1 Introduction

For the past decades, rare-earth (RE) ions doped infrared-to-visible frequency upconversion (UC) luminescence materials have attracted much attention due to their versatility in various optoelectronic devices, such as optical sensors, lasers, solar cells, and compact light sources [1–10]. High-quality red, green and blue waveguide-typed irradiation light sources with high power density and compact structure are extremely desired for their potentials to replace conventional lighting sources in medical and biochemical [11–17]. Among the trivalent RE ions with 4 f n electronic configuration, Ho3+ and Er3+ doped materials have drawn much attention, as their visible emissions in the red and green regions caused by successive absorption of infrared photons, leading the population to higher energy levels by UC [18–22]. Yb3+ ion is generally employed as a sensitizer to increase the emission intensity and pump efficiency for Ho3+ and Er3+ ions doped materials under ∼980 nm commercial diode lasers [23–25].

In developing practical waveguide-typed optoelectronic devices, at present, related researches have been carried out on oxide glass host that possesses excellent chemical stability and preferable acid resistance [26–28]. Of the oxide glasses, tellurite and heavy-metal glasses are attractive for their lower phonon energies [29–31]. However, the ultralow transition temperature and the large crystallization tendency limit their possibilities in making favorable K+-Na+ ion-exchanged planar waveguides [32]. Fortunately, germanium-containing glasses with good capacity for dissolving RE ions, relatively lower phonon energies (∼820 cm−1) and perfect photosensitivity enable the developing of effective UC from infrared to visible and integration of waveguide light source [33–40]. As is known, photon quantification is of benefit to selecting potential optoelectronic materials. Previous relative methods adopting spectroradiometer and goniophotometer can reflect fluorescence characteristics to some extent [41]. In spite of that, absolute evaluation for UC photons of RE ions is paid attention and it will give an intuitive reference for the improvement and advancement on optical properties of practical optoelectronic devices.

In this work, the quantitative characterization of visible UC photons is carried out in Ho3+/Yb3+ and Er3+/Yb3+ doped waveguide-adaptive aluminum germanate (NMAG) glasses using an integrating sphere for absolute measurements, including spectral power distribution, photon distribution and quantum yield (QY). Efficient two-photon red and green UC emissions generated from Ho3+ and Er3+ in samples have been investigated under 975 nm laser excitation, and the relation of UC luminescence intensity and the pumping power was discussed. The QYs of red and green (660 and 548 nm) UC are 2.41 × 10−5 and 0.17 × 10−5 in Ho3+/Yb3+ doped NMAG glasses, and the ones (665 and 548 nm) in Er3+/Yb3+ doped NMAG glasses are 4.26 × 10−5 and 1.44 × 10−5 when the excitation power density is 1227 W/cm2. The macroscopic quantization of UC photons in Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses reveals a potential application prospect in developing visible optoelectronic devices.

1.1 Experiments

NMAG glasses were prepared from high-purity Na2CO3, MgO, Al2O3, and GeO2 powders according to the molar composition 23:2:22:53. Additional 0.8 wt% Ho2O3 and 2.4 wt% Yb2O3, and 0.8 wt% Er2O3 and 2.4 wt% Yb2O3 were added based on the host weight. First, the raw materials were well grinded in agate mortar and preheated in pure alumina crucible at 1200 °C for 4 h, then at a higher melting temperature of 1550 °C for 3 h. Second, the molten glasses were poured onto an aluminum plate and annealed at 550 °C for 4 h, and afterwards allowed to cool down slowly to room temperature. Before preparing the K+-Na+ ion-exchanged waveguides, Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glass substrates were optically polished and cleaned. Slab waveguides were fabricated by thermal ion-exchange processes at 370 and 390 °C for 4 h with pure molten KNO3, respectively. After cooling down to room temperature, the ion-exchanged samples were rinsed carefully for further optical and spectral measurements.

The densities of Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glass samples were both measured to be 3.21 g⋅cm−3 by Archimedes method, and the number densities of Ho3+, Er3+ and Yb3+ ions are derived to be 0.793×1020, 0.784×1020 and 2.282 × 1020 cm−3. Using the Metricon 2010 prism coupler, the refractive indices of Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses were measured to be 1.5805 and 1.5798 at 635.96 nm, and 1.5624 and 1.5617 at 1546.9 nm, respectively. At all other wavelengths, the refractive indices of the two samples can be derived by Cauchy equation \(n=A+B/{{\lambda }^{2}}\) [42] with A = 1.5587 and 1.5580, and B = 8824 and 8785 nm2, respectively. Absorption spectra were recorded with a PerkinElmer UV/VIS/NIR Lambda 950 double-beam spectrometer. The spectral power distributions of glass samples were measured using integrating sphere (Labsphere) with 3.3-inch diameter, which was connected to a CCD detector (Ocean, USB4000) with 600 μm-core optical fiber, and the fiber pigtailed 975 nm laser was used as the pump source. The schematic diagram of experimental setup is depicted in Fig. 1. The luminescence pictures were taken using a Sony DSLR-A200 digital camera. All the measurements were carried out at room temperature.

