Up-conversion of red light into blue light in thulium doped fluorozirconate glasses

LUMINESCENCE

Journal of Luminescence 50 (1992) 317—332

JOURNAL OF

Up-conversion of red light into blue light in thulium doped fluorozirconate glasses E.W.J.L. Oomen Philips Research Laboratories, P.O. Box 80 000, 5600 JA Eindhoven, The Netherlands Received 23 May 1991 Accepted 10 October 1991

3~doped fluorozirconate glasses results in blue light. Up-conversion of red of light a wavelength of about in Tm This blue light consists twowith emission bands; one at 450650 nm nm which is ascribed to the ‘D 3H 2 —~ 4 transition, the other at 475 3H nm is ascribed to the 104 —~ 6 transition. The emission intensities of both bands vary quadratically with the excitation power. The up-conversion mechanisms for these bands are elucidated from the results3+ofconcentrations. the (up-conversion) For glasses emission with anda Tm3~ concentration 0.2 mol% and greater, cross-relaxation processes occur which excitation spectra andofdecay time measurements of fluoride glasses with different Tm seriously decrease the up-conversion efficiency. These cross-relaxation processes turn out to be very efficient in Tm3 doped fluoride glasses.

1. Introduction The conversion of long-wave into short-wave radiation by multiphoton mechanisms is known as up-conversion. In the up-conversion process energies of absorbed photons are added into the energy level scheme of a rare earth ion by excited state absorption (the ESA process) or by energy transfer (APTE, which is an abbreviation of the French expression “Addition de Photons par Transfert d’Energie”), resulting in the population of a higher excited state than can be accompushed by one-photon absorption. Subsequent emission from that higher excited state yields photons with higher energy than the energy of the absorbed Up-conversion was60sextensively studied photons. in particular in the late and early 70s; for excellent reviews of the work during that period see refs. [1—4].Afterwards the interest in up-conversion diminished because of the lack of practical applications and suitable excitation means in the infrared region. However, the recent demand for compact, efficient and reliable laser devices that operate in the blue—green spectral region (for use in optical data storage, color 0022-2313/92/$05.00 © 1992

—

displays, laser printing, barcode reading, etc.) and the availability of powerful laser diodes have led to a renewed interest in up-conversion [5—181. Furthermore, heavy metal fluoride glasses became available with similar up-conversion efficiencies as fluoride crystals which are the most efficient up-conversion materials known so far

[5,6]. Laser action in the visible upon up-conversion pumping with infrared light was already predicted by Johnson and Guggenheim in 1971 [191. Nevertheless, it was not until 1987 that the first up-conversion laser was reported by Macfarlane and coworkers [7]. They observed green laser action at 550 nm upon pumping a cooled (77 K) YAIO 33~crystal simultaneously with 792.1 and 839.8 Er Shortly thereafter laser action at liquid nitronm. gen temperature (LNT) was reported for other erbium doped crystals [8,13]. Recently blue upconversion lasers operating at LNT were reported [14—16]and even a first CW green up-conversion laser operating at room temperature has been described [17]. Several up-conversion lasers (among which the last mentioned) have been realized in a rare earth doped, single mode,

Elsevier Science Publishers B.V. All rights reserved

318

E. W.J.L. Oomen

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Up-cont’ersion in thulium doped fluorozirconate glasses

ZBLAN fluoride glass fibre [16,17] (ZBLAN stands for a fluoride glass made of ZrF4, BaF2, LaF3, A1F3 and NaF). These results show that up-conversion lasers form a new and rapidly developing field with a compact visible laser as a potential application. The latter statement is justified because up-conversion pumping occurs at(and wavelengths forinvestiwhich laser diodes are available still under gation) and CW laser action at room temperature has been realized. Of course, it will be advantageous for device construction if only one pumping wavelength (i.e. one laser diode) is required for efficient up-conversion. It is also clear that rare earth doped single mode fluoride glass fibres are very suitable materials for efficient up-conversion and reveal good laser characteristics. Furthermore, fibres are very convenient for device construction and offer long absorption lengths [20]. The interest in studying up-conversion processes in rare earth doped fluoride glasses arises, therefore, from the high up-conversion efficiencies of these materials and because several of these glasses, especially fluorizirconate glasses like ZBLA and ZBLAN, can be readily drawn •into fibres of high optical quality (see refs. [21—23], and references therein). Up-conversion of 800 nm light in Er3~ doped fluoride glasses has been investigated and results mainly in green light [11,12]. This is an efficient up-conversion process and high power 800 nm laser diodes are available, Up-conversion of 980 nm into green light in fluoride glasses codoped with Er3~and Yb3~is even more efficient (due to enhanced energy transfer probabilities from Yb3~to Er3~ in glasses with well-chosen Er3~and Yb3~concentrations), but lacks, so far, the availability of reliable, high power laser diodes at 980 nm. Green laser action has not yet been achieved for Er3~ or Yl~3~ Er3~ doped fluoride glasses. Ho3~ doped ZBLAN glasses convert red light (about 650 nm) into green light (about 550 nm) [17]. Although this process yields CW green laser light at room temperature, it is not a convenient process for practical application mainly because of the small reduction of the wavelength. Another disadvantage is that laser diodes around 650 nm have to be used. These laser diodes have only recently

