Ho3+/Yb3+ codoped glass ceramic was prepared by melt-quenching and subsequent thermal treatment. Under a 980 nm diode laser excitation, upconversion emissions from Ho3+ ions centered at 540, 650, and 750 nm were greatly enhanced compared with those in the precursor glass. Especially, the short-wavelength upconversion emissions centered at 360, 385, 418, 445, and 485 nm were successfully obtained in the glass ceramic. An explanation for this phenomenon is given based on the fluorescence decay curve measurements. In addition, an optical temperature sensor based on the blue upconversion emissions from 5F2,3/3K8→5I8 and 5F1/5G6→5I8 transitions in Ho3+/Yb3+ codoped glass ceramic has been developed. It was found that by using fluorescence intensity ratio technique, appreciable sensitivity for temperature measurement can be achieved by using the Ho3+/Yb3+ codoped glass ceramic. This result makes the Ho3+/Yb3+ codoped glass ceramic be a promising candidate for sensitive optical temperature sensor with high resolution and good accuracy.
©2012 Optical Society of America
In recent years, the study on the upconversion (UC) materials have attracted considerable attention because of their wide applications such as color displays, short-wavelength lasers, IR quantum counter detectors, optical sensing, optical communications, and biomedical diagnostics [1, 2]. UC, i.e., photoexcitation at a long wavelength followed by luminescence at a shorter wavelength, is typically observed in compounds containing rare earth (RE) ions. The unique optical properties for RE ions arise from the weak interaction of the 4f elections with the ligand field due to the shielding effect of the outer 5s and 5p orbitals, which results in sharp intra-4f transitions. In addition, the RE ions possess abundant electronic states at various energies, which afford them the ability to absorb one or more low-energy near infrared (NIR) photons and subsequently convert them to high-energy emissions . Thus, the generation of UC fluorescence can be easily realized by ample availability of well-matching, cost-effective, and high-power near infrared diode lasers. Many RE ions, including Er3+, Ho3+, Tm3+, and Pr3+, have been extensively reported for their UC luminescence [4–8]. However, short-wavelength UC emissions (from ultraviolet to blue) from RE ions, especially from Ho3+, induced by NIR lasers have rarely been investigated presently.
It is well known that Ho3+ has abundant energy levels in the spectroscopic range of ultraviolet to NIR . Although the short-wavelength UC emissions of Ho3+ was reported in Ho3+ doped LiYF4 and YF3 via synchrotron radiation, these emissions induced by NIR laser excitation are not observed . And it is known that Ho3+ sensitized by Yb3+ not only can generate strong visible UC emissions, but also can result in visible photon avalanche UC emissions under NIR laser excitation . Green and red UC emissions from Ho3+/Yb3+ codoped materials under 980 nm laser excitation have been widely studied, and photon avalanche for visible UC emissions in Ho3+ doped or Ho3+/Yb3+ codoped materials have also been studied under 750 or 840 nm excitations [10, 11], but the short-wavelength emissions are scarcely observed. So far, just very a few works have obtained the UC emissions of Ho3+ ions in the ultraviolet-blue spectral range excited by NIR laser [7, 8, 12]. However, the weak chemical and physical stability of the fluoride hosts significantly limits their practical applications [7, 8]. It, therefore, is of great scientific interest and importance to obtain the short-wavelength UC emissions of Ho3+ ions in an appropriate host matrix.
As is well known, the oxyfluoride glass ceramic has been proven to be one of the most promising luminescent materials for its advantages combined with low phonon energy for fluorides and desirable mechanical and chemical performances for oxides [13, 14]. This kind of material is generally fabricated through controlling crystallization of the defined fluoride nanophase from the precursor glass by thermal treatment. And by incorporating abundant RE ions into the fluoride nanocrystals, efficient UC emissions are readily obtained . In addition, the suitability of the oxyfluoride glass ceramic to be fibered has been demonstrated . This characteristic makes this kind material compatible with a range of optical schemes and highlights the potential applications on optical amplifiers and laser systems.
