Up-Conversion Photoluminescence in Thulia and Ytterbia Co-Doped Yttria-Stabilized Zirconia Single Crystals

: ZrO 2 is an attractive host matrix for luminescence material because of its excellent physical properties, such as low phonon energy and wide band gap. In this work, the highly transparent Tm 2 O 3 and Yb 2 O 3 co-doped yttria stabilized zirconia (YSZ) (abbreviated as Yb/Tm: YSZ) single crystals were grown by the optical ﬂoating zone method. The Yb/Tm: YSZ samples were stabilized in the cubic phase at room temperature when Yb 3+ and Tm 3+ replaced Y 3+ . The inﬂuence of Yb 3+ co-doping on the up-conversion luminescence properties of the crystals was systematically studied. A total of 0.5 mol% Tm 2 O 3 and 2.0 mol% Yb 2 O 3 co-activated YSZ single crystal (abbreviated as 2.0Yb/Tm: YSZ) has the maximum luminous intensity. There were seven absorption peaks located at around 358, 460, 679, 783.3, 850–1000, 1200, and 1721.5 nm that were observed in the absorption spectrum of the 2.0Yb/Tm: YSZ single crystal. There were three up-conversion peaks at around 488, 658 and 800 nm that were observed when the Yb/Tm: YSZ samples were excitated at 980 nm. The ﬂuorescence lifetime of Tm 3+ for the 1 G 4 → 3 H 6 transition of the 2.0Yb/Tm: YSZ sample is 7.716 ms as excited with a 980 nm laser. In addition, the oscillator strength parameters Ω λ ( λ = 2, 4 and 6) of this sample were derived by the Judd–Ofelt theory to evaluate the laser performance of the host materials. The ratio Ω 4 / Ω 6 of this sample is 0.80, implying its excellent laser output. Therefore, the 2.0Yb/Tm: YSZ single crystal is a considerable potential material for laser and luminescence applications.


Introduction
Rare earth ions activated up-conversion luminescence material can convert nearinfrared light into visible light by absorbing two or more low-energy photons and emitting one high-energy photon.This phenomenon violates the Stokes law, called the anti-Stokes luminescence or the up-conversion luminescence [1].As they can emit short wavelength (high energy) light under the excitation of long wavelength (low energy) light source, the up-conversion luminescent materials have attracted great attention in the field of the infrared anti-counterfeiting, the anti-stokes cold light refrigeration, the up-conversion laser, the up-conversion 3D display, the sensing, and the medical applications [2][3][4][5][6][7].
As the emitted photon energy is greater than that of the excited ones, the energy levels between the activator and the sensitizer have to be very close to realize the continuous absorption of photons and energy transfer in the up-conversion luminescence process.Among the trivalent rare earth ions, Tm 3+ is an important up-conversion luminescence activator.It has a ladder-like schema of energy level, and its 1 D 2 → 3 F 4 and 1 G 4 → 3 H 6 transition wavelengths are around 450 and 480 nm, respectively.The blue up-conversion luminescence process of Tm 3+ can be excited under the pumping at 650, 800, and 980 nm.Tm 3+ has been extensively studied due to its suitability for commercial InGaAs (940-990 nm) laser diode pumping.Yb 3+ has the electronic configuration of 4f 13 , and its energy state structure only contains one excited state( 2 F 5/2 ), which is slightly higher than the metastable excited state of Tm 3+ ( 3 H 5 ).Besides, Yb 3+ has a strong absorption cross-section at 980 nm.The effective energy transfer can occur between Yb 3+ and Tm 3+ , achieving efficient upconversion luminescence.The co-doping of Yb 3+ and Tm 3+ can substantially improve the up-conversion luminescence efficiency [8][9][10][11][12], which was widely adopted in investigating and utilizing light radiation.
The efficiency of up-conversion luminescence material depends not only on the characteristics of rare earth ions, but also on the host material [13][14][15][16].Seeking appropriate host materials to achieve high efficiency, high sensitivity, and a stable up-conversion luminescence laser remains a challenge.Among various inorganic host materials, fluorides were widely studied because of their lower phonon energy and higher up-conversion emission efficiency.However, their chemical and thermal instability limits the potential applications in different environments.Besides, the sensitivities of present optical thermal sensors decline rapidly with increasing temperature.Oxide materials have attracted extensive attention because of their stable physical and chemical properties, such as good thermal stability, oxidation resistance, high mechanical strength, and being green and pollution-free.
Among many oxides, zirconia (ZrO 2 ) is proven to be an excellent matrix material for the trivalent lanthanide ions doping due to its low phonon energy (470 cm −1 ) [8], wide band gap (5.0 eV) [9], high density, low thermal expansion, and large chemical stability.However, the ZrO 2 has three phases, including monoclinic (lower than 1170 • C), tetragonal (1170 • C-2370 • C), and cubic (larger than 2370 • C) [9].The phase transformation of ZrO 2 often comes with a volume change, leading to cracking.It is necessary to keep ZrO 2 stable in the cubic phase.One of the important ways is adding the stabilizers to ZrO 2 to form a stable solid solution.The more common way is adding 8 mol% Y 2 O 3 to ZrO 2 to form the yttria zirconia solid (YSZ) solution, which has a cubic structure from room temperature to the melting point.
It is difficult to prepare high quality YSZ single crystal by conventional methods due to its high melting point (2700 • C).The optical floating zone technology can concentrate the light at the same point to increase the heating temperature to 3000 • C. Also, it can be used to grow metal oxide single crystals with a high melting point as no crucible is needed in the growth process.Our research group has successfully grown Y 3 Al 5 O 12 [17][18][19][20] and ruby [20,21] single crystals by this method.Therefore, in this work, the YSZ single crystals co-activated by various concentrations of Yb 3+ and Tm 3+ were grown by the optical floating zone technology.The structure and up-luminescence properties of the samples were characterized.Moreover, we utilized the Judd-Ofelt theory to analyze the spectral parameters of Tm 3+ and Yb 3+ in the samples.

