Enhanced upconversion lasing and luminescence is obtained in a transparent compound fluorosilicate glass co-doped with Yb3+ and Er3+ ions. The sample is prepared by a conventional melt-quenching technique followed by a heat treatment, and a very high upconversion efficiency (quantum yield >1%) is achieved. The physical processes involved start from a phase-separated, as-melt fluorosilicate glass consisting of fluorine-rich domains, which provide the low phonon energy environment for Er3+ ions. In addition, visible green lasing in a microsphere resonator with a diameter of 58 μm is observed with a relatively low lasing threshold of circa 52.5 μW. Importantly, when the input power increases to 16.8 mW in a microsphere with a diameter of 110 μm, the colors of the luminescent emissions became yellow-green and then yellow-red when the pump power is increased further to 40 mW. This presents a significant improvement in the development of visible light sources for microphotonics applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Infrared upconversion to red, green and violet light in Er3+ and Yb3+ ions has been widely investigated over the past three decades [1–5].The addition of multiple dopants can enhance the utility of laser glasses by increasing the range of spectral emission whilst simultaneously reducing the lasing thresholds of devices fabricated from these materials. One such class of device which has shown attractive optical properties is the rare-earth doped microspherical whispering gallery mode (WGM) laser [6–9]. This combination of active medium and resonant cavity has resulted in much research focusing on the characterization of doped microspheres and integration with structures for all-optical processing. The vast majority of work on active glasses found in the literature deals with bulk samples, waveguides, and to a lesser extent, microcavities, but most of them are based on silica (SiO2) host glass material [10,11].Traditionally, the suitability of compound fluorosilicate glasses as rare-earth host materials has been well-known as their low phonon energies determine multiphonon relaxation rates and, ultimately, the efficiency of upconversion processes [12,13].
Compared to fluoride glasses, most oxide glasses, e.g. SiO2, are much more chemically and mechanically stable, and are more easily fabricated into various shapes such as rods, optical fibers, or planar waveguides. Unfortunately, silica glasses are very inefficient for infrared upconversion because of their large phonon energy . It has also been shown that the addition of oxides into fluoride glasses to improve their stability is not ideal since even a small amount of oxides significantly quenches the upconversion luminescence . Rare-earth doped fluoride glasses generally have low phonon energy and superb optical emission efficiency and, when combined with an oxide glass matrix, namely fluorosilicate glass, they can provide high optical transparency and chemical/thermal stability, thereby becoming an optimum system for luminescence and laser applications [11,16,17].
In this work, we present an Er3+/Yb3+ co-doped transparent compound fluorosilicate glass with a very high upconversion efficiency (higher than 1%). The glass is shown to have improved chemical and mechanical stabilities with respect to similar fluorozirconate glasses and is an excellent host for rare-earth ions [17,18]. In the first study of fluorescence emissions from Er3+/Yb3+ co-doped compound fluorosilicate glass based microsphere resonators, we identified emissions over a spectral range extending from 400 nm to 700 nm, as well as lasing in the visible wavelength range. The visible lasing characteristics and upconversion spectra were investigated using a single-mode 980 nm laser diode pump with a spectral width of about 1 nm and under CW pumping conditions. Microspheres of diameter 50-150 μm were fabricated using a CO2 laser heating method. To achieve efficient coupling of the pump into the microsphere and efficient collection of the infrared lasing spectrum, a 980 nm single mode fiber was adiabatically tapered to 1 μm diameter using a “flame brushing” heating technique and used as a coupling waveguide . Green lasing emissions from a microsphere with a diameter of 58 μm were exceptionally strong when the intensity of the pump power was 9.6 mW and were clearly visible with the naked eye, making it interesting for biological applications. We also observed tunable multicolor luminescent emissions but only from a microsphere with a diameter of 110 μm, and the color, which was initially yellow-green, turned to yellow-red when the pump power was increased from 16.8 mW to 40 mW. This optical characterization might pave the way for the development of visible light sources for microphotonics applications. Also this Er3+/Yb3+ co-doped gain compound fluorosilicate glass microsphere offers substantial miniaturization, greatly reduces the power required for fluorescence and visible lasing, and allows ease of integration via a fiber taper-microsphere coupling system.