Meanwhile, due to the strong dependence of transitions from the elections in 4f systems of RE ions on the temperature conditions [17, 18], the thermal behavior of UC fluorescence becomes increasingly important for further understanding the UC mechanisms in general and finding the potential uses, especially, on the optical thermal sensing [19–21]. Thus, in the present work, we synthesized Ho3+/Yb3+ codoped oxyfluoride glass ceramic through high temperature solid-state reaction and the subsequent thermal treatment. Under a 980 nm diode laser excitation, the UC emissions in the ultraviolet-blue range were successfully obtained in the glass ceramic. Analysis based on the luminescence decay curves was performed to explain this phenomenon. In addition, the temperature dependent blue emissions from Ho3+/Yb3+ codoped glass ceramic were investigated at temperatures ranging from 303 to 643 K. A much higher sensitivity for temperature measurement was achieved here.
The glass ceramic is prepared with the following starting composition ratios (in mol %): 50SiO2-50PbF2-0.05Ho2O3-0.5Yb2O3. The fully mixed chemicals are melted in a covered alumina crucible at 1000 °C for 1 hour in a SiC-resistance electric furnace in air. Then, the melt is cast into a preheated stainless steel mold and naturally cooled down to room temperature to form the precursor glass. The precursor glass is heated to 460 °C with a heating rate of 10 K/min, and then hold for 3 hours to form transparent glass ceramic through crystallization. The resulting samples are cut and polished into the size of 10 mm × 8 mm × 2 mm for optical measurements.
Differential thermal analysis (DTA) measurement was carried out by the differential thermal analyzer (Rigaku TG-DTA TG8120) to confirm the glass transition temperature (Tg) and the crystallization peak temperature (Tc). X-ray diffraction (XRD) patterns were acquired using a Rigaku D/max-γB diffractometer with Cu Κα radiation (λ = 0.15418 nm). The UC luminescence spectra were recorded by Zolix-SBP300 grating spectrometer equipped with a CR131 photomultiplier tube. A 980 nm diode laser is used as the pump source. The luminescence decay curves are measured by the 980 nm diode laser modulated through square-wave electric current and recorded by Tektronix DPO 4140 oscilloscope. When measuring the luminescence decay curves for the infrared emissions, the light signal was detected by InGaAs photodiode. To investigate the temperature dependent UC luminescence, the sample was placed in a mini furnace and its temperature was increased from 303 to 643 K. The temperature of sample was monitored by a copper-constantan thermocouple and controlled by a proportional-integral-derivative loop feedback temperature control system.
To investigate the temperature dependent UC luminescence, the sample was put into a quartz tube and then placed in a home-made mini Si-C furnace. The sample was heated from 303 to 643 K and the temperature of sample was monitored by a copper-constantan thermocouple with an accuracy of ± 1.5 K and controlled by a proportional-integral-derivative loop feedback temperature control system. The UC luminescence signal of the sample generated by the excitation of 980 nm laser was finally coupled into the grating spectrometer through a quartz lens.
3.1. Upconversion emissions
Figure 1 shows the DTA curve of the glass. It can be observed that the glass transition occurred at around 409 °C and distinct crystallization peak began at 450 °C, centered at 489 °C and ended at 539 °C. It was found that the glass heated at 489 °C lost easily its transparency. To get transparent glass ceramic, the heat treatment temperature was selected to be 460 °C based on the experiment experience. The XRD patterns of the glass and the glass ceramic are shown in Fig. 2 . There are only two humps in the XRD curve for the glass, indicating its amorphous structure. However, after thermal treatment, intense diffraction peaks attributed to β-PbF2 nanocrystals  emerge in the XRD patterns, reflecting the crystallization during thermal treatment.