been developed [24] and it is questionable whether these laser diodes can reach high powers. In summary, several methods, each which its own advantages, can be chosen in order to develop a green compact up-conversion laser. For obtaining a blue up-conversion laser fewer options 3are available. Fluoride + and Tm3 + reveal blueglasses light ofcodoped 480 nm with Yb upon 980 nm pumping [6,10]. However, in addition to the disadvantage that 980 nm laser diodes must by used, the efficiency of this up-conversion process is low because it is a three-step up-conversion process. Recently, conversion of red light of 647 and/or 676 nm into blue light of 450 and 480 nm in Tm3~doped fluoride glasses has been reported (even yielding blue laser light at LNT) [16,18]. This indicates that pumping with one laser diode around 650 nm (this pumping wavelength is a disadvantage) might result in blue emission as a result of a two-step up-conversion process. Blue laser action is, in principle, easy to obtain because 450 nm emission involves a transition to an excited state of the Tm3~ion which is empty at room temperature. This makes population inversion easy to achieve. Providing that red diode lasers with sufficient power become available in the future, this offers the most promising way to realize a compact blue up-conversion laser. The mechanism of this up-conversion process has been investigated to a small extent in one fluoride glass fibre only [18]. In this work, up-conversion of red light into blue light is extensively investigated in fluorozirconate glasses with different Tm3~ concentrations. The up-conversion mechanism is elucidated, optimal excitation wavelength and thulium concentrations are determined and consequences for constructing a blue up-conversion laser are discussed. 2. Experimental The following Tm3~ doped fluorozirconate glasses are prepared: 53 ZrF 4—20 BaF2—(4 —x)LaF3—3 A1F3—20 NaF—x TmF3, abbreviated as ZBLAN-Tm(x) with x 0.05, 0.1 and 1, 56 ZrF4—33 BaF2—(7 —x)LaF3—AIF3—x TmF3 or ZBLA—Tm(x) with x 0.1, 0.2, 0.5 and 1, —

=

—

=

E. WJ.L. Oomen

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Up-conversion in thulium doped fluorozirconate glasses

55 ZrF~—33BaF2—8 TmF3—4 A1F3 or ZBTmA. The thulium content in the glasses was analyzed by ICP mass spectrometry and corresponded with the weighted-in amount of thulium for all sampies except for ZBTmA in which the actual Tm content was only 6 mol%. The glasses were pre—

319

2 for dye laser excitation and about 50 1000 ~im Ar or Kr laser excitation). The about p~m2for emission signal in the wavelength region 350—850 nm was scanned through a monochromator and detected with a photomultiplier tube. An emission band around 1600 nm (3H 3H 4 6) was detected using a Ge photodiode. This emission band was measured without using a monochromator, but the wavelength was selected by using several filters. Decay curves were measured upon excitation with 647 and 363 nm using the above-mentioned equipment in which the excitation light was modulated by a chopper. The decay curve of the 1600 nm emission band was recorded by using a signal averager. The other decay curves were obtained by using a gated photon counter (Stanford Research Systems SR 400). —*

pared by standard methods for fluoride glass preparation using CC14 as reactive atmosphere [25,26]. The starting materials were the metal fluorides with a purity of at least 99.9% (BDH, Fluortran). Optical measurements were performed on polished glass plates with a thickness of about 1.5 mm. All measurements were performed at room temperature. Absorption spectra were measured on a Perkin-Elmer Lambda 9 spectrophotometer. Excitation spectra from 620 to 680 nm and emission spectra upon 650 nm excitation were obtamed by using an Ar laser pumped dye laser (Coherent 599) supplied with DCM dye as excitation means. Further measurements were performed for 363 nm excitation by using an Ar laser (Spectra Physics 171) and for 647 and 676 nm excitation by using a Kr laser (Spectra Physics 164). In all measurements the excitation light was focused into the glass plates (focus spot size was

3. Results 3.1. Absorption measurements The absorption spectrum of ZBTmA is given in fig. 1. The absorption bands can be ascribed to

1.0 0.9 <

0.8 0.7 0.6 684

200

400

600

1212

800

1000

1200

1400

1600

1800

2000

X(nm) 101og(I/1 Fig. 1. Absorption spectrum of ZBTmA glass (thickness

=

1.5 mm). A is the absorption (— (in nm).

0)) and A is the wavelength

320

E. W.J.L. Oomen

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Up-conversion in thulium doped fluorozirconate glasses

level

absorption bands of rare earth ions are due to electronic transitions within the inner 4f shell, which are not very sensitive to chemical sur-

______________

28000

1)

E(cm-

1G 4

3F 3F3 4

be calculated from the absorption spectrum using [27—29]

21400

f= (mc/Ne2IT)fa(P)

15200 14600

_____________

12800

3H 3H 5 4

5800

3H U3t

The energies of the Fig. 2. of levels Energy Tm3’~’level in ZBTmA scheme ofglass Tm are obtained from the absorption maxima,

(1) 3~ions per cm3, in iswhich N is the number of Tm the frequency and a(v) is ln(I 1~/I)/d(d is the absorption path length). The other symbols have their ordinary meaning. A more convenient formula is obtained by assuming the surface of the absorption band to be equal to its maximum multiplied by its full width at half maximum (FWHM): f= 4.318 x 109Emax x FWHM, (2) in which ~max is the molar extinction coefficient (in cm mol 1) at the maximum of the absorption band and FWHM is taken in cm’. -

transititions from the ground state (3H to The the 3~ion (see fig.6)2). excited states of theof Tm absorption spectra all Tm3~doped fluorozirconate glasses had a similar pattern with only small differences in absorption maxima and mutual intensities. Of course, the absorption intensities for the different samples decreased for lower Tm concentrations. The absorption maxima and the mutual intensities of the absorption bands are presented in table 1. The minor shifts in wavelengths of the maxima are expected because the

3H 3H 3H6 -. 3H~ 4 6 -~ —~ 3F 3H 3F, 4 3H6 —a 3F 6 —a 2 ‘04 —a ‘D2

-

The theoretical oscillatortheory strength be calculated by the Judd—Ofelt [30].can Assuming only electric transitions, this reveals [27—29,31,32]

f

(v/2J + 1)~/n2(8~r2/3h)

Amax (nm)

mt

Amax (nm)

mt

1682 1191 1212,

0.55 1.05

1664 1180 1212,

0.60 0.89

2 (t=2,4,6), (3) 1U(t) in which J is the spin—orbit quantum number of the original level; x (n(n2 + 2)2/9); n is the refractive index; the term 8’rr2m/3h represents several physical parameters and is equal to 3.62 s/cm2 f1~ are the Judd—Ofelt parameters (in cm2) and U(t) are tabulated matrix elements [31,32] which are assumed to be independent of the host material. The value of n at the wavelength in question is calculated using the relation n A + B/A2 taking A 1.50583 and B 3478.14 nm2 for ZBLA glasses [33,34]. The Judd—Ofelt parameters are obtained by fitting eq. (3) to the experimental absorption bands. The

791, 684 778

0.67 1.00

791, 779 684

0.63 1.00

refractive are taken equal indicestofor those all of fluorozirconate ZBLA, whichglasses intro-

