CHAPTER-2. OPTICAL STUDIES OF Er 3+ DOPED GLASSES IN PRESENCE OF Yb 3+ ION

Transcription

1 CHAPTER-2 OPTICAL STUDIES OF Er 3+ DOPED GLASSES IN PRESENCE OF Yb 3+ ION -3-

2 2.1. General Introduction The study of triply ionized lanthanides doped glasses are growing its importance due to their various applications in photonics, lasers, sensors, upconversions, intensity enhancement, optical fiber amplification and optoelectronic systems [1-10]. Glass hosts doped with triply ionized lanthanides having no positional order over a macroscopic distance gives inhomogeneousely broadened emission profiles. Due to this, the dopant ion occupies a variety of environments and therefore experiences different crystal fields. This makes the rare earth doped glasses attractive for developing tunable solid state upconversion lasers. Lanthanides doped heavy metal oxide glasses possessing high thermal stability and chemical durability are considered to be promising glass hosts for photonic applications [1, 8]. Recently, the attention has been devoted to the frequency upconversion of near infrared to ultraviolet / visible light due to its applications in color displays, upconverters, biomedical diagnostics, amplifiers and telecommunications etc. [11-15]. Solid state erbium based laser systems pumped by high-power diode lasers have high efficiencies and are now standard tools for many applications [16]. The I 13/2 I 15/2 emission of Er 3+ at 1.5 m has been extensively studied for the purpose of developing pulse amplifiers for telecommunication devices made of fiber glasses [17, 18]. Telecommunication wavelength, 1.5 m for erbium-doped fiber lasers (EDFLs), pumped by solid-state source, make thermal management easier since fibers are long and thin. The sub-picosecond pulse from passively mode locked fiber laser has shown high energy pulses (nj) and low cost, comparable to the solid state Ti: Sapphire lasers [19]. Also the I 11/2 I 13/2 emission of erbium at 2.7 m in fluoride glasses, constitutes a very promising system to build all solid-state lasers emitting near 3.0 m to be applied as surgical tools [20, 21]. Triply ionized erbium is most popular as well as one of most efficient lanthanide ion because of its favourable energy level structure with the I 11/2 I 15/2 transition in the near infrared spectral region that can easily be excited using 976nm diode laser as an excitation source [22-2]. With the commercialization of laser diodes, the triply ionized erbium among the lanthanides has been found popular as well as one of the most efficient ion in fabricating the Er 3+ doped fiber, waveguide lasers and amplifiers. --

3 The upconversion emissions in Er 3+ doped different glass hosts have been studied by several workers [15, 25-33]. The frequency upconversion in Yb 3+ - Er 3+ codoped PbO-GeO 2 glass containing silver nanoparticles upon the optical excitation with a 980nm diode laser has been studied and explained on the basis of energy transfer and local field effect [15]. Room temperature near infrared (NIR) to green upconversion (UC) emissions in the region nm and red UC emission in the region nm of the Er 3+ ions doped in Y 2 O 3 phosphor have been observed upon direct excitation into the I 11/2 level using ~972nm laser radiation of nanosecond pulses [27]. Two green upconversion emission bands centred at ~53nm and ~59nm in the Er 3+ doped silicate glass using an excitation of 978nm from a diode laser source have been observed and based upon the fluorescence intensity ratio (FIR) of the green upconversion emissions the maximum sensitivity and temperature resolution have been estimated approximately K -1 and 0.8K respectively [29]. Dwivedi et al. [30] monitored the near infrared (NIR) to intense white light conversion followed by the efficient energy transfer in the Pr 3+ -Er 3+ -Yb 3+ codoped TeO 2 -BaF 2 glass. Halide glasses doped with the rare earth ions possess good upconversion characteristics, but due to its hygroscopic property their applications are very much limited. Therefore the glasses which have lowest phonon energies, nonhygroscopic in nature, high refractive index combined with the high resistance etc. may be suitable hosts [31, 32]. The energy transfer in Er 3+ : Sm 3+ codoped TeO 2 Li 2 O glass has been studied upon excitation with 532nm laser radiation and explained on the basis of fluorescence intensity and lifetime measurements [33]. The search for new host materials and sensitizers has attracted the researchers very much for their growing importance in photonic applications. Tellurite based glasses viz. Tellurite lead oxide, tellurite zinc oxide, tellurite germanate glasses are of particular interest. The optical properties viz. high linear and non-linear indices of refraction, long transmission window, relatively low phonon energy, large dielectric constant, high optical damage threshold, small absorption coefficient, many valence states of tellurium, a low-bonding strength of Te O, low glass transition, corrosion resistance, thermal, mechanical and chemical durability make them promising candidates for fiber laser and optical amplifier applications [1, 7, 25, 26, 3-1]. Additional alkaline dopants modify the glass structure, its units and network, with advantages for optical applications [2, 3]. The present chapter describes the optical studies of Er 3+ doped (TeO 2 -PbO and TeO 2 -ZnO) glasses in presence of Yb 3+ ions. The NIR to visible frequency upconversion emissions in the Er 3+ / -5-