Crystal Growth
A series of polycrystalline ceramic rods were prepared through a solid-state reaction process before the single crystal growth.The oxides ZrO 2 (99.99%),Y 2 O 3 (99.99%),Tm 2 O 3 (99.99%),and Yb 2 O 3 (99.99%)powders (Aladdin, Shanghai, China) were employed as raw materials.The appropriate amounts of powders were weighed at specified mole ratios (as seen in Table 1) and mixed homogeneously by a magnetic stirrer for 24 h.Then, each mixture was oven-dried at 85 • C for 24 h and grounded in an agate mortar for 0.5 h.The prepared powder was packed in a long rubber balloon, vacuumed, sealed, and isostatically pressed under a pressure of 68 MPa to obtain the feed and seed rods.The obtained rods were sintered at 1550 • C for 10 h in the air to be compact and uniform polycrystalline ceramic rods.Single crystals of Tm 2 O 3 and Yb 2 O 3 co-doped YSZ were grown using an optical floating zone furnace (FZ-T-12000-X-VII-VPO-GU-PC, Crystal Systems Co., Yamanashi, Japan).During growth, a counter-rotated seed of 10 rpm of the seed and feed rods, a flow rate of 4 L/min of air was maintained, and the crystal growth rate was about 5 mm/h.
The as-grown Yb/Tm: YSZ single crystals were colorless and about 5 × 80 mm in size, as shown in Figure 1.The crystals had a smooth surface and were free from cracks.The chips of about 1.0 mm in thickness were cut from the Yb/Tm: YSZ crystal rods and then double-sided polished.Single crystals of Tm2O3 and Yb2O3 co-doped YSZ were grown using an optical floating zone furnace (FZ-T-12000-X-VII-VPO-GU-PC, Crystal Systems Co., Yamanashi, Japan).During growth, a counter-rotated seed of 10 rpm of the seed and feed rods, a flow rate of 4 L/min of air was maintained, and the crystal growth rate was about 5 mm/h.
The as-grown Yb/Tm: YSZ single crystals were colorless and about 5 × 80 mm in size, as shown in Figure 1.The crystals had a smooth surface and were free from cracks.The chips of about 1.0 mm in thickness were cut from the Yb/Tm: YSZ crystal rods and then double-sided polished.

Characterization
The X-ray diffraction (XRD) patterns were collected by a diffractometer (DX-2700, Haoyuan, Dangdong, China) using Cu-Kα (λ = 1.5406Å) radiation in the 2θ range of 20°-80° with a step of 0.02°.The existing phases of as-prepared crystal chips were investigated also by Raman scattering.The Raman spectra in the wavenumber range of 150-950 cm −1 were obtained by a confocal Raman spectrometer (inVia Reflex, Renishaw, London, UK) with the laser excitation at 532 nm.
The absorption spectra of crystal chips were collected on a UV-Visible-NIR spectro photometer (UV-3600, Shimadzu, Kyoto, Japan) with a resolution of 1 nm.The steady emission spectra were measured by a photoluminescence (PL) spectrometer (ZLF325, Zolix Instruments Co.Ltd, Beijing, China) with a 980 nm laser diode as an excitation source.Fluorescence decay curves were recorded with a fluorescence spectrometer (FLS920, Edinburgh Instruments, Edinburgh, UK) excited with a 980 nm laser as the excitation light.