Figure 3 shows the room temperature UC emission spectra for the Ho3+/Yb3+ codoped precursor glass and glass ceramic under 980 nm excitation with the pumping power of 650 mW. Both spectra exhibit green, red, and NIR emissions bands of Ho3+ ions originated from the following three transitions: 5S2/5F4→5I8(~540 nm), 5F5→5I8(~650 nm), and 5S2/5F4→5I7(~750 nm). In comparison with the precursor glass, the intensity for these UC emissions in the glass ceramic is greatly enhanced, and the green, red, and NIR emissions are respectively enhanced to be about 26, 20, and 5 times. Especially, the short-wavelength UC emissions ranging from ultraviolet to blue centered around 362 nm, 390 nm, 420 nm, 445 nm, and 485 nm, which are not detected in the precursor glass, are successfully obtained in the glass ceramic. Such emissions can be assigned to the transitions from 5F2, 3/3K8, 5F1/5G6, 5G5, 5G4/3K7, and 5G5/3H6 of Ho3+ to the ground state 5I8, respectively [7, 8]. In addition, the UC emission bands in the glass ceramic exhibit obvious Stark splits, which indicates the successful incorporation of RE ions into the β-PbF2 nanocrystals after thermal treatment.
3.2. Upconversion mechanisms
To understand the UC mechanisms, the pump power dependences of emissions for the glass ceramic were investigated. It is known that at the unsaturation condition, the UC emission intensity (IUP) depends on the excitation power (P) and follows the below relationshipFig. 4 by a log-log plot. As illustrated in Fig. 4(a), slope n values of the linear fittings are 1.34, 1.51, and 1.28 for the green, red, and NIR UC emissions, respectively. The n values suggest two-photon process for these UC emissions. Meanwhile, as illustrated in Fig. 4(b), the slopes of the linear fitting are 2.28 for 5F2,3/3K8→5I8, 2.45 for 5F1/5G6→5I8, 2.42 for 5G5→5I8, 2.63 for 5G4/3K7→5I8, and 2.66 for 5G5/3H6→5I8 transition, indicating three pumping photons are required to populate the 5F2,3/3K8, 5F1/5G6, 5G5, 5G4/3K7, and 5G5/3H6 levels. Figure 5 shows the energy level diagrams of Ho3+ and Yb3+ ions, as well as the proposed mechanisms to explain the short-wavelength UC emissions. The pumping photons of 980 nm can be efficiently absorbed by Yb3+ ions, and the 5S2/5F4 and 5F5 energy levels can be populated through two successive energy transfers from Yb3+ to Ho3+ ions. Subsequently, one energy transfer from excited Yb3+ can promote the Ho3+ ions on the 5S2/5F4 or 5F5 onto the 5G5/3H6 or 5G4/3K7 states. Alternatively, the 5G4/3K7 levels can be populated via the multiphonon nonradiative relaxation from 5G5/3H6. Then, the 5F2,3/3K8, 5F1/5G6, and 5G5 levels are populated by the multiphonon nonradiative relaxations from their upper levels. In addition, some possible cross-relaxation (CR) processes may also occurs [22, 23], as illustrated in Fig. 5.
3.3. Analysis of the luminescence decay curves
Note that the green, red, and NIR UC emissions of Ho3+ ions were enhanced dramatically after thermal treatment on the precursor glass, and the short wavelength emissions from ultraviolet to blue were successfully obtained in the glass ceramic. As is well known, a significant factor affecting the UC efficiency of RE ions is the multiphonon nonradiative relaxation rate WNR, which follows the below equation Equations (2) and (3) suggest that medium with lower phonon energy results in a smaller multiphonon nonradiative relaxation rate and consequently improves the luminescence efficiency.
