Thermal and thermo-mechanical properties

The thermal and thermo-mechanical properties of the proposed cooling material are essential for investigating the integration of numerous optical parameters of an optical cooler. The DSC traces recorded on the 30SiO 2 -15Al 2 O 3 -(29-x)CdF 2 -22PbF 2 -4YF 3 -xYbF 3 (mol%) glasses for various Yb3+ ion concentrations are shown in Fig. 2(a). The glass transition temperature (T g , ±2 °C), the onset temperature of crystallization (T x , ±2 °C) and peak crystallization temperature (T p , ±1 °C) were determined from the thermograms as well as the corresponding glass thermal stability against crystallization criterion (ΔT = T x − T g , ±4 °C). Among the Yb3+-doped samples, a slight increase of their glass characteristic temperatures T g , T x and T p is observed in Fig. 2(a,b) with increasing Yb3+ concentration. First, the increase of glass transition temperature (from 412 to 445 °C) with increasing Yb3+ content (from 0 to 20 mol%) shows here that the Yb3+ ions are well incorporated into the glass network. Whereas addition of RE ions like Yb3+ into a glassy material usually tends to decrease its stability by altering its network reticulation (and thus decreasing its T g ), it seems here that the addition of Yb3+ reinforces the glass network. The heavy level of doping attained (up to 20 mol%) supports this assumption. Further investigation is required to understand the structural role played by Yb3+ in such heavily doped glasses.

Figure 2 (a) DSC traces of the undoped and Yb3+-doped 30SiO 2 -15Al 2 O 3 -(29-x)CdF 2 -22PbF 2 -4YF 3 -xYbF 3 oxyfluoride glasses as a function of Yb3+ concentration (x). The thermograms were vertically shifted for better comparison. (b) Variation of T g , T x , T p and ΔT of the samples under study as a function of Yb3+ concentration. Full size image

A progressive shifting towards higher temperature of the crystallization peak can be observed on the DSC traces (Fig. 2(a)) with increasing Yb3+ concentration. This slight increase of T x and T p can be directly correlated with the strengthening of the glass network after the replacement of Cd2+ cations by Yb3+ ions, as above mentioned. The shape of the crystallization peak also evolves with increasing Yb3+ concentration, as can be seen in Fig. 2(a). From the SYb05 to the SYb20 sample thermograms, the peak is broadening and clearly consists of two contributions: a sharp intense one at lower temperature, and a broad weak shoulder at higher temperature. It can be assumed that the first peak is related to the crystallization of β-PbF 2 crystals, as already reported in refs 23, 24, 25 while the second contribution can be associated with the formation of new crystalline phase or even phase transformation. Further investigation focused on the crystallization kinetics and identification of the crystalline phase structure would be required to fully describe the crystallization process in these SiO 2 -Al 2 O 3 -CdF 2 -PbF 2 -YF 3 -YbF 3 glasses. Nevertheless, it is worth mentioning that the undoped sample exhibits the highest temperatures of crystallization (both onset and peak) and the largest thermal stability against crystallization (ΔT = 69 °C, see Fig. 2(b)). In addition, one can see in Fig. 2(a) that its crystallization peak is weaker and flatter than those on the other DSC traces, indicating therefore that the undoped glass is less prone to crystallization. Such result was expected as it is well-known that addition of RE ions like Yb3+ into a glassy material tends to decrease its stability vs crystallization, as previously reported in many works in the literature23,24,26,27.

As above mentioned, the knowledge of the thermo-mechanical material properties such as thermal conductivity, specific heat capacity and thermal expansion coefficient (TEC) is crucial for a proper design of an optical cooler. The heat transfer rate within the cooled material is proportional to changes in temperature, thermal conductivity and heat capacity after excitation with a suitable laser28. Thermo-mechanical analysis (TMA) was performed on the samples SYb02 and SYb12. The TEC determined for these samples (in the temperature range of 100-350 °C) are 11.3x10−6/K and 13.7x10−6/K, respectively. The theoretical description of the TEC has been reported for Yb3+-doped phosphate laser glasses elsewhere29. The TEC values are higher than those reported for Li 2 O–Al 2 O 3 –SiO 2 glasses (4.6–7.5 × 10−6/K)30, phosphate glass (LiPO 3 –Al(PO 3 ) 3 –Ba(PO 3 ) 2 –La 2 O 3 , 9.8 × 10−6/K)31, Yb3+:YAG crystal (8.06 × 10−6/K)32 but lower than that of ZBLAN fluorozirconate glass (16.4 × 10−6/K)33 and comparable to that of silicate laser glasses (12.7–13.4 × 10−6/K)34. The TEC values for the investigated glasses are between those of laser cooled materials such as Yb3+:YAG crystal32 and Yb3+:ZBLAN glass33.