658 471, 464 357

0.12 0.19 0.48

658 472, 463 356

0.12 0.20 0.47

duces a very small error [25]. The Judd—Ofelt parameters calculated for the glasses ZBTmA and ZBLAN-Tm (0.2 mol%) are given in table 2

Table 1 Absorption maxima (Amax) and mutual intensities (Int) of the absorption bands of ZBTmA and ZBLAN-Tm(0.2 mol%). The absorption at 684 nm is set to 1. Transition

d~,

i’

8400

6

roundings of the rare earth ions. The oscillator strength, f, of a transition can

ZBTmA

ZBLAN-Tm(0.2)

xEn

=

=

=

=

E. WJ.L. Oomen Table 2 Judd—Ofelt parameters (in

10—20

/

Up-conversion in thulium doped fluorozirconate glasses

2) for Tm3” in several

cm

fluoride glasses. The matrix elements U(t) are taken from ref. [30].

___________________________________________ Composition3~ BIZYT-Tm ZBLA-Tm3~ BATY-Tm3~ BZYT-Tm3” ZBLALi-Tm3~

Q2 2.07 2.72 2.46 1.14 2.80

114 1.54

ZBTmA 3” ZBLAN-Tm

2.39 2.57

1.82 1.41 1.57 1.91

116 1.12 0.99 1.13 1.13 1.01

Reference [27,281 [28] [28] 16] [29]

1.60 1.90

0.89 0.84

This work

together with Judd—Ofelt parameters for Tm3” in other fluoride glasses as reported in the literature. It appeared that for all Tm3 doped fluorozirconate glasses studied in this work the Judd—Ofelt parameters are very similar. These values are comparable to those for Tm3”’ doped ZBLA and ZBLALi glasses reported elsewhere (see table 2), +

glass and the glass structure is similar for these glasses [35,36]. This is obviously not the case for indicating that the incorporation of Tm3 in the fluoride glasses other than fluorozirconate glasses such as BIZYT (BaF 2, InF3, ZnF2, YF3, ThF4), BATY (BaF2, AIF3, ThF4, YF3) and BZYT (BaF2, ZnF2, YF3, ThF4) (see table 2). This is in agreement with current structural models for fluoride glasses [22,37]. The Judd—Ofelt parameters obtained can be used to calculate emission transition probabilities +

to calculate, among other things, radiative lifetimes, branching ratios and cross-relaxation probabilities (see section 4.1). [35,36,38,39]. These probabilities can then be used 3.2.

321

bands at 450 and 475 nm are attributed to the 1D 3H 1G 3H 2 -~ 4 and 4 —~ 6 transitions, respectively. Furthermore, two other weak emission 3H bands can be discerned, viz,3Hthe ‘D2 —~ 5 band at 510latter nm band and the ‘D2 -~ 6 band at 360 The is only weakly observed duenm. to instrumental limitations; a rough correction reveals that this band has an intensity comparable to that of the 1D 3H 2~ 4 band. This is in agreement Thewith excitation reportedspectra results of[27,29,40]. the two main emission bands are depicted in fig. 4. Clearly, the excitation spectrum of the 450 nm emission band differs from that of the 475 nm emission band. This accounts for the different intensity ratios of the 450 and 475 nm emission bands with varying excitation wavelengths. The intensity ratio did not change with varying excitation power. This is a consequence of the quadratic dependence of the

a

1

b

100 ~

100

;~,

= 6

50

0 430

450

‘

470

‘

490

610

430

450

‘

470

490

510

490

510

d

C

100 ~

100

50

= 6

50

Emission and excitation spectra

The up-conversion emission spectra 3”of for a ZBL.AN glass doped with 0.1 mol% Tm several excitation wavelengths around 650 nm are shown in fig. 3. In all cases, two main emission bands are observed with maxima at about 450 and 475 nm. The 475 nm band is somewhat distorted upon 676 nm excitation which might be due to a self-absorption effect. The emission

430

450

470

490

510

430

450

470

—

Fig. 3. Up-conversion emission spectra of ZBLAN-Tm(0.1) glass upon excitation with 647 nm (a); 650 nm (b); 676 nm (c) and 647+676 nm (d). In the latter case about 70% of the excitation light consists of 647 nm radiation. I denotes the emission intensity in arbitrary units. Spectrum (b) is recorded with different equipment than the other spectra.

322

E. W.J.L. Oomen 680 100

670

660

650

640

/

Up-conversion in thulium dopedfluorozirconate glasses

The above-mentioned results are observed for a ZBLAN sample doped with 0.1 mol% Tm3”’, The other samples reveal the same emission and excitation bands. However, the intensity ratio be-

630

tween the two main blue emission bands at 450 and 475 nm decreases with increasing Tm3”’ concentration. This becomes clear from table 3 in which the intensity ratio and the absolute emis-

50

0

little ~‘/\~em=450nm 14.7 15.0 680

670

660

15.5

650

16.0 ~ (nm) 640

630

Tm) inefficient very reveal that(see up-conversion section 4.3). in Therefore, this sample these is samples are not investigated further.

b

—

are given. The absolute emission intensities have sion intensities for different Tm concentrations quantitative value when comparing the different samples because of differences in scattering, polishing and shape of the glass plates which affect the absolute emission intensity. Nevertheless, the values for the ZBTniA sample (6 mol%

50

__________________________________________ 0’

a.

0

~

SI 0l I 0~

__________________ ~J\~m=47~475nm

0 14.7

C,;

I

I

15.0

15.5

I

16.0 —.

E (10~cm~) Fig. 4. Up-conversion excitation spectra of the emission intensity of the blue up-conversion emission band at 450 nm (a) and 475 nm (b) for ZBLAN-Tm(0.1). Both spectra are normalized and corrected for the wavelength dependence of the output of the dye laser.

