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Journal of Quantitative Spectroscopy &Radiative Transfer

Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1594–1600

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Laser induced optical heating from Yb3þ/Ho3þ:Ca12Al14O33 and itsapplicability as a thermal probe

R.K. Verma, S.B. Rai n

Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India

a r t i c l e i n f o

Article history:

Received 11 February 2012

Received in revised form

3 April 2012

Accepted 3 April 2012Available online 11 April 2012

Keywords:

Heating effect

Thermal probe

Luminescence

CIE diagram

Upconversion

Nanoheater

73/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.jqsrt.2012.04.001

esponding author. Tel.: þ91 542 230 7308;

1 542 236 9889.

ail address: sbrai49@yahoo.co.in (S.B. Rai).

a b s t r a c t

Yb3þ/Ho3þ co-doped calcium aluminate phosphor has been synthesized using solution

combustion process. Multicolored (blue, green and red) strong upconversion emission

(lexc¼980 nm) due to Ho3þ ion is observed which shows a color tunability (from green

to red) with a variation in input laser power. The color tunability has been attributed to

be due to the induced heating in the local volume of the sample and the temperature

produced has been estimated using the fluorescence intensity ratio (FIR) method. The

sample shows temperature sensing behavior and more importantly the temperature

could be sensed through two pairs of thermally coupled levels, one lying in the green

region (5F4/5S2-5I8) and the other in the blue region (5G4/5G5-

5I8). The temperature

sensing through the blue pair of levels is novel in itself. The material thus prepared

serves as temperature sensor as well as a source for the production of heat in a localized

volume.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The upconversion (UC) luminescence properties of rareearth doped materials on near infrared pumping have beena subject of focus in recent years due to their broad range ofpotential applications such as biosensors, magnetic reso-nance imaging (MRI), color display devices, temperaturesensors, magnetic field sensors, solar cell, etc. [1–6]. The UCemission from Yb3þ/Ho3þ codoped materials has beenachieved in different host matrices like glasses, phosphors,polymers, etc. by several groups [7–9]. Among these,aluminate phosphor material has been grown by severalgroups recently owing to their novel properties and its vastapplications in various fields [10,11]. Calcium aluminatesare a high-quality host matrix for UC due to their inter-mediate phonon frequency, good chemical durability andthermal stability [12,13]. The moderately low phonon

ll rights reserved.

frequency (�800 cm�1) of the host matrix would increaseradiative transition probability and results in a relativelygood quantum yield. However, in some materials, particu-larly at higher pump powers, a part of absorbed energy isconverted into heat due to electron–phonon coupling (non-radiative relaxation) and as a result material gets heated.The heat generated is appreciably large if the crystal size isin nano-range. Such materials are termed as a nanoheater[14]. There are many devices available for measurements oftemperature. However, each method suited for only certainapplications. In optical materials where the temperaturearise due to electron–phonon coupling inside the material innano-region, when it is irradiated with appropriate wave-length of light. This rise in temperature cannot be measuredwith normal thermometric processes and we need anadvanced technique that has high sensitivity in nano-regime. Yb3þ/Ho3þ co-doped phosphor can be used as anano-thermometer because it can detect such heat andmeasure the temperature inside the host matrix. Suchmeasurements require the presence of two close lying levelswhich are thermally coupled and the emission/absorptionproperties of these are temperature dependent. A variation

R.K. Verma, S.B. Rai / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1594–1600 1595

in the relative intensities of the two peaks (FIR) can be usedto measure the temperature [15–17]. The optical heatingcan also be used in medicine for local hypothermal treat-ments of cells externally, for drilling nanoholes in organicsand soft materials [18,19]. This may be possible becausethe heat generation can easily be tuned with variation inlaser power.