2. Glass fabrication and characterization of bulk samples
To fabricate the gain microspheres, we first fabricated the host fluorosilicate glass with the compositions (mol %): 70SiO2-15KF-15ZnF2, then we mixed the both Er2O3 and YbF3 with the host glass materials (mol %): 1.5YbF3-xEr2O3(x = 0.1, 0.2, 0.3, 0.4, 0.5). When progressively doping with 1.5 and x mol% of YbF3 and Er2O3, respectively, other chemical components including SiO2, KF and ZnF2 in the above glass formula were decreased proportionally. The obtained glass is further identified according to the doping levels 1.5 and x of their precursor glasses as 1.5Yb-xEr, respectively.
A 50 g reagent grade stoichiometric mixture of SiO2 (99.999%), KF (99.999%), ZnF2 (99.999%), YbF3 (99.999%) and Er2O3 (99.999%) was mixed thoroughly in an agate mortar and melted in a covered platinum rhodium crucible at 1600°C for 30 mins. The molten liquid was subsequently cast onto a brass plate and was pressed immediately with another cold plate. The resulting glasses were annealed at a temperature of 450 °C for 3 hrs. Finally, the bulk glass samples were cut and polished for further optical measurements. The refractive index of the compound glass sample was measured as 1.52, using an ellipsometric measurement system (VUISEL Plus, Horiba, Japan). Fluorosilicate glass fibers were drawn from the glass melt in a platinum rhodium crucible using a diamond tip, similar to that described in Ref . The glass fibers were prepared by quickly drawing at an appropriate glass melt viscosity and they were initially cleaned ultrasonically, followed by a number of hydrofluoric acid washes.
For characterizing the absorption properties of the Er3+/Yb3+ co-doped fluorosilicate glasses, the transmission spectra were measured using an ultraviolet-visible-NIR spectrophotometer (Lambda-900, PerkinElmer, USA). Figure 1 shows the transmittance of the Er3+/Yb3+ co-doped fluorosilicate glass sample with a composition of 70Si-1.5Yb3+-0.2Er3+; one can see that the fundamental absorption edge, due to inter-band transitions, is located around 260 nm. Sharp 4f–4f up-transitions due to absorption in the inner shells of Yb3+ and Er3+ are clearly seen, and each absorption peak is attributed to the electronic transition from the ground-state 4I15/2 to the indicated excited state of Er3+. The main absorption peak is located at 976 nm of Yb3+. Images of the Er3+/Yb3+ co-doped fluorosilicate glass samples are given in the inset to Fig. 1. The change in the transparency of the fabricated glass samples with different Er3+ doping concentrations is clearly visible, with the color varying from transparent to a light pink color when increasing the concentration of the Er3+ ions in the glass samples.
The emission spectra of the Er3+/Yb3+ co-doped fluorosilicate glasses were measured using an FLS920 fluorescence spectrometer (Edinburgh Instruments, UK). Figure 2 shows typical emission spectra of Er3+/Yb3+ co-doped fluorosilicate glasses with different Er3+ and Yb3+ concentrations under 980 nm excitation. From Fig. 2, one can see that the emission intensity is highly sensitive to the Er3+ dopant concentration; At relatively low doping concentration (0.2 mol%) the trend of the emission intensity reaches a maximal value. As the concentration of Er3+ increases, the emission intensity decreases, following saturation phenomena due to cross-relaxation of the excited energy. It can be also seen that there are three typical emission bands around 523 nm (Green1(G1) band), 545 nm (Green 2 (G2) band), and 660 nm (Red(R) band), which are transitions, respectively, from 2H11/2, 4S3/2, and 4F9/2 to the ground state, as illustrated in the energy level diagram (Fig. 3). Note that in Fig. 2, the intensities of the emission peaks have been normalized in order to have relative comparisons of these emission bands among different glass samples. Overall, the emission of the R band shows the highest intensity among the three emission bands. In order to get a closer physical insight into the upconversion luminescence microscope mechanisms, the emission bands may be explained using the upconversion luminescent mechanism as outlined in Fig. 3. Initially, Yb3+ ions transfer the absorbed 980 nm photon energy to Er3+ ions via energy transfer processes, leading to a population of the 4I11/2 level. Absorption of a second 980 nm photon by an Er3+ ion or another energy transfer from an Yb3+ ion can populate the 4F7/2 level of the Er3+ ion. The Er3+ ions first relax non-radiatively to the 4H11/2 level. Moreover, blue emission corresponding to the 2H9/2 to 4I15/2 level transition is possible. Because of the very narrow energy gap between the 4S3/2 and 2H11/2 levels, the Er3+ ions immediately relax to the 4S3/2 level as shown in Fig. 3. As soon as the 4S3/2 level is populated, the 2H11/2 level is then thermally populated owing to Boltzmann’s distribution and the green emissions of 2H11/2 → 4I15/2 (G1 band) and 4S3/2→4I15/2 (G2 band) occur. Alternatively, the ions can further relax and populate the 4F9/2 level, leading to the 4F9/2 → 4I15/2 transitions (R band) and having two-photon excitation. The 4F9/2 level may also be populated from the 4I13/2 level of the Er3+ ion by absorption of a 980 nm photon, or energy transfer from an Yb3+ ion, with the 4I13/2 state being initially populated via the non-radiative relaxation of 4I11/2.