4 Er 3+ : Yb 3+ doped / codoped tellurite glasses upon excitation at 976nm and 808nm have been investigated. The spectroscopic parameters that provide the information about its radiative properties have been estimated by using Judd-Ofelt theory. The addition of Yb 3+ ions in the Er 3+ doped glasses enhanced their upconversion emission intensity by several times and the possible mechanisms involved in the upconversion emissions are explained in detail Experimental details For Er 3+ doped TeO 2 -PbO glass The molar compositions used to prepare the doped / codoped glasses were the following: (95-x-y) TeO 2 + 5Pb 3 O + xer 2 O 3 + yyb 2 O 3 ; where x = 0.3, 0.5, 0.7 & 1.0 mol% and y = mol %. The well mixed raw materials were heated for 60 minutes in an alumina crucible at C using an electric furnace. The melt was stirred properly to get the homogenous molten mass. The glasses were obtained by pouring the molten mass into a preheated brass mould. Several pieces of glasses were prepared for each combination. These glasses were polished and used for the further optical measurements. For the absorption spectra, a double beam UV-Vis-NIR spectrophotometer was utilized. The photoluminescence spectra were recorded by using the excitation with a diode laser operating at ~ 976nm with a monochromator attached with a photomultiplier tube (PMT). All the measurements were made at room temperature For Er 3+ doped TeO 2 -ZnO (TZO) glass The Er 3+ / Er 3+ -Yb 3+ doped / codoped tellurite zinc oxide (TZO) glass samples were formed by melting and quenching technique. The starting materials with the compositions (80-x-y) TeO ZnO + x Er 2 O 3 + yyb 2 O 3, (where x= 0.3, 0.5, 0.7 and 1.0 mol %, y= mol%} were weighted in proper amounts and crushed in an agate mortar to obtain homogenous mixture. The homogenously mixed powder for each composition was fused separately at ~750 0 C into an alumina crucible in an electric furnace until the material was converted into transparent liquid. Then the transparent liquid was poured into a preheated brass mould and pressed with another brass plate for quenching process. The material was left in that stage for some time for cooling. The obtained -6-

5 samples were cut in rectangular shapes of 15mm x 10mm x 2mm sizes and carefully polished in order to avoid the roughness of the surface. The optical absorption spectra of the samples in the region nm were measured by using a UV-Vis-NIR double beam spectrophotometer with a resolution of 0.2 nm. The frequency upconversion emission spectra of the samples were monitored by using ~976nm and 808nm continuous wave (CW) excitation wavelength from diode lasers. All the experiments were carried out at room temperature Results and discussion Absorption study of Er 3+ /Er 3+ - Yb 3+ doped / codoped TeO 2 -PbO (TPO) glass The absorption spectra of the doped samples have been recorded in the UV-NIR region of the electromagnetic spectrum. The ground state of the Er 3+ ions is I 15/2. There appear several Fig.2.1: Absorption spectrum of the 1.0 mol% Er 3+ doped TeO 2 -PbO glass. Inset spectrum shows the NIR absorption peaks of Yb 3+ ions. -7-

6 absorption bands spreading from nm region (Fig.2.1). These absorption bands are ascribed due to the transitions from the ground state to different G 9/2, G 11/2, 2 H 9/2, F 3/2, F 5/2, F 7/2, 2 H 11/2, S 3/2, F 9/2, I 9/2 and I 11/2 excited states peaking at about ~380 nm, ~00 nm, ~2 nm, ~51 nm, ~65 nm, ~88 nm, ~522 nm, ~5 nm, ~65 nm, ~798 nm, and ~976 nm respectively (Fig. 2.1). In the case of Er:Yb codoped samples, all the Er 3+ ions f-f absorption peaks are observed along with a broad peak centred at ~978 nm. This peak is accompanied with three low intensity broad peaks at ~920 nm, and ~955 nm and a sharp intense peak at ~976 nm (inset of Fig. 2.1). These peaks are ascribed to the transitions between the Stark levels of the 2 F 7/2 and 2 F 5/2 levels of the Yb 3+ ions. Similar features have been observed for the other doped / codoped samples with their relative intensities Frequency upconversion in the Er 3+ doped TeO 2 -PbO (TPO) glass The photoluminescence spectra were recorded in the nm wavelength region upon excitation with a diode laser lasing at ~ 976 nm (Fig 2.2). The photoluminescence intensity has been optimized by varying the erbium and ytterbium contents in the glass. The samples with 1.0 mol% of Er 3+ and 1.0 mol% Er mol% Yb 3+ were found to exhibit the best result. The photoluminescence intensities were found to reduce appreciably beyond these concentrations due to the concentration quenching phenomena. It is probably due to the increasing interaction between the Er 3+ - Er 3+ ions at higher concentrations. During this interaction, the excited Er 3+ ions transfer its excitation energy to the unexcited Er 3+ ions and this process will be continued upto a defect level through which this energy will be finally dumped through nonradiative process. Three prominent upconversion emission bands were observed at ~527 nm, ~58 nm and ~660 nm respectively corresponding to the 2 H 11/2, S 3/2 I 15/2 and F 9/2 I 15/2 transitions in the Er 3+ doped TeO 2 -PbO glass. The observed upconversion transitions can be understood on the basis of following mechanisms: For the green emissions in the first step, the I 11/2 level is directly excited with a ~976 nm (1026 cm -1 ) laser radiation through the ground state absorption (GSA). A part of the population stored in the I 11/2 level is again re-excited through the excited state absorption (ESA) and promoted to the F 7/2 level by the same incident photon. As the energy separation between the F 7/2 and 2 H 11/2 level is ~1500 cm -1, where as the cut-off phonon energy of the present host material is ~750 cm -1. This energy gap (~1500 cm -1 ) is filled by the two phonons. Therefore, the 2 H 11/2 and S 3/2 levels are -8-

