Seven-photon ultraviolet upconversion emission of Er³⁺ induced by 1,540-nm laser excitation

Abstract

The multi-photon ultraviolet upconversion emission properties and synergistic effect are investigated in BaSr₂Y₆O₁₂:Er³⁺ phosphor. The deep-ultraviolet emissions centered at 274, 297 and 324-nm are observed under the 1,540-nm excitation, which results from a seven-, six- and six-photon upconversion process, respectively. A synergistic effect is found, which shows that the red emission intensity under 351- and 1,540-nm dual excitation is 4.7 % time stronger than the sum of red emission intensities under the 351 and 1,540-nm single excitation. This phenomenon is attributed to the ⁴I_13/2 and ⁴I_11/2 levels of Er³⁺ from non-radiative transition process under the 351-nm excitation are excited again to ⁴F_9/2 level by absorbing 1,540-nm photon in the 351- and 1,540-nm dual-excitation process.

1 Introduction

In recent years, the upconversion luminescent properties research has been dominated by the conversion of infrared to visible region [1–3]. While the ultraviolet upconversion emission under 1,540-nm excitation is still greatly restricted, populating the high-energy states needs a multi-photon upconversion process. In the previous reports, the deep-ultraviolet upconversion emissions of Gd³⁺ and Nd³⁺ were detected under the excitation of near-infrared or visible laser [4–7], and the ultraviolet upconversion emission in the region of 270–330 nm from Er³⁺ ions which was sensitized by Yb³⁺/Gd³⁺ in fluoride material hosts under 1,560-nm excitation has also been reported. These high-lying excited levels were populated through excited state absorption (ESA) and complex energy transition (ET) processes [8, 9].

Among the rare-earth ions, trivalent erbium ions (Er³⁺) provide several long-lived intermediate levels as well as metastable high energy levels, and the energy gap between the ground level ⁴I_15/2 and the first excited level ⁴I_13/2 is 6,400 cm⁻¹, which matches well with the absorption of 1,540-nm photon [8, 9]. Also, the wavelengths around 1,540 nm have a relative strong peak in the air-mass 1.5 global (AM1.5G) spectrum [10]; therefore, investigating the upconversion emission properties under the excitation of 1,540 nm may have a potential application value to reduce the solar cells’ sub-bandgap losses and optical data storage, color display, infrared sensor and so on [11–17].

In this paper, the single-phase BaSr₂Y₆O₁₂:Er³⁺ powders are prepared by solid-state reaction method for the first time, and the deep-ultraviolet upconversion emissions are detected under 1,540-nm excitation. Particularly, red emission combines downshifting with upconversion under 351- and 1,540-nm dual-wavelength excitation is also investigated. The properties of material combined downshifting with upconversion effect have been studied in the previous reports [18–20]. We discovered the synergistic effect between ultraviolet downconversion and infrared upconversion by employing the dual-wavelength (369 and 980 nm) excitation source. Further, in our work, the synergistic effect is also explored in BaSr₂Y₆O₁₂:Er³⁺ powders using the 1,540-nm infrared and 351-nm ultraviolet dual wavelength as excitation source.

2 Experimental

Powder samples of BaSr₂Y₆O₁₂:Er³⁺ are synthesized via conventional high-temperature solid-state reaction in air. Analytical reagents BaO and SrO, high-purity Y₂O₃ (99.99 %) and Er₂O₃ (99.99 %) are used as raw materials. Each sample is weighed according to the nominal compositions BaSr₂Y_6(1−x)O₁₂:6xEr³⁺(x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.20) and then mixed thoroughly. The samples are filled in alumina crucibles and kept at 1,350 °C for 7 h in air atmosphere before decreasing temperature to ambient temperature.

The crystal structures of obtained samples are investigated by X-ray diffraction (XRD, Cu tube with Kα radiation at 1.54056 Å for 2θ = 10−80°). The Raman spectrum is measured by a laser micro-Raman spectrometer system. The excitation spectra and emission spectra of samples are measured by an Omni-λ3007 spectrophotometer with a photomultiplier tube with a CW Xe lamp (150 W) and a 1,540-nm variable power laser (0–150 mW) as the excitation light source. Among them, the 351-nm light source origin from a Xe lamp equipped with an optical grating monochromator. All the measurements are taken at room temperature.