Structure Analysis
The as-grown Yb/Tm: YSZ single crystals were crushed and ground into a powder to obtain detailed structure information.The XRD patterns of Yb/Tm: YSZ powders are shown in Figure 2a.For all the samples, six peaks were detected.It is impossible to identify the cubic and the tetragonal ZrO2 as they share very similar X-ray reflection [12,22].Raman spectroscopy is widely used to distinguish the three possible structures of ZrO2 [23][24][25].The Raman spectra of Yb/Tm: YSZ crystal chips are shown in Figure 2b.The

Characterization
The X-ray diffraction (XRD) patterns were collected by a diffractometer (DX-2700, Haoyuan, Dangdong, China) using Cu-Kα (λ = 1.5406Å) radiation in the 2θ range of 20 • -80 • with a step of 0.02 • .The existing phases of as-prepared crystal chips were investigated also by Raman scattering.The Raman spectra in the wavenumber range of 150-950 cm −1 were obtained by a confocal Raman spectrometer (inVia Reflex, Renishaw, London, UK) with the laser excitation at 532 nm.
The absorption spectra of crystal chips were collected on a UV-Visible-NIR spectro photometer (UV-3600, Shimadzu, Kyoto, Japan) with a resolution of 1 nm.The steady emission spectra were measured by a photoluminescence (PL) spectrometer (ZLF325, Zolix Instruments Co., Ltd, Beijing, China) with a 980 nm laser diode as an excitation source.Fluorescence decay curves were recorded with a fluorescence spectrometer (FLS920, Edinburgh Instruments, Edinburgh, UK) excited with a 980 nm laser as the excitation light.

Structure Analysis
The as-grown Yb/Tm: YSZ single crystals were crushed and ground into a powder to obtain detailed structure information.The XRD patterns of Yb/Tm: YSZ powders are shown in Figure 2a.For all the samples, six peaks were detected.It is impossible to identify the cubic and the tetragonal ZrO 2 as they share very similar X-ray reflection [12,22].Raman spectroscopy is widely used to distinguish the three possible structures of ZrO 2 [23][24][25].The Raman spectra of Yb/Tm: YSZ crystal chips are shown in Figure 2b.The Raman spectra of all samples are characterized by one strong peak in the wave-number range of 150-950 cm −1 .The Raman peak centered at around 625 cm −1 , corresponding to the F 2g mode of the cubic phase [26], is a little bigger than that of 0.5 Tm 2 O 3 :YSZ single crystal [12], and it may attributed to the presence of co-doped Yb 2 O 3 .The diffraction patterns of all the samples match well with the standard card of cubic ZrO 2 (JCPDS No.97-008-9429), belonging to the Fm-3m (225) space group.No extra peaks of a secondary phase were observed, indicating that Tm 3+ and Yb 3+ successfully entered the YSZ lattice.In addition, the cell parameters decrease slightly with the increase of Yb 2 O 3 content, as seen in Table 2.This is due to the Yb 3+ preferentially substituting for Y 3+ , and the radius of Yb 3+ (0.985 Å) is smaller than Y 3+ (1.019 Å).
Crystals 2023, 13, x FOR PEER REVIEW 4 of 12 Raman spectra of all samples are characterized by one strong peak in the wave-number range of 150-950 cm −1 .The Raman peak centered at around 625 cm −1 , corresponding to the F2g mode of the cubic phase [26], is a little bigger than that of 0.5 Tm2O3:YSZ single crystal [12], and it may attributed to the presence of co-doped Yb2O3.The diffraction patterns of all the samples match well with the standard card of cubic ZrO2 (JCPDS No.97-008-9429), belonging to the Fm-3m (225) space group.No extra peaks of a secondary phase were observed, indicating that Tm 3+ and Yb 3+ successfully entered the YSZ lattice.In addition, the cell parameters decrease slightly with the increase of Yb2O3 content, as seen in Table 2.This is due to the Yb 3+ preferentially substituting for Y 3+ , and the radius of Yb 3+ (0.985 Å) is smaller than Y 3+ (1.019 Å).