Linear refractive index

The refractive indices of the glass samples were measured by the prism coupling technique with a resolution of ±0.001, and plotted in Fig. 3 as a function of wavelength and Yb3+ concentration. The values reported here were obtained for the transverse-electric (TE) mode of the incident laser radiation while no significant difference was observed in the transverse-magnetic (TM) mode, confirming the absence of birefringence, as expected in isotropic glass materials. First, one can observe in Fig. 3 a decrease of the refractive index with increasing the wavelength for each glass sample, showing thus their respective chromatic dispersion. The Sellmeier’s dispersion relation was used to fit the experimental data and facilitate their reading. Then, if we do not consider the undoped sample, one can observe that their refractive index decreases with increasing Yb3+ concentration. Such behavior is quite unusual. Indeed, glass doping with RE ions which are heavy elements compared to traditional components used to form glass (e.g. SiO 2 ), generally results in increasing its refractive index. Here, the ytterbium fluoride (YbF 3 , molar mass = 230.04 g/mol) is incorporated into the glass by substituting for the cadmium fluoride, which is lighter (CdF 2 , molar mass = 150.41 g/mol), following the composition law 30SiO 2 -15Al 2 O 3 -(29-x)CdF 2 -22PbF 2 -4YF 3 -xYbF 3 (mol%). Therefore, an increase of refractive index and density could be expected with increasing the Yb3+ concentration. However, while the density increase is observed with increasing Yb3+ concentration (as shown in Fig. S1) as expected, an opposite trend is observed for the refractive index in our glasses, as shown in Fig. 3. Interpreting the refractive index change of glasses as a function of their chemical composition is relatively complex. Indeed, it essentially depends on two factors, i.e. the glass molar volume (related to its density and molar mass) and the polarizability of its constituents. A tentative explanation can be as follows. First the high refractive index of these glasses is mainly governed by their large concentration of heavy metals with large electronic densities. Then, it is known that F− anions possess a lower polarizability than O2− anions35. The progressive replacement of CdF 2 by YbF 3 in these glasses implies an increase of its fluorine content to the detriment of its oxygen content, as presented in the Table 1. This results then in a decrease of the glass average polarizability. Therefore the observed decrease in glass refractive index can be ascribed here to the dominant role played by its decreasing polarizability whereas its density, which increases with increasing Yb3+ concentration (see Fig. S1), has a lower impact. Last, one can also notice in Fig. 3 that refractive index was accurately measured at 972 nm for the undoped sample while no value was obtained by the prism couling method at that wavelength on the Yb3+-doped samples. We assume that it is related to the strong absorption of Yb3+ ion in this spectral region. Then, the refractive index of the undoped sample (as a function of wavelength) is comprised between those of the SYb08 and SYb12 samples, illustrating once again the complexity to represent its dependence on the glass chemical composition. Following the same reasoning as above, we would have indeed expected a higher refractive index for the undoped glass than for those doped with Yb3+ (because of a lower fluorine content). But it is clearly not the case here as one can see in Fig. 3. It can be assumed here that the density of the undoped glass, which is significantly lower than those of the Yb3+-doped glasses (Fig. S1), plays a more significant role. Further structural investigation is required to elucidate such behavior.

Figure 3: Measured linear refractive index (n) of the undoped and Yb3+-doped glasses as a function of wavelength. The data were fitted by using the Sellmeier’s dispersion formula. Full size image

Table 1 Elemental quantitative analysis (EPMA) of the undoped and Yb3+-doped samples compared with the theoretical values. Full size table

Electron probe micro analysis.