3 Results and discussion

3.1 Sample synthesis

In our experiment, the BaSr₂Y₆O₁₂ powders with different Er³⁺ concentrations are obtained by simple solid-state method. Figure 1a shows the XRD patterns of BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ as a function of Er³⁺ concentration. The XRD patterns of BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ match well with the standard data in JCPDS Card file of NO. 72-0023, and the doping of Er³⁺ ions does not cause any significant change in the BaSr₂Y₆O₁₂ host structure. The Raman spectrum is excited by a 488-nm argon laser of BaSr₂Y₆O₁₂, which is shown in Fig. 1b. It is found that the spectrum shows several strong peaks corresponding to phonon modes with energy 217, 324, 379, 410 and 485 cm⁻¹. It is known that the host material with low phonon energies would have a high upconversion efficiency. The dependence of unit cell parameters a(Å), c(Å) and v(ų) on Er³⁺ concentration in BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ is shown in Fig. 2. The unit cell parameters a, c and v decrease linearly with the increasing Er³⁺ concentration, which accords with the Vegard’s rule [21]. The linear decreasing trend of unit cell parameters is due to the substitution of Y³⁺ ions by small Er³⁺ ions in the host lattice. All these results indicate that the hexagonal phase BaSr₂Y₆O₁₂ is fully developed and a small amount of Er³⁺ ions successfully doped into the host.

Fig. 1

a XRD pattern of as-prepared BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.20) and JCPDS Card (NO. 72-0023) of BaSr₂Y₆O₁₂. b The Raman spectrum of BaSr₂Y₆O₁₂

Fig. 2

Unit cell parameters a(Å), c(Å) and v(ų) dependence on Er³⁺ concentration in BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.20)

3.2 Photoluminescent properties

Figure 3 shows the upconversion emission spectra of BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ (x = 0.02) under 1,540-nm excitation. The ultraviolet and visible upconversion emissions are obtained. In particular, the ultraviolet emissions in the range of 260–350 nm are detected in these samples, as depicted in Fig. 3a. These emission peaks centered at 274, 297 and 324 nm come from the transitions of ⁴G_9/2, ²K_13/2 and ²P_3/2 → ⁴I_15/2 of Er³⁺ ions, respectively. All these ultraviolet emission peaks of Er³⁺ ions are rarely detected under the 1,540-nm excitation in the previous reports. In addition, it should also be noted that peaks centered at 382 and 408 nm are assigned to the ⁴G_11/2, ²H_9/2 → ⁴I_15/2 transition, as shown in Fig. 3b. Figure 3c depicts the upconversion emission spectrum in the range of 500–900 nm; these peaks are assigned to the following transitions: Near-infrared emission centered at 808 nm, red emission centered at 659 nm and green emission centered at 550 and 560 nm are assigned to the ⁴I_9/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2 and ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ions, respectively. Figure 3d gives the upconversion emission in the infrared region of 900–1,100 nm, the emission peak centered at 980 nm corresponding to the ⁴I_11/2 → ⁴I_15/2 of Er³⁺. It should be pointed out that ultraviolet and blue emissions are measured with a higher voltage value of photomultiplier tube due to the weak emission (the same blow).

Fig. 3

UC luminescence spectrum of BaSr₂Y₆O₁₂:Er³⁺ (2 %) powder (λexc = 1,540 nm) in the range of a 250–350 nm, b 350–450 nm, c 500–800 nm and d 900–1,150 nm

To investigate the contribution of Er³⁺ ions toward the photoluminescent (PL) properties of BaSr₂Y₆O₁₂:Er³⁺ powders, the emission intensities under 1,540-nm excitation are recorded for samples with different Er³⁺ ion concentrations, as shown in Fig. 4. The emission intensity is highly dependent on the Er³⁺ concentration. With the increasing Er³⁺ ion concentration, the intensity of all the emission regions increases firstly and reaches a maximum and then decreases remarkably when Er³⁺ concentration is further increased. This phenomenon is due to the Er³⁺ ions at high concentration that leads to the decrease in energy transfer effect, which is known as concentration quenching effect [22]. It is noted that the maximum emission intensity from all the regions of spectrum is observed for a critical concentration of 8 mol % Er³⁺.