In this paper, the UC photoluminescence of Yb3þ/Ho3þ

doped in calcium aluminate phosphor material excited by980 nm diode laser has been investigated. The emissionfrom Ho3þ is observed in the NIR, red, green (strongest one),blue and UV regions. These emissions are further enhancedin presence of Yb3þ . Besides the pair of levels emitting ingreen region and used as temperature sensor, we reportanother pair of levels emitting in blue region which can alsobe used to serve as a temperature sensor with highersensitivity than the green pair. The luminescence from thephosphor has been recorded from room temperature to500 K. The tunable luminescence is observed through laserpower variation and increase in the temperature insidesample is estimated using FIR.

2. Fluorescence intensity ratio (FIR) method for thetemperature sensor (temperature detection)

The ratio of fluorescence emitted by the two thermallycoupled levels can be used for sensing the temperature.This could be understood by considering an energy leveldiagram having two closely spaced (only a fewkT�800 cm�1) upper levels decaying radiatively to acommon lower level as shown in Fig. 1. The excited levels2 and 1 are populated through direct excitation orthrough non-radiative relaxation from higher levels. Thepopulation decay rates for different levels can be written

Fig. 1. Mechanism of temperature sensor in terms of two thermally

coupled levels through direct and indirect excitations. NRE: non-radia-

tive emission, NRA: non-radiative absorption.

as [18]

dN1

dt¼N2A21�N1ðw12þA10ÞþN0w01

dN2

dt¼N1w12�N2ðA21þA20ÞþN0w02

dN0

dt¼N1A10þN2A20�N0ðw01þw02Þ ð1Þ

where Aij and wij are the spontaneous emission and theabsorption rates, respectively, of the ions from the level i

to j. Thermalization occurs between the excited levelsand b21¼ (g2/g1)e�DE21/kT represents the ratio of thepopulation in levels 2 and 1 in the equilibrium, g1 andg2 are the respective degeneracy. k is the Boltzmannconstant and T is the temperature.

The intensity emitted at a particular wavelength dueto transition from level i-f is proportional.

IifpNioif Aif ð2Þ

where oif is the angular frequency of radiation. Thus if I10

and I20 are the fluorescence intensities of the bandsemitted from the two closely spaced levels (1 and 2 inFig. 1) at a particular temperature T, the intensity ratio ofthe transitions from levels 1, 2 to 0 will therefore be givenas [15]

R¼ I20I10¼

N2o20A20N1o10A10

¼o20A20g2o10A10g1

exp �DE21

kT

� �¼ Bexp �DE21

kT

� �

where B¼ o20A20g2o10A10g1

ð3Þ

Thus if R and B are known T can be calculated.

3. Experimental

The composition used for the preparation of phosphorwas

50CaOþ(50�x�y) Al2O3þxYb2O3þyHo2O3

where x¼0, 3, 6 mol%, and y¼0.6,2.0,5.0 mol%.

All materials taken from Merck were of reagent grade(99.9%). In order to prepare the phosphor material,stochiometric ratios of these reagents were taken anddissolved in small amount in de-ionized water. Urea wasthen added and the mixture was stirred and heated at333 K in a beaker until the material converted into gelform. The gel was taken in a platinum crucible and kept in aclosed furnace maintained at fixed temperature 873 K. Anautomatic ignition occurred within few minutes (�2 min)which resulted phosphor material in the form of powder[19]. The X-ray diffraction patterns of the materials calci-nated at different temperatures were recorded. Transmis-sion electron microscope (TEM) images were recordedusing Technai 20G2 (Philips) microscope operated at accel-erating voltage 200 kV. A NIR diode laser emitting at980 nm was used to excite the material (in the form of adisk with 1 mm thickness and 2 mm radius), and fluores-cence was recorded using an iHR320 Jobin Yvon spectro-meter equipped with R928 photon counting photo-multiplier tube. FTIR spectra were recorded using PerkinElmer, spectrum RXI, spectrometer.