7 populated via the nonradiative relaxations from the F 7/2 level. The excited triply ionized erbium ions in the 2 H 11/2 and S 3/2 levels again relaxes radiatively to the I 15/2 level giving two photons in the green region. For the emission in the red region due to the F 9/2 I 15/2 transition, the remaining population stored in the I 11/2 level relaxes non-radiatively to the I 13/2 level. The excited ions in the I 13/2 level absorb the same incident photon and transits upward to the F 9/2 level, from where they relax radiatively to the ground state ( I 15/2 ) by emitting a photon in the red region. The excited state absorption is found to be the dominant process for the upconversion emissions lying in the green and red regions. Fig.2.2: Photoluminescence spectra of singly doped 1.0 mol % Er 3+ and 1.0 mol % Er mol % Yb 3+ codoped tellurite glass on 976 nm laser excitation Effect of co-doping with the Yb 3+ ions in the Er 3+ doped TeO 2 -PbO glass As the Yb 3+ ion has a strong absorption band corresponding to the 2 F 7/2 2 F 5/2 transition peaking at ~976nm (i.e. ~10, 26 cm -1 ). On the other hand, the I 11/2 level of Er 3+ ions lie at -9-

8 ~10, 26 cm -1 and the absorption corresponding to the I 15/2 I 11/2 transition is very weak, hence the upconversion emissions observed from the upper levels due to the direct excitation into the I 11/2 level by using ~976 nm laser are not very strong. It can be anticipated that at certain ionic separation an energy transfer between the two ions (i.e. between Yb 3+ Er 3+ ) may be possible. The Fig. 2.2 shows the frequency upconversion emission spectrum in the nm region for 1.0 mol% Er mol% Yb 3+ codoped in TeO 2 -PbO glass. In the case of codoped glasses, usual green and red emission peaks at ~527 nm, ~58 nm and ~660 nm are observed. However, these peaks are observed to enhance by several folds compared to the singly Er 3+ doped glass. The increasing content of Yb 3+ enhances the intensities of the green and red emissions. Both the curves (as shown in Fig 2.2) in the lower and upper part were measured at the same sensitivity. The maximum enhancement is noted for 2.0 mol% of Yb 3+ ion concentration in the codoped samples. There appears three upconversion emission bands peaking at ~527nm, ~58nm and ~660nm respectively lying in the green and red regions. The maximal intensities for both the green and red upconversion emissions were obtained as about ~0 and 90 times higher than that of the 1.0 mol % Er 3+ doped TeO 2 -PbO glass respectively. It is worthwhile to mark out that the red upconversion emission was observed easily by naked eyes even at low pump power (~50 mw) of diode laser. Along with these, two more peaks lying at ~92 nm and 97 nm were also observed in the blue region. This peak is due to the cooperative sensitization of the Yb 3+ and assigned to be the F 7/2 I 15/2 transition of Er 3+ ions. These peaks are rarely observed in the Er 3+ / Er 3+ -Yb 3+ doped / codoped solid host materials. In this case, these peaks are not observed in the case of singly Er 3+ doped glass. In case of codoping with Yb 3+ ions, depending upon the relative separation between two neighboring Yb 3+ ions, a short range dipole-dipole interaction takes place and hence a pair of Yb 3+ ions lose their excitation energy to the ground state Er 3+ ions and promoted them to the F 7/2 level, from where through the radiative relaxation emits a photon corresponding to the F 7/2 I 15/2 transition in the blue region. In the Er 3+ : Yb 3+ codoped samples however, though the aforementioned processes are also possible, the Yb 3+ -Er 3+ energy transfers are known to be the most significant contribution for the upconversion emissions of Er 3+ ions with 976 nm excitation. The detail of the same can be easily understood by looking the energy level diagram (Fig. 2.3). -50-