Fig. 1
figure 1

Schematic diagram of experimental setup

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2 Results and discussion

2.1 Thermal ion-exchange applicability

The refractive indices as a function of diffusion depth at 635.96 nm for Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses slab waveguides are presented in Fig. 2a, b, which were obtained from the measured mode indices using an inverse Wentzel–Kramer–Brillouin (IWKB) method [43]. Figure 2c, d depict typical measurement results of slab waveguide, of which the peaks represent the detected transverse electric (TE) modes of waveguide. Here, at least three complete TE modes at 635.96 nm are demonstrated both in Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glass slab waveguides, which means that NMAG glass substrate is feasible to fabricate visible waveguide devices.

Fig. 2
figure 2

Intensity of reflected light versus index value with 635.96 nm laser source for a Ho3+/Yb3+ doped NMAG glass slab waveguide, and b Er3+/Yb3+ doped NMAG glass slab waveguide. Index profiles at 635.96 nm for c Ho3+/Yb3+ doped NMAG glass slab waveguide by ion-exchanging at 370 °C for 4 h, and d Er3+/Yb3+ doped NMAG glass slab waveguide by ion-exchanging at 390 °C for 4 h

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The indices of the slab waveguide surface n 0 and substrate n sub were derived to be 1.5884 and 1.5805 at 635.96 nm for Ho3+/Yb3+ co-doped NMAG glass slab waveguide, and 1.5867 and 1.5798 at 635.96 nm for Er3+/Yb3+ co-doped one. The maximum refractive index change \(\Delta {{n}_{\text{max}}}={{n}_{0}}-{{n}_{\text{sub}}}\) in the two cases was calculated to be 0.0079 and 0.0069, respectively. Effective diffusion depth d was fitted by the formula \(n(x)={{n}_{\text{sub}}}+\Delta {{n}_{\text{max}}}\times \exp (-{{x}^{2}}/{{d}^{2}})\), where x is the depth from surface of the glasses. The effective diffusion coefficient D e was calculated by the equation \(d=2\sqrt{{{D}_{\text{e}}}t}\), where t is the diffusion time. Table 1 lists K+-Na+ ion-exchanged parameters of various glasses, and D e values for Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses are smaller than those in Corning 0211 borosilicate [44] and NMAP [45], larger than those in BK7, PYREX7740 [46] and Microscope slide (Fisher Premium) glasses [44], and close to the values in NMAG: Dy3+ glasses [47], indicating that K+-Na+ ion-exchange process is temperate and controllable in the NMAG glasses.

Table 1 Ion-exchange parameters of various glasses
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2.2 Absolute spectral power and upconversion (UC) photon quantification

Integrating sphere coupled with CCD detector measurement was applied for absolute spectral parameter characterization, which provides external QY to assess luminescence materials. Powers of 975 nm laser were selected to be 375, 501, 603, 686, 770 and 853 mW, respectively, and excitation power densities of laser at the point of incidence on sample surface were identified as 539, 721, 867, 987, 1108 and 1227 W/cm2. The spectral power distributions of UC in Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses are presented in Figs. 3 and 4. Three visible emission bands at 548, 660 and 755 nm in Ho3+/Yb3+ co-doped NMAG glasses are observed, in which the 660 nm red one is far greater than the other two bands and plays a dominant role in UC emissions. In Er3+/Yb3+co-doped NMAG glasses, the 548 nm green emission band is attribute to 2H11/2+4S3/2 → 4I15/2, and the larger 665 nm red one corresponds to the 4F9/2 → 4I15/2 transition [48–51]. The integrated spectral powers are labeled in Table 2, in which the luminous intensity is identified as a positive correlation with laser power density. When excited laser density is up to 1227 W/cm2, emitting powers of green and red emissions are 2.36 and 28.03 µW for Ho3+/Yb3+ doped NMAG glasses, and 20.15 and 47.86 µW for Er3+/Yb3+ doped NMAG glasses, respectively. Although the ratios between red and green emission intensities are much smaller in the Er3+/Yb3+ doped case, the integrated spectral powers of red emissions in Er3+/Yb3+ doped NMAG glasses are obviously larger than those in Ho3+/Yb3+ doped NMAG glasses.