R.K. Verma, S.B. Rai / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1594–16001596

4. Results and discussion

4.1. Structural investigation

Fig. 2(a) shows the XRD pattern of calcium aluminatephosphor material calcinated at 1473 K. The XRD patternsof as-synthesized and the samples heated at lower tem-peratures can be seen in our earlier work [12,13]. Thecrystallite size were estimated using the Scherrer formula[20,21]

t¼0:9lbcosy

where l is the wavelength of the incident X-ray [CuKa(0.154056 nm)], b is the full width at half maximum(FWHM) and y is the diffraction angle for (h k l) plane.The calcium aluminate sample crystallizes in single phase(Ca12Al14O33), and crystallinity increases as annealingtemperature increases due to increase of the particle size.The calcium aluminate is cubic with a cell parametera¼1.982 nm and belongs to the I43d space group. TheCa12Al14O33 phase is well known as an oxygen trap and isalso an electronic conductor. The hkl planes for differentpeaks are as follows: (211), (321), (400), (420), (422),(521), (611), (640), and (642). In the case of the sampleheated at 1473 K, the average crystallite size was�45 nm. Fig. 2(b) shows the TEM image of the materialwhich clearly indicates the particles agglomerated andtheir average size �100 nm.

4.2. Infrared spectra

Fourier transform infrared (FTIR) spectra of sampleswere recorded in the 4000–400 cm�1 regions. Fig. 2(c)shows the IR spectra of the sample calcinated at 1473 Kalongwith as-synthesized one. FTIR spectra shows thepresence of conventional impurities (NO3� , OH�) in the

Fig. 2. (a) XRD pattern of host matrix heated at 1473 K, (b) transmission

electron microscope picture and (c) Fourier transform infrared spectra of

as-synthesized and heat treated (1473 K) samples.

host material which decreases with calcinations. Thebroad band observed near 3400 cm�1 is due to OH.Similarly the broad band near 844 cm�1 is ascribed toAl–O–Al stretching mode while the band near 525 cm�1

is related to Ca–O and the bending mode of Al2O3. Theband near 1390 cm�1 is due to the presence of NO3�

group as impurity from the fuel used and its intensitydecreases with the calcination temperature [12,13].

4.3. Up-conversion

The partial energy levels of Ho3þ and Yb3þ are shown inFig. 3. As is clear from the figure, Ho3þ ions are goodcandidates for infrared to visible UC owing to their favorableenergy levels. However, the absorption cross section ofHo3þ is poor in itself for a 980 nm source. Thus excitationof Ho3þ requires doping of another rare earth which shouldhave not only high absorption cross section for 980 nmradiation but also have capability to transfer energy to Ho3þ

ions. Yb3þ ion in this respect is most appropriate since ithas high absorption cross section for 980 nm radiation andemits intense cooperative UC emission in the blue region[22]. The cooperative blue emission at 488 nm preciselymatch with the 5F2 level of Ho3þ ion and transfers energyradiatively to it. As a result very intense upconversionemission is observed in green and red. Thus when thesample containing only Ho3þ ions is excited by 980 nm,Ho3þ ion absorb 980 nm laser radiation and promoted tothe 5I6 level. A re-absorption of 980 nm radiation by ions in5I6 promoted to the 5F4, 5S2 levels through excited stateabsorption (ESA) and emit a weak green and red emission.However, if Yb3þ ion is also present with Ho3þ ion, onexcitation with 980 nm radiation, it populates the 5F4 and5S2 levels of Ho3þ and 2F5/2 of Yb3þ . Two Yb3þ ions in the2F5/2 level cooperatively emit a photon at 488 nm and at thesame time transfer energy to Ho3þ in ground state populatethe 5F4 and 5S2 levels. This process of energy transfer from

Fig. 3. Energy level diagram of Ho3þ and Yb3þ codoped in calcium

aluminate and possible upconversion mechanism.

Fig. 5. (a) Effect of calcinations on the upconversion emission,

(b) variation of emission intensity of green and red with pump power,

(c) color tunability with diode laser power, and (d) CIE diagram and the

color purity of emission at different pump powers. (For interpretation of

the references to color in this figure legend, the reader is referred to the

web version of this article.)

