Indian Journal of Pure & Applied Physics Vol. 42, February 2004, pp. 136- 141

Excitation and emission spectra of anti-Stokes luminescence of Tn13+ in glass

• I

ceramics doped with various concentrations of sensitizer .

R Kathuria', B P Chandra l, M Ramrakhiani 3 & D P Bisen'

'Department of Physics, D. N. Jain College, Gole Bazar. Jabalpur 48200 I ' PI. Ravishankar Shukla University, Raipur 492010

JDepartment of Postgraduate Studies & Research in Physics & Electronics Rani Durgavati University, Jabalpur 482 00 I

Received 9 illlgllst 2003. accepted 17 November 200J

In certain rare-earth doped glass ceramics luminescence emission has been observed at wavelength shorter than the exciting wavelength. Thi s is known as anti-Stokes luminescence or up-conversion, which is due to accumulation of excitation energy by rare-earth ions. In Tm» and Yb" doped glass ceramics, Tnrl+ acts as an activator and Yb}' acts as a sens itizer. The activator concentration was kept constant at 0.2 mol % and the sensitizer Yb"" concentration was varied from 0.0 mol% to 20 mol%. In emission spectra of glass ceramics doped with TmJ• and Yb" , under infrarcc e)(citation (966 nm) one peak of high illlensity was found at diflerent wavelengths between 400 to 500 nm for different concenlrations of sensi ti zer. The peak is slightly shined towards shorter wavelength with increas ing concentrations of the semitizer. Thi s reveals that 3-photon up-conversion is prominent and presence of Yb}' ions slightly shifts the energy levels of Tm J

•. In the excitation spectra of glass ceramics doped with YbJ

• and Tm" , initially the emission intensity increases wi th increasing wavelength, attains an optimum value for 920 nm, 930 nm, 950 nm and 960 nm and then it decreases with further increase in the wavelength. These photon energies may correspond to energy difference between levels of 'I'm" or Yb". Both in the excitat ion spectra and emission spectra, initially the anti-Stokes luminescence intensity .increases with sensiti ze r concentration. attains an optimum value and then it decreases with f1ll1her increase in the sensitizer concentration.

[Keywords: Anti -Stokes luminescence, Rarc-eaJ1h doped glass ccramic, Excitation and emission spectra, Luminescence of Tm}' 1

IPC Code: C09 11/07

1 Introduction

Lumine. cence is the non-equilibrium phenomenon of

excess light emission over and above the thermal emission

of a body, in which emission has a duration considerably

cxceeding the period o f oscillations. In othcr words, it is a

process which involves at least two steps, the excitation of

the electronic system of solid and the subsequent emission

of photons. In certain rare-earth doped glass ceramics, the

wavelength of emitted light is shorter than the wavelength

of exc iting light, and such type of luminescence is known

as anti-Stokes luminescence or up-conversion . This unique

process named either summation of photons, infrared to

v isible conversion or frequency up conversion, anti-Stokes

luminescence, energy transfer up-conversion (ETU),

quantum counting through energy transfer, or addition of

photons by transfer of energy CAPTE), has received a great

deal of attention from solid state scientific community . The

direct conversion o f infrared radiation to visible light is

possible in a number of rare-ea rth ion doped crystal

phosphors and glass ceramic contrary to the empirical Stokes

law. The study of anti-Stokes luminescence due to excita ti on

energy accumulation by RE3+ ions, has been carried out

initially by Bloembergen ' , Ovsyankin 2, Feofilov 3 and

Auzel4

. Severa: models have been proposed to explain the

phenomenon of anti -Stokes luminescence. Number of

researchers have reported anti-Stokes luminescence in rare­

earth ions during recent pas!'" 8.

There is a good interest in glass ceramics for the

conversion of infrared radiation into v isible light. Thi s

phenomenon is useful for detection of infrared radiation by

changing the light to a spectral region , where detectors ha ve

higher efficiency. In addition, these compounds have some

advantages because they present a high tl-ansparency from

the UV to IR and relatively large amount o f trivalent rare­

earth ions can be introduced into the hosl.

The present paper reports the excital.ion and emission

spectra of the anti-Stokes luminescence of Tm 3+ in g lass

ceramics doped with various concentrations of sensitizer.