+

—0.5 +

o

I

—

I I

•

I I

I

—1.0

® I •

.. I I

intensities of both emission bands on excitation power as shown in fig. 5. In addition to the up-conversion emission spectra, the emission bands on the lower energy side of the excitation wavelength are also measured. Figure 6 shows a doublet-structured band around 800 nm with a shoulder on the high-energy side. The doublet band is attributed to the —~3H 6 transition 3Hand the shoulder is probably due to the ‘G4—’ 5 transition [27,29]. The intensity of the doublet band varies linearly with the excitation power. The same accounts for the measured emission between 1500 and3H2000 3H nm which is assumed to originate from the 4 6 emission band only.

I

+

‘C

I

5?

-1.5

I 0

I

#

I +

—2.0 -0.3

0

0.5 1.0 Iogø~(ø~inkW/cm”) Fig. 5. Log—log plot of the emission intensity of the blue up-conversion emission bands versus the excitation density at 650 nm, ‘Dr,. Data are for ZBLAN-Tm(0.1) (450 nm band = 475 nm band = +) and ZBLAN-Tm(1) (450 nm band = ‘,480 nm band =0). Full lines have a slope of 2, the dashed line has a slope of 1.5.

E. W.J.L. Oomen

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Up-conversion in thulium doped fluorozirconate glasses

323

100

50 9

0 740

760

780

800

820

8

7

6

840

5

4

3

2

1

to

~0I~

?~.(nm) Fig. 6. Emission spectra from 740 to 840 nm for ZBLANTm(0.l) upon 647 nm excitation.

For all samples a linear dependence of the intensities of the infrared emission bands (3F4 —~ 3H 3H 3H 6 and 4 -~ 6) upon the excitation power is observed. The quadratic excitation power dependence of the blue emission bands at 450 and 4753’1’nm, however, only apparent for concentrations up tobecomes 0.2 mol%. Above this Tm concentration the intensities approach a more

(~ )3/2

dependence (see fig. 5). Emission spectra are also obtained upon direct excitation into the mD2 level using 363 nm light, CXC

Table 3 Integrated emission intensities of the blue bands at 3”’ emission doped ZBLA(N) 450 nm (I45~)and 475 nm (1475) of Tm glasses and mutual intensity ZBTmA ratios glass R(= upon 145~/I475).CTm 647 nm excitation represents and the Tm concentration (in rnol%); for CTm = 0.1 and 1 the average values for ZBLAN-Tm and ZBLA-Tm are taken. All experiments are performed in the same set-up under equal conditions,

~

8

7

6

5

4

3 2 1 to (ms) 3F Fig. 7. Decay plots (ln(I) versus t with 1= emission intensity in arbitrary units and t = time in milliseconds) of the 4 level (790 nm emission)(a)upon nm excitation(b).forThe the vertical glasses ZBLAN-Tm(0.1) and 647 ZBLAN-Tm(1) dashed line indicates the time t 0, at which the excitation light is shut off; the horizontal dashed line indicates the dark current of the photomultiplier. Full lines represent one-exponential (a) and two-exponential (b) decay patterns. For further explanation see text.

1D These spectra reveal only at emission level,i.e. a strong emission 450 nmfrom (mD the3H 2 3H 2 5). ~-9 This 4) and a weaker shows that theone 1Gat 510 nm (1 D2 —4 4 level is not populated by 1D relaxation or any other process from the 2 level.

CTm

R

‘450

I

3.3. Decay times

0.05 0.1 0.2 0.5 1 6

2,5±0,2 2.4±0.1 1.8±0.1 1.2±0.2 1.2 ±0.2 0.3 ±0.1

49± 3 100± 3 296±10 600±50 1050 ±50 20± 5

19± 2 46± 2 164±10 490±40 850 ±20 68±15

1D 1G 3F Decay times of the 2, 4 and 4 excited states upon 647 nm excitation are measured. Typical decay plots are shown in fig. 7; both one-exponential (fig. 7(a)) and two-exponential curves (fig. 7(b)) are obtained. A two-exponential curve

324

E. W.J.L. Oomen

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Up-conversion in thulium dopedfluorozirconate glasses

Table 4 Decay times of the ‘D

3F 3~ 2, 04 and 4 excited states of Tm in ZBLA(N) fluoride glasses upon 647 nm excitation. CTm denotes the Tm concentration (mol%); for 0.1 and 1 mol% the average value for Tm doped ZBLAN and ZBLA glass is given. All decay times are obtained from a two-exponential curve except those denoted with (1-exp); in that case a one-exponential curve was measured. ~ and i’~ denote the slow and fast decay times, respectively. Furthermore, the contribution of the slow component to the total emission intensity at t = 0, (j~)S/J~,is given. CT,,,

~

(p.s)

~

(i.~s)

2 level under 363 nm excitaion. For further explanation see caption table 4. Cm~ 0.05 0.1 0.2 0.5 1

r1 (p.s) r~(p.s) 57±2(1-exp) 56±2(I-exp) 55 ±2 (1-exp) 41±3 54±4 37 ±3 55 ±4

(bo)s/1o — — —

0.67±0.07 0.45 ±0.05

(J~)s/J,,

Decay times ‘D2 level 0.05

50±5

150±30

0.07±0.02

0.5 1

54+5 54±5a)

147±20 132±20

0.27±0.04 0.27±0.04

Decay times G4 level 0.05 550±100(l-exp) 0.1 480± 50(1-exp) 0.2 510± 50 (1-exp) 0.5 340±30 510±50 1 310±30 480±50

Table Decay 5times of the ‘D

— -

0.44±0.05 0.39±0.04

Decay times “F4 level 0.05 1600±40(1-exp) — 0.1 1600±40(1-exp) — 0.2 1560±40(1-exp) — 0.5 1020±40 1520±100 0.67±0.07 1 810±30 1550±100 0.45±0.05 a) Two decay times can be discerned; 54 ± 5 p.s and 45 ±10

p.s. indicates that two processes compete in depopulating the excited state involved. These processes are characterized by their decay times r~(for the slower process) and T~ (for the faster process). The contribution of the slower process to the total decay at time t0 (the time the excitation is stopped) is given by (Io)S/Io (see fig. 7(b)). The decay times obtained are presented in 3F table 4. The iG and 4 excited states show a single-exponential decay for Tm concentrations up to 0.2 mol%. For higher Tm concentrations a two-exponential decay curve is found, indicating that another process with a shorter decay time, whose contribution increases for higher Tm con centrations, becomes active for Tm concentrations above 0.2 mol%. The decay curve for the ‘D2 excited state is two-exponential for all Tm

concentrations; for 1 mol% even a third decay process can be discerned. The contribution of the slower decay process to the total decay increases with increasing Tm concentration. The decay of the ‘D2 excited state was also measured with direct excitation (363 nm). The results are shown in table 5. The slow decay process observed upon 647 nm excitation has vanished. Up to 0.2 mol% Tm a single-exponential decay, with a decay time of about 55 ~s is

—

C

4

3

2

10

‘

‘

20

3H

t(ms) -______ Fig. 8. Decay plots of the 4 level (emission 1500—2000 nm) upon 647 nm excitation for ZBLAN-Tm(0.2). The drawn line represent the radiative decay.