R.K. Verma, S.B. Rai / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1594–1600 1597

Yb3þ to Ho3þ is very efficient. As a result of this stronggreen emission from 5S2, 5F4 levels to ground level of Ho3þ

is observed. A part of the ions in 5F4 level also relax to the5F5 level. A transition from 5F5 to the ground level gives redemission. Fig. 4 shows the UC spectrum of Yb3þ/Ho3þ

doped phosphor material under a 980 nm diode laserexcitation. The inset in Fig. 4 shows UV emission whichappears due to three photon absorption. The emission peaksare observed due to 5G2-

5I8 (365), 3K7-5I8 (410 nm),

5G4-5I8 (467 nm), 5G5-

5I8 (492 nm), 5F4/5S2-5I8

(548 nm), 5F5-5I8 (665 nm), and 5S2 -5I7 (758 nm) elec-

tronic transitions, respectively. Interestingly, the violet andthe ultraviolet UC bands in the case of Ho3þ have rarelybeen observed. The appearance of this in the present case isdue to lattice effect. The emission spectra from samplescalcinated at different temperatures are shown in Fig. 5(a).Brightest emission is observed from the sample heated at1473 K temperature. This is due to reduction of quenchingcenters like, OH� , NO3� .

The power dependence of emission intensity of Ho3þ

ion in the green and red regions has been recorded withthe 980 nm diode laser power varying from 0.0 to 2.0 Wand is shown in Fig. 5(b). Power dependence showsinteresting features. At low pump power it shows ava-lanche effect and the number of photons involved are verylarge. At moderately higher power, it shows a normalbehavior of I p Pn, where n is the number of photonsneeded to pump these levels and its value is �2. Forhigher values of pump power it show saturation. Thecolor tunability with power variation has also beenobserved on pump power variation and is shown inFig. 5(c). With the increase in pump power, though thereis not significant change in green emission however thered emission increases rapidly. This changes the overallcolor of emission to yellow, to orange red, and finally tored. The CIE diagram corresponding these values is shownin Fig. 5(d). The CIE diagram reveals the color tunability

Fig. 4. Ultraviolet and visible emission spectrum from Ho3þ/Yb3þ ions

in calcium aluminate, on excitation with 980 nm CW diode laser.

with high purity of color. As is clear from the diagram, allCIE points are very close to color boundary with colorcoordinates (0.32, 0.67), (0.34, 0.65), (0.37, 0.62), (0.39,0.60), (0.41, 0.58) and (0.47, 0.52). The material onexcitation with 980 nm emits strong green, it becomesyellowish when the pump power is 0.75 W. At still higherpump power though more and more ions are pumped tothe 5F4, 5S2 levels (responsible for green emission) but inthis condition the intensity of green emission decreasesand that of red increases. This is possible only when therelaxation from 5S2, 5F4 to 5F5 is very fast which is possibleat higher temperatures. Spectra have been recorded inboth the modes (i.e. during increasing as well as decreas-ing pump power). It is found that in the two modes it doesnot follow exactly the same path. In case of reverseprocess (i.e. decreasing pump power) intensity is slightlyreduced. This is due to loss of optical energy in heatgeneration inside the host matrix.

4.4. Temperature dependence

The effect of temperature on the intensity of UC emissionbands has also been studied. In triply ionized form, the rareearth ions have three types of energy distribution: (i) the twolevels are very close to each other i.e. energy separation iso200 cm�1 (nano-coupled), (ii) the two levels are separatedin the 200–1000 cm�1 range (thermally coupled), (iii) thetwo levels are separated by 41000 cm�1 (uncoupled), andthey are discussed in detail by several workers [15,23]. Thetwo green levels of Ho3þ ion in the present host is inthermally coupled regime and can be used as a temperaturesensor [9,15]. The use of two thermally coupled levels to