KATHURIA el al. : RARE-EARTH DOPED GLASS CERAMICS LUMINESCENCE 137

2 Experimental Procedure

The g lass ceram ics were prepared by heati ng the

mixture of glass forming oxides - germanium oxides (GeOz)

and tungsten ox ides (W03) with lead fluoride (PbF). For

doping hi gh purity (99.99%) rare-earth ox ides - yttribium

ox ides (YbP3) and thulium oxides (Tmp3) were also added

to the initi a l mixture which was heated and melted inside a

muffle furnace at II OO°C for 30 min9. The sample was then

obtained by sudden cooling of the melt. In Tm3+ and Yb3

+

doped g lass ceramics, Tm3+ acts as an act ivator and Yb3

+

acts as a sensiti zer. The act ivator (Tm3+) concentrati on was

kept constant at 0 .2 mol % a nd the sens iti zer (Yb3+)

concen trati on was va ried to 0.0 mol%, 8 mol %, 10 mol %,

12 mol %, 15 mol % and 20 mol % . The g lass ceramics

prepared for this investigation were:

69.8 PbF) + 20 Ge02 + 10 W03 + 0.2 Tmp3 69.8 PbF2 + (20-xI2) Ge01 + (10-xI2) W03 + x YbP3 +

0.2 Tmp3

x = 8, 10, 12, 15

64.8 PbF2 + 10 Ge02 + 4 W03 + 20 YbP3 + 0.2 Tmp3

The experimenta l set-up used for recording the anti­

Stokes luminescence intensity and spectra is shown in

Fig. I . The main units are li ght source (IR lamp of250 W),

grating monochromator, constant deviation spectrometer

and detector unit consisting of RCA 931 photomultiplier

tube, high voltage power supply and digital picoammeter.

For the measurement of ant i-Stokes luminescence, a

powder sample of glass ceramics was spread on a quartz

MONOCHROMATOR

plate using araldite as a binder. The qua rtz plate was then

placed near the exi t slit of the grating monochro mator. This

was placed at an angle of 45° to the incident rays coming

from grating monochromator. For measuring the anti-Stokes

emiss ion spectra, the drum of grating monochromator was

fixed at 966 nm, while the drum of constant de viation

spectrometer was varied from 420 nm to 700 nm . Fo r

measuring the excitati o n spectra of g lass ceramics doped

with Yb3+ and Tm3+, the sample was excited by li ght ha vi ng

wavelength ranging from 800 nm to 1000 nm and emi ssion

was measured at those wave lengths where the anti -S tokes

luminescence emission was maximum .

3 Results

Fig. 2 (a,b) shows the up-convers ion em ission spectra

of glass ceramics doped with Yb3+ and Tm3

+ under infrared

excitation (966 nm) in the wavelength range of 420 nm to

700 nm . In this case, the concentration of Tm3+ io ns was

kept constant at 0.2 mol %, while that of Yb3+ was vari ed

from 0.0 mol %, 8.0 mol %, 10.0 mol %, 12 mol (l,fJ, 15 mol %

and 20 mo l%. It is observed that main peaks are obtained

between 400 and 500 nm at different wave lengths for

different concentrations of sensitizer Yb3+.

Fig. 3(a,b) shows the exc itat ion spectra of th e samples

with different concentrations of Yb3+ . The sample was

excited by li ght havi ng wavelength ranging from 800 nm to

1000 nm and e mission was measured at the main peak in

the emiss ion spectrum of the con'esponding sample. It is

seen that initially the e mi ssion intensity increases with

SAMPLE

CONSTANT

DEVIATION

SPECTRO·

METER

PMT

RCA

931

DIGITAL

PICOMETER

OUTPUT

Fig. 1- Experimental set up of the measuremenl of anli-Stokes luminescence:

138

170

160

150

140

13

~ 120 c :> 110 .ri

~100 >-t-- 90 Vi z w 80 t--

~ 70

z 2

60 '" '" ~ 50 w

40

30

20

10

140

130

120

1',J

'" .~ 100 OJ

.ri 90

" >- 60 t--

~ 70 w I-

Z 60

Z 2 SO

'" '" ::r 1,0 w

30

20

0

INDIAN J PURE & APPL PI-IYS. VOL 42. FEBRUA RY 2004

lR EIo4ISSION SPECTRA

o - I - Tm]· : 0 . 2 mol 'I" Yb]' : 0 mol'l,

.- II - Tm]· : 0 .2mol'I" ytf' : 8 mol 'I,

A -III - Tm)' : 0 .2 mol 'I .. Yb]· : 10mol'I,

WAVELENGTH(nm)

I R EIo4ISSION SPECTRA

0 - I-Tm]> : 0 . 2mol'I" Yb]' : 12mol'I,

• - 11 - Tm]·: 0 .2 mol 'I .. Yb]' : 15mo!'I,

"" -III - Tm]> : 0 . 2 mol 'I.. Yb]' : 20mo!'/,

10

OL-~~~~-L~~~~~L-~~dL~-L~~~~~~~~ 740

WAVELENGTH (nm)

Fig. 2- (a.h) - Up-conversion emi ssion spectra of glass cerami cs doped with Yb)' and Tm'+ under in frared excitation (966 nm) in the range of 420 to 700 nm at T=300 K for constant (0.2 mol%) Tm J

• concentration (a) Curves I. II and III correspond to YbJ+:O mol%. Yb·'+ :8 mol%. Yb J+: 10 mol%. respecti vely. (b) Curve I. II and III correspond to Yb": 12 mol%. YbJ+: 15 mol%. YbJ+:20 11101%. respecti vely.