9 Fig.2.3: A schematic energy level diagram of the Er 3+ and Yb 3+ ions. The observed transitions and the possible energy transfer pathways on 976 nm diode laser excitation. In the energy transfer process, the Yb 3+ ions excited to the 2 F 5/2 level transfer its excitation energy to the ground state triply ionized erbium ions and promoting it to the I 11/2 level. A part of the excited triply ionized erbium ions in the I 11/2 level through the energy transfer from the Yb 3+ jumps to the F 7/2 upper level. The excited Er 3+ ions in the F 7/2 level relax rapidly via the nonradiative transitions to the 2 H 11/2 and S 3/2 levels. This is the dominant process, because the Yb 3+ ions show a strong absorption cross-section compared to that of Er 3+ ions (Fig. 2.1). Also the oscillator strength of the Yb 3+ ions corresponding to the 2 F 7/2 2 F 5/2 transition is larger compared to that of the I 15/2 I 11/2 transition of the Er 3+ ions. Therefore, the pump energy efficiently absorbed by the Yb 3+ ions is transferred to the Er 3+ ions. The radiative relaxation from the 2 H 11/2 and S 3/2 levels to the I 15/2 ground level produces photons in the green region. For the energy transfer (ET) process, the probability depends on the product between lifetimes of the levels involved and the energy transfer process. Thus, it is required that the I 11/2-51-

10 level has a smaller lifetime than the 2 F 5/2 level so that the Yb 3+ Er 3+ predominate. In fact, as the concentration of Yb 3+ increases with respect to Er 3+, the intensity of the upconversion emission bands in the green and red region varies. This confirms that the Yb 3+ Er 3+ energy transfer is more efficient. The fast increase in the integrated intensity of the F 9/2 I 15/2 transition in the red region might be due to the energy transfer process from the excited Yb 3+ ions to the Er 3+ ions. This is possible due to the longer lifetime of the I 13/2 level [1]. This makes this process stronger over the other processes. The enhancement observed for the upconversion emission bands in the green and red regions in the Yb 3+ : Er 3+ codoped TeO 2 - PbO glass is maximum compared to other hosts [15, -6]. Also another important characteristic of the Er 3+ green upconversion emission appears from the fact that the populations of the thermally coupled 2 H 11/2 and S 3/2 levels are highly dependent on temperature, and hence this may be used for making the optical temperature sensors [5, 7]. In order to get the clear information about the number of photons in the proposed upconversion mechanisms, the integrated green and red emission intensities are measured at different pump powers. The log-log plot of the same yields a quadratic behavior for all the transitions, thereby showing that the two pump photons are involved in the upconversion process Absorption study of the Er 3+ /Er 3+ - Yb 3+ doped / codoped TeO 2 -ZnO (TZO) glass The absorption spectrum of Er 3+ ions doped in TeO 2 -ZnO (TZO) glass is shown in Fig. 2.. There appears seven absorption bands peaking at ~86 nm, ~519 nm, ~51 nm, ~66 nm, ~797 nm, ~967 nm and ~1527 nm and attributed as absorption transitions of Er 3+ ions to the F 7/2, 2 H 11/2, S 3/2, F 9/2, I 9/2, I 11/2 and I 13/2 excited states from the I 15/2 ground state. Similar features have been observed for all the doped samples except their intensity variations. The intensities of absorption peaks show a linear behavior with concentration of the Er 3+ ions. The absorption transitions are inhomogeneously broadened due to site to site variations in the crystal field strength. -52-

11 Absorbance (Arb. Units) Absorbance ( Arb. Units) Chapter-2. Optical studies of Er 3+ doped glasses in presence of Yb 3+ ion I15/2 F7/2 3 I15/2 2 H11/2 I15/2 S3/ F 7/2 2 F 5/2 I15/2 F9/2 2.5 Yb 3+ peak I15/2 I9/2 I15/2 I11/ I15/2 I13/ Wavelength (nm) Wavelength (nm) Fig. 2.: Absorption spectrum of Er 3+ doped TeO 2 -ZnO glass. The extra Yb 3+ peak observed in codoped glass is shown in the inset figure Judd-Ofelt calculations The recorded absorption spectrum of the Er 3+ ions doped in TZO glass has been utilized to find out the spectroscopic parameters of the Er 3+ ions by using Judd-Ofelt analysis [8, 9]. The expressions used to calculate the various radiative parameters viz. radiative transition probabilities, branching ratios and radiative lifetimes have been estimated using the standard relations [50] and their numerical values along with the assignments of the bands are given in Table

12 Table 2.1: Radiative transition probabilities, branching ratios and radiative lifetimes in the Er 3+ : TZO glass. SLJ S L J A T (s -1 ) r τ r (ms) 2 H 11/2 S 3/2 F 9/2 I 9/2 S 3/2 F 9/2 I 9/2 I 11/2 I 13/2 I 15/2 F 9/2 I 9/2 I 11/2 I 13/2 I 15/2 I 9/2 I 11/2 I 13/2 I 15/2 I 11/2 I 13/2 I 15/ A T =185.61s A T = s A T = s A T = s -1-5-