Figure 6 shows the luminescence decay curves for 5I7, 5I6, 5F5, and 5S2/5F4 states in the glass and the glass ceramic. It is observed that the lifetimes for these states are increased in the glass ceramic compared with those in the precursor glass. This is mainly because that in the precursor glass, Ho3+ ions are coupled to the non-bridging oxygen on the strong O-Si bonds with the phonon mode around 930 cm−1 ; in the glass ceramic, most of RE ions are incorporated into the fluoride nanocrystals . In such a case, Ho3+ ions are shielded from the direct coupling with the nonbridging oxygen of O-Si and coupled to a weaker phonon mode of 230 cm−1 for Pb-F . The lower phonon energy environment of the fluoride nanocrystals can weaken the multiphonon nonradiative relaxations and contribute to the UC emissions of Ho3+ ions in the glass ceramic. As the intermediate states, the longer lifetimes for 5S2/5F4 and 5F5 states also contribute to the population of the higher energy levels. Due to the undetected ultraviolet-blue short wavelength UC emission signals in the precursor glass, the comparison between the lifetimes for the upper energy levels of Ho3+ ions in the precursor glass and the glass ceramic are not performed. However, longer luminescence lifetimes for 5F2, 3/3K8, 5F1/5G6, 5G5, 5G4/3K7, and 5G5/3H6 states in the glass ceramic can also be expected.
In addition, according to the Dexter theory , the energy transfer rate between the RE ions is proportional to R−6, where R is the distance between a sensitizer and an acceptor. Since most of Ho3+ and Yb3+ ions are concentrated into β-PbF2 nanocrystals in the glass ceramic, the distance between Ho3+ and Yb3+ ions becomes much shorter than that in the precursor glass. As a result, the energy transfer rate from Yb3+ to Ho3+ ions will be enhanced. The energy transfer rate WYb→Ho from Yb3+ to Ho3+ can be estimated from the following relation
3.4. Optical thermometry based on the blue upconversion emissions of Ho3+ ions
Accurate and reliable measurement of temperature is a major requirement in the monitoring of changes in chemical, physical, and biological processes. Fluorescence temperature sensing technique has been attracted great interest for its high sensitivity and good accuracy . Compared with the conventional technique, the fluorescence based sensors are especially advantages in the applications in the electromagnetically and/or thermally harsh environments, such as the electrical power station, oil refineries, coal mines, and building fire detection . An optical temperature sensor is generally based on the fluorescence intensity ratio (FIR) technique, which is independent of signal losses and fluctuation in the excitation intensity and has the potential to further improve the measurement accuracy. Generally, the FIR utilizes the temperature dependent fluorescence from two thermally coupled energy levels of one type RE ions, and the energy gap ΔE between these two levels should be 200 cm−1≤ ΔE ≤ 2000 cm−1. As is known, the fluorescence of RE ions can be generated via UC and downconversion process by proper pumping source. But the former is more popular in the case of optical thermometry not only for the commercial excitation sources, but also for the eliminating of stray light which is easily introduced by downconversion excitation source. Till now, Er3+, Pr3+, Nd3+, and Yb3+ have been developed as the activators for optical temperature sensors [20, 28–34].
For optical thermometry, it is of great importance to understand the rate at which the FIR changes for a change in temperature T. This value is known as the sensitivity. To allow the comparison between the sensitivities obtained from the FIR of different RE ions, the absolute sensitivity S is defined as [20, 21]Equation (5) suggests that using pairs of levels with larger energy difference are in favor of higher sensitivity. Currently, the largest sensitivity for optical temperature sensors based on the UC fluorescence is just limited in the materials with Nd3+ ions, the energy gap for the thermally coupled 4F3/2 and 4F5/2 levels being about 1000 cm−1 . But to satisfy the thermometry requiring a higher sensitivity, the finding of the thermally coupled levels with much larger separation, from which luminescence can be easily produced via proper pumping, is becoming increasingly important.
