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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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