Electron probe micro analysis (EPMA) was carried out to identify and quantify the elemental composition of the prepared glasses. The experimental results along with the theoretical data are presented in Table 1. The synthesis process was performed at the same temperature (1100 °C) but with varying duration (1h30, 2h, 2h30, 3h, 3h30 and 4h) of the glass melting with increasing Yb3+ concentration. Note that the results presented in Table 1 are the mean value of five independent measurements on the same sample at different positions. It is worth mentioning that both theoretical and experimental F contents increase with increasing Yb3+ concentration whereas an opposite trend is observed in the case of the O content. To show the reproducibility of the synthesis process, the same glass (SYb02) was prepared three times by keeping all the conditions strictly identical (melting temperature and duration of glass melting are 1000 °C and 1 h, respectively) and the results are presented for the three samples in Table 2. The obtained maximum errors (%) in experimental results between the three samples when compared to theoretical values, indicate here an excellent repeatability of the sample preparation in the given conditions.

Table 2 Elemental quantitative analysis (EPMA) of three SYb02 samples prepared under identical conditions. Full size table

Absorption spectra

The UV-visible-near-infrared (NIR) absorption spectra of the undoped and SYb02 samples are presented in Fig. 4, showing a broad absorption band for the SYb02 sample centered at a wavelength of 975 nm which corresponds to the Yb3+:2F 7/2 → 2F 5/2 transition. The transmission spectra obtained for the other samples (see the supplementary information, Fig. S2) show very similar profiles with the same Yb3+ absorption band shape, except for its intensity which depends on the Yb3+ concentration, as plotted in the inset of Fig. 4. The inhomogeneously broadened absorption bands are due to the electronic transitions between the Stark sublevels of the ground (2F 7/2 ) and the excited (2F 5/2 ) levels as well as the strong electron-phonon interaction characteristic to the glassy host36. The quasi-linear variation of the integrated absorption band intensity observed with increasing Yb3+ concentration (inset of Fig. 4) indicates the presence of a similar local environment around the Yb3+ ions in all the investigated glasses.

Figure 4: UV-visible-NIR absorption spectra of the undoped and SYb02 samples. Inset shows a variation of the linear absorption coefficient, α (•, at 975 nm) and the integrated absorption band intensity (■, from 900 to 1100 nm) related with the Yb3+ band absorption as a function of its concentration in the SYb sample. Full size image

Photoluminescence quantum yield (PLQY)

The PLQY measurements were performed inside an integrating sphere coupled to an optical spectrum analyzer (OSA) with a multimode optical fiber and then determined using the method reported in refs 22, 25. The absolute photoluminescence spectra of the samples obtained under a laser excitation at 920 nm (510 mW of power), are presented as a function of their Yb3+ concentration in Fig. 5. As can be seen from Fig. 5, luminescence quenching is observed for Yb3+ concentration higher than 2 mol%, due to either an increase in the energy transfer or reabsorption by the Yb3+ ions. Reabsorption or radiation trapping effects are usual when dealing with Yb3+-doped glasses because of the overlap of their absorption and emission bands, directly related to the Yb3+ ion concentration, the sample thickness (2.3 mm for our samples) and the optical path length of the photons in the medium37,38.

Figure 5 Absolute photoluminescence spectra under laser excitation at 920 nm (laser power of 510 mW) of the Yb3+-doped oxyfluoride glasses as a function of Yb3+ concentration (the sharp peak at 920 nm corresponds to the laser excitation line). Full size image

The absorption and emission spectra of the SYb02 sample (measured inside and outside the integrating sphere) are presented in Fig. 6. As can be seen in Fig. 6, reabsorption effect is observed even for the SYb02 sample and is more predominant for samples with higher Yb3+ concentration (as shown in the supplementary information, Fig. S3). The emission peak position shifts towards longer wavelength and broadens due to reabsorption. The reabsorption and luminescence quenching effects observed for all the Yb3+ concentrations are illustrated in Fig. 7(a,b). At lower concentrations, the interaction or radiation exchange between the Yb3+ ions is significantly reduced and may become negligible. In Fig. 7(a), the absorbed radiation from the pump laser is re-emitted in the form of luminescence (photons) without heat generation, resulting in higher luminescence intensity. At higher concentrations, the interaction between the Yb3+ ions becomes stronger and their energy exchange leads to a decrease in photoluminescence intensity due to non-radiative (phonons) emission resulting in luminescence quenching effect, as schematized in Fig. 7(b). This also induces the Yb3+ ions luminescence reabsorption by the Yb3+ neighboring ions, resulting in a redshift of the emission band.

