ORTEC photon counting system which is connected to a computer for the spectral measurements or a SR430 multichannel scaler to record the fluorescence decays. The obtained bywith exciting the 3H infrared spectra3 +were around 790 nm the CW 4 level of the Tm Ti:sapphire a Jobin Yvon laser H20and JR monochromator they were analysed equipped with with a 300 groove/mm grating blazed at 1.5 p.m and detected with a liquid-nitrngen-cnoled PbS photodiode. The infrared fluorescence lifetimes were detected through the same monochromator as above by a Judson model JlO-D InSb cell or an ADC. model 403 HS germanium photodiode (with time responses of about 500analysed ns) cooled at liquid-nitrogen temperature and were with a LeCroy 9400 digital oscilloscope interfaced with a computer.

3. Materials The yttrium orthosilicate Y2SiO5 (or YSO) is a monoclinic biaxial system with a C~h(C 2/C) space group and the lattice parameters were given in Refs. [19,20]. There are two types of yttrium sites with C1 local symmetry for which the rare earth dopants can substitute. Because of the biaxial character of the system, three principal axes of polarization can be defined: I) the <010> direction which is a two fold symmetry axis of the crystal and 2) the Dl and D2 directions perpendicular to each other and to the <010> direction and which correspond to cxtinction directions [14,20] when the sample is viewed inThese the <010> direction crossed polarizers. directions werebetween determined exactly with respect to the crystallographic axis by X-ray analysis and shown in Ref. [20]. The silicate oxyapatites CaY 4(Si04)3O (or CYS) and SrY4(SiO4)3O (or SYS) are hexagonal uniaxial systems with a C~h (P63/m)isomorphously space group [12]. 3~ dopants substitute for The the RE yttrium ions and there are two nonequivalent positions for the yttrium ions with point groups Clh and C 3. This leads to the formation of two types of activator centers, each of them having a particular energy level scale. The relative occupancy

C. Li et a!.

Journal of Luminescence 62 (1994) 157 171

of the two sites was not determined. However, according to Refs. [12,13], the intensities of the optical transitions within the Clh symmetry centers significantly exceed that of the C3 symmetry ones.

159

50C

25C

the C3 to the Cih active centers. With fusion temperatures of about 2000°C,the YSO, CYS and SYS crystals are refractory crystals. Single crystals of each material can be grown by the Czokralski technique in large dimension and with good optical quality. These crystals have good chemical and thermomechanical properties and they be activated by large amounts of rare earth can dopants. The samples used in the present study were powders and a crystal of YSO:10% Tm3 + cut in the form of a small parallelepiped with each of its faces oriented perpendicularly to one of the principal directions <010>, Dl and D2 of the optical indicatrix.

4. Room temperature infrared emission properties of Tm3

+

doped YSO, CYS, SYS

The infrared emission properties of the Tm3 + ions after 3H 4 level excitation 800nm are particularly important for the 2around p.m diode pumped laser application. It is well-known that the 3F 4 level may be populated via CR 3H a phonon-assisted 3H 3F energy transfer process 4(l) + 6(2) —~ 4(1) + 3F 4(2) where 1 and The 2 are the labels for twoprocess neigh3 + ions. effectiveness of this boring Tmwith the Tm3 + ion concentration and is increases usually high already for 1% Tm3 + ion-doped systems. The room-temperature emission spectra between 1300 and 2200 nm of the YSO, CYS and 3 + SYS ions powafter dered samples doped with 1% Tm 3H 4 excitation are shown in Fig. 1. The emission bands found beyond l600nm are assigned to the 3F 3H 4 6 transition and the to bands around 1480nm are assigned a 3Hobserved 3F 4-+ 4 transition. We note thatthan the relative intensity of the latter is smaller in YSO in the other systems. It —÷

YSO~1%Tm

1 N V~

150 100 ~

~

~Jf’ ~oo

J

‘N...

SYS:1%Tm

1 ~ CYS:1%Tm

!~$I&~,J

i~00 15b0

1600

1~00 l~00 1900

2d00

2100 2200

Wavelenglh(flm)

Fig. 1.3HRoom temperature infrared emission spectra observed 3~doped YSO, after 4 excitation around 790 nm of l°~ Tm SYS and CYS powders.

means either more efficient cross-relaxation energy transfers or more efficient non radiative multiphonon relaxations between the excited states in Tm3~doped YSO. The former assertion will be confirmed in the following by analyzing the 3H 4 fluorescence lifetimes of the different 3 + ions.systems doped various Tm The with shapes of theamounts emissionof spectra of the 1 o~/~ Tm3 + doped SYS and CYS are very similar. They peak around 1.75 p.m and their intensities are vanishingly small beyond 2 p.m. In the case of YSO: 1% 3’~,in addition to the peak around 1.75 p.m, Tm is another peak with about the same magnithere tude around 1.81 p.m and a significant bump appears around 2.04 p.m, a region in which we recently obtained very efficient room temperature CW laser action [15]. emission of +these systems dopedThe with 5% andspectra 10% Tm3 ions same were also recorded. In that case the bands around 1480 nm disappear and only those corresponding to the 3F 3H 4-+ 6 transition beyond l600nm subsist. 3H 3F The 4 and 4 fluorescence were after also measured in the different decays materials 3H 3F 4 and/or 4 pulsed 3H laser excitation at about 790 and 1620nm. The 4 fluorescence decays are non3+ exponential and strongly shorten when the Tm ion concentration is increased; this YSO. is shown 3 + doped This in Fig. 2 in the to case Tmenergy transfer process behaviour is due theofCR