E. W.J.L. Oomen

/

Up-conversion in thulium dopedfluorozirconate glasses

observed. For higher concentrations a faster decay contributes to the total decay. 3H The decay themeasured. 4 levelA upon nm excitation was ofalso typical647decay curve is shown in fig. 8. Similar decay curves are observed for all Tm3 ± concentrations. The shape of this curve is clearly different from that of a one- or two-exponential decay and can only be explained by the presence of a build-up process which is necessary to populate the 3H 4 excited state after excitation with 647 nm. The characteristic time of this build-up process has to be shorter than the radiactive decay time of the 3H 4 level, otherwise the build-up will appear as a second decay process, similar to a two-exponential decay curve. For all Tm concentrations, the same build-up time and radiative decay time are found, viz. = 3.0 ±0.3 ms and 6.3 ±0.3 ms. The build-up time is characterized by the time at which the difference between the measured intensity and the extrapolated intensity for purely radiative decay (see fig. 8) has decreased to 1/e times the value at t 0 ms. (T~~)

(Tr)

Tr

=

=

325

Table 6 Radiative emission probabilities, A~ 1(in s”), and 3”radiative fluorozirconate glasses, calculated from eqs. (5) and (6)doped using decay times of the excited states, ~r(i), of Tm the matrix elements tabulated in ref. [311and ~2 = 2.7 pm2, 114 = 1.8 pm2 and 116 = 1.0 pm2. Only those states from which no fast nonradiative relaxation takes place are considered. The wavelengths A of the transition in question are also given. Transition A (nm) A.~(~‘1) Tr(l)(~L5) ‘D —a 104 1475 115 56.7 3F —a 3F2 780 698 —a 3F 3 747 621 3H —~ 3H 4 651 933 —* 5 506 70 —a 4 453 8590 —a 357 6586 3F 3F ‘G4— 2 1657 8 774 —a 3F 3 1513 35 —a 3H4 1164 126 —a 3H5 770 398 —a 3H4 654 99 471 628 3F —a3H 6 4 —a 3H5 2277 19 1310 —a 3H 4 1495 60 791 683 3H —a 3H6 4—a

6

1680

166

6020

4. Discussion probabilities the radiative decay time of a level i, Tr(l), can be calculated by

4.1. Radiative transition probabilities Radiative transition probabilities can be calculated from Judd—Ofelt theory using the parameters obtained in section 3.1. The probability for a radiative emission transitions from state i to state j, A~3,is given by [27—36]: 4/(2J+ 1) x ~,3e2/3hc3X~fl,U(t)2, A.3 64’rr =

(4) for t 2, 4, 6. The symbols have the same meaning as in eqs. (1)—(3). A more convenient formula is 31/(2J+ 1)A3 Xx~f2~U(t)2, A,1 = 7.23 x 10 (5) =

in which A is the wavelength of the transition in question (in nm). The calculated transition probabilities are given in table 6. From these transition

1/~~A~1. (6) The radiative lifetimes obtained are also given in table 6. In view of the errors in the Judd—Ofelt parameters, wavelengths, refractive indices, etc., times it is clear that the calculated radiative decay agree well with the one-exponential decay times for ‘G 3F 4, 4 and ‘D2 (upon 363 nm excitation) 3H and with the radiative 4 decay time (see section 3.3). In addition to these errors and inaccuracies in the measurements, discrepancies between measurements and theory can also arise from shortcomings in the Judd—Ofelt theory (see, e.g. ref. [41]). Furthermore, magnetic dipole or electric quadrupole interactions, which have been neglected, may have a small contribution to several transition probabilities [39].Nevertheless, the rather good agreement between calculated and Tr(l)

=

326

E. W.1L. Oomen

/

Up-conversion in thulium dopedfluorozirconate glasses

measured radiative decay times allows one to conclude that nonradiative contributions from the ‘D2, 1G4, 3F4 and 3H4 levels of Tm3”’ in ZBLA and ZBLAN glasses can be neglected. From table 6 it can be deduced that the 1D 2 3H 4 transition 1D 3H probabilty is somewhat larger than the 2 6 transition probability. This is in agreement with the observation that the inten sities of both emission bands in question are of the same order of magnitude. Because our interest in mainly in a high blue emission intensity 1D 3Hat 450 nm, a high contribution of the 2 4 transition to the total decline of the ‘D2 state is favorable. Nieuwesteeg [39] calculated that this contribution increases for (simultaneously) decreasing values for 126/Q2 and 114/02. Table 2 shows that, with respect to this contribution, fluorozirconate glasses are to be preferred above BATY, BIZYT or similar Furthermore, the values for A.~in table 6glasses. make plausible that the ‘G 3H 4 3F—÷ 53Ht, emission is observed emission band (fig.as6).a shoulder on the 4

4 450 nm

3

104

—*

475 nm

—-a

-______________

4.2.