Absorption Spectra and Judd-Ofelt Analysis
The optical absorption spectra of x Yb/Tm: YSZ single crystals between 300 and 2000 nm were tested, as shown in Figure 3.There is no characteristic absorption related to d-d transition in the range of 300-2000 nm because the d orbit of Zr 4+ is empty.Therefore, the absorption peaks of all the samples are from the transitions between Tm 3+ and Yb 3+ .There are four visible and three IR absorption peaks, centering at 358, 460, 679, 783.5, 1200, and 1721.5 nm which correspond to the transitions of Tm 3+ : 3 H6→ 1 D2, 3 H6→ 1 G4, 3 H6→ 3 F2,3, 3 H6 → 3 H4, 3 H6→ 3 H5, and 3 H6→ 3 F4, respectively.The strongest absorption peak is at around 850-1000 nm, corresponding to the transition of Yb 3+ : 2 F7/2→ 2 F5/2, which indicated that Yb 3+ has a large absorption of the 980 nm laser excitation and is suitable as a sensitizer in the up-conversion luminescence of Tm 3+ .

Absorption Spectra and Judd-Ofelt Analysis
The optical absorption spectra of x Yb/Tm: YSZ single crystals between 300 and 2000 nm were tested, as shown in Figure 3.There is no characteristic absorption related to d-d transition in the range of 300-2000 nm because the d orbit of Zr 4+ is empty.Therefore, the absorption peaks of all the samples are from the transitions between Tm 3+ and Yb 3+ .There are four visible and three IR absorption peaks, centering at 358, 460, 679, 783.5, 1200, and 1721.5 nm which correspond to the transitions of Tm 3+ : 3  3 H 6 → 3 H 5 , and 3 H 6 → 3 F 4 , respectively.The strongest absorption peak is at around 850-1000 nm, corresponding to the transition of Yb 3+ : 2 F 7/2 → 2 F 5/2 , which indicated that Yb 3+ has a large absorption of the 980 nm laser excitation and is suitable as a sensitizer in the up-conversion luminescence of Tm 3+ .
Optically, the Judd-Ofelt (J-O) theory, through an analysis of the oscillator strengths, the emission branching radio and the radiative lifetime [27][28][29] is used for analyzing the possible transition mechanisms of the rare earth ions that are affected by the host materials.
Based on the absorption spectrum of 2.0Yb/Tm: YSZ single crystal chip, the oscillator strengths for an induced electric dipole transition from the initial state to the final state are calculated according to the Equation (1) [28,29].
where m, c, σ, h, J, and n are the electron mass, velocity of light, transition energy, Planck constant, total angular momentum, and index of refraction, respectively.Ω λ (λ = 2, 4 and 6) is oscillator intensity parameter and U λ (λ = 2, 4 and 6) is the reduced matrix element.The values of average wavelength and the measured and calculated oscillated strengths for the 2.0Yb/Tm: YSZ single crystal are collected in Table 3. Optically, the Judd-Ofelt (J-O) theory, through an analysis of the oscillator strengths, the emission branching radio and the radiative lifetime [27][28][29] is used for analyzing the possible transition mechanisms of the rare earth ions that are affected by the host materials.
Based on the absorption spectrum of 2.0Yb/Tm: YSZ single crystal chip, the oscillator strengths for an induced electric dipole transition from the initial state to the final state are calculated according to the Equation (1) [28,29]. ( where m, c, σ, h, J, and n are the electron mass, velocity of light, transition energy, Planck constant, total angular momentum, and index of refraction, respectively.Ωλ (λ = 2, 4 and 6) is oscillator intensity parameter and Uλ (λ = 2, 4 and 6) is the reduced matrix element.The values of average wavelength and the measured and calculated oscillated strengths for the 2.0Yb/Tm:YSZ single crystal are collected in Table 3.According to the J-O theory, the absorption oscillator strengths of Tm 3+ ion are calculated, as shown in Table 3.The Judd-Ofelt parameters of Tm 3+ in different host materials are listed in Table 4.The root mean square (RMS) is 0.077 × 10 −20 cm 2 , indicating the high reliability of the obtained oscillator strengths.The intensity parameters Ω λ (λ = 2, 4, 6) of Tm 3+ are 0.41 × 10 −20 cm 2 , 0.12 × 10 −20 cm 2 , and 0.15 × 10 −20 cm 2 .It is well known that the Ω 2 parameter is sensitive to crystal structure and related to the covalency of the RE 3+ sites.While Ω 4 and Ω 6 depend on the viscosity and rigidity of the host material.The value of Ω 2 is larger than that of Ω 4 , revealing the relatively low symmetry of the Tm 3+ site and the covalence that exists between the Tm 3+ ions and anions as well as the asymmetry around the metal ion site [30].According to the previous analysis, oxygen vacancies can be formed when the Y 3+ , Tm 3+ and Yb 3+ ions occupy the sites of the small Zr 4+ .Based on the theoretical calculation, the oxygen vacancies are located at the nearest neighbor of Zr 4+ , leading to the seven-fold coordination and the symmetry reduction of the Zr 4+ sites.It could be concluded that the Tm 3+ occupy the Zr 4+ site with seven coordination after entering YSZ lattice.Meanwhile, Ω 4 /Ω 6 is one of the important parameters for the spectral characterization of the host materials.The value of Ω 4 /Ω 6 for 2.0Yb/Tm: YSZ single crystal is 0.8, which is larger than 0.68, 0.389, and 0.15 in LLF [31], YVO 4 [32], and SrWO 4 [33], revealing that the sample is a potential material for laser output.Using the Ω λ values, the radiative properties such as spontaneous transition probability A ed , branching ratios β, and radiative lifetime τ rad for the transitions of Tm 3+ in 2.0Yb/Tm: YSZ single crystal are calculated and listed in Table 5.