Fig. 4

The dependence of UC emission intensity at red, green and UV light on Er³⁺ doping concentration in BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.20)

To better understand the physical mechanism responsible for the PL properties, the emission intensities are measured as a function of pumping power under 1,540-nm excitation. The upconverted emission intensity (I_em) depends on the pumping laser power density (P) according to the following equation [23]:

$$ I_{\text{em}}\, \alpha \, P^{n} , $$

(1)

where I _em is the fluorescence intensity, P is the pumping power density of the 1,540-nm laser, the power density is calculated from the ratio between the laser power and the area of light spot, and n is the number of pumping photons absorbed per upconverted photon emitted. Figure 5 shows the logarithmic plot of the integrated emission intensities of the upconverted emission bands in the region of ultraviolet light, violet, green and red as a function of the pumping laser power density. The output slopes (n) for ultraviolet and violet upconversion emissions centered at 274, 297, 324, 382 and 408 nm are 6.87, 5.84, 5.87, 4.96 and 3.94, respectively, indicating that seven-, six-, six-, five- and four-photon processes are responsible for the populating of ²P_3/2, ⁴G_11/2, ²K_13/2, ⁴G_9/2 and ²H_9/2, states of Er³⁺ ions. When the power density increases to about 90 mW/mm², the output slope (n) of log-log graph is out of shape, and the corresponding slopes of linear fittings are reduced to 3.01, 2.28, 2.94, 2.53 and 2.26 for 274, 297, 324, 382 and 408 nm, respectively. The numbers of photons decrease rapidly when the excitation power exceeds digital, owing to the competition between the linear decay and the upconversion processes for the depletion of the intermediate excited states [24]. In addition, the n values for red and green emission bands are all around 3, indicating that populating the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels needs three 1,540-nm photons and is a three-photon process. For the emission peaked at 808 and 980 nm, the n value is 2.22 and 2.11, respectively, indicating two-photon process.

Fig. 5

Dependence of the upconversion emission intensity on excitation power density in BaSr₂Y₆O₁₂:Er³⁺. a Ultraviolet and violet emission, b visible and infrared emission

The photoluminescence excitation spectrum (PLE) of Er³⁺: ⁴F_9/2 → ⁴I_15/2 emission (659 nm) is shown in Fig. 6a, in which an excitation peak in UV region centered at 351 nm is observed, which corresponds to the ⁴I_15/2 → ²G_7/2 transition of Er³⁺ ion. Figure 6b shows the downshifting emission spectrum of sample under 351-nm excitation, the red region emission centered at 659 nm is assigned to the transition of ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ion, and the infrared characteristic emissions of Er³⁺ from ⁴I_11/2 (980 nm) and ⁴I_13/2 (1,540 nm) levels are presented here clearly. The insert of Fig. 6 shows the dependence of red emission intensities on Er³⁺ doping concentration under the excitation of 351 nm. It is noticed that the emission intensity of red emission increases rapidly with the increasing Er³⁺ doping concentration; however, a decreasing trend is observed clearly when concentration is set to more than 8 mol % due to concentration quenching.