In order to better characterize the highly efficient upconversion and compare it to that of other gain compound glass samples, a number of different Er3+/Yb3+ co-doped glass and glass ceramic samples were fabricated:
- 1) NaYF4 glass ceramic (GC) sample: An Er3+ and Yb3+ codoped nano-scaled oxyfluoride glass-ceramics with composition of (mol%) 40(SiO2)-25(Al2O3)-18(Na2CO3)-10(YF3)-7(NaF) was prepared.
- 2) LaF3 GC sample: A LaF3 glass-ceramics sample with composition of 45SiO2-12Na2O-23Al2O3-20LaF3 (mol%) was prepared.
- 3) Tellurate glass (GL) sample: A tellurate glass sample with composition of 60TeO2-30WO3-10La2O3 was prepared.
In order to verify the results in Fig. 4, we also measure the quantum yield of fluorosilicate glass samples. The resulting upconverted emission was collected using an integrating sphere and excitation was at 980 nm. Here, the upconversion quantum yield is defined as the total number of visible-wavelength photons emitted divided by the number of NIR photons absorbed. The quantum yield as a function of the excitation power is shown in Fig. 5. One can see that the quantum yield increases as a function of increasing excitation power with an excitation density of 20 ± 0.5 W/cm2; however, the quantum yield reaches a maximum of about 1.04% when the excitation power is 211.2 mW. This quantum yield is one order of magnitude higher than the quantum yield reported in Ref .
3. Theoretical analysis
The Yb3+/Er3+co-doped fluorosilicate glass shows highly efficient upconversion, indicating that the structures of fluorosilicate glass provide a local environment with low phonon energy for the luminescence center of Er3+ . To investigate the structures of such fluorosilicate glass, we employed customized molecular dynamics (MD) simulations to reproduce the structural evolution of a 70SiO2-15KF-15ZnF2 glass system. A cubic cell containing 3652 atoms was defined in the calculation and was subjected to periodic boundary conditions. The cubic cell has a volume of (26.845 Å)3 based on the experimental density of 2.703 g/cm3. We used a Born-Mayer interatomic pair potential of the form :24]. To avoid atoms overlapping, the initial atomic coordinates were randomly distributed with constraints between ion pairs. The time step for this simulation was set at 1 fs. The glass was first equilibrated at 4000 K with 20.000 time steps, then quenched to 293 K with 100,000 time steps. A final relaxation, with 20,000 time steps, was carried out at room temperature (293 K). The glass structural information, such as coordination number and bond length, can be extracted from the last configuration of the simulated model.
First, we simulated the distribution of fluorine (F) and oxygen (O) elements, as shown in Fig. 6 (a) and (b). We find that the F element is heterogeneously distributed inside the glass matrix, while the distribution of the O element is homogeneous. The size of the F-rich region is around 2 nm, and it is expected to be quenched from the melt, starting from a liquid–liquid phase separation . Figures 6 (c) and (d) show the MD results of the bond length and coordination number of Si-F and Si-O bonds. A study of the coordination feature of the glass indicates that the glass network is mainly composed of the structural units of [SiO4] and [SiOX]F4-X. For determining the amorphous or crystalline phase of the F-rich heterogeneous structures, X-ray diffraction patterns were measured, see Fig. 7. There is no evidence of sharp diffraction peaks originating from the glass, indicating that the F-rich heterogeneous structures are amorphous.