Fig. 3
figure 3

a–f UC spectral power distributions of Ho3+/Yb3+ doped NMAG glasses under various laser powers densities. Inserted photos: visible fluorescence of the samples under related excitation powers densities

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Fig. 4
figure 4

a–f UC spectral power distributions of Er3+/Yb3+ doped NMAG glasses under various laser powers densities. Inserted photos: visible fluorescence of the samples under related excitation powers densities

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Table 2 Integrated spectral power for each emission band
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Net absorption and emission photon distributions derived from net spectral power distributions of Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses are shown in Figs. 5 and 6, and the integrated values are listed in Tables 3 and 4, respectively. With excitation power densities become larger, the net absorption photons are on the increase, and total net emission photons are also rise from 2.28 × 1013 cps to 10.03 × 1013 cps and 4.75 × 1013 cps to 21.57 × 1013 cps for Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses, respectively. In all pumping cases, the tendency of net radiation photon distributions is the same both in Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses, as the red emission is consistently the dominant one. When excitation density reaches to 1227 W/cm2, the net absorption photons are 3.84 × 1018 and 3.85 × 1018, and the net radiation photons of the red emissions at 660 and 665 nm for Ho3+ and Er3+ are 9.26 × 1013 and 16.03 × 1013, respectively. Under all the laser power densities, the ratios between red and green emission photon numbers in Ho3+/Yb3+ doped case are above four times larger than the values in Er3+/Yb3+ doped case, which results in the visual differences.

Fig. 5
figure 5

a–f Net UC radiation photon distributions of Ho3+/Yb3+ co-doped NMAG glasses under various laser powers densities. Inset: related net absorption photon distributions

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Fig. 6
figure 6

a–f Net UC radiation photon distributions of Er3+/Yb3+ co-doped NMAG glasses under various laser powers densities. Inset: related net absorption photon distributions

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Table 3 Net absorption and emission photon numbers in Ho3+/Yb3+ co-doped NMAG glasses
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Table 4 Net absorption and emission photon numbers in Er3+/Yb3+ co-doped NMAG glasses
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The relationship between integrated UC emission photons and excitation power densities appear to be straight lines in a log–log plot as presented in Fig. 7. The slopes of three transition emissions at 548, 660 and 755 nm for Ho3+/Yb3+ doped NMAG glasses are 2.02, 1.79 and 2.02, respectively, and the values at 548 and 665 nm for Er3+/Yb3+ doped NMAG glasses are 2.15 and 1.75, respectively, which demonstrate these above UC transitions are attribute to two-photon excitation process. In Ho3+/Yb3+ and Er3+/Yb3+ co-doping cases, Yb3+ acts as an effective sensitizer for enhancing the UC emission efficiency during radiation pumping at the wavelength of 975 nm, because Yb3+ possesses a large absorption cross-section and a broad absorption band at around 980 nm, and its excited energy level 2F5/2 matches with the excited energy level 5I6 of Ho3+ and 4I11/2 of Er3+. As a consequence, Yb3+ can absorb 975 nm radiation energy effectively and transfer a part of energy to Ho3+ and Er3+ in the two-photon excitation processes.

Fig. 7
figure 7

Dependence of net emission photon on excitation power density: a Ho3+/Yb3+, b Er3+/Yb3+ doped NMAG glasses

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2.3 Radiative parameter and quantum yield

To further explore the radiative transition properties of Ho3+ and Er3+, Judd–Ofelt (J–O) theory analyses were carried out based on the absorption spectra of Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses, as depicted in Fig. 8a, b, respectively. Derived from the electric-dipole contributions of the experimental oscillator strengths by a least-square fitting approach, the J-O intensity parameters Ω t (t = 2, 4, 6) of Ho3+ are obtained to be 7.43 × 10−20, 2.96×10−20 and 1.00×10−20 cm2, and the J-O intensity parameters Ω t (t = 2, 4, 6) of Er3+ are 7.17×10−20, 1.98×10−20 and 0.85×10−20 cm2, respectively. The intensity parameter Ω 2 has been identified to be associated with the asymmetry and the covalence of lanthanide sites [52–55]. In Ho3+/Yb3+ and Er3+/Yb3+ doped NAMG glass system, the Ω 2 value is relatively larger than those of silicate, soda-lime silicate, borate and tellurite glasses, indicating strong asymmetrical and covalent environment around Ho3+ and Er3+ ions in the NAMG glasses [56–60].