13 The radiative transition probabilities and branching ratios of the 2 H 11/2 I 15/2, S 3/2 I 15/2, F 9/2 I 15/2 and I 9/2 I 15/2 transitions are more significant than other transitions. These values of radiative transition probabilities and branching ratios for the 2 H 11/2, S 3/2 I 15/2 and F 9/2 I 15/2 transitions should theoretically support the radiative emissions of the corresponding transitions Upconversion studies of the Er 3+ ions doped in TeO 2 -ZnO (TZO) glass under 976nm excitation The upconversion emission spectrum of the 1.0 mol% Er 3+ doped TZO glass upon excitation at 976nm wavelength from a diode laser is shown in Fig. 2.5 (a). There appears four upconversion emission peaks centred at ~532 nm, ~556 nm, ~670 nm and ~800 nm and attributed to the 2 H 11/2 I 15/2, S 3/2 I 15/2, F 9/2 I 15/2 and I 9/2 I 15/2 transitions respectively. In the Er 3+ doped TZO glass samples; the upconversion emission peak at ~556 nm corresponding to the S 3/2 I 15/2 transition appears stronger in intensity than other emission peaks. The intensity of upconversion emission peaks increase with increase in the Er 3+ ions concentration and shows maximum intensity for 1.0 mol% of the Er 3+ ions concentration. The upconversion emission intensity shows a linear behaviour with the Er 3+ ions concentrations. The upconversion emissions observed in the green, red and near infrared (NIR) regions exhibit the quadratic behaviour with the laser pump power. This indicates that two near infrared (NIR) laser photons are contributing for the upconversion emissions. The observed upconverted emission was very bright green and was efficiently detectable even at very low pump power (~15mW) of the diode laser. The brightness increases with increasing the laser intensity. The excited state absorption (ESA) seems to be the dominant mechanism responsible for the upconverted emission in the Er 3+ doped TZO glasses. The mechanism involved in the observed upconversion emissions of the Er 3+ ions can be well understood from the schematic energy level diagram (Fig. 2.6). The ground state Er 3+ ions are -55-

14 Upconversion Intensity (Arb. Units) S 3/2 I15/2 Upconversion Intensity (Arb. Units) Chapter-2. Optical studies of Er 3+ doped glasses in presence of Yb 3+ ion b a 2 H11/2 I15/ F9/2 I15/2 a 1Er 3+ : TZO b 1Er Yb 3+ : TZO I9/2 I15/2 b a 2 H11/2 I11/ Wavelength (nm) Wavelength (nm) Fig. 2.5: Upconversion emission spectra of 1.0 mol % Er 3+ / 1.0 mol% Er mol% Yb 3+ doped / codoped TeO 2 -ZnO glasses. The inset shows the upconversion spectra around 800nm for both doped and codoped glasses. Black line (a) indicates for the Er 3+ doped TeO 2 -ZnO glass & red line (b) indicates for the Er 3+ Yb 3+ codoped glass. excited to the I 11/2 state by the ground state absorption process (GSA) through 976 nm diode laser excitation. A part of the excited Er 3+ ions in the I 11/2 state is re-excited to the F 7/2 state by the absorption of second NIR laser photon followed by the first excited state absorption (ESA). The excited erbium ions in the F 7/2 state decay non-radiatively to the 2 H 11/2, S 3/2 states via the emission of phonons. After that, the radiative transitions from the 2 H 11/2 and S 3/2 states to the I 15/2 state radiate the photons in the green region. The electronic interaction between the two Er 3+ ions into the I 11/2 state may participate in the energy transfer process at higher Er 3+ ions concentrations. One ion (donor) contributing its own energy to another Er 3+ (acceptor) ion decays to the ground state immediately and the acceptor after receiving the energy from the donor jumps to the F 7/2 level. But -56-

15 the log-log plot of upconversion (UC) intensity versus Er 3+ ions concentration shows a linear behaviour, therefore the energy transfer between the Er 3+ ions is not possible within the concentration range of our samples. The GSA process excites the ground state Er 3+ ions to the I 11/2 level, some Er 3+ ions nonradiatively decay to the I 13/2 level. The excited ions in the I 13/2 state are again excited through ESA process and the F 9/2 level is populated. The excited Er 3+ ions in the F 9/2 state relax radiatively to the ground state ( I 15/2 ) and a photon corresponding to the F 9/2 I 15/2 transition is emitted in the red region. From the Table 2.1, the calculated radiative transition probabilities and branching ratios of the 2 H 11/2 I 15/2, S 3/2 I 15/2, F 9/2 I 15/2 and I 9/2 I 15/2 transitions have been found larger than the other transitions, which supports the observed upconversion emissions upon 976nm excitation Effect of the Yb 3+ ions on the upconversion emissions of the Er 3+ ions doped in TZO glass The upconversion emission spectrum of 1.0 mol% Er mol% Yb 3+ codoped TeO 2 -ZnO (TZO) glass is shown in Fig. 2.5 (b). The positions of four upconversion emission bands corresponding to the 2 H 11/2, S 3/2 I 15/2, F 9/2 I 15/2 and I 9/2 I 15/2 transitions observed in the Er 3+ - Yb 3+ codoped glass are identical with the Er 3+ doped glass. Except these upconversion emission bands, another upconversion emission with very low intensity approximately at ~ 850nm has been seen (shown in the inset of Fig. 2.5) and assigned as the 2 H 11/2 I 11/2 transition. The blue upconversion emission band except these green and red upconversion emission bands was observed upon excitation with NIR radiation in the Er 3+ : Yb 3+ codoped host matrices [51-53]. The above fact suggests that the local field around the rare earth ions affect the spontaneous emission rates and hence the origin of upconversion emissions [5, 55]. The luminescence intensity enhancement from the codoped lanthanides is an interesting topic in recent days because of their utility in the field of upconverters, sensors, photonics, lasers and nonlinear optics purposes. The codoping of Yb 3+ ions with other rare earth ions doped materials enhances the upconversion intensity several times. The upconversion intensity enhancement in the codoped hosts have been confirmed due to the energy transfer from the Yb 3+ ions to the Er 3+ ions [51, 56]. In the presently Er 3+ -Yb 3+ codoped glass, the intensity of green and red upconversion emission bands have been enhanced by 8 and 9 times respectively, where as weak infrared upconversion emission centred at ~800nm has been enhanced by 3 times only. The maximum enhancement in the UC emission intensity is marked for the 1.0 mol% Er mol% Yb