It is well known that, in comparison with the oxide environment, the fluoride environment shows lower phonon energy; this is beneficial to the highly efficient upconversion of Er3+ ions . In order to confirm if Er3+ ions are distributed within the F-rich environment, we performed the Judd-Ofelt (J-O) theory to analyze the local environment around the Er ions. The calculation method for the J-O theory has been described elsewhere [27,28]. According to the absorption spectra, the J-O intensity parameters Ωλ (λ = 2, 4, 6) were calculated to be 2.19 × 10−20 cm2, 0.86 × 10−20 cm2, and 1.2 × 10−20 cm2, respectively. The root-mean-square value is on the order of 10−7, indicating that the J-O analysis was performed in good accordance. The parameter Ω2 is associated with the local environment configuration, and it is especially sensitive to the covalency of the chemical bond. In a fluoride environment, the host ions (such as Er3+ in our case) are mainly coordinated by F- ions. A more ionic bond environment of the host ions can be expected in comparison with that in an oxide environment. Hence, the value of Ω2 in a fluoride environment should be smaller than that in an oxide environment . In our case, Ω2 of the Er-doped glass shows a substantially smaller value of 2.19 × 10−20 cm2 compared with the values reported for Er3+ doped oxide glass (3.0-6.0 × 10−20 cm2) . Therefore, we can conclude that the Er3+ ions should be incorporated into the F-rich heterogeneous structures, and the low phonon energy provided by this structure facilitates Er3+ ions to perform highly efficient upconversion.
Furthermore, using the relative J-O parameters, we calculated the electric-dipole transition probability (AED) ratios for the visible Er3+ transitions (2H9/2 → 4I15/2; 2H11/2 → 4I15/2; 4S3/2 → 4I15/2; and 4F9/2 → 4I15/2), as presented in Fig. 8. The 2H11/2→4I15/2 transition shows the highest AED in the glass matrix. This result also suggests that the Er3+ ions are distributed within the fluoride environment, thereby reducing the probability of non-radiative relaxation from 2H11/2 to other levels (e.g. 4S3/2, 4F9/2) and improving the population density of the 2H11/2 level.
4. WGM lasing
To fabricate the Er3+/Yb3+ co-doped fluorosilicate glass microsphere, an Er3+/Yb3+ co-doped fluorosilicate glass fiber was first prepared by vertically suspending it on a three-dimensional adjustable mount. A small region of the fiber was heated by a CO2 laser beam focused to about 150 µm. The laser power was adjusted until the glass fiber rapidly elongated into a taper. The waist diameter of the taper was controlled by adjusting the distance between the glass fiber and the focus of the CO2laser beam; a taper diameter of about 3 µm was selected and the fiber was cut at this region by illuminating it with the focused spot of the CO2 laser. Next, the tip of the half-taper was heated by adjusting the position of the CO2 laser beam until it was close to the melting temperature of the fluorosilicate glass, about 1550°C. The surface tension of the fluorosilicate glass creates a microsphere at the tip. Using this method, we made Er3+/Yb3+ co-doped fluorosilicate glass microspheres with a range of diameters from 50 to 150 µm, with the final diameters of the microspheres being determined by the size of the diameter of the initial fiber tip. Finally, the Er3+/Yb3+ co-doped fluorosilicate glass microspheres were used as gain matrices as well as resonant cavities to realize up-conversion lasers.
Figure 9 shows a typical lasing spectrum (as collected from the output pigtail of the coupling fiber taper) recorded on an optical spectrum analyzer (Advantest Q8341, Tokyo, Japan) with a limited resolution of 0.05 nm. The left inset shows the bright green emission image of the Yb3+/Er3+codoped fluorosilicate glass microsphere (diameter circa 58 µm). From this figure, one can see that a single ring encircling the equator of the sphere suggesting that the pump light was coupled to a fundamental sphere mode (m = l). The spatial distribution of the pump modes defines the active gain region and provides a potential mechanism for wavelength selection in the laser cavity since only those laser modes with maximum spatial overlap with the pump modes can achieve enough gain to initiate lasing. In general, a fundamental pump mode is most likely to excite a fundamental laser mode. Further understanding of the ring modes of the pump wave could be worth investigating in the future. The right inset in Fig. 9 demonstrates the occurrence of a saturation process for the generation of the upconversion; note that the green lasing appears to be only in the outer ring, but may purely be an artefact of the saturation of the central sections of the microsphere resonator.