Fig. 8
figure 8

Absorption spectra of a 0.8 wt% Ho2O3 and 2.4 wt% Yb2O3, and b 0.8 wt% Er2O3 and 2.4 wt% Yb2O3 doped NMAG glasses

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Using the Ω t values, some important radiative properties, including spontaneous transition probabilities (\({{A}_{\text{ed}}}\) and \({{A}_{\text{md}}}\)), branching ratios (\(\beta\)), and radiative lifetime (\({{\tau }_{\text{rad}}}\)) for the optical transitions of Ho3+ and Er3+ in NMAG glasses are calculated and listed in Tables 5 and 6. For Ho3+ ions in NMAG glasses, the branching ratios for the transitions of 5F4 → 5I8, 5S2 → 5I8 and 5F5 → 5I8 are 76.4, 54.2 and 77.2%, respectively, which are much larger than the other transitions. For Er3+ ions in NMAG glasses, the branching ratios of 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 are 67.3 and 90.9%, respectively. Thus, it supports the obtaining of efficient green and red emissions in the Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses under the suitable excitation.

Table 5 Predicted emission probabilities, branching ratios, and radiation lifetimes of Ho3+ in NMAG glasses
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Table 6 Predicted emission probabilities, branching ratios, and radiation lifetimes of Er3+ in NMAG glasses
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The photoluminescence quantum yield (QY) for the conversion efficiency of net absorption photon number into net emission photon number is served as a key external parameter for evaluating luminous characters of material [61, 62]. Here, absolute photon identification method is applied for QYs, and original data for each emission peaks in Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses are given in Table 7. In Ho3+/Yb3+ co-doped NMAG glasses, with the pump power densities increase from 539 to 1227 W/cm2, QYs become larger especially for the green (548 nm) and red (660 nm) emissions, which rise from 0.70 × 10−6 and 12.44 × 10−6 to 1.69 × 10−6 and 24.11 × 10−6, respectively. Under all the laser power densities, the QYs of red emissions are over 14 times higher than the green emissions, illustrating the dominant position of the red emission. For Er3+ ions in NMAG glasses, when excitation power densities increase from 539 to 1227 W/cm2, QYs of green and red emissions change from 5.46 × 10−6 and 21.84 × 10−6 to 14.39 × 10−6 and 42.64 × 10−6, respectively. Although the red emissions are strong in Er3+/Yb3+ doped NMAG glasses, the ratios of red/green emission QYs are smaller, which results in the green fluorescence to naked eyes. Overall, in the NMAG glasses with relatively lower phonon energies, the red emission QYs of the Er3+/Yb3+ doped case are about 1.7 times higher than that in the Ho3+/Yb3+ doped one under all the pumping power densities. The obtained QYs comparison of Ho3+ and Er3+ in lower phonon-energy NMAG glasses in this work is helpful to improve and advance optical properties of visible optoelectronic devices.

Table 7 Quantum yields of Ho3+/Yb3+ and Er3+/Yb3+ doped NMAG glasses excited by 975 nm laser
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For comparing the QY of Er3+ in different glasses, the UC efficiencies of other oxide and fluoride glasses are listed in Table 8. It can be seen that the QY (green) in NMAG glasses is higher than those in silicate and phosphate glasses [63], lower than that in fluoride glasses [63], and comparable to the values in calcium aluminum germanate [64] and lead tellurium germanate glasses [65]. The absolute quantitative parameter of UC photons in Er3+ doped NMAG glasses provides a reliable evidence for comparing the capacity of luminous radiation in various Er3+ doped luminescence materials.

Table 8 Quantum yields of Er3+ in various glasses
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3 Conclusion

Visible upconversion (UC) photon quantification has been performed in Ho3+/Yb3+ and Er3+/Yb3+ doped waveguide-adaptive aluminum germanate (NMAG) glasses by absolute spectral power test. Efficient red and green UC emissions of Ho3+ and Er3+ were captured and illustrated in contrast under the excitation of 975 nm laser with different power density. The QYs are positively correlated with laser power densities, and when pump power density is at the maximum 1227 W/cm2, the corresponding QYs for red and green UC emissions are calculated to be 2.41 × 10−5 and 0.17 × 10−5 in Ho3+/Yb3+ doped NMAG glasses, and the ones in Er3+/Yb3+ doped NMAG glasses are deduced to 4.26 × 10−5 and 1.44 × 10−5, respectively. The accurate quantification of UC photon demonstrates that Ho3+- and Er3+-doped waveguide-adaptive NMAG glasses hold great promise for developing compact waveguide-typed irradiation light sources, tunable visible lasers, and solid-state display devices.