Luminescence Properties
Figure 4 shows the emission spectra of the YSZ: 0.5 mol%Tm 2 O 3 , x mol%Yb 2 O 3 (x = 1.0, 2.0, 3.0, 4.0, 5.0) in the wavelength range of 400-900 nm under the 980 nm laser excitation at room temperature.Clearly, under the excitation of 980 nm, the bright blue luminescence was observed by the naked eye, as shown in Figure 5.The emission spectra consisted of three main emission bands at around 488 (blue), 658 (red), and 799 nm (NIR), corresponding to the of 1 G 4 → 3 H 6 , 1 G 4 → 3 F 4 , and 3 H 4 → 3 H 6 transitions of Tm 3+ , respectively.Among these, the NIR emission corresponding to 3 H 4 → 3 H 6 at 799 nm is the dominant one, and is consistent with the literature.As the Yb 2 O 3 concentration increases, the position and shape of the emission peaks is similar, indicating the same surrounding of Tm 3+ .In other words, the structure of YSZ crystals does not change significantly after Yb 2 O 3 co-doping.The structure of the crystal affected the luminescence properties directly.The blue up-conversion luminescence of Tm 3+ in cubic ZrO 2 is composed of three peaks, while it is a single peak in monoclinic ZrO 2 .The blue emission band in Figure 4 is mainly composed of three peaks, indicating that the crystal structure is cubic, and it is consistent with the XRD and Raman spectroscopy results.
Figure 6 shows the intensities of the emission peaks of the samples.The luminescence intensity increases with an increase in Yb 2 O 3 concentration up to 2.0 mol%, and then decreases dramatically, due to the concentration quenching effect.According to the Blasse theory, the process of non-radiative energy transfer depends on the radiative reabsorption or the electric multipolar interaction of the activated ion.However, the radiation reabsorption process only occurs in the case of a large overlap between the excitation spectrum of the sensitizer and the emission spectrum of the activator.As can be seen in Figure 4, the emission peaks of Tm 3+ are located at 488, 658, and 799 nm, while the absorption peak of the Yb 3+ is located at around 980 nm, and the peak overlap is very small.Therefore, the concentration quenching in Yb/Tm: YSZ single crystals is induced by the electric multipolar interaction.Figure 6 shows the intensities of the emission peaks of the samples.The luminescence intensity increases with an increase in Yb2O3 concentration up to 2.0 mol%, and then decreases dramatically, due to the concentration quenching effect.According to the Blasse theory, the process of non-radiative energy transfer depends on the radiative reabsorption or the electric multipolar interaction of the activated ion.However, the radiation reabsorption process only occurs in the case of a large overlap between the excitation spectrum of the sensitizer and the emission spectrum of the activator.As can be seen in Figure 4, the emission peaks of Tm 3+ are located at 488, 658, and 799 nm, while the absorption peak of the Yb 3+ is located at around 980 nm, and the peak overlap is very small.Therefore, the concentration quenching in Yb/Tm: YSZ single crystals is induced by the electric multipolar interaction.Figure 6 shows the intensities of the emission peaks of the samples.The luminescence intensity increases with an increase in Yb2O3 concentration up to 2.0 mol%, and then decreases dramatically, due to the concentration quenching effect.According to the Blasse theory, the process of non-radiative energy transfer depends on the radiative reabsorption or the electric multipolar interaction of the activated ion.However, the radiation reabsorption process only occurs in the case of a large overlap between the excitation spectrum of the sensitizer and the emission spectrum of the activator.As can be seen in Figure 4, the emission peaks of Tm 3+ are located at 488, 658, and 799 nm, while the absorption peak of the Yb 3+ is located at around 980 nm, and the peak overlap is very small.Therefore, the concentration quenching in Yb/Tm: YSZ single crystals is induced by the electric multipolar interaction.To examine the up-conversion mechanisms, the emission spectra of (0.5 mol%Tm2O3, 2.0 mol%Yb2O3): YSZ single crystal under 980 nm excitation at different power are plotted in Figure 7.The luminescence intensity (I) increases with the power (P), following the multiphoton formula, I ∝ P n .Here, n is the number of photons to populate the emitting states and is obtained by the slope of the linear fits of lg I-lg P curves.The slopes are 3.13, To examine the up-conversion mechanisms, the emission spectra of (0.5 mol%Tm 2 O 3 , 2.0 mol%Yb 2 O 3 ): YSZ single crystal under 980 nm excitation at different power are plotted in Figure 7.The luminescence intensity (I) increases with the power (P), following the multiphoton formula, I ∝ P n .Here, n is the number of photons to populate the emitting states and is obtained by the slope of the linear fits of lg I-lg P curves.The slopes are 3.13, 3.30, and 2.32 for the blue emission centered at 488 nm, red emission centered at 658 nm, and NIR emission centered at 799 nm, respectively.This indicates that the red and blue emissions involve three photons while the NIR emission needs two photos.Although a photo avalanche (PA) is a possible mechanism for up-conversion luminescence, it is excluded because no inflection was observed in the power curve.Therefore, the upconversion luminescence of Yb/Tm: YSZ single crystal is realized by the energy transfer (ET), the ground state absorption (GSA), and the excited state absorption (ESA) processes.To examine the up-conversion mechanisms, the emission spectra of (0.5 mol%Tm2O3, 2.0 mol%Yb2O3): YSZ single crystal under 980 nm excitation at different power are plotted in Figure 7.The luminescence intensity (I) increases with the power (P), following the multiphoton formula, I ∝ P n .Here, n is the number of photons to populate the emitting states and is obtained by the slope of the linear fits of lg I-lg P curves.The slopes are 3.13, 3.30, and 2.32 for the blue emission centered at 488 nm, red emission centered at 658 nm, and NIR emission centered at 799 nm, respectively.This indicates that the red and blue emissions involve three photons while the NIR emission needs two photos.Although a photo avalanche (PA) is a possible mechanism for up-conversion luminescence, it is excluded because no inflection was observed in the power curve.Therefore, the up-conversion luminescence of Yb/Tm: YSZ single crystal is realized by the energy transfer (ET), the ground state absorption (GSA), and the excited state absorption (ESA) processes.Figure 8 is the energy level diagram of the Yb 3+ -Tm 3+ co-doped system.The transition processes in this up-conversion luminescence can be explained by the following.First, the electron of Yb 3+ is excited from the ground state 2 F7/2 to the 2 F5/2 energy level by absorbing one 980 nm photon.Then, the Yb 3+ transfers the energy to the ground state of neighboring Tm 3+ according to the ET process.After that, the electron in the 3 H6 state of Tm 3+ is excited to the 3 H5 state via the GSA process.The 3 F4 state is populated by the fast non-radiative relaxation.Following this, the Yb 3+ absorbs the second photon and transfers the energy to Figure 8 is the energy level diagram of the Yb 3+ -Tm 3+ co-doped system.The transition processes in this up-conversion luminescence can be explained by the following.First, the electron of Yb 3+ is excited from the ground state 2 F 7/2 to the 2 F 5/2 energy level by absorbing one 980 nm photon.Then, the Yb 3+ transfers the energy to the ground state of neighboring Tm 3+ according to the ET process.After that, the electron in the 3 H 6 state of Tm 3+ is excited to the 3 H 5 state via the GSA process.The 3 F 4 state is populated by the fast non-radiative relaxation.Following this, the Yb 3+ absorbs the second photon and transfers the energy to the Tm 3+ , leading to the excitation of the electron from the 3 F 4 state of Tm 3+ to the 3 F 2,3 state via the ESA channel.Subsequently, the non-radiative transition between two activator ions will populate the 3 H 4 state.Finally, the Yb 3+ absorbs the third photon and transfers the energy to the 3 H 4 state, leading to the electron excitation to the 1 G 4 state.Then, the up-conversion luminescence is generated by the transitions from the high to the low-energy states.Therefore, the emission of blue (488 nm), red (658 nm), and NIR (799 nm) are observed according to the 1 G 4 → 3 H 6 , 1 G 4 → 3 F 4 , and 3 H 4 → 3 H 6 transitions, respectively.