The up-conversion process

Two blue emission bands are obtained 1D upon 3H excitation around 650 nm, one is1Gthe 3H 2 4 band at 450 nm, the other is the 4 6 band at 475 populating 1Dnm. The up-conversion3”’process is obvious. First, the 2 excited state of Tm3F 3F absorption populates the 2 and 3 excited states followed by fast nonradiative relaxation to 3F the 4 state. Then a second photon is absorbed, 3”’ ion in the 1D bringing the Tm 2 excited state from where 450 nm emission takes place. The first absorption transitions have their at 684 nm (strong absorption) and at 658 maxima nm (weak absorption, see fig. 1), while the maximum of the second absorption step is estimated at 650 nm. The excitation band of the 450 nm emission band (fig. 4(a)) will be an overlap of these separate absorption bands. This makes its shape and the maximum at 656 nm plausible. The origin of the ‘G 3H 4 6 emission is less obvious. The ‘G4 state is not populated by some relaxation process from the ‘D2 state because the decay of the ‘D2 state is entirely due to radiative decay to lower lying levels and no 475 nm emis—*

—*

—-a

2

4

1

3H

-~

—-a

4

3

______________

H6 0 Fig. 9. Up-conversion excitation for glasses. 450 and 475 nm 34 dopedroutes fluoride emission in Tm

sion 1D becomes apparent upon excitation into the 2 state. Therefore, it is concluded that the ‘G4 process is3H populated by an up-conversion process from the 4 excited state, This process is likely to occur because the maximum of the transition 3H 4 ‘G4 is at about 645 nm, which is close to the excitation wavelength for the 450 nm emis3H sion, and the 4 level3Hhas a1Glong lifetime of about 6 ms,the Before 4 —o 4 transition can take place, 3H the 4 level has to be populated first by 450 nm emission from the ‘D2 level. This excitation mechanism explains the shape of the excitation spectrum of the 475 nm emission (fig. 4(b)) which can be interpreted as a superposition of the band for 450 and 3H excitation 1G~absorption bandnm at emission 645 nm. The the 4 —o proposed up-conversion processes are shown in fig. 9 and are in agreement with a model proposed earlier [16,18]. With this model also the quadratic excitation power dependence of both blue emission bands for samples with a low Tm3”’ concentration ( 0.2 mol%) can be understood (see fig. 5). The power —.

dependence for samples with a higher concentration will be discussed in the next section. The quadratic power dependence of 1D the 450 nm emission is obvious because the 2 excited state is populated by two successive absorptions [1—

E. W.J.L. Oomen

/

Up-conversion in thulium doped fluorozirconate glasses

4,11,12]. To explain the quadratic dependence of the 475 nm emission band, the rate equations for the 3H4 and 104 excited states (levels 1 and 3 in fig. 9, respectively) are considered: dN1/dt

=

aA4iN4 —A,0N1

dN3/dt

=

7PN1 —

5A30N3

— yPN1 =

0,

=

0,

(7a) (7b)

327

level more slowly than its radiative decay. This build-up process cannot be due to an excited state absorption (ESA) process because these processes stop immediately after the excitation beam is shut off. Therefore, the build-up process is attributed to another up-conversion process, the so-called APTE process. This is an energy3 ions (both in transfer process between Tm in one Tm3’1 3Ff),two resulting the in firstthe excited state ion ground state and the other in a higher excited state, in this case the ‘D 2 state [1—3].The increasing contribution of this process to the pop1D ulation of the 2 state with increasing Tm concentration (table 4) is expected and confirms its assignments. From table 4 it can be concluded that the contribution of the APTE process in populating the ‘D2 level is rather small (even for the highest Tmexcitation concentrations). This means that upon 647 nm the D~state is mainly populated by the ESA process, in agreement with ref. [18]. Because the 3H 4 state is only populated by emission from the iD2 state, a build-up pro±

in which IV~ stands theis population of 9), level (the number of eachfor level shown in fig. A~i is the radiative probability from state i to state ,j 1 y is the ESA coefficient, a and 5 are the frac1D tions of photons from the 3H2 (or 3H ‘G4) state which decay radiatively to the 4 (or 6) state, and P is the excitation power. Radiationless processes from levels 1, 2, 3 and 4 and other radiative processes involving levels 1 and 3 (for exampie ‘D2 —o iG4) are neglected in view of the resuits of the decay and table 6. 2, eq.measurements (7a) can be rewritten as (for Because N4 a P not too low excitation powers) 2/(A, N1 aP 0 + yP). (8) Because A10 is very small (166 s”, see table 6), eq. (8) can be approximated by: N1

a~

(9)

This is confirmed by3H the measured linear power 3H~,emission. Equadependence of the 4 tions (7b) and (9) reveal: 2, (10) N3 aP which is observed. This confirms also the proposed model for populating the 1G 4 state. The results of the decay time measurements are in agreement with this model. In this section only the results for Tm concentrations up to 0.2 mol% will be3Fdiscussed. The one-exponential decay for the 4 level upon 647 nm excitation is due to a 3F very fast nonradiative relaxation from 3F 3 and 2 to this level, which has the consequence that only the radiative decay is observed. The same accounts for the ‘D2 decay excitation with 363 nm. This is in contrast with the decay from the same level upon 647 nm excitation for which anThis additional process is observed. process slower has todecay originate from the two-step absorption process and is assumed to be due to a build-up process which feeds the 1D 2 —-a

cess becomes apparent. In this case the build-up process is faster than the radiative decay, resulting in a decay plot as shown in fig. 8. For the mG4 decay upon 647 nm excitation no build-up is observed, although this level is populated from the 3H 4 level. This indicates that the3H‘G4 level is only populated by ESA from the 4 level, a process which stops off. immediately after thefor excitation light is shut Some evidence this assumption appears from the excitation spectrum of 475 nm emission band which has a more sudden cut-off on the low-energy side than the corresponding spectrum of the 450 nm emission band (fig. 4). This means that ground state absorptions are less important in populating the ‘G4 level compared to the ‘D2 level. However, it should also be remarked that, because of the low accuracy of the ‘G4 decay curve due to the rather low 475 nm emission intensity, weak build-up processes cannot be discerned in the decay plot. From these results it is clear that APTE is not a major process for populating theexcitation blue emitting 3 upon with ‘D2 light. and 104 of Tmdifferent for up-converred Thislevels is clearly sion of 980 nm into visible light in Yb3”’, Er3’1’ 3”’, Tm3” codoped systems [1—6]and for and Yb +