From the emission spectra of Ho3+/Yb3+ codoped glass ceramic, it is noted that the ΔE between 5F2, 3/3K8 and 5F1/5G6 is about 1853 cm−1, which satisfies the situation of thermally coupled levels and is much larger than that of Nd3+ ions. That is to say, the blue emissions from Ho3+: 5F2, 3/3K8 and 5F1/5G6 states can be utilized for optical thermometry with higher sensitivity . To understand the characteristics for temperature sensing, the blue UC emissions from Ho3+/Yb3+ codoped glass ceramic were studied as a function of temperature ranging from 303 to 643 K. Figure 7 shows the temperature dependent blue UC emissions for Ho3+/Yb3+ codoped glass ceramic. The pumping power of 980 nm laser is set as 650 mW. From Fig. 7, it can be seen that with the increase of temperature, the position for 485 nm fluorescence is not changed, but its intensity decreases gradually. In contrast, the intensity for 5F1/5G6→5I8 transitions is slightly enhanced, and about 1.4 times enhancement is achieved at 583 K. In addition, the center wavelength for 5F1/5G6→5I8 changes from 445 to 453 nm as temperature increases. Such phenomenon is interesting. This may be due to that the peak of the hypersensitive 5I8→5G6 absorption band, which was measured to be around 453 nm at room temperature, exhibits blue-shift as temperature increases. In this case, the reabsorption effect could be weakened for the 453 nm emission but enhanced for the emission at 445 nm, so that the red-shift of the 440-460 nm emission peak occurs with the increase of temperature. Of course, the underlying mechanisms may be complex and require further study.
The change in the intensity ratio between the two blue emissions is originated from the small energy gap between the 5F2, 3/3K8 and 5F1/5G6 levels, which allows the ions on the 5F2, 3/3K8 to be thermally populated onto the 5F1/5G6 levels. It has been well known that the relative population of the thermally coupled levels follows a Boltzman type population distribution, and according to the theory by Wade et al. , the FIR for the emissions from the thermally coupled levels of RE ions is modified asFigure 8 presents the FIR between the shorter and the longer wavelength blue emissions from Ho3+/Yb3+ codoped glass ceramic as a function of temperature. The experimental data are fitted by Eq. (6). It can be observed that the fittings agree well with the experimental data. C and B are fitted to be about 4.22 ± 0.73 and 0.043 ± 0.002, respectively, and ΔE is fitted to be about 1544.6 ± 81 cm−1. Several temperature circles have been tested and a good repeatability was obtained. In addition, to understand whether the heating effect generated by the absorption of Yb3+ ions at 980 nm influences the quasi-thermal equilibrium of the sample, the temperature dependent ratios were recorded when the sample was excited with a lower power of 330 mW. The result illustrates that the ratios can also be well fitted by the Boltzman distribution of Eq. (1) with slight change in the values of the fitted parameters, indicating the quasi-thermal equilibrium condition of the sample is hardly affected by pump absorption heating. And based on the above experiment results, such heating effect on the optical thermometry actually can be eliminated by the offset factor B in Eq. (1). Table 1 shows the expression of the sensitivity for Ho3+/Yb3+ codoped glass ceramic. For comparison, the sensitivities for the other RE ions doped materials are also presented. It is observed that optical temperature sensors with excellent sensitivity can be realized by using Ho3+/Yb3+ codoped glass ceramic. This result is attributed to the large energy gap between 5F2, 3/3K8 and 5F1/5G6 states, which is also in favor of a high resolution.Fig. 9 . The SR keeps increasing with temperature and can be expected to reach its theoretical maximum value (about 1.02 × 10−3 K−1) at 1119 K. For comparison, the theoretical maximum value SR-MAX and their corresponding temperatures for other temperature sensors in the literature [20, 28–30] are also shown in Table 1. From the table, one can conclude that Ho3+ ions are appropriate to be used as the activators for high temperature measurements.
In conclusion, Ho3+/Yb3+ codoped glass ceramic was prepared. Under 980 nm excitation, the short-wavelength UC emissions in the spectral range of ultraviolet-blue were successfully obtained. The luminescence decay curves were measured, and it was found that the weakened multiphonon nonradiative relaxations and the enhanced energy transfer from Yb3+ to Ho3+ in the glass ceramic played an important role in such phenomenon. Furthermore, the thermal behavior of the blue emissions from Ho3+/Yb3+ codoped glass ceramic at temperatures ranging from 303 to 643 K was investigated. It was observed that by using the FIR technique, appreciable sensitivity for temperature measurement can be achieved by using the Ho3+/Yb3+ codoped glass ceramic as the sensing medium This result suggests that the Ho3+/Yb3+ codoped glass ceramic is a good medium for optical temperature sensors with high sensitivity, high resolution as well as good accuracy.
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