Luminescence Decay Kinetics
Figure 9a presents the fluorescence decay curve of the emission line 1 G 4 → 3 F 4 (658 nm) of Tm 3+ in x Yb/Tm: YSZ crystal (x = 1.0, 2.0, 3.0 mol%), excited by 980 nm at room temperature.From a single exponential function fitting, the average fluorescence lifetime τ of the 1 G 4 → 3 F 4 transition of the x Yb/Tm: YSZ crystal (x = 1.0, 2.0, 3.0 mol%) is equal to 0.949 ms, 0.972 ms, and 0.953 ms, respectively.The 2.0Yb/Tm: YSZ single crystal has the longest lifetime.As the concentration of Tm 3+ remains the same, the excited state lifetime of Tm 3+ is affected by the energy transfer process between Yb 3+ and Tm 3+ .In addition, the fluorescence decay curve for Tm 3+ for the 1 G 4 → 3 H 6 (488 nm) transition in 2.0Yb/Tm: YSZ single crystal is shown in Figure 9b.The average fluorescence lifetime is 7.716 ms.The decay time is larger than that of Tm: LGYSO [37] (1.669 ms), Tm: YVO 4 [38] (1.9 ms), Tm: SSO [39] (1.14 ms), revealing the suitability of the YSZ crystal as a host material for luminescence of Tm 3+ .
the Tm 3+ , leading to the excitation of the electron from the 3 F4 state of Tm 3+ to the 3 F2,3 state via the ESA channel.Subsequently, the non-radiative transition between two activator ions will populate the 3 H4 state.Finally, the Yb 3+ absorbs the third photon and transfers the energy to the 3 H4 state, leading to the electron excitation to the 1 G4 state.Then, the upconversion luminescence is generated by the transitions from the high to the low-energy states.Therefore, the emission of blue (488 nm), red (658 nm), and NIR (799 nm) are observed according to the 1 G4→ 3 H6, 1 G4→ 3 F4, and 3 H4→ 3 H6 transitions, respectively.