R.K. Verma, S.B. Rai / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1594–16001598

sense temperature using emitted fluorescence intensity ratiohas many advantages. First, the physics of thermally coupledlevels is well known and easy to discuss. The other advantageis that the population of thermally coupled levels is propor-tional to the total population which depends on pump powerand temperature. Therefore, any change in population cangive information about the variation in laser power as well astemperature. As mentioned, though both the factors affectthe population of thermally coupled levels but their depen-dences are distinctly different from each other. Whereas anincrease in laser power affects the population of both thelevels and the intensity of both the bands increases withpower. However, in case of increase of temperature, theintensity of one peak increases whereas that of the otherdecreases. Two pairs of thermally coupled levels have beenobserved in the Yb3þ/Ho3þ codoped sample which can beused as a temperature sensor. These two also show theirdistinct features. One pair of levels emitting in the greenregion (which is also reported in other hosts, see Ref. [24]) issuitable for low temperature region because it appears atvery low pump power and its intensity decreases with theincrease of temperature. While, the other sensing pair 5G4

and 5G3 with a separation �881 cm�1 emitting in the blueregion can sense higher temperature because it is relativelyweak and appears at high temperatures. The green peaksseen at 542 nm and 552 nm (separation �333 cm�1) whichinvolve transitions from the 5F4 and 5S2 levels to the groundstate (5I8). It is observed that at lower temperatures 552 nmpeak has higher intensity than 542 nm peak. As the tem-perature is increased, the intensity of 552 nm peak decreasesslowly and that of 542 nm peak increases. The ratio ofintensity of the two i.e. I542/I552 thus increases. This is dueto promotion of Ho3þ ions to higher level due to rise intemperature (see Fig. 6(a)). As is clear from Fig. 6(a) at roomtemperature I5524I542. At 473 K the intensity of both thepeaks are nearly equal but for higher temperatures theintensity of the peaks inverts and 542 nm peak becomes

Fig. 6. (a) Dependence of fluorescence intensity of bands on pump

power, (b) dependence of fluorescence intensity on temperature, and

(c) Variation of fluorescence intensity ratio with temperature.

more intense than 552 nm peak. The variation of fluores-cence intensity ratio with temperature is shown in Fig. 6(b).Fluorescence intensity ratio for these two peaks with tem-perature is shown in Fig. 6(c). Similarly the variation inintensities of the two peaks, at 467 nm and 492 nm is shownin Fig.7(a), and (b).

Further, the sensitivity (the rate at which the fluores-cence intensity ratio changes with the change in tem-perature) is one of the important parameter for devicesbased on such measurements. It is discussed by Rai [23]as well as by Singh and Rai [24] as

S¼1

R

dR

dT¼

DE

kT2

This equation suggests that larger is the energy differ-ence between two thermally coupled levels (as is the caseof blue pair of bands), larger must be the sensitivity.However, this is true only to some extent. In the case oflarge energy separation, the population and the fluores-cence intensity ratio to the thermally coupled levelsdecrease rapidly. In case of very large difference thepopulation in the upper level will be negligibly small (orthe two levels will be thermally decoupled). For largersensitivity the energy difference between the two levelsmust be in the range �200–800 cm�1 [15,24]. If theseparation between the two levels o200 cm�1 it comesunder the category of nano-coupled levels and results acomplex feature. It was also observed that when thepump power is increased, though the intensity of theradiative emission is increased but at the same timenonradiative relaxation is also increased appreciably.The non-radiative relaxations in the case of CW lasersgenerate heat. This heat increases lattice vibration due towhich radiative emission decreases. The induced tem-perature in nano-system cannot be sensed by othersensing methods like thermocouple, thermometer, etc.Only optical thermocouple method is best suited and willbe discussed in detail in Section 4.5.