328

E. W.J.L. Oomen

/

Up-conversion in thulium dopedJluorozirconate glasses

up-conversion of 800 nm or 1.5 p.m in Er3”’ doped systems [11—13,42,43]. 4.3. Dependence on Tm concentration

cesses increase with increasing Tm concentration. Evidence will be given that these processes are mainly cross-relaxation processes. Cross-relaxation processes have already been found to be very efficient in quenching the 3F 4 andfluoride ‘G4 excited 3 doped BIZYT glass states in Tm [27,28,40]. In this glass cross-relaxation starts to be effective for Tm concentrations between 0.1 and 0.5 mol%, in agreement with the concentrations reported here. Cross-relaxation is an energy-transfer process between two Tm3”’ ions resulting in depopulation of the highest excited states. Therefore, cross-relaxation processes decrease the up-conversion processes. Cross-relaxation processes which appeared to be efficient in Tm3”’ doped BIZYT glasses are [27,28,40]: ±

The up-conversion mechanism described in the previous section explains all measurements for glasses with a Tm concentration up to 0.2 mol%. However, for glasses with higher Tm concentrations some phenomena appear which cannot be explained with this model. The most striking phenomena in this respect are: (i) For Tm concentrations above 0.2 mol% the quadratic dependence of the blue emission intensities on the excitation power disappears and a dependence of about (Pexc)3”2 is measured. (ii) The ratio of the intensities of the 450 nm emission and the 475 nm emission decreased with increasing Tm concentrations, starting at a concentration of 0.2 mol% (table 3). (iii) For glasses with a Tm concentration above 0.2 mol% an additional decay process becomes apparent for the 1G ‘D2 state 3F upon 363 nm excitalion and forThe thecontribution 4 and 4ofstates upon 647 nm excitation. this process to the total decay increases with increasing Tm concentration. Its decay time is shorter than the radiative decay and decreases with increasing Tm concentration (tables 4 and 5). Obviously, the up-conversion efficiency ~ (for a two-photon up-conversion process ~i (blue) 2) decreases emission intensity/[absorbed with higher Tm concentrationspower] for a Tm concentration of 0.2 mol% and more. This is clear from the decay measurements showing that decay processes other than the radiative processes compete in declining the excited states. Furthermore, it can also be descerned from table 3 assuming that, because of the small absorption length and the low Tm concentration in the glass plates, the absorbed power varies linearly with the Tm concentration. Therefore, it is concluded that for Tm concentrations of 0.2 mol% and more, processes competing with the up-conversion processes become active. These processes are different from those already active in glasses with a low Tm concentration (i.e. nonradiative processes). Furthermore, the probabilities of these new pro=

Tm(’G

3Ht,)

4) Tm(’G4)

+ Tm(3H + Tm( 6) —

Tm(3F 3H 2) ++ Tm(3H Tm( 4), 3F~) Tm( 5),

3F 3Ht,) 2 Tni(3H Tm( 4) + Tm( 4). in the ground 3” ion In all isthese processes, state involved. This aisTm not a necessary condition, but it increases the efficiency of the cross-relaxation process. Cross-relaxation processes between two excited rare earth ions have been observed, but their efficiencies are weak [12]. Cross-relaxation processes between two Tm3 ions involving the Tm(’D stateishave not been reported yet (as far2)asexcited the author aware). Cross-relaxation is assumed to be mainly based on dipole—dipole energy-transfer mechanisms which can well be described by the Föster—Dexter equation [44]. This expression has been modified by Kushida for energy transfer between two rare earth ions. This results in the following equation for the probability of energy transfer (or cross-relaxation), between two rare earth ions (A and B) [45]: ±

NCR’

~CR

=

2/{3(2JA

+

1)(2JB

4/hR6HO” X 2’rre

+

1)) (11)

in which h and e are the well-known physical constants, J~is the spin—orbit quantum number of the level of ion i in question, R is the distance

E. W.J.L. Oomen

/

Up-conversion in thulium dopedfluorozirconate glasses

between the two rare earth ions involved and 0” is the overlap integral of the transitions involved in the energy-transfer process (varies between 0 and 1). Finally, H

=

( Ecl,u(t)

2

~n1u(t) )

2, 4, 6), (12) which is a product of the Judd—Ofelt parameters (t

Table 7 Cross-relaxation parameters for given Tm concentration in ZBLAN glasses. II * = I1/(2JA + 1X2J~+ 1); ~E is the enmismatch between the transitions on Tm(A) and Tm(B), 0 represents the overlap and (PCR)rei is the relative cross-re-

ergy

laxation probability. The values are calculated from eqs. (12)

2

)A(

=

and the matrix elements for the two transitions concerned. Assuming a homogeneous distribution of the rare earth ions in the glass results in 3

CRE = 3V,~/4’rrNr (13) with CRE being the rare earth concentration, r half the distance between the two rare earth ions, V is the molar volume and N is Avogadro’s number. Combining eqs. (11) and (13) results in ,

and (14). For further information see text. Process 11” 4) (cm~) i~E 0 (pm TmA(’D 3H 1G 2)+TmB( 3H 6)_a TmA(3F 4)+TmB(3F 4) 0.043 450 0.2

=

C”H/(2JA

+

1)(2JB

+

1)

x 0(CTm)

(14)

(PCR)rel

8.6

TmA(3F2)+TmB(3F4) TmA(3F 3)+TmB(3F 4) TmA(3F4)+TmB( 3F3) TmA( 3H4)+TmB( 2)

0.016 0.012 0.002 0.013

150 430 430 150

0.6 0.2 0.2 0.6

9.6 2.4 0.4 7.8

TmA( 4)+TmB(’G4) 3H TmA(’G4)+TmB( 6) —a 3F 3H TmA( 2)+TmB( 4) ~

0.008

450

0.2

1.6

0.005 ~

100

0.6

3.0

480

~

0.002

100

0.6

0.1

0.023

500

0.1

2.3

2,

~CR

329

TmA(3H

3F

4)+TmB( 2) 3H TmA( 4)+TmB( 6)—a 3H 2 Tm( 4) 3F

in which C” is a constant and 0 represents the overlap between the transitions of both ions, which depends on the energy difference between transitions, ~E, and on the broadness of the bands of the transitions involved, For a given Tm concentration the mutual intensities for ~cR are calculated from the values of J,, H and 0. These calculated values are given in table 7. Because the point of interest in this case is restricted to efficient cross-relaxation processes between a Tm ion in the ground state and the 3F other in the iD2 iG4 and 4 excited states are calculated. The values for ~E are obtained by taking the excitation energies from the absorption maxima and by assuming the corresponding emission 1transitions to have anoverlap energy0which is 300 lower [27—29].The is obtained em” by assuming Gaussian band shapes and a FWHM (full width at half maximum) of 500 cm for all transition bands. These approximations are rather rough, although they are as close as possible to experimental results. Moreover, average values have to be used as a consequence of inhomogeneous broadening in the glasses. Therefore, the values in table 7 are approximated average values and serve only as a guide. However, it can be concluded from table 7 that cross-relaxation from —

