1 Introduction

The possibility of the upconversion photoluminescence from rare earth ions has been previously reported [1]. It includes the conversion of absorbed low-energy photons to emitted photon(s) with higher energy via participation of single or multiple RE ions. Thanks to the variety of RE ions with different luminescence properties, this offers the opportunity to tune the emitted photons in wide range of light spectrum. It is known that the electronic properties of most RE ions characterize by partially unfilled 4f energy level. This allows the 4f electrons to undergo radiative transitions with emitting photons in ultraviolet, visible, and infrared regions. This opens potential applications in several fields such as color displays, solid-state laser, optical storage, Infrared sensors, and bio-medical diagnostics. In addition, the concept of upconversion photoluminescence holds the promising approach of seeking light-emitting materials in visible spectrum using the available IR lasers as excitation sources. In this regard, two common mechanisms have been proposed to explain the upconversion emission from the higher excited energy levels [2]. The first one, called excited state absorption mechanism (ESA), considers the upconversion excitation a result of subsequent absorption of two or more excited photons by single ions. The second one, called energy transfer mechanism (ET), deals with the absorption of two or more excited photons by different ions followed by energy transfer between them to excite the higher energy levels in a single ion. However, the two mechanisms are competitive and can coexist in the same material.

In the field of upconversion, lanthanide ions exhibit great attention owing to their richness with different optical transitions covering the entire light spectrum. Among lanthanides, Nd3+ is considered as the most widely RE ion used in the infrared–visible light conversion. Nd3+ riches with upper emitting states that can be involved in this upconversion process results in visible and even UV emission. Thus, Nd3+ has been subjected to several investigations in different hosts [3,4,5]. It also showed a special interest as a candidate for upconversion lasers [6,7,8]. The optical characteristics and upconversion mechanisms of Nd-doped in YAG and glass-based material have been widely investigated. However, little attention is given to Nd-doped Y2O3 ceramic host despite its high stability and good thermo-mechanical properties. It is also characterized by competitive optical properties with the well-known YAG ceramic garnet. Unlike YAG host, Nd-doped Y2O3 undergoes specific Stark level splitting providing higher probability for the two emission lines at 914 nm and 945 nm, which are adapted for different applications [9].

In this work, the infrared to visible upconversion emission from Nd-doped Y2O3 ceramic was achieved using 1064-nm pulsed laser as the excitation source. The down conversion and PLE have been also studied to analyze the luminescence properties of the prepared ceramic. In addition, the photoluminescence excitation power dependent and TRPL have been performed to figure out the upconversion mechanism.

2 Experimental Section

1% Nd-doped Y2O3 nanocrystals were prepared via co-precipitation method. Aqueous solution from Y(NO3)3∙6H2O and Nd(NO3)3∙6H2O have been mixed and the precipitant solution was prepared using ammonium hydrogen carbonate (NH4HCO3) of 0.5 M concentration. The chemical precipitation was performed by the reverse-strike method accompanied by stirring at ambient temperature. Then, the suspension was filtrated and washed several times with distilled water to remove residual ammonia and finally ultrasonically with ethanol. The obtained product has been dried using a microwave oven operated at (400 W) for 5 min then it calcinated in air for 2 h at temperature of 1000 °C to grow the nanocrystals. The X-ray diffraction (Philips Analytical X-Ray, 0.154 nm for CuKα) was used to investigate the crystal structure of the nanopowder. The surface morphology of the ceramic sample was examined by scanning electron microscopy (SEM). The Nd-doped Y2O3 ceramic has been fabricated using the Spark Plasma Sintering technique, SPS, (Japan, Fuji Electronic Industrial, Model 515 S). During the sintering process, the Nd-Y2O3 nanopowder was subjected to pressure around 80 MPa and heating rate of 100 °C/min until maximum temperature of 950 °C. Then the ceramic pellet underwent polishing process for the optical measurements. The down conversion photoluminescence (PL) measurements and the photoluminescence excitation (PLE) spectroscopy were carried out with a spectrofluorometer (Fluorolog-Horiba) equipped with a cooled silicon-based CCD camera. The upconversion PL measurement was performed using 1064 nm line of a YAG:Nd laser with 40 ns, 70 mJ and 10 Hz pulse duration, pulse energy, and repetition rate, respectively. Then, the upconversion emission was collected using an optical fiber and detected by a compact spectrometer (Thorlabs-CCS100). For time-resolved photoluminescence measurement of the UCPL and DCPL, the sample was pumped by 1064 nm and 532 nm lines of YAG:Nd laser, respectively. The emitted light was detected by a fast photomultiplier tube (Hamamatsu r1328) and analyzed by GHz Oscilloscope.