KATHURIA ('/ al. : RARE-EARTH DOPED GLASS CERAM ICS LUMINESCENCE 139

10

60

so

40

30

20

10

I R EXCITATION SPECTRA

o - 1 - Tm]· : 0 . 2 mot '/., Vb)- : 0 mol'l •

• - a - Tm1+: 0 . 2 mol 'I. t Vb)- : 8 mol'l.

Il> -111- Tm" : 0 2 mol 'I,. Vb" : 10mol'I,

WAVF' I FNr.TH (nm)

>­... iii z

60

50

~ 30 z

z o ~ 20 ::r UJ

800

I R nCITATION SPECTRA

o - 1- Tm)' : 0 . 2 mol" •• Vb)' : 12 mol ' I,

• - 11 - Tm)' : 0 . 2 mol ' I. t Yb'· : lSmol'/.

~ - 10 - Tm)': 0 . 2 mot ' ,., Vb)' : 10mo( ' I.

820 840 860 820

WAVELENG TH(nm )

Fig. 3- (a.b ) - Exci tat ion spectra of glass ceramics doped with Ylr" and Tm" under infrared (/ R) exci tation for constant (0.211101%) Till" concentration at room temperature (a) Curves I. II and III correspond to YbJ+:O 11101%. YbJ+:8 mol%. Yb" :IO 11101 %. respectively: (b) Curves I. II and III correspond to YbJ' :12 mol~ . YIy": IS Illol ')f . YbJ+:20 11101%. respecti vely.

increas ing wavelength . attains an optim um value for a

parti cular excit ing wavelength ranging from 920 nm and

940 nm, different for different samples and then it decreases

with further increase in the wavelength.

Fig. 4 shows the sensi ti zer concen trati on dependence

of the anti-Stokes emission and exc itation spectra o f glass

ceramics doped with Yb)+ and Tm 3+. It is seen that initi ally

luminescence emission intensity increases wi th increas ing

concentration of sensitizer Yb>+ in both cases 0" emi ss ion

as well as exc itati on spectra. Lum inescence intensity I S

max imum for 10 mol% in case of emission spectra and for

8 mol% in case of exc itation spectra . It decreases with further

increase in the sensiti zer concentration.

4 Discussion

At the present time, several mechanisms are known for

summing the energy o f simple pumping or RE3+ ions that

leads to direct conversion or infrared radiat ion to vis ible

140 INDIA I J PURE & APPL PII YS, VO L. 42. FEBR UA RY 2004

170 • Emission spectra

16 0 • E XCitatio n spec Ira

150

140

.~ 130 ~ :0

120

-e ~ 110

?: 100 'iii

~

" 90 .£ c 80

. ~ .. 70 v "E-

GO w

50

40

JO

20

10

10 1 2 IS 20

Fig . 4- CO ll cent ration dependence o r emi ssion ;I nd cxriUlt ion spect ra or Yh,(),:Yh " .TIll '· under inrrared e)(cilalion 1'01' nJl1~tant O . ~ 1l1ol'lr Tm,O , doped concen trat ion at i'(H) 1ll te 111 pC I·;l!url'.

lG4

20

'E u

MO

15 )F 2

>-lF

J <.) ]F, cr w z 1/;" 1FS/ w 10

~ lHs

lH,

o J.

Vb TFT' ).

Fig . .'i- Energy level diagram o r Yh '+ and 'I'm'" ions and schematic processes ror rour-. three-. and two-photon processes .

li ght. Hi storicall y, thc first to be exa mined wa s the

mec hani sm of sequenti al (stepw ise) absorpti on of several

IR photons in the sa me rare-earth ion w hich thereuponm;lde

the transi tion to a higher energy state. Such a scheme was

proposed by B loembergan ' for an IR photon counter. T he

dillerent models used to ex plain the phcnomenon ()f anti ­

Stokes luminescence ;lre ( i) scqucntial ab~() rpti()n , (ii)