As seen in Fig. 9, the wavelength of the single-mode lasing peak is around 545.57 nm. Lasing emission from the microsphere was observed by tuning the pump wavelength to a pump band resonance of the cavity and increasing the pump power to an approximate lasing threshold of 52.5 μW (total pump power absorbed and scattered by the microsphere, namely, Pin-Pout, which collected by the fiber taper). This threshold is overestimated since the absorbed pump power (circa 0.55% of the total input pump power of 9.6 mW) was calculated without accounting for losses due to scattering. Future work should focus on refining the experimental setup to allow for more precise measurements of the loss mechanisms occurring in the microsphere-fiber taper system, especially surface scattering from the microsphere, as it appears to be by far the dominant loss mechanism for the pump. To obtain a high power transfer to the cavity from the fiber taper, light coupling into the microsphere should be improved. For this work, improving the coupling coefficient of the taper-sphere system relies on the phase matching condition, i.e., simply obtaining size matching between the microsphere resonator and the fiber taper, as discussed in Ref . The polarization state of the pump source was adjusted using a polarization controller. However, the exact polarization state at the coupling fiber waist could not be determined with complete certainty due to the uncertainties surrounding the light polarization at the waist of ultrathin fibers. The output laser power collected from the fiber taper as a function of estimated total pump power absorbed is presented in Fig. 10. Below the pump threshold power at 52.5 μW, the light emission from the microsphere presents as spontaneous radiation.
Figure 11 shows the luminescent emission images of a different Yb3+/Er3+ co-doped fluorosilicate glass microsphere (diameter circa 110 µm) at different pump powers from a980 nm laser diode. One can see that, when the pump power increases to 16.8 mW, the colors of the luminescent emissions from the microsphere became yellow-green, see Fig. 11 (b), yellow when the excitation power is increased to 25 mW and yellow-red when the pump power is increased further to 40 mW. The luminescent emission spectra output from the fiber taper are presented in Fig. 12 and were acquired using an optical spectrum analyzer (Advantest Q8341, Tokyo, Japan). It follows that the color of the emitted light signal can be controlled by varying the 980 nm pump source power. In the case of both the two and three-photon contributions to the red emission, the Yb3+-Er3+ interaction favors populating the red emitting state, and hence the emission from 70SiO2-15KF-15ZnF2: Er3+ (0.2%) is predominantly red. As a traditional way to tune the color of emission, by reducing the Yb3+ concentration, and/or incorporating small amounts of Er3+, it is possible to increase the yellow-to-red intensity ratio. Alternatively, the emission can be changed from yellow (at the low 980 nm pump power) to red (at the high 980 nm pump power) as a result of the dominance of the red emission at the higher power pump output. Moreover, the blue emission centered at 408 nm (due to the 2H9/2→4I15/2 transition) becomes more intense in the emission spectrum C of the microsphere when the pump power reaches 40 mW. It is expected that the behavior of the tunable up-conversion to multiple colors of the compound glass of this investigation has potential applications in color displays, bio-labels and bio-imaging.
In conclusion, a transparent 70SiO2-15KF-15ZnF2 fluorosilicate glass co-doped with Er3+/Yb3+ was fabricated and characterized in bulk and as a WGM resonator. The upconversion luminescence of Er3+/Yb3+ co-doped in this compound glass was significantly enhanced in comparison with a number of glass ceramic and glass samples, such as NaYF4, LaF3 glass ceramics and tellurate glass. A single-mode green lasing emission with a low lasing threshold of 52.5 μW was observed from a fluorosilicate glass microsphere resonator with a diameter of 58 μm. In addition, tunable multicolor luminescence emissions have also been obtained for a microsphere resonator with a diameter of 110 μm. Owing to the presence of a high concentration of SiO2 material, this glass material is expected to be more chemically and mechanically stable and, importantly, also has a higher laser damage threshold than single crystal or conventional fluoride glass, such as ZBLAN and ZBLA glass [31,32]. Furthermore, unlike fluoride glass or crystals, this transparent fluorosilicate glass with demonstrates a high upconversion efficiency can be easily fabricated using a traditional melting-quenching method in a standard laboratory since it does not need any inert atmosphere protection. Further enhancement of the upconversion efficiency in this compound fluorosilicate glass may be expected through optimizing the composition along with refining the melting-quenching fabrication method, thermal treatment conditions, and simultaneous multi-wavelength excitation. Finally, due to the ultra-low absorption loss of this novel fluorosilicate glass over the wavelength ranges of UV-VIS-NIR, it is expected to have potential applications in fiber laser devices involving long optical path lengths.
National Key R&D Program of China (2016YFE0126500); National Natural Science Foundation of China (NSFC) (61575050); Key Program for Natural Science Foundation of Heilongjiang Province of China (ZD2016012); Open Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2016KF03); 111 project(B13015).
P. Wang gratefully acknowledges the Recruitment Program for Young Professionals (The Young Thousand Talents Plan). This work was partly supported by Okinawa Institute of Science and Technology Graduate University.
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