3 Results and Discussion

3.1 Structural Properties

Figure 1a shows the FTIR spectrum of the prepared Nd-doped Y2O3 nanopowder. It exhibits the vibrational features and the formation of yttria phase of the prepared material. The broad band locates around 3426 cm−1 is assigned to stretching of O–H bonds in water molecules.

Fig. 1
figure 1

a FTIR and b XRD pattern of Nd-doped Y2O3 nanopowder calcined at 1000 °C/2 h. c SEM image of the as-sintered Nd-doped Y2O3 ceramic sample

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The bands at 1414 cm−1 and 1523 cm−1 are matching to the symmetric and asymmetric stretching modes in the carboxylate group. On the other hand, the intense and very narrow peaks at 498 cm−1 and 573 cm−1 can be ascribed to starching vibration of Y–O bonds in Y2O3 molecules, which indicates the formation of Y2O3 phase [10, 11]. Thus, to confirm the growth of Y2O3 nanocrystals, the XRD has been measured. Figure 1b shows the XRD of the prepared Nd-doped Y2O3 nanopowders. The diffraction pattern reveals the crystalline features of single phase of cubic Y2O3 structure in agreement with the standard JCPDS 41–1105 reference. Only the diffraction peaks corresponding to cubic yttria are observed. No other impurity phases could be detected in the XRD result. This indicates the high phase purity of the prepared yttria nanocrystals. In addition, the average crystallite size has been calculated using Scherrer’s equation. It is found that the average sizes of the nanocrystals are around 30–37 nm by using the most intense diffraction peaks. This small nanocrystal size is preferred for good sintering condition in ceramic fabrication [12]. Then, the prepared nanocrystals underwent the sintering process. Regarding the microstructure, the SEM, Fig. 1c, of the as-sintered Nd-doped Y2O3 ceramic shows the fracture surface of irregular polyhedral morphology. The average grain sizes are ranging from 300 nm to 1 µm. A fracture surface can be found in between the grains, which led to the nearly absence of pores in or between the grains.

3.2 Optical Properties

Figure 2a shows the near-infrared (NIR) PL spectra of Nd-doped Y2O3 ceramic sample excited by different wavelengths. The PL responses exhibit the same spectral features regardless of the excitation wavelength. It can be also observed that the excitation by 590 nm exhibits the strongest PL intensity. The PL spectra consist of three NIR emission bands, centered around 890 nm, 1080 nm, and 1350 nm, with sub-split lines. The origin of these PL bands is attributed to the electronic transitions from 4F3/2 to (4I9/2, 4I11/2, and 4I13/2) in Nd3+ ions. The splitting feature of the PL bands can be considered as a sign of a crystalline local environment around the active Nd3+ ions. This local environment induces Stark splitting due to the crystal field interactions [13]. This is contrary to the reported un-split PL bands from Nd.3+ ions that exist in amorphous materials. [14, 15].

Fig. 2
figure 2

Down conversion PL spectra excited at different wavelengths and (a) PLE spectra monitored at emission lines 892 nm, 1079 nm, and 1357 nm (b) of the as-sintered Nd-doped Y2O3 ceramic sample

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This finding agrees with our XRD result that shows the crystalline nature of the prepared nanopowder before sintering process. It can be also noticed that the PL intensity related to 4F3/24I9/2 transition is higher than the two other main transitions. This indicates that Nd-doped Y2O3 sample has favorable emission transition around 0.9 µm, which meets particular applications compared to Nd-doped other hosts like YAG that shows preferentially intense PL band around 1064 nm[16]. To explore the transition levels, the photoluminescence (PLE) monitoring the fundamental lines in the NIR bands has been measured. Figure 2b shows the PLE spectra monitor the PL lines at 892 nm, 1079 nm, and 1357 nm. The PLE spectra show several excitation bands and peaks with slightly higher intensity in the case of monitoring the 892 nm emission line with respect to the monitored 1079 nm emission. This is in line with the higher PL intensity of 892 nm peak. Also, the similarity of the PLE spectral shapes infer that all upper excited levels, assigned as shown on the spectra, contribute in the excitation of all monitored emission peaks. In other words, Nd3+ ions excited from the ground state 4I9/2 to the upper levels, and then relaxed to the meta-stable state 4F3/2 subsequent by the transition to ground states 4I9/2, 4I11/2, and 4I13/2 with emission of photons at around 892 nm, 1079 nm, and 1357 nm, respectively.