Figure 6: Normalized absorption and emission bands of the SYb02 sample showing their overlapping responsible for the reabsorption effect. The emission spectra were measured outside and inside the integrating sphere under laser excitation at 920 nm. Outside the sphere: the sample was excited at a depth of 1 mm from the sample surface and the luminescence was collected with a multimode fiber and measured with an OSA. Inside the sphere: the sample was excited within the integrating sphere and the absolute luminescence was collected with a multimode fiber and measured with an OSA. Full size image

Figure 7 Representation of the luminescence quenching effect of Yb3+ ions (•) in solids at (a) low and (b) high Yb3+ concentration. Excitation: solid red arrow, Emission: curved red arrow, and Non-radiative transition (heat generation): zigzag. Full size image

PLQY is evaluated by using both Eqs (2) and (5), giving similar values for each SYb sample as a function of Yb3+ concentration, as summarized in Table 1. The standard deviation of measurements is around ±0.11, which is typical of absolute PLQY measurements. Moreover, the acquisitions were repeated 5 times to ensure the consistency of the results. The highest PLQY value (0.99) was obtained for the SYb02 sample. Further increase of the Yb3+ concentration results in a PLQY decrease, owing to the concentration quenching effect. Then, the PLQY obtained for the SYb02 sample is comparable to that of Yb3+:YAG single crystal (containing 3 at.% of Yb3+)12 and Yb3+:ZBLANP glass6 in which optical cooling has been already demonstrated. It is worth mentioning that a high PLQY close to unity is one of the most important conditions in order to achieve a better cooling efficiency by removing successfully heat from the sample in every cooling cycle6. The background absorption coefficient (α b ) of the samples, measured with a 1300 nm wavelength laser by the calorimetric method described in our earlier works12,25 is reported in Table 3. One observes a background absorption increase with increasing Yb3+ concentration.

Table 3 Photoluminescence quantum yield (PLQY) of the Yb3+-doped samples is determined by using the Eqs (2) and (5), pump wavelength (λ P ), limiting wavelength (λ cutoff ) which separates the integration of N ipj and N epj (j = a,b and c), mean fluorescence wavelength (λ f ) which takes into account reabsorption, background absorption (α b ) determined at 1300 nm laser by using the calorimetry method described in refs 12,25. Full size table

The mean fluorescence wavelength (λ f ) is calculated by using Eq. (4). The laser cooling/reduced heating can be expected when the samples are excited at or above λ f 6. As can be seen from Table 3, the λ f value increases with increasing Yb3+ concentration due to reabsorption. The λ f is found to be 1003(1) nm for the SYb02 sample which is larger than that reported for the Yb3+:ZBLANP (995 nm, 1 wt% of Yb3+)6. Hence, as the SYb02 sample exhibits high PLQY and low background absorption when compared with the other investigated samples, it appears to be the best candidate for laser cooling application besides serving as a reference sample for PLQY measurements in the near-infrared region.

Decay curves

The luminescence decay curves of the Yb3+:2F 5/2 → 2F 7/2 transition were measured by exciting with 940 nm wavelength laser and monitoring above the 975 nm wavelength emission, as shown in Fig. 8. The luminescence lifetime (τ) of the Yb3+:2F 5/2 excited level was evaluated from a single exponential fit. It is observed that the τ of the Yb3+:2F 5/2 excited state shortens from 1.52 to 0.19 ms in the investigated glasses when Yb3+ concentration increases from 2 mol% to 20 mol%. These results indicate that the decrease in PLQY with increasing Yb3+ concentration is not only due to reabsorption but also to concentration quenching. The quenching of lifetime may be either due to multiphonon relaxation, energy transfer among the Yb3+ ions (diffusion limited)39 or direct coupling with OH− groups37. In the present study, since the amount of OH− groups is expected to be relatively constant in all the samples, it is assumed that the most dominant mechanisms for lifetime quenching are the energy transfer among the Yb3+ ions, as well as the multiphonon relaxation. The longest lifetime measured here is 1.52 ms for the SYb02 sample, which is longer than that reported for the Yb3+:YAG crystal (1.1 ms, for a concentration of 2.5 at.% Yb3+)40 but shorter than that measured for the Yb3+:ZBLAN glass (1.82 ms, for a concentration of 2 mol% Yb3+)41. The high PLQY, which is a key parameter for laser cooling process, of the SYb02 sample with lower Yb3+ concentration (2 mol%) indicates its higher potential for laser cooling. Longer lifetime is not an obstacle for cooling, but it is not desirable, since it can slow down the cooling process.