16 composition. Such an enhancement is due to the efficient energy transfer from the Yb 3+ ions to the Er 3+ but independent of surfactant. The decay time in the energy transfer process depends on the decay of intermediate state. The decay time for the I 11/2 state of Er 3+ ion is smaller than that of the 2 F 5/2 state of Yb 3+ ion. Also the presence of Yb 3+ ions avoid the clustering between the Er 3+ ions and thereby increasing all the optical properties [57, 58]. As the pumping wavelength is in resonant with the 2 F 7/2 2 F 5/2 absorption band of the Yb 3+ ions, therefore the excitation radiation at ~976nm due to the larger absorption crosssection of the Yb 3+ ions corresponding to the 2 F 7/2 2 F 5/2 absorption transition is firstly, absorbed by the ground state ytterbium ions. After Yb 3+ excitation, an energy transfer from excited Yb 3+ ions to the ground state erbium ions takes place through the cross relaxation 2 F 5/2 2 F 7/2 : I 15/2 I 11/2 energy transfer mechanism. Also the I 11/2 level of Er 3+ ion is energetically resonant with the 2 F 5/2 level of the Yb 3+ ion. Therefore, the energy transfer from the Yb 3+ ions to the Er 3+ ions is highly efficient. After that the second photon absorbed by the Yb 3+ ion can be transferred to the previously excited Er 3+ ions promoting them to the higher multiplets. This second step energy transfer may take place via two different processes ET 1 and ET 2 by the 2 F 5/2 2 F 7/2 : I 11/2 F 7/2 cross-relaxation energy transfer process (Fig. 2.6). After being excited to the I 11/2 level, some Er 3+ ions decay to the I 13/2 level and others are re-excited to the F 7/2 level by the above mentioned process. The Er 3+ ions in I 13/2 level are re-excited to the F 9/2 level through the 2 F 5/2 (Yb 3+ ) + I 13/2 (Er 3+ ) 2 F 7/2 (Yb 3+ ) + F 9/2 (Er 3+ ) energy transfer process. The erbium ions from the F 7/2 level non-radiatively decay to the 2 H 11/2 and S 3/2 levels. The erbium ions in the 2 H 11/2, S 3/2 and F 9/2 levels relax radiatively to the ground I 15/2 level, emitting photons in the visible region. The Er 3+ ions in the 2 H 11/2 level relax radiatively to the I 11/2 level giving a photon in the near infrared (NIR) region. The population of 2 H 11/2 level is decreased in populating the S 3/2 level too. This explains why the upconversion luminescence intensity corresponding to the 2 H 11/2 I 11/2, I 15/2 transition is smaller than that of the S 3/2 I 15/2 transition. This is supported by the transition probability and branching ratio calculated using the Judd-Ofelt theory (Table 2.1). As the transition probability and branching ratio for the 2 H 11/2 I 15/2 transition is about ~10 times larger than that of the 2 H 11/2 I 11/2 transition, therefore, the intensity of the upconversion emission band corresponding to the 2 H 11/2 I 11/2 is many times smaller than that of the 2 H 11/2 I 15/2 transition. -58-

17 976nm 976nm 532nm 556nm 670nm 800nm Energy ( x10 3 cm -1 ) 850nm Chapter-2. Optical studies of Er 3+ doped glasses in presence of Yb 3+ ion ET 2 F 7/ H 11/2 S 3/2 15 GSA ESA F 9/2 ET 1 I 9/ F 5/2 I 11/2 I 13/2 5 CR 0 2 F 7/2 I 15/2 Yb 3+ Er 3+ Fig. 2.6: Schematic Energy level diagram of the Er 3+ : Yb 3+ ion. It is observed that slopes for the log of green and red upconversion luminescence intensity as a function of the log of NIR pump power are 1.85 and 1.67 respectively (Fig. 2.7). This confirms the contribution of two photons absorption process. This is due to the fact that most of the Er 3+ ions in the I 11/2 level decay to the I 13/2 level non-radiatively and do not have larger chance to be reexcited to the F 7/2 level. This explains why the value of slope corresponding to the red upconversion emission is smaller than that of the green emission. Moreover, a change in the relative intensity of the 2 H 11/2 I 15/2 and S 3/2 I 15/2 transition with increase in the NIR pump power has been monitored. It is marked that intensity ratio of the 2 H 11/2 I 15/2 to the S 3/2 I 15/2 transition shows an increasing trend with increasing the pump power. It appears that such behaviour may be of significant importance to be used in monitoring the temperature. Not only that, fluorescence intensity ratio corresponding to the 2 H 11/2 I 15/2 and S 3/2 I 15/2 transition in the Er 3+ : Yb 3+ codoped glass is found to be larger than that in the Er 3+ doped glass. It is observed to be 0.28 and 0.32 for the doped and codoped samples respectively. This is due to the smaller energy gap (~ 800 cm -1 ) between the 2 H 11/2 and S 3/2 level, which are thermally coupled and follow a Boltzmann -59-