1D 1G the 2 and 4 levels has a 3Fhigher probability than cross-relaxation from the 4 level. The most efficient cross-relaxation processes are shown in fig. 10. As mentioned before, cross-relaxation appears for Tm concentrations of about 0.2 mol% and more. These processes decrease the population3Fof the ‘D2 and 104 and, to a lesser extent, 4 excited states. Each excited state is depopulated 3H differently. Several states, especially 4, are populated by cross-relaxation. Therefore, it is plausible that cross-relaxation explains the decreasing intensity ratio of the 450 and 475 nm emission bands with increasing Tm concentration (table 3). It accounts also for a less than quadratic dependence on the excitation power because the cross-relaxation probability increases with increasing population numbers of the excited (blue emitting) states. Furthermore, the cross-relaxation processes account for the additional decay process because the cross-relaxation probability and, therefore, its contribution to the total decay increases with increasing Tm concentration. The

330

E. W.J.L. Oomen

/

Up-conversion in thulium dopedfluorozirconate glasses

5. Conclusions

I

The conversion of red light of about 650 nm doped fluoride glasses into blue studied. light inThe Tm3’1 has been blue light is mainly emitted

—

1

in an emisssion band around 450 nm (‘D 2 but a rather strong emission at 475 nm (‘G4—’ 3H~)is also observed. The ‘D 2 and ‘G4 excited states are populated by up-conversion processes. The mechanisms of these up-conversion pro3F levels; cesses are elucidated. The ‘D2 level is excited by a two-step absorption process via the the second excitation being mainly an ESA pro—*

3F 3F23 4

.\___

~

j~

~\

3H 5 3

— ________

H4

—

________

6

—

\ \

\,~

\ \~ ~

_______________

___JL_.... ________

________

L,=—=,,..-—--~~) L~~—~-—---~.J

Fig. 10. Most efficient cross-relaxatk~n processes ~n Tm3” doped ZBLA(N) glasses between a Tm ion in the H 1D 3F 6 state and the other in the 2 state (a), ‘G4 state (b) or 4 state (c).

1D third observed decay process of the 2 isdecay of ZBLAN-Tm(1) with 647 nm excitation probably also due to cross-relaxation. For the sample doped with 0.5 mol% Tm this contribution is obviously too weak to be discerned in the two-exponential decay curve. In summary, cross-relaxation processes appear in Tm34’ doped ZBLA(N) glasses for Tm concentrations of 0.2 mol% or more. Using a density of these glasses of about 4.5 g/cm3 [46] results in a distance between the Tm3”’ ions of about 35 A for a Tm concentration of about 0.2 mol%. Generally the critical distance for very efficient cross-relaxation between two rare earth ions is 20 A [47,48]. The exceptionally efficient cross-relaxation between Tm ions in fluoride glasses is not well understood at the moment. A possible explanation might be the clustering of the rare earth ions in fluoride glass as suggested before [49,50]. This explanation would also account for the observation of both the radiative and the cross-relaxation decay for glasses with a relatively high Tm required concentration. However, experiments are to elucidate this more problem which is beyond the scope of this work.

cess, but APTE contributes also the occupation ~ntet 655 nm. The population of the 104 excited state is somewhat more complex. After emission of a 450 nm photon 3H from the D2 level, the long-living (6.3 ms) 4 state is populated. Then ESA from this level takes place resulting in the occupation of the ‘G4 state. The dependence 3”’ of concentration the up-conversion effihas been ciency upon For the concentrations Tm investigated. of 0.2 mol% and higher the efficiency decreases with increasing Tm3”’ concentration, It is shown that this decrease is caused by cross-relaxation processes which are very efficient in Tm3”’ doped fluoride glasses. Up-conversion in Tm3”’ doped fluoride glasses is investigated in order to study the possibilities of constructing a blue up-conversion laser, consisting of a laser diode pumped Tm3”’ doped fluoride glass fiber cavity. Laser action at 450 nm seems promising because the final state (3H 4) is not occupied at room temperature and, there3H fore, population inversion between iD2 and 4 should be possible. However, the lifetime of the 3H 4 level (about 6 ms) is much longer than that of the ‘D2 level (about 50 p.s), which makes operation of a 450 nm up-conversion laser only possible 3H in a pulsed mode unless the lifetime of the 4 level is considerably shortened. In principie this can be achieved in 3twodoped different glass ways with [49].ion One is to causes codopeenergy the Tmtransfer from Tm3”’ an which in the 3H 4 excited state and, therefore, quenches +

E. WJ.L. Oomen

/ Up-conversion in thulium doped fluorozirconate glasses

its lifetime (an example is Th3’1- [51]). The other possibility to shorten the 3H 1G 4 lifetime is to empty this level by ESA to the 4 state. This “trick” is often obtaining 2.7 p.m lasingwill in 3”’ used dopedfor materials [52,53]. ThisCW process Er be more efficient for higher excitation powers (i.e. in a single mode fibre). In conclusion, Tm doped fluoride glasses can be used to convert red into blue light which might result in a blue up-conversion laser. Further research is required to obtain an efficient and, possibly, CW blue laser. Some results concerning this work will be reported in the near future [~~L

Acknowledgements The author is grateful to A. de Rijke for glass preparation, to Y. Kessener and T. Brandsma for their contribution to the results, and to A. van Dongen, E. Lous, P. le Gall and R. Raue for interesting discussions. Special thanks are due to 0. Rikken for offering the measurement facilities, for building the optical set-ups and for interesting discussions.

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Up-conversion in thulium dopedfluorozirconate glasses

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