4.5. Power dependence and estimation of temperature

The power dependent emission spectrum of the sampleannealed at 1473 K was recorded for pump power up to2.0 W using 980 nm laser radiation. It was observed thatbelow 0.75 W power, the sample emits in the green andred regions through upconvesion. Above 0.75 W pumppower, bands in the blue and in the ultraviolet region

Fig. 7. (a) Variation of fluorescence intensity of peaks with pump power,

(b) dependence of fluorescence intensity of peak on temperature.

Fig. 9. Estimation of the temperature at different pump powers compar-

ing the blue pairs of lines fluorescence intensity.

R.K. Verma, S.B. Rai / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1594–1600 1599

starts appearing through three photon process. Interest-ingly, it was observed that with an increase of pumppower, the FIR (I542/I552) value also increases similar tothe one observed in temperature variation case. Thisindicates that at higher pump power the induced heatthrough nonradiative relaxation becomes prominent andthe material starts behaving as nanoheater (heating due tonon-radiative process). Since 5F4 and 5S2 are thermallycoupled levels and the energy difference between them isonly 333 cm�1, so, as the temperature increases it popu-lates the higher level. As a result the ratio of I542/I552

increases. These increases in FIR are an indicative of heatgeneration in the sample with laser pump power and areshown in Fig. 8. A plot of the FIR versus temperature hasbeen taken as the standard to calculate the temperaturegained by the sample during the laser power variation (seeFig. 8). The Y-axis in left-hand side represents the pumppower while the fluorescence intensity ratio (FIR) for greenpair on the X-axis. The Y-axis in right-hand side indicatesthe corresponding temperature. The temperature gainedby the sample at a particular pump power can be calcu-lated using the FIR value easily. The temperature rise in thesample at a particular value of laser power can be under-stood from the nanocrystalline nature of the material.Nanocrystalline material can efficiently release heat underoptical excitation. The laser electric field strongly drivesthe charged mobile carriers inside the nanocrystals, andthe energy gained by the electrons turn into heat throughelectron–phonon coupling (non-radiative channels). Heatthus generated spreads away from the nanocrystals andled to an increase in temperature of the surroundingvolume. Thus, a calcium aluminate nanocrystal hostingthe dopants Ho3þ/Yb3þ plays an important role in releaseof absorbed thermal energy into surrounding medium dueto their large surface/volume ratio. In addition to thisquantum confinement of phonons and therefore enhancedelectron–phonon interaction takes place in these nanopar-ticles, which results in extra heating of nanoparticles. Theelectron–phonon interaction increases with the increase intemperature and in some cases give incandescent emission[14,25,26]. Yb3þ/Ho3þ doped in calcium aluminate thusbehaves as multifunctional system capable of generatingheat and sensing the temperature. This dual behavior (heat

Fig. 8. Variation of fluorescence intensity ratio with pump power and an

estimation of temperature at different pump powers comparing the

green pair of lines fluorescence intensity ratio.

generation and temperature sensor) is important for manyapplications.

The same result of heat generation is also observed forpair of levels giving transition in the blue region andestimation of temperature is shown in Fig. 9. It is worthnoting that the laser power plays an important role in Yb3þ/Ho3þ codoped calcium aluminate phosphor material and ithas low threshold value for UC as well as optical heating.

5. Conclusions

Strong UC luminescence from Yb3þ/Ho3þ codoped cal-cium aluminate phosphor material has been observed in theultraviolet, visible and NIR regions. The material has beenfound suitable for sensing temperature up to moderate valueof T using two pairs of close lying levels. Avalanche as well ascolor tunability in emission has been observed with pumppower. An infrared laser induced heating has also beenobserved in this material. The heating and up-conversionluminescence essentially co-exists in this host and dependson the laser threshold value. Induced temperature inside thehost material has been evaluated by comparing it with thestandard value of the FIR ratio. The nanoheater may be usedin medicine for local hypothermal treatment of cells, fordrilling nanoholes in organic solids and soft materials.

Acknowledgments

Authors are grateful to University Grants Commission forfinancial assistance. One of the authors (R.K. Verma) wouldlike to thank CSIR New Delhi for senior research fellowship.

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