Fig. 6

a The PLE spectra of Er³⁺: ⁴F_9/2 → ⁴I_15/2 emission (659 nm) in BaSr₂Y₆O₁₂:2 %Er³⁺. b The visible and infrared photoluminescent spectrum of sample under 351-nm excitation. The inset shows the dependence of red emission intensity on Er³⁺ ions doping concentration under 351-nm excitation in BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.20)

As described above, the emission band of Er³⁺ ions centered at 659 nm is observed under 1,540- and 351-nm single excitation, respectively. However, under the 1,540- and 351-nm dual excitation, a synergistic effect is witnessed in these samples, which shows that the red emission intensity under the 1,540-nm and 351-nm dual excitation is stronger than the sum of red emission intensity under the 351- and 1,540-nm single excitation. To quantitatively describe the synergistic effect, the absolute enhancement rate (ψ) of red emission can be defined as follows [20]:

$$ \psi = \frac{{I_{ 3} - (I_{ 1} + I_{ 2} )}}{{I_{ 1} + I_{ 2} }}, $$

(2)

where I ₁, I ₂ and I ₃ are the integrated intensity of red emission bands under 1,540-nm excitation, under 351-nm excitation and under 1,540- and 351-nm dual excitation, respectively. The red emission bands, the dependence of red emission absolute enhancement rate on 1,540-nm power density and Er³⁺ doping concentration are shown in Fig. 7a, b and inset, respectively. When the maximum value (ψ = 4.7 %) is witnessed, the excitation power density of 1,540-nm semiconductor laser is adjusted to 115 mW/mm² and the excitation power density of 351 nm is fixed at 0.8 mW/mm² in 8 mol %Er³⁺-doped BaSr₂Y₆O₁₂ sample.

Fig. 7

a The red emission spectra of BaSr₂Y₆O₁₂:Er³⁺ under 351-nm excitation, under 1,540-nm excitation and under 1,540- and 351-nm dual excitation. b The dependence of absolute enhancement rate (ψ) of red emission on Er³⁺ concentration in BaSr₂Y_6(1−x)O₁₂:6xEr³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.20), on 1,540-nm laser power density when 351-nm power density fixed at 0.8 mW/mm² (the inset)

3.3 Upconversion process and synergistic effect mechanism

It is no doubt that ET, excited/ground-state absorption (ESA/GSA) and non-radiative relaxation (NR) processes can be accounted for the upconversion emission mechanisms in BaSr₂Y₆O₁₂:Er³⁺ under the excitation of 1,540-nm semiconductor laser [25, 26]. The possible upconversion procedure is schematically illustrated in Fig. 8. As to the ultraviolet emission, for populating ²K_13/2 and ⁴G_9/2 levels as examples, a possible upconversion process is provided as follows:

$$^{4}{\text{I}}_{15/2} \xrightarrow{\text{GSA}} ^{4}{\text{I}}_{13/2} \xrightarrow{{\text{ESA}}/{\text{ET}}}^{4} {\text{I}}_{{9/2}} \xrightarrow{{{\text{ESA}}/{\text{ET}}}}^{2} {\text{H}}_{{11/2}} \xrightarrow{{{\text{NR}}}}^{2} {\text{S}}_{{3/2}} \xrightarrow{{{\text{ESA}}/{\text{ET}}}}^{2} {\text{H}}_{{9/2}} \xrightarrow{{{\text{NR}}}}^{4} {\text{F}}_{{3/2}} \xrightarrow{{{\text{ESA}}/{\text{ET}}}}^{2} {\text{G}}_{{7/2}} \xrightarrow{{{\text{ESA}}/{\text{ET}}}}^{2} {\text{K}}_{{13/2}} \xrightarrow{{{\text{NR}}}}^{2} {\text{P}}_{{3/2}} \xrightarrow{{{\text{ESA}}/{\text{ET}}}}^{4} {\text{G}}_{{9/2}} . $$

Populating ²K_13/2 and ⁴G_9/2 are assigned to six- and seven-photon upconversion processes, respectively, as the above process, which accords with the calculated n as shown in Fig. 5a.

Fig. 8

Energy-level diagram of Er³⁺, upconversion and downshifting process under excitation of 1,540 and 351 nm, respectively, and the possible mechanism of synergistic effect for red emission under the 351- and 1,540-nm dual excitation

The mechanism resulting in upconversion emission occurs via two distinct processes acting concomitantly: ESA and ET processes. It is well known that both processes are dependent upon doping concentration. While the ET rate is stronger depending on the distance between the ions involved than the ESA process, and once over a critical distance, the efficiency of energy transfer mechanism will be decreased rapidly [27]. The different slopes of intensity in different regions of emission are observed with the increasing concentration as shown in Fig. 4; the slope of red emission is greater than the others; we deduce that ET process is the dominant mechanism for populating the low energy levels, while the ESA process becomes the dominant mechanism, which is responsible for populating the high energy level of Er³⁺.