18 Upconversion intensity (arbitrary Units) Chapter-2. Optical studies of Er 3+ doped glasses in presence of Yb 3+ ion distribution at ordinary temperature. Due to this effect, the variation in temperature changes the population density of each green emitting level and hence the relative intensity of the bands. Particularly, it has been verified that the ratio between green and red emission bands is approximately constant within the range of the pump power used Green emission Red emission Slope for green emission: Slope for red emission: Log [Pump Power (Watt) ] 1 Fig. 2.7: Log-log variation of observed upconversion emission intensity versus NIR pump power Upconversion emissions in the Er 3+ doped TeO 2 -ZnO (TZO) glass using 808 nm excitation The upconversion emission spectrum of the 1.0 mol% Er 3+ doped TZO glass under 808nm excitation is shown in Fig. 2.8 (a). There appears six upconversion emission bands at ~18 nm, ~80 nm, ~537 nm, ~55 nm, ~571 nm, ~600 nm and are assigned as the 2 H 9/2 I 15/2, F 7/2 I 15/2, 2 H 11/2 I 15/2, S 3/2 I 15/2, F 3/2 I 13/2, F 5/2 I 13/2 transitions respectively

19 The green upconversion emission band in the erbium doped TZO glass appears stronger in intensity than the other emissions. The maximum upconversion emission intensity has been observed for the 1.0 mol% Er 3+ doped TZO glass. The blue and green upconversion emission intensities with the variations of laser pump powers show the quadratic behaviour, whereas it follows a linear trend with concentration of the rare earth ions. This confirms that the dual photon absorption process is responsible for the observed upconversion emissions. As the upconversion emission intensity varies linearly with the rare earth ions concentrations (Fig. 2.9), thereby indicating the possibility of excited state absorption process. In the Er 3+ doped TZO glass the ground state Er 3+ ions are pumped to the I 9/2 level by the 808nm excitation. Two excited Er 3+ ions through exchange of their energies in the I 9/2 level may promote one ion (acceptor) to the 2 H 9/2 excited level and other (donor ) to the ground state. Other possible ET processes may populate the F 7/2 and S 3/2 states via the following mechanism. 1). I 11/2 + I 11/2 F 7/2 + I 15/2 2). I 13/2 + I 11/2 S 3/2 + I 15/2 However, energy transfer (ET) mechanism is not much effective in the excitation process within the concentration range of our samples. Furthermore, the ET process is ruled out due to linear behaviour of UC emission intensity with the Er 3+ ions concentration. A part of the excited Er 3+ ions in the I 9/2 level relax nonradiatively to the I 11/2 and I 13/2 levels followed by the phonon assisted lattice vibrations. Second photon absorption process (i.e. ESA) populates the Er 3+ ions to upper states viz. 2 H 11/2, S 3/2 and F 5/2, 7/2, etc. of Er 3+ ion. The rest of the excited Er 3+ ions in the I 9/2 level through the excited state absorption (ESA) transit upward to the 2 H 9/2 level. The F 3/2 level is populate via the non-radiative relaxation from the 2 H 9/2 level. Afterwards, the excited Er 3+ ions decay radiatively to low lying levels producing upconversion emissions in the blue, green and red regions. -61-

20 (upc Intensity (Arb. Units) Chapter-2. Optical studies of Er 3+ doped glasses in presence of Yb 3+ ion H9/2 I15/2 F7/2 I15/2 (upc Intensity (Arb. Units) H9/2 I15/2 F7/2 I15/2 2 H11/2 I15/2 S3/2 I15/2 b: 1Er Yb 3+ : TZO F3/2 I 13/2 F5/2 I 13/ Wavelength (nm) 2 H11/2 I15/ Wavelength (nm) S3/2 I15/2 F3/2 I 13/2 a: 1Er 3+ : TZO F5/2 I 13/2 Fig. 2.8: Upconversion spectra of (a) Er 3+ and (b) Er 3+ - Yb 3+ codoped zinc tellurite glasses under 808nm diode laser excitation. -62-

21 Log (Upconversion Intensity) Chapter-2. Optical studies of Er 3+ doped glasses in presence of Yb 3+ ion a 0. b a: Slope for F 7/2 b: Slope for S 3/2 I 15/2 = I 15/2 = Log (Er 3+ ions Concentration) Fig 2.9: Variation of upconversion emission intensity with concentration of the Er 3+ ions Effect of codoping by the Yb 3+ ions on upconversion emission intensity of the Er 3+ ions The energy transfer from Yb 3+ ions to other rare earth ions has been intensively researched in the field of upconversion luminescence [15, 59-62]. The Yb 3+ ion consists of only two energy levels namely 2 F 7/2 (ground) state and 2 F 5/2 (excited) state with its energy gap of about ~10, 26 cm -1, which suits for a NIR photon absorption and energy transfer between the rare earth ions [63]. The upconversion emission intensity enhancements in the Er 3+ doped TZO glass for different radiative transitions in presence of the Yb 3+ ions can easily be seen (Fig. 2.8). If integrated intensities of radiative emissions in the 1.0 mol % Er 3+ doped glass are represented by I d and the corresponding radiative emissions in the 1.0 mol% Er mol% Yb 3+ codoped glass are represented by I c ; then the ratios I c /I d in the codoped glass corresponding to the 2 H 9/2 I 15/2, -63-

22 Energy ( x10 3 cm -1 ) 18nm 80nm 537nm 55nm 571nm 600nm 808nm 808nm S 3/2 Chapter-2. Optical studies of Er 3+ doped glasses in presence of Yb 3+ ion F 7/2 I 15/2, 2 H 11/2 I 15/2, S 3/2 I 15/2, F 3/2 I 13/2 and F 5/2 I 13/2 transitions peaking at ~18 nm, ~80 nm, ~537 nm, ~55 nm, ~571 nm and ~600 nm are observed to be ~29, ~37, ~13, ~11, ~16 and ~20 respectively. The enhancement observed in the upconversion emission intensities for the codoped glass is basically due to the efficient energy transfer from the Yb 3+ ions to the Er 3+ ions G 7/ G 9/2 G 11/2 2 H 9/2 20 F 3/2 5/2 7/2 2 H 11/2 15 F 9/2 I 9/2 ET 10 I 11/2 2 F 5/2 5 I 13/2 ET 0 Er 3+ I 15/2 3+ Yb 2 F 7/2 Fig Energy level diagram of the Er 3+ - Yb 3+ glass system and mechanism of upconversion by 808nm excitation. To get the complete information regarding the excitation of both ions to upper levels and energy transfer from the Yb 3+ ions to the Er 3+ ions can be understood from energy level diagram (Fig. 2.10). The ground state Yb 3+ ions pumped by the 808nm excitation is excited to the 2 F 5/2 level followed by the emission of three phonons. The excited Yb 3+ ion transfer its energy to the ground state Er 3+ ions. The Er 3+ ion, after accepting the excitation energy from the Yb 3+ ion, jumps to the I 11/2 level. As the lifetime of the I 13/2 level is larger than that of the I 11/2 level, it relaxes nonradiatively to the I 13/2 level. In the second step, due to the energy transfer from the Yb 3+ to Er 3+, the Er 3+ ions from the I 13/2 level are excited to the F 9/2 level. Since the radiative lifetime of the F 9/2-6-

23 level (1.07 ms) is smaller than that of the I 9/2 level (7.82 ms). The erbium ions from the F 9/2 level relaxes non-radiatively to the I 9/2 level. Thus the I 9/2 level is populated via the ground state absorption (GSA) as well as the non-radiative relaxation from the F 9/2 level. Thus the excited levels are populated by the energy transfer from the Yb 3+ ions to the Er 3+ ions along with the aforementioned processes. The radiative transitions from the excited levels to the ground state and low lying levels produce emissions corresponding to the 2 H 9/2 I 15/2, F 7/2 I 15/2, 2 H 11/2 I 15/2, S 3/2 I 15/2, F 3/2 I 13/2 and F 5/2 I 13/2 transitions. 2.. Conclusion The Er 3+ and Yb 3+ doped/codoped TeO 2 -PbO (TPO) and TeO 2 -ZnO (TZO) glasses have been successfully prepared by melt and quenching technique. The large values of calculated radiative transition probabilities and branching ratios of the Er 3+ ions using Judd-Ofelt theory support the corresponding upconversion emissions. The excitation state absorption (ESA) process is dominant in single Er 3+ doped glassy systems, whereas the efficient energy transfer from the Yb 3+ to Er 3+ ions suited more for the upconversion emission enhancement in the Er-Yb codoped glassy systems. The results confirm that the Yb 3+ ion behaves as a sensitizer by using a suitable laser excitations (i.e. 976 nm and 808 nm). The intensity ratio of the 2 H 11/2 I 15/2 to the S 3/2 I 15/2 transition of Er 3+ ions shows an increasing trend with increasing the pump power. This confirms that such behaviour may be of significant importance to monitor the temperature. Consequently, the Yb 3+ :Er 3+ codoped TPO and TZO glasses may be suitable for making the upconverter and optical temperature sensor. -65-

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