160

C. Li et a!.

Journal of Luminescence 62 (1994) /57 171

mentioned above and will be discussed in more details in Section 5.3. The 3F4 fluorescence decays 3 + ion concentration is inalso shorten as the Tm as for 3H creased but not so much They also are 3 + ion 4.concentrations non-exponential foronly Tmslightly and just after the higher than 1% but exciting laser pulse. This behaviour is usually associated with an efficient energy migration process among the excited Tm3 + ions mediated by resonant energy transfers followed by trapping at various quenching centers. We report in Table 1 the 1/c values of the 3H 3F 4 and fluorescence in the three4systems dopedtime withconstants 1%, 5% measured and 10% Tm3 + ions. We note that the apparent fluorescence lifetime of the 3H 3F 4 level, as that of the 4 metastable, is longer in YSO than in theasother systems; however, it decreases more rapidly the Tm3 + ion

_______________________________

YSO:Tm .~

34.

0,2%1%

10 3

\ \ ~

\ \

2

~ 10

50/

\ \io°,’0 0

50 3H T

me

1~0 (~)

150

Fig. 2. Room 3~(x temperature —0.2, 1,5 4and fluorescence 10) after pulsed decay excitation at 800 nm at in YSO:nm. x% Tm 654

concentration is increased; as anticipated above it meansinthat CR energy are and moreCYS. efficient Tm3the + doped YSO transfers than in SYS 3F In the end, itinisTm3 worthdoped notingYSO, that the 4 fluorescence decays SYS and CYS are significantly shorter than in the well-known Tm3~doped YAG or YLF [21] which might be due to more important radiative and/or multiphonon transitions. This will be clarified in the following section. 3 ± doped YSO 5. General optical properties of Tm In this section we report the results of polarized absorption and fluorescence measurements made 3+ at low oriented and high temperatures 10% doped crystal and 0.2%,on 1%,a 5% andTm 10% Tm3 + doped powdered samples. From the data obtained with the single crystal, the positions of the various Stark components of 3 3 the H 6 ground and F4 excited multiplets corres3 + centers in ponding the maximum different types of Tm YSO andtothe splitting of the various higher lying excited states 3H 3H 3F 3F 5, 4, 1, 2, ‘G4 and ‘D2 are determined. Then a Judd Ofelt [22,23] analysis of the polarization averaged absorption transition intensities is made to determine the Q, parameters, the radiative transition probabilities and radiative lifetimes of the main emissions and their branching ratios. Fig. 3 displays the room-temperature absorption spectrum of the crystal recorded between 300 and 2000nm for light polarized parallelly to the Dl direction, a polarization which is particularly interesting in Fig. for 4) since diodeitlaser leadspumping to the strongest around 800 absorption nm (see lines.

Table I t/e time constants of the fluorescence decays of the 3H

3F 4 and 4 levels measured at room temperature SYS: Tm3~ CYS: Tm3

3~ Excited levels

3H 3F 4 4

YSO: Tm 10o

50o

1000

100

500

71 ~ss 1.6 ms

6 .ts 1.1 ms

2 Is 0.6 ms

65 p.s 738 p.s

10 p.s 163 p.s

boo

47p.s 2.6 p.s

i0o

500

100o

325 40 p.s is

1028 p.s

27 2 p.s p.s

C. Li et a!.

/ Journal of Luminescence 62

3H

1.4

(1994) 157 171

3 5

161

Y

.

0.E

12

3~

2SiO5:Trri E//(010)

Y2SO5T,~,

3F

1.C

4 p0.8

I

3F

1D

~

~F2,

2

3

0:

I

4

6 0

~04

I

800 1 1 00 Woo0Iengih(or,,)

1400

1600

1800

-Do

Fig. 3. Room temperature 3~crystal EIID1 (sample polarized thickness: absorption 1.3 mm). spectrum ofa YSO: 10% Tm

I

I

I

12

1

5.1. Judd Ofelt analysis Os

Details on the theory and the method have been well described earlier [24,25] ; hence, only a short summary will be given here. In calculating the line strengths, the selection rules for electric dipole and magnetic dipole transitions must be considered. The selection rules for electric dipole transitions are: Al

±1,

—

AS

=

0,

IALI,IAJI

~

21

where, for the lanthanides, I = 3. The selection rules for magnetic dipole transitions are:

0.4 0 ‘71,0

760 780 800 820 Wcivelength(nm)

Fig. 4. Room temperature polarized absorption spectra of a YSO: 10% Tm3~crystal (sample thickness: 1.3 mm) around 800

Thus, it is found AS = AL

=

0.

AJ = 0. + 1 (but not 0

—*

—

0)

—~

=

8 ~ 2 mc Sd 2h(2J + 1)2

with ~

=

6

(1)

and

Fmd =

27.25 x 10 ~ for 2

—

8390 cm

1

5) ~0.39 X

10 20 cm 2 The bntegratlons of the absorption bands (up to the ‘D2 level) werepolarizations. made for eachBeof the E~<010>, E~D1and EMD2 .

As a consequence, all the ground-state 3” ion are absorpelectric tion transitions within the Tm dipole (ED) except for the 3H 3H 6 5 transition which contains a substantial magnetic dipole (MD) moment. As this magnetic dipole contribution 5md does not depend significantly on the host material (in first order), it is obtained from the oscillator strength value Fmd which was calculated by CFR [24] knowing that: Fmd

3H

Smd(3H 6

.