coo pcrati ve se nsit izat io n, ( iii ) sequen tial sensi ti zat ion

(stcpwise) model , and ( i v) muitiphonon-ass i" ted ant i -St okes

exc itation l11odel 'o." . Th e present oher va t ion ca n be

exp lai ned by sequentia l absorpti on. scquentia l sensiti za ti on

and phonon ass isted energy transfers. Th ,~ f irst in frared

photon brings a system into some intermediate metastable

state from whi ch upon absorption o f a sec()nd phot()n. it

goes to thc upper leve l. T he tra ns iti on scheme ror Yb , •

sensit izer and Tm3• acti vator ill a g lass ceramic host is shown

in rig. 5. T he IR photons transfer the acri vator Tm3+ from

31-1 (, to -' /-1 5 and if sensiti zer Yb 1. is present it is also excited

I· 'F ' ,- -rl I 'f" rYb'·' r · I I . rom ' "712 to ' ""2' lC I ell l11 C 0 . "',,: IS arge. l ence 11

transfers th e energy to 'I'm '· .1 H , ass isted w ith ph onon

emi ss ions. These exci ted 'I'm h Ions relax to "11 .) leve l by

non-rad iative transiti ons which is aga in a mctastable state.

T he 'I'm" ion is then exci ted to ' r 2 or .1 F , levels either by

ab~orpti () n by ano th er IR ph oton or by anoth er ph() non

ass isted energy transrcr from exc ited Y b.1, ion. then they

may relax to dilTerent lower leve ls Jr , an '1:" respecti ve ly.

Trans ition 1'1'0 111 3F.) tn .1 /-1 (, gives red el11 i ~s i () n. but at the

same time TmJ+ ions in ' F.j k: vclmay absorb third I R photon.

or acquire energy from cxc ited Y b.1, ions and go to IG.j Icve l.

Radia ti ve relaxa tion o f Tm " from IG.j to 111(, g i ves blue

emiss ion and to .1 /-1 .) gives red emi ss ion. 1\ part o f popul at ion

o f IG.) leve ll11ay be cxc itedto a ID2 leve l b:, mea ns of photon

ab~urpti on or energy transfer from exci ted Y b ,. i()n~. Radiati ve relaxation from ID2 to ' H(, gives v io let e m i~~ion and to .1 HJ gi ves blue emi ss ion.

In our samples the bluc emission is prrl minent due to

IG.j to 31-1 (, transit ion whi ch is three photon up-con\'e rsion.

Sensi t izer Yb.1+ increases the population o f ' II .j ' .1 F.j and IG.1

levels o f Tm.1+ and hence increasing the in tensity o f blue

em i ss i ()n. Thi s enect initiall y i ncre; , ~ es w ith Vb '·

concentr;ltion. For higher va l ue of concen t ra t ion o f sensi t izer

Yb>+. the qucnching o f thi s emi ss ion ma) be due to back

energy transfer from Tm3+ to Yly'+ ions and energy dilTu sion

bel ween V b'· ions.

Thus. the anti -S tokes luminescence intensity is optimulll

for a parti cu lar concentrat ion o f the sensiti zer.

5 Conclusions

T he mai n co nc lu sions d rawn f rom th e studi es llr exc it ati o n and emiss ion spec tra o f the a nt i -~ t okes

luminescence ofTm.1+ in g lass ceramics c1 eoped w ith various

concentrations of Yb.1+ sensiti zer are:

( i) In the emi ssio n spectra of gl ass 'era l11i cs doped

w ith Tm'+ and Yb.1 .. , one peak o f high intensity was

found at di rferent wave lengths be[wee n 400 and

500 nl11 for different co ncentrations of sensitizer.

T he peak i s sli ghtl y shifted to vard s sho rt er

KATH URI A ('/ al. : RA RE-EARTH DOPED GLASS CERAMICS LU M INESCE ICE l<-ll

wavelength w ith increasi ng concentrations o f the

sen sit i ze r. Thi s revea l s that 3-phot o n up­

conversion is prominent and presence o f Yb1+ ions

slightl y shifts the energy levels o f Tm'+.

( ii ) In the exc itation spec tra o f glass ceramics doped

with Yb·h and Tm"+, initiall y the emiss ion intensity

increases with increasing wavelength , attains an

optimum value for 920 nm, 930 nm, 950 nm and

960 nm and then it decreases w ith further increase

in the wave length. These photon energies Ill ay

correspond to energy el i fference between the levels of Tm}+ or Ybh

(i ii ) Both in theexcitation spec tra and emi ss ion spectra,

initi al ly the ant i -Stokes lum inescence intensity

increases w ith sensit izer concentration, attains an

optimum value and then i t decreases w ith further

increase in the sensiti zer concentration.

( iv) The up-conversion of Tm,h glass ceramics doped

with Yb" , in vo l ves four-photon and th ree-photon

absorpti on processes for violet (363 nm) and blue

(478 nm) emi ss ion bands, respecti vely. The t\\'o­

photon absorpti on process is req uired for reel (680

nm, 698 nm, 779 nm ) emissions.

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