Figure 8 Decay curves for the Yb3+: 2F 5/2 → 2F 7/2 transition of the SYb samples as a function of Yb3+ concentration, under laser excitation at 940 nm. Full size image

Pump power dependence PLQY and lifetime studies

The pump power dependence of PLQY and lifetime measurements were performed on the Yb3+-doped glasses. Boconilli et. al. have reported on the pump power dependence studies of upconversion (UC, process consisting in the absorption of two or more photons of low energy followed by the emission of one photon of higher energy) PLQY in Er3+:β-NaYF 4 nanocrystals by considering the effect of reabsorption for solar cell applications42. This is the first time to the best of our knowledge that the pump power dependence PLQY of Yb3+-doped glasses for laser cooling prospective is reported by considering. The PLQY (and the intensity of NIR emission as well) always follows a linear dependence with the pump power in our Yb3+-doped oxyfluoride glasses, as presented in Fig. 9. The PLQY was found to be as high as 0.99 for 510 mW of laser power measured at the entrance of the integrating sphere. Three regions are distinguished in Fig. 9, the first region for the low powers, the second one for intermediate powers and the third one for high excitation powers. The PLQY follows a progressive increase at low excitation powers whereas it remains unchanged at the intermediate excitation powers. This can be explained by the fact that there is no influence of the absorbed power by the Yb3+ ions which means that reabsorption may play a crucial role for this behavior providing a low fluorescence escape efficiency. At higher powers, the PLQY progressively increases due to the enhanced absorption of Yb3+ ions within the unit area for a fixed concentration. Moreover, the bleaching occurring at high pump powers can decrease significantly reabsorption, inducing further increase in PLQY43. The upconversion effects from the Tm3+ and Er3+ ions present as impurity traces (detailed discussion in the next section) at high pump powers may also cause a slight deviation from a straight line.

Figure 9 Variation of PLQY with pump power for the 2 mol% Yb3+-doped glass. Full size image

The power dependence of lifetime for the Yb3+-doped glasses is shown in Fig. 10. As can be seen in Fig. 10, there is a small but consistent decrease of lifetime with increasing pump power, especially at higher Yb3+ concentration (20 mol%). This may be due to either a decrease in the lifetime and PLQY because of lower reabsorption due to bleaching effects43. The decrease in lifetime (Fig. 10) indicates that non-radiative and also upconversion processes should play an important role which means the non-radiative rate, W nr , increases with increasing the Yb3+ concentration. The most important channels on the non-radiative rate increase are energy migration among Yb3+ ions, followed by transfer to impurity centers: trapping by defects such as OH− radicals, radiation trapping of energy among Yb3+ ions, interaction between Yb3+ ions and the glassy host defects.

Figure 10 Power dependence of lifetime for SYb samples with different Yb3+ concentrations. Full size image

Upconversion luminescence

The UC emission spectra of glasses recorded under laser excitation at 975 nm (80 mW power) are shown in Fig. 11 while a photograph of the UC luminescence from the SYb05 sample is shown in inset. It is clear that a higher UC intensity at 478 nm is obtained for the SYb05 sample than for the other samples. All the samples exhibit UC emissions at 478 nm (1G 4 → 3H 6 ) and 800 nm (3H 4 → 3H 6 ) originating from Tm3+ impurity as well as 410 nm (2H 9/2 → 4I 15/2 ), 539 nm (2H 11/2 , 2S 3/2 → 4I 15/2 ) and 647 nm (4F 9/2 → 4I 15/2 ) originating from Er3+ ions also present as an impurity. Those ions are excited thanks to energy transfer (Addition of Photons by Transfer of Energy: APTE effect) from the Yb3+ ions which act as a sensitizer44. It was evidenced that the APTE effect is 105 times more efficient than the cooperative luminescence which is usual in Yb3+-doped samples at high concentrations for the same Yb3+-Yb3+ distances45,46. Due to this reason no cooperative luminescence was observed in these glasses even at high Yb3+ concentration. The pump power dependence of the UC luminescence is shown in Fig. S4. It is worth mentioning that no UC emission was observed from the Yb3+ free sample (as shown in Fig. S5). Therefore, it is clear that these Tm3+ and Er3+ traces (contents of respectively less than 10 and 8 ppm according to the certificate of analysis provided by the chemical supplier) originate from the YbF 3 starting powder, in spite of its relatively high purity (99.99%). Complete separation of RE ions during their manufacturing process to obtain ultra-high purity raw materials is indeed a well-known issue in the industry.