Luminescence Decay Kinetics
Figure 9a presents the fluorescence decay curve of the emission line 1 G4→ 3 F4 (658 nm) of Tm 3+ in x Yb/Tm: YSZ crystal (x = 1.0, 2.0, 3.0 mol%), excited by 980 nm at room temperature.From a single exponential function fitting, the average fluorescence lifetime τ of the 1 G4→ 3 F4 transition of the x Yb/Tm: YSZ crystal (x = 1.0, 2.0, 3.0 mol%) is equal to 0.949 ms, 0.972 ms, and 0.953 ms, respectively.The 2.0Yb/Tm: YSZ single crystal has the longest lifetime.As the concentration of Tm 3+ remains the same, the excited state lifetime of Tm 3+ is affected by the energy transfer process between Yb 3+ and Tm 3+ .In addition, the fluorescence decay curve for Tm 3+ for the 1 G4→ 3 H6 (488 nm) transition in 2.0 Yb/Tm: YSZ single crystal is shown in Figure 9b.The average fluorescence lifetime is 7.716 ms.The decay time is larger than that of Tm: LGYSO [37] (1.669 ms), Tm: YVO4 [38] (1.9 ms), Tm: SSO [39] (1.14 ms), revealing the suitability of the YSZ crystal as a host material for luminescence of Tm 3+ .

Luminescence Decay Kinetics
Figure 9a presents the fluorescence decay curve of the emission line 1 G4→ 3 F4 (658 nm) of Tm 3+ in x Yb/Tm: YSZ crystal (x = 1.0, 2.0, 3.0 mol%), excited by 980 nm at room temperature.From a single exponential function fitting, the average fluorescence lifetime τ of the 1 G4→ 3 F4 transition of the x Yb/Tm: YSZ crystal (x = 1.0, 2.0, 3.0 mol%) is equal to 0.949 ms, 0.972 ms, and 0.953 ms, respectively.The 2.0Yb/Tm: YSZ single crystal has the longest lifetime.As the concentration of Tm 3+ remains the same, the excited state lifetime of Tm 3+ is affected by the energy transfer process between Yb 3+ and Tm 3+ .In addition, the fluorescence decay curve for Tm 3+ for the 1 G4→ 3 H6 (488 nm) transition in 2.0 Yb/Tm: YSZ single crystal is shown in Figure 9b.The average fluorescence lifetime is 7.716 ms.The decay time is larger than that of Tm: LGYSO [37] (1.669 ms), Tm: YVO4 [38] (1.9 ms), Tm: SSO [39] (1.14 ms), revealing the suitability of the YSZ crystal as a host material for luminescence of Tm 3+ .

Color Chromaticity Coordinates
The color chromaticity characteristics of the Yb/Tm: YSZ single crystals were analyzed by the Commission International de l'éclairage (CIE) chromaticity coordinates diagram as shown in Figure 10 and Table 6.The color points of all the samples are in cyan region, and the purity of the blue emission is 79.5% for the 2.0Yb/Tm: YSZ single crystal.

Figure 1 .
Figure 1.Yb/Tm:YSZ single crystal rods grown by the optical floating zone method.

Figure 1 .
Figure 1.Yb/Tm:YSZ single crystal rods grown by the optical floating zone method.

Figure 2 .
Figure 2. (a) XRD patterns for powders that were obtained by grinding x Yb/Tm: YSZ single crystals, (b) Raman spectra of x Yb/Tm: YSZ single crystals.

Crystals 2023 ,
13,  x FOR PEER REVIEW 7 of 12 a single peak in monoclinic ZrO2.The blue emission band in Figure4is mainly composed of three peaks, indicating that the crystal structure is cubic, and it is consistent with the XRD and Raman spectroscopy results.

Figure 6 .
Figure 6.Intensities of emission peaks as a function of x in the x Yb/Tm: YSZ single crystal.

Figure 6 .
Figure 6.Intensities of emission peaks as a function of x in the x Yb/Tm: YSZ single crystal.

Figure 7 .
Figure 7. Up-conversion emission spectra of 2.0 Yb/Tm: YSZ single crystal under the 980 nm excitation with different powers.The inset is the relationship between the up-conversion emission intensity and excitation power for the 2.0 Yb/Tm: YSZ single crystal.

Figure 7 .
Figure 7. Up-conversion emission spectra of 2.0Yb/Tm: YSZ single crystal under the 980 nm excitation with different powers.The inset is the relationship between the up-conversion emission intensity and excitation power for the 2.0Yb/Tm: YSZ single crystal.

Figure 8 .
Figure 8. Energy level diagram of Tm 3+ −Yb 3+ energy transfer in the YSZ crystal under the 980 nm excitation.ET = energy transfer upconversion; GSA = ground state absorption; ESA = excited state absorption.

Figure 8 .
Figure 8. Energy level diagram of Tm 3+ −Yb 3+ energy transfer in the YSZ crystal under the 980 nm excitation.ET = energy transfer upconversion; GSA = ground state absorption; ESA = excited state absorption.

Figure 8 .
Figure 8. Energy level diagram of Tm 3+ −Yb 3+ energy transfer in the YSZ crystal under the 980 nm excitation.ET = energy transfer upconversion; GSA = ground state absorption; ESA = excited state absorption.

Table 1 .
Chemical compositions of Tm 2 O 3 and Yb 2 O 3 co-activated YSZ single crystals.

Table 1 .
Chemical compositions of Tm2O3 and Yb2O3 co-activated YSZ single crystals.

Table 3 .
Values of average wavelength and the experimental and calculated oscillated strengths for the 2.0Yb/Tm: YSZ single crystal.

Table 3 .
The Judd-Ofelt parameters of Tm 3+ in different host materials are listed in Table4.The root mean square (RMS) is 0.077 × 10 −20 cm 2 , indicating the high

Table 3 .
Values of average wavelength and the experimental and calculated oscillated strengths for the 2.0Yb/Tm: YSZ single crystal.

Table 5 .
Calculated radiative transition rates, branching ratios, and radiative lifetimes for different transition levels of 2.0Yb/Tm: YSZ single crystal.

Table 6 .
Values of CIE coordinates calculated for the various samples.Grant No. 2022GXNSFBA035447, and the National Natural Science Foundations of China under Grant No. 12105055 and 12265005.