In addition, the highest intensity in the PLE peaks is coming from the two manifolds 4G5/2 and 2G7/2 at around 580 nm, which is consistent with the strongest PL intensity excited by this wavelength (see Fig. 2a). This probably points to the high excitation cross-sections of these two levels, which could play a role in the upconversion process. In this direction, the UCPL measurement has been performed under excitation by pulsed laser at the 1064 nm line of Nd:YAG laser. The UCPL spectra were measured in the visible range at room temperature under different laser excitation energies (see Fig. 3a). The UCPL spectra exhibit emission lines that cover most of the visible region with intense lines in the blue-green part. This leads to a visible color in the cyan zone as depicted in the CIE diagram with color coordination at (0.1567, 0.4241) (Fig. 3b). This also holds the promising approach of generating blue light emission pumped by infrared wavelength at 1064 nm of the commercial available Nd:YAG laser.

Fig. 3
figure 3

Upconversion PL spectra excited by 1064 nm at different laser powers (a) and the CIE chromaticity diagram of the upconversion emission (b) of the as-sintered Nd-doped Y2O3 ceramic sample

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It is worth to mention that the assignment of the UCPL lines is in good agreement with the reported UCPL transitions from Nd3+ in different host materials [3, 4, 17, 18]. In the present study, the assignment of the blue, green, and yellowish-orange upconversion emissions will be discussed in the light of the measured PLE compared with the reported work in [3, 4, 17,18,19]. Figure 4a shows the superimposition of one PLE spectrum with one UCPL spectrum. Good overlapping between the PLE bands and the UCPL lines with a slight relative blue shift in the PLE peaks, as guided by the dashed lines, can be observed. This blue shift can be attributed to the Stoke shift between the excited states and the emitted states owing to nonradiative relaxation of the excited ions to these emitting states. This matching between the excitation and emission bands points to their contribution in the UCPL process. However, the exact assignments of these transitions are quite difficult due to the complexity of the energy levels of Nd3+ and the small separations between the sublevels, especially when interact with the crystal field. Nevertheless, a schematic diagram can be proposed based on the PL and PLE results compared with the reported energy diagrams of Nd3+ [4, 19,20,21]. Figure 4b shows the schematic energy levels diagram and the proposed transitions responsible for the UCPL. The blue, green, and orange emission can be assigned to transitions (4G11/2, 2G9/2, 2K15/2–4I9/2), (4G11/2, 2G9/2, 2K15/2, 4G9/2, 4G7/2, 2K13/2–4I11/2, 4I9/2), and (2G9/2,2K15/2, 4G9/2,4G5/2, 2G7/2–4I13/2, 4I11/2, 4I9/2), respectively. On the other hand, it is worth to mention that Sukul et al. [22] attributed the UCPL spectra in the green region to energy transfer between Nd3+ and unavoidable traces of Er ions in the raw materials. However, in this work, the optical contribution from Er ions in our UCPL signal is excluded. Er ions characterize, unlike Nd3+, by energy level match with the excitation by 980 nm [22]. Therefore, UCPL measurement on our sample has been performed under excitation by 980 nm laser (Fig. 4c). It can be observed that No UCPL signal can be detected, which could exclude the possible contribution from Er ions, if any, in the UCPL from Nd ions.

Fig. 4
figure 4

Upconversion PL spectrum, excited by 1064 nm, superimposed with PLE spectrum monitored at 1079 nm emission line (a), schematic energy level diagram of the proposed up and down conversion transitions (b), and Upconversion PL spectrum excited by 980 nm (c) of the as-sintered Nd-doped Y2O3 ceramic sample

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Regarding the UC mechanism, two possible mechanisms in Nd-doped materials have been proposed. The first one, called excited state absorption mechanism (ESA), considers the upconversion excitation a result of subsequent absorption of two or more pumping photons by single ions. The second one, called energy transfer upconversion (ETU), deals with the absorption of two or more pumping photons by different ions followed by energy transfer between them to excite the higher energy levels in one ion. However, both mechanisms can coexist. It is worth to mention that the ETU mechanism requires interaction between at least two Nd3+ ions with populated 4F3/2 levels after pumped by the IR photons. Then, one of the two ions relaxes to the ground states (e.g., 4I9/2, 4I11/2) and transfers its energy to the other excited ion, which receives the energy and goes to the upper excited states (e.g., 4G9/2, 4G11/2). On the other side, the ESA mechanism occurs when the Nd3+ ion absorbs one IR pumping photon to populate 4F3/2 state, then successively absorbs a second pumping photon to go to the upper excited levels (4G9/2, 4G11/2). In this work, the excitation wavelength 1064 nm considers as nonresonant excitation to the metastable state 4F3/2. Hence, the excitation to the upper excited states, for the upconversion emission, can be proposed by successively absorption of 1064 nm photons from the pump as reported in [21]. This process requires high pumping power, as in our case, where the visible upconversion emission starts to be realized at (0.5 MW peak power). This high pumping regime can be also manifested in the power-dependent behavior of the upconversion emission. In this power dependence relation, the UCPL intensity (I) is proportional to Pn, where P is the pumped power and n is the number of absorbed photons involving in the conversion process.