840

cause they cannot be distinguished 3F 3Fclearly at roomtemperature the levels 2 and 3 were treated together to reduce the experimental Thus, six line strengths (Sed) for eachuncertainties. polarization were measured and they are reported in Table 2 with the mean wavelengths Aof the corresponding absorption transitions (A = 2I~.)d2/$I(2)d2).

$

ducedpolarization Then by giving equal averaged weights linetostrengths each of the werethree depolarized line strengths and they are reported in Table 3. In the case of the 3F 3F 3, 2 level mixing, the line strength of the band was calculated by using an expression that we already discussed in a previous

162

C. Li eta!.

Journalof Luminescence 62 (/994) 157 /7/

Table 2 3~at room temperature for bight polarized parallel to the ~0l0), DI and D2 Measured absorption tiansition intensities in Y directions and measured and calculated polarization 2SiO5.Tm averaged transition intensities Transition 3H 6 —.

E] (010>

1F 3H4 3H5 4 3F 3F 2 + 3 ‘G4

E Dl

E D2

Polarization averaged

~~[nm]

S~°~’ 20cm2J [10

A[nm]

S~)’°’ [10 20cm2]

z[nm]

S~°’ [10 20cm2]

S~f”20cm2] [10

S~’~ [10 20cm2]

1683.9 1182.4 784.4 (682.5) 679.9 (658.5) 467.2 356.7

2.81 0.90 1.42

1677.8 1181 780.1 (680) 679.8 (655.5) 465.8 356.8

2.66 1.06 1.31

1685.9 1184 788.2 (682.5) 680 (653.8) 468 355.9

2.59 0.92 1.15

2.68 0.96 1.29

272 108 1.16

0.76

1.27

1.77

0.31 0.63

0.32 0.73

0.26 0.61

1.39 0.37 0.84

1.66 0.30 0.73

3 than in be generally more important in mentioned YSO: Tm above. 3 doped crystals theKnowing other Tm the ‘l~parameters and the matrix elements 1< Ut>~2[30], the probabilities of the electric and magnetic dipole transitions (Aea and Amd) between each level of the Tm3”’ ion (up to the ‘D 2 level) were also calculated. They are reported in Table 3 with their branching ratio fiR and the expressions: radiative lifetimes ‘r~of the excited levels given by the +

paper [20]. The three JO intensity parameters Q 2, Q4 C26 were determined a leastsquare and fit using the then measured electricfrom dipole line strengths and the 1< Uw> 2 reduced matrix elements given by CFR [26] and reported also in Table 2. It is found: cm 2 2 1.74 x 10 20 cm 0.66 x 10 20 cm2. .-,..,

= =

x

20

~,

,

+

.

.

.

f3JJ

These JO parameters are used in turn to derive the theoretical line strengths of the six absorption lines and these calculated values are compared to the measured line strengths in Table 2. The rms deviation found equal to good 0.13 20cm2value which5 is is satisfactory but is less x 10 what was obtained in case of Er3~doped YSO than [20]. These parameters are doped smallerphosthan those found three in theJO case of the Tm3 phate studied in Ref. [27] (Q 2, 5.88 x 10 20cm2) 20 cm = 2.88 x 10 20cm2 Q 2 6 = 0.71 x 10 but of the orderBaF than those found [28] with 3 same doped the Tm glass 2, Q 2 ThF4 fluoride 2, 26 (Q2 = x 2.02 20 cm 4 = 1.56 x 10 than 20 cmthose = 1.1 10 x 2010cm2) and equal or larger 3 doped YAG [25] and YAP [29] found (YAG: in Q Tm 2, Q 2, 2 = 0.7 x 10 20 cm 4 1.2 x 10 20 cm = 0.5 x 10 20 cm2 YAP: Q 2, = 2.3 x 10 20 ~2 Q 2 0.67 x 10 20 cm 6 = 0.71 x 10 20 ~2) It means that the radiative transition probabilities will +

—

+

+

=

.

T~ 5A(J—s J),

>~A(J—~ J’)

(2)

i

with I

I

(3) We note first that the radiative lifetime of the 3F 4 level is very short compared theand lifetimes 3~dopedwith YAG YLF measured in the 1%YAG Tm and 22 ms for YLF) [21]. systems (11.3 ms for So the stimulated emission cross-section for the 3F 3H 4—~ 6 transition of interest for infrared laser action could be higher than in YAG and YLF. On the other hand comparable the values oftothethat branching fiR are found found inratios the BaF 2 ThF4 fluoride glass [28] 5.2. Spectral analysis and energy levels A(J~J)Aed(J~J)+Amd(J*J).

—

Absorption and emission were 3H measurements 3F made in the region of the 6 4 infrared laser ~

C. Li et a!.

/ Journal of Luminescence

62 (1994) 157 171

163

Table 3 Values of the emission transition probabilities, of the radiative lifetimes and of the branching ratios in Y Average frequency 3H 4—~ 6 3H 3H 5—’. 3F 6 4 3H 3H 4 —. 3F 6 4

A,d

[s

1]

Am,~[s

1]

3+

2SiO,: Tm TR [ms]

3F

3F

5944 cm 1(1682.5 nm)

266.8

8457 cm 1(1182.5 nm) 2513cm 1(9793 nm)

241.3 28.7

94.3 1.8

12 752cm 1(784.2 nm) 6808cm 1(1468.9 nm) 4295cm 1(2328.3 nm)

1002.9 95.8 30.6

33.8 9.3

14 669cm 1(681.7 nm) 8725cm 1(1146.1 nm) 6212cm 1(1609.8 nm) 1917cm 1(5216.5 nm)

2017 45.5 295.3 4.1

15 246cm 1(6559 nm) 9302cm 1(1075 nm) 6789cm 1(1473 nm) 2491 cm 1(4010 nm)

490.5 532.4 226.7 14

21 413 cm 1 (467 nm) 15 469cm 1 (646.Snm) 12956cm 1(771.8 nm) 8661cm 1(1154.6 nm) 6744cm 1(1482.8 nm) 6167 cm 1 (162l.Snm)

1053.5 141.3 510.7 181.9 46.6 16.9

28050cm 1 (356.Snm) 22 19 593cm 106cm 11(510.4 (452.4 nm) nm)

10851.4 13898.6 83.9

15 298cm 1(6537 nm) 13 381 cm 1(7473 nm) 12804cm 1 (781 nm) 6637cm 1(1506.7 nm)

1318.2 1012.3 1178.1 179.4

3.75

1.00

2.73

0.92 0.08

0.85

0.86 0.11 0.03

0.41

0.83 0.05 0.12 0

0.79

0.39 0.42 0.18 0.01

0.47

0.49 0.07 0.30 0.10 0.02 0.01

0.035

0.38 00.48

3H 3 —.

6 3H 3H5 4 3H —o 3F 6 3H4 3H5 4 1G 3H 4 -.o 6 3H 3H5 3F 4 3 3H ‘D2—~3F 6 3H, 4 3H 3F 4 3 1G 4

transition at high (Fig. 5) and low (T~10K) temperatures (Fig. 6) for the three polarizations mentioned above, with attention given primarily to the long-wavelength region in which the laser should operate. The room temperature absorption and emission spectra show that absorption is negligible for the wavelengths beyond 1.95 p.m, that a pronounced emission peak exists around 2.04 p.m in the Dl polarization, thus the possibility of a broad wavelength laser tunability in the 2 p.m region. It is clear also that the laser should operate more efficiently with the light polarized along either the Dl or the D2 direction and that, in case of longitudinal

66.7 0

12 131.3 33.2 3.5

94.7 57.7

0.05 0.04 0.04 0.01

optical pumping, the crystal rod axis should be parallel to the <010> direction. These room temperature spectra also allowed us in a preliminary article [15] to determine the corresponding absorption and stimulated emission cross-section spectra by using the method of reciprocity [31,32] or the Fuchtbauer Ladenburg formula [33 35] which are based on the relation between the Einstein coefficients A and B and knowing the radiative lifetime of the emitting level. A careful examination of the low temperature absorption spectra (Fig. 6) also allowed us to distinguish 18 lines which agree well with the (2J + 1) x 2 = 18 Stark components of the

164

C. Lie! a!.

Y2SiO5:Tm3~1,~j E//<010>

0

::

1

‘Th—~

~

$

E//D1

1.2

Os

0.8

0.4

0.4

0

I

i1~’i~V~r’—

i

0

E//D2

..d

1.2

________

~°

1.6

3~

25iO5~Trn E//

________

0.8

Y

1.6

E//D2

Pt

0.4

12

I’

0.8

Journal of Luminescence 62 (1994) 157 17/

1.2

El//Di

~, L

0

122

0.8

o:

—

0. ~~II I I 0 1400 1500 1600 2000 2200 Wavelength (rim) Fig. 5. Room temperature absorption and emission spectra of a YSO: boo Tm3~crystal (sample thickness: 1.3 mm) around 1800 nm.

3F 3 + sites. In emission, 4 manifolds for thepossible two Tmcoincidences between because of the many the transitions, the analysis was more difficult. Nevertheless, by analyzing the evolution of the lines with the temperature, the (2J + 1) x 2 = 26 lines corresponding to the the transitions to the various Stark components in 3H 6 3ground + sites state couldmanihave folds for the two Coincidences types of Tm were found between been determined. two absorption and emission lines located at about 1746.1 nm (5727 cm 1) and l734nm (5767 cm i).

1400 .

1600

1800

2000

\~~Javelength(nm)

2200

.

Fig. 6. Absorption and emission spectra of a YSO: l0°~Tm3 crystal (sample thickness. 1.3 mm) around 1800 nm at T— 10 K.

They are assigned to optical transitions between the lowest energy Stark components of each manifold 3H 3F 3 + sites. 6 and 4 within Tm already The corresponding energyeach level type scalesofwere reported in Ref. [15]. They show in particular that the highest Stark components of the 3H 6 ground state manifolds lie up to 850 1 which is 3 + 1000cm doped systems such higher than in the other Tm as the well-known YAP and YVO4 single crystals [36]. TheseYAG, high energy components are very interesting because the 2 p.m laser system approaches the operating condition of a four level laser system even better.

C. Li et a!.

/ Journal of Luminescence 62

Low temperature absorption spectra were also recorded in the region of the 1H6 —o 3H5, ~H4,3F3, 3F 2, ‘G4 and iD2 optical transitions. From these spectra we could derive the overall energy diagram reported in Fig. 7. This diagram allowed us to interpret the emission spectra that we registered in the visible domain, in particular to distinguish between the many coincident emissions originating from the various excited states that we report now. Emission spectra of powdered samples doped 3”’ ions were recorded in with 1%, 5% and 10% Tm the 400 1300 nm range after pulsed laser excitation of level 1D 2 (for various excitation pump powers) 1

2

3

and levels G4 and ( F2, F3). The results obtained between 400 and 850nm after iD2 excitation at about 355 nm are displayed in Fig. 8. Apart from the weak band around 520 nm observed 3” at high excitation pumpallpowers and/or ion concentration the emission 10% Tm

165

(1994) 157 17]

a 2

1

A I ~

40 ~iJ

ç

4 l’ ~

500 ~5o”~o1’~ -~

2

b

.

c _

4~J I

~

4~J

E(xlO3cm_1)

C

30. 1~

-~

25.

H 3

400~tJ ____

_______

1G 4

20

~ 0

0

Fig. 8. Room temperature 3~doped visible YSO emission powders spectra observed of 1%after (a),

,

E

E E

——

-

—

—

-

—

—

6007

‘o°IaveIeng~h(nm)

0

is

5

3F 3F 2, 3 ____

______

—

—

10

3H 5 3F —

—

_________

5% (b) and 10% (c) Tm various excitation powers of level iD2 at 355 nm.

________

4

5

Fig. 7. Energy level diagram of Tm3 + in YSO and main emission transitions experimentally observed,

bands of these spectra can be easily identified by using the diagram of Fig. 7. These spectra show 3H of the emission band around that the 1D intensity 450nm ( 2 —* 6) decreases while that of the others around 480, 520, 650 and 800excitation nm increase as 3 + ion concentration or the pump the Tm power is increased. Except that around 520 nm, the variations of these transition intensities with the Tm34 concentration can be easily explained by cross-relaxation energy transfers of the types presented in Fig. 9(a). We shall see in the next section that the strong non-exponential decay mode of the

166

C Li ci a!.

Journal of Luminescence 62 (/994) /57 171

(a)

1D 2

Ref. [39]. This hypothesis is quite probable since 1D the energy gaps between the state 2 and the

~

A BIC D I — _+._~. I II I I I I

f

—

=_

:

3H 4

=

3H 5

— ______

__

=

=~

—

~ ......L.

I I

I

~.

I

.

—~

II ____

______

A B ‘~ IC II

states 1I~ and iG4 are very comparable, which temperature, make the energy andtransfers becausealmost the 520nm resonantemission at room

(b)

—+—÷

A B~CD’

F~,F3

3F

__ .~— .

I

I

A’ B C D

intensity then varies with the dopant concentration pected to1D do. From this point of view, in the case of and the 2 excited 3~doped state sample, population a relatively as it ishigh cxthe 10% Tm excitation pump power was necessary to produce high enough iD2 (thustype 116) energy excitedtransfers state populavarious cross-relaxation mention to compensate the energy losses caused by the a

3H 6

3°C1)

Tm3°12) ~—

- ______

Tm34.(1)

______

~—

Tm3”(2) —-—-‘--

Tm

3F 3F 213H 3

_______ _______

_______

~

3H

________

5 _______

_______

3”(l)

6 Tm

Tm3’.(2)

tioned previously and tosignal. obtain a good enough (antistokes) fluorescence The‘Gspectra obtained between 600 and l200nm after 3F 4excitation(~i475nm)and 3F around 800nm after 2, 3 excitation (654 nm) are displayed in Figs. 10, 11 and 12. Because the shapes of the bands observed around 650 and l200nm after iG excita3F 3F tion and around _______________________________ 800nm after 2, 3 excitation

(C)

2

Fig. 9 Various cross relaxation mechanisms involved after (a) ‘D 1G 3H 2, (b) 4 and (c) 4 excitations.

1D 3F~transition) fluorescence around 450nm ( 2 —‘. behaviour of the intensity thenature band and around is a good illustration of that.ofThe the 520 nm is more puzzling. It cannot be assigned to a iD ~ 3H 5 transition because its intensity does not behave as that of the 450 nm band. It probably 3 + energy level originates from a higher lying Tm such as ~ 1 that we could not determine in YSO but with other systems [37,38], around 34600 cm that we can locate approximately, by comparison (~289 nm), thus in the UV domain. In this3Fcase,3F the band would be assigned to a 116 -“s 2, 3 transition and the 116 level would be populated via 1G energy transfers iD2(l) + iD2(2) 3~ up-conversion 1~~[) + 4(2) between neighbouring Tm ions labelled 1 and 2 primarily in their ‘D 2 excited states. Such an emission around 510 nm can be also 3~but it is even weaker observed [39] in LaF3:Tm 3”’ and is not mentioned in than in YSO:Tm

o:21

I

‘~ 4-

C C

4, U o2 4’

0

~I

600

700

800

900

________________________

4, I-

0

~ 1

1100

1200

1300

\oJaveIenq~h(nm)

aFig. 10o 10. Tm3’~doped YSO powder 1G spectra of Room temperature near observed infrared after emission 4 excitation at 475 nm.

C. Li et al.

/

Journal of Luminescence 62 (1994) 157 17/

167

is more ambiguous since it probably comes from various contributions (see in Fig. 7). 5.3. Cross-relaxation energy transfers

C

~i. O

4, 4, I..

,

700

I

800 Wavelengl~h(nmn)

3H populations The time dependences when the levels of the are‘D2, directly excited 3H 3F ‘G4 and 4 were studied by considering the decays of the flu3F 3H orescences 4—~ 4), 450nm (iD~ 4) at and 1480nm l200nm ((‘G4—o 4) and re-

I

900

Fig. 11. Room temperature 3’ neardoped infrared YSOemission powder spectrum observed around after 3F 800 3Fnm of a 1°oTm 2, 3 excitation at 654 nm,

Y5O:Tm3~ :2

~ io~

101

I

0

These results can be all interpreted by the crossrelaxation mechanisms represented in Fig. 9; these mechanisms been proved to of occur many 3”’ dopedhave materials. Because the innon-exTm ponential character of the decays whatever the Tm3”’ concentration considered (0.2%, 1%, 5% and 10%) we are not dealing with very fast energy diffusion regimes among the Tm3 + ions but with what are called “static” and diffusion limited energy transfers. Thus, assuming dipole-dipole interactions between the ions, we used the Yokota Tanimoto [40] expression to describe our experimental data, i.e.:

U) 4, I. 0 IN O

ported in the Figs. 2, 12 and 13, respectively. These decays are Tm3” non-exponential and strongly shorten when the ion concentration is increased.

10%

N(t)

12

.

TIme(~s)

Fig. 12. Room temperature 1D 3~’(x = 0.2, 1, 52 and 10) after pulsed excitation fluorescence decay at 450 nm in YSO: x% Tm at 355 nm,

—~

~

(4)

where b

do not vary significantly with the Tm3”’ ion concentration they are assigned respectively to ~G 3F 1G 3H 3H 3H 4 —~ 4, 4 —o 4 and 4 6 transitions. As above, however, their intensities decrease and their decays level shorten very significantly as thefluorescence Tm3 + ion doping is increased, which is interpreted now by the CR relaxation energy transfers sketched in Figs. 9(b) and (c) (the latter being already mention in Section 4 and illustrated in the fluorescence decays of Fig. 2) and which will be analyzed in the next section. The assignment of the band observed around 800 nm after 1G 4 excitation

— N(0) exp [ 1 + 10.87x + 15.5x2 ( I + 8.743x )314],

— ~it312CTmR~RT

(5)

if2

and ,~

=

D(C~D~) i/2t2/3

=

DRcR2t113t213.

(6)

t is the donor fluorescence lifetime in the absence 3 + ion concentration of energy transfer, the Tm and RCR the criticalCTm radius for the cross-relaxation energy transfer. CDA stands for the donor acceptor transfer constant and D is the diffusion constant which is given by:

D

=

kc413 = 1 4m Tm

2

“~4/3

(~CTm)

CDD,

(7)

168

C. Li ci al.

Journa! of Luminescence 62 (1994) 157 17/

_________________________________ 3~

16C i4~

YSOTm

~d~Tm ________

,,

bC

~ io2

,2/~

61

1

4C I) (I) ~ 4,

C L 0

50/ \ 10%

12(

6C 2CC

0

_12C

.2

100

200

300

400

700

~

p

800

900

______

IYSO1%Tml c5iO0

0

50I

I 100

150

Time (~j.s) Fig. 13. Room temperature ‘G 3~(x 0.2, 1,54 fluorescence and 10) afterdecay pulsedatexcitation 1200 nm in 475 YSO:nm. X°o Tm at

I~

4i .rI

21

where CDD is the donor—donor transfer constant and is defined as: — R~t 1 (8)

c

c _________________________________ • e 500 600 700 icc U 100 200

with R0 the critical radius for the diffusion. 3H The best fits of expression (4) to the 4 fluorescence decays are shown in Fig. 14. They were obtamed with the three parameters: ‘r (167 ±5) p.s, RCR 6s — (10.1 + 0.3) and k — (4.7 1 (or RD~c7A). ±0.3) Thosex concerning 10 ~ cm the 1D tG 2 and 4 fluorescence decays are not reported but they are found as much satisfactory and they were obtained with the three parameters: 1G for 4: ‘r (610 + 50) p.s, RCR — (12.7 6s ±0.4) 1 (or and k = (1.5 ±0.2) x 10 ~ cm RD~7.2A). for 1D 2: t = (24 ±1) p.s, RCR ±0.3) and 6s —i (9.1 (or RD~~~6A). k The (13.6 ±0.8) x 10 ~ cm critical diffusion radius can be derived too from the spectra according to the expression:

w ~

U

8C

A,

A,

A

R~=

3cr 4

8m n

2 $5em(2)aabs(1t)t1’~,

(9)

where ‘r is the donor lifetime (in the absence of energy transfer) and aem(2) and aabs(2) are the emis0em spectra sion and absorption cross sections. The

W C)

& 5C

\~~lYS0:5%TmI

02C

rl

1C

~o

in

~o

~o

h

~i:i

~i1

~fl

80

& & ~S0: i0%Tm 4C 3C 2C

Co

8

Time

i’O

12

14

16

(lJ.s)

Fig. 14. Fits (full lines) ofexpression (6) in the text with the room temperature 3H Tm3~(v 0.2, 41.fluorescence 5 and 10). decay data (squares) in YSO: ~°o

C. Li et a!.

/ Journal of Luminescence 62

can be derived either from the emission spectra and the radiative lifetimes of the emitting levels or from the absorption spectra by using the method of reciprocity mentioned previously in Section 5.2. From the polarized absorption spectra and the t values derived above we then found the following polarization critical diffusion radii: 4.l for averaged 1D 1G RDi~ 2, RD~i4.7Afor 4, k~~7.2A 3H for 4. The critical diffusion radii determined by 3H the two methods are only1Dequal for1Gthe 4 fluorescence. In the case of the 2 and 4 fluorescences the radii determined via the spectral method are smaller than those determined via the fluorescence decay modes. This discrepancy could be due to an underestimation of the phonon-assisted diffusion in the spectral method.

A

5.4. Multiphonon relaxations

In a previous paper [20], the predominance of the non-radiative multiphonon relaxation processes between the was various excited states of We the Er3 + ion in YSO clearly demonstrated. 3 + doped system to check reexamine this in the Tm if the results were consistent. To do that, however, we had to start with fluorescence decay data which were not spoiled by the influence of the numerous UP and/or CR energy mentioned sample. above, thus by working with a transfers lower concentrated 3” doped powdered sample was So, 0.2%theTm useda and fluorescence decay of each excited state was recorded after direct excitation of each of them. The various tluorescences decay almost exponentially with a slight short component at the shortest times after the exciting laser pulse which can be seen, as examples, in the Figs. 2, 12 and 13. This short component results predominantly from multipolar long range “static” energy transfers (without diffusion) between the Tm3”’ ions and the long timefluorescence exponential portion associated witht the intrinsic decay ofis time constant of the emitting level. These t values are reported in Table 4, along with the radiative lifetimes ‘rR derived from our JO analysis of the absorption data, and they are used in the expression: 1

1 =

+ WNR

(10)

(1994) 157 171

169

Table 4 Fluurescent~eand radiative lifetimes, noniadiative rates and 3~ energy gaps measured in Y 2SiO5: YSO: Tm3~ 1D 1G Tm 3H 3F 2 4 4 4 r

20gs 432 ~ts 351is 470p.s 2.14 x iO~ 187 6900 5880

tR WNR [s 1] LsE [cm ‘1

134 jis 850iis 6286 4300

1.6 ms 3.75 358 5700

to derive the nonradiative deexcitation rate WNR of each excited state, the one is given by the wellknown empirical expression: WNR

=

Ce

(11)

~

where C and ~ are constants and AE is the mean energy separation between two 1G successive 3H 3Fmanifolds. The WNR values of the 4, 4 and 4 are close to the values which were expected; it is shown in Fig. 15 where we 3have plotted values of + and Tm3 + the doped YSO. WNR fit versus L~Efor Er (11) to the 1G 3H The of expression 4, 4 and 3F 4 data is good and yields the parameters: ~ = 2.43 x 10 ~cm and C — 3 x 108 ~ 1 These are On very the close contrary, to that found 3 +values dopedwhich YSO. the in WNRErvalue of the ‘D 2 level with AEis >much 6000 larger cm 1 determined by the expression (10) than that expected. The difference probably arises from the closure approximation made in the JO theory which often gives bad results in the case of the high energy levels of the rare earth ions [41]. Indeed, in the theory, it is assumed that the radiative transition probability ‘rR 1 is proportional to the energy separation A between the barycenters of the ground and excited configurations 4f12 and 4f1’5d; so, in the case of level 1D 2 the energy of 1°configurawhichthe is above the barycenter of the 4f tion, value used for A is probably too small and the calculated radiative transition probability underestimated which leads, following expression (10) to an overestimated multiphonon relaxation rate WNR. We finally notice that the 3F 4 infrared fluorescence lifetime is about half the associated radiative value; it means a fluorescence quantum efficiency, expressed by ~ T/TR, of about 50% —

170

C. Ii ci a!

Journal of Luminescence 62 (1994) 157 /71

junction with their fluorescence decay modes as a function of the Tm3 + ion concentration and, in some cases, of the excitation pump power. The results have shown that most of the excited states are subject to important non-radiative multiphonon relaxations but, also, to very efficient cross relaxation energy transfers and various theories

100000

10000

have been used to model the observed behaviours. z

1000 YSO:Tm

~

Acknowledgements

Fig. 15. Plots of the nonradiative multiphonon rates WNR derived in Er3 * and Tm3 + doped YSO as a function of the energy gaps AE.

Thanks are expressed to J.Y. Rivoire (UA 442, U. Lyon I) for his help in the experiments and S. Lecoq (Lab. Minéralogie, Cristallographie, U. Lyon I) for the orientation of the YSO crystal. We also are very grateful to the LETI/DOPT crystal growth team for preparing most of the samples used in this study.

which is fairly low and will certainly have detrimental consequences on the laser performance.

References

100~4~~4400

4~0

‘

5~O

6000

~E (cm_b)

6. Conclusion The infrared emission properties of the Tm3 + doped YSO, CYS and SYS systems have been analyzed and compared with those of other systems. Because of larger transition strengths and broader optical bands around 800 nm as well as 2 p.m, these systems seem to be particularly promising for the semiconductor diode pumped tunable laser application. However, because of important non radiative multiphonon relaxations, these advantages might be counterbalanced by a low infrared quanturn efficiency. The optical properties of the YSO:Tm3 + have been studied in more details. Polarized absorption and emission spectra have been recorded at low as at high temperatures in the visible as in the infrared to have the best description of all the energy levels. These spectra have been analyzed within the framework of the Judd Ofelt theory to get values for the various excited state radiative lifetimes and to have an estimate of the branching ratios of the various fluorescences. They also have been analyzed in con-

[1] P. Targ, Mi. Kavaya, R.M. Huffaker and R.L. Bowles, Appi. opt. 30 (1991) 2013. [2] J.C. Petheram, J.F. Shanley, J.T. Sroga, R.C. Vitz, A.B. Wissinger and T.R. Lawrence, Proc. SPIE. 1062 (1989) 274. [3] SW. Henderson, R.M. Huffaker, Mi. Kavaya, C.P. Hale, JR. Magee and L.E. Myers, Proc. SPIE. 1222 (1990) 118. [4] M.J. Kavaya, SW. Henderson, E.C. Russell, R.M. Huffaker and R.G. Frehlich, AppI. Opt. 28 (1989) 840. [5] G. Huber, E.W. Duczynski and Klaus Petermann. IEEE J Quant. Elect. QE24 tl988) 920. [6] R.C. Stoneman and L. Esterowitz, Opt. Lett. 15 (1990)486. [7] R.S. Chang, H. Hara, S. Chaddha, S. Sengupta and N. Djeu, IEEE Photon. Technol. Lett. 2 (1990) 695. [8] H Saito, S. Chaddha, R S F. Chang and N Djeu, Opt. Lett. 17 (1992) 189. [9] KS. Bagdasarov, A.A. Kaminskii, AM. Kevorkov, AM. Prokhorov, SE. Sarkisov and TA. Tevosyan, Soy. Phys. Dolk, Tkachuk, 18 (1974) 664. [10] AM. AK. Przhevusskii, L.G. Morozova, A.V. Poletimova, M V. Petrov and A.M Korovkin, Opt. Spectrosc. 60 (1986) 176. [11] AM. Morozov, MV. Petrov, V.R. Startsev, AM. Tkachuk and PP. Feofllov, Opt. Spectrosc. 41(1976) 641. [12] KB. Steinbruegge, T. Henningsen, RH. Hopkins, R. Mazelsky, NT. Melamed, E.P. Riedel and G.W. Ro land, AppI. Opt. 11(1972) 999. [13] A.O. lvanov, L.G Morozova, IV. Mochalov and V.A. Fedorov, Opt. Spectrosc. 42 (1977) 311.

C. Li ci a!.

/ Journal of Luminescence 62

[14] R. Beach, M.D. Shinn, L. Davis, R.W. Solarz and W.F. Krupke, IEEE J. Quant. Elect. QE-26 (1990) 1405. [15] C. Li, R. Moncorgé, J.C. Souriau and Ch. Wyon, Opt. Commun. 101 (1993) 356. [16] C. Li, R. Moncorgé, J.C. Souriau, C. Borel and Ch. Wyon, Opt. Commun. 107 (1994) 61. [17] J.C. Souriau, R. Romero, C. Bore!, C. Li and R. Moncorgé, App!. Phys. Lett. 64 (1994) 1189. [18] G.H. Rosenblatt, G.J. Quarles, L. Esterowitz, M.H. Randles, i.E. Creamer and R.F. Belt, in: Proc. Advanced Solid-State Lasers and Compact Blue-Green Lasers of OSA, ATu2-1/147, 1 4 February 1993, New Orleans, Louisiana, USA. [19] JCPDS (ASTM) file no. 36-1476. [20] C. Li, Ch. Wyon and R. Moncorgé IEEE J. Quant. Elect. QE-28 (1992) 1209. [21] C. Li, iC. Souriau and R. Moncorgé, J. de Phys. (France) IV 1 (1991) c7 371. [22] B.R. Judd, Phys. Rev. 127 (1962) 750. [23] G.S. Ofelt, J. Chem. Phys. 37(1962)511. [24] W.F. Krupke, IEEE J. Quant. Elect. QE-7 (1971) 153; QE-lO (1974) 450. [25] M.J. Weber, T.E. Varitimos and B.H. Matsinger, Phys. Rev. B 8(1973) 47. [26] W.T. Carnall, P.R. Fields and K. Rajnak, J. Chem. Phys. 49 (1968) 4412; 4424. [27] R. Reisfeld and Y. Eckstein, J. Chem. Phys. 63(1975)4001.

(1994) 157 171

171

[28] D.C. Yeh, R.R. Petrin, WA. Sibley, V. Madigou, J.L. Adam and M.J. Suscavage, Phys. Rev. B 39 (1989j 80. [29] W.F. Krupke, In: Proc. IEEE Region VI Conf., 24 26, April 1974 Albuquerque New Mexico, [30] N. Spector, R. Reisfeld and L. Boehm, Chem. Phys. Lett. 49 (1977) 49. [31] D.E. MeCumber, Phys. Rev. 136 (1964) 954. [32] B.F. Aull and H.P. Jenssen, IEEE J. Quant. Elect. QE-18 (1982) 925. [33] P.F. Moulton, J. Opt. Soc. Amer. B 3 (1986) 125. [34] W.F. Krupke, M.D. Shinn, i.E. Marrion, iA. Caird and SE. Stokowski, J. Opt. Soc. Amer. B 3(1986)102. [35] W. Koechner, Solid-State Laser Engineering (Springer, New York, 1986) p. 17. [36] A.A. Kaminskii, Laser Crystals, Springer Series in Opt. Sci. Vol. 14 (1981) pp. 145 146. [37] G.H. Dieke, Spectra and Energy Levers of Rare Earth Ions in Crystals, (Interscience, New York, 1968). [38] J. Sanz, R. Cases and R. Alcala, J. Non-Cryst. Solids 93 (1987) 377. [39] S. Huang, ST. Lai, L. Lou, W. Jia and W.M. Yen, Phys. Rev. B 24 (1981) 59. [40] M. Yokota and 0. Tanimoto, J. Phys. Soc. Jpn. 22 (1967) 779. [41] C. Li, Y. Guyot, C. Linares, R. Moncorgé and M.F. Joubert, OSA Proc. Advanced Solid State Lasers 1993, Vol. 15, eds. A. Pinto, T.Y. Fan (1993) p. 91.