In addition, the possible mechanism of synergistic effect is also proposed, as shown in Fig. 8. Under the 1,540-nm excitation, the Er³⁺ ions are excited to the ⁴I_9/2 level through successively absorbing two 1,540-nm photons, some of Er³⁺ ions at ⁴I_9/2 level into the ⁴I_11/2 level through a non-radiative relaxation process. Then, the cross-relaxation process from Er³⁺(⁴I_13/2) to Er³⁺(⁴I_11/2) and ESA process occur simultaneously: Er³⁺(⁴I_13/2 → ⁴I_15/2), Er³⁺(⁴I_11/2 → ⁴F_9/2), which leads to the red upconversion emission (⁴F_9/2 → ⁴I_15/2). Under the 351-nm excitation, ²G_7/2 level of Er³⁺ ion is populated through GSA process, and then the red luminescent centered at 659 nm due to a NR process from ²G_7/2 to ⁴F_9/2 [28, 29]. Besides, the ⁴I_11/2 and ⁴I_13/2 levels are also populated through non-radiative relaxation process in downshifting process. In general, some of the energy in the ⁴I_11/2 and ⁴I_13/2 levels of Er³⁺ ions is dissipated via thermal energy; however, under the 351- and 1,540-nm dual excitation, the Er³⁺ ions in the ⁴I_11/2 and ⁴I_13/2 levels from NR process can be excited again (EA) to the ⁴F_9/2 level by absorbing 1,540-nm photons as follows:

$$ ^{{\text{4}}} {\text{I}}_{{{\text{15/2}}}} \xrightarrow{{{\text{GSA}}}}^{{\text{2}}} {\text{G}}_{{{\text{7/2}}}} \xrightarrow{{{\text{NR}}}}^{{\text{4}}} {\text{I}}_{{{\text{11/2}}}} \xrightarrow{{{\text{EA}}}}^{{\text{4}}} {\text{F}}_{{{\text{9/2}}}} \xleftarrow{{{\text{EA}}}}^{{\text{4}}} {\text{I}}_{{{\text{11/2}}}} \xleftarrow{{{\text{NR}}}}^{{\text{4}}} {\text{I}}_{{{\text{9/2}}}} \xleftarrow{{{\text{EA}}}}^{{\text{4}}} {\text{I}}_{{{\text{13/2}}}} \xleftarrow{{{\text{NR}}}}^{{\text{2}}} {\text{G}}_{{{\text{7/2}}}} \xleftarrow{{{\text{GSA}}}}^{{\text{4}}} {\text{I}}_{{{\text{15/2}}}} . $$

Therefore, the energy of non-radiative relaxation is excited again, resulting in the synergistic enhancement effect of red emission (⁴F_9/2 → ⁴I_15/2) under the dual excitation.

4 Conclusion

In summary, the preparation and photoluminescence properties of single-phase BaSr₂Y₆O₁₂:Er³⁺ are investigated systemically for the first time. Power-dependent studies reveal that ultraviolet emissions of Er³⁺ under 1,540-nm excitation result from a seven-, six- and five-photon process, and the ultraviolet upconversion mechanism is discussed based on complex ESA and GSA/ESA processes. Additionally, a synergistic effect combined ultraviolet downshifting with infrared upconversion, which shows that the red emission intensity under the 1,540- and 351-nm dual excitation is stronger than the sum of red emission intensities under the 351- and 1,540-nm single excitation, and the maximum absolute enhancement rate is 4.7 %. It is believed that the ⁴I_13/2 and ⁴I_11/2 levels of Er³⁺ from non-radiative relaxation process under 351-nm excitation are excited again to ⁴F_9/2 level by absorbing 1,540-nm infrared photon in the 351- and 1,540-nm dual-excitation process, which is responsible for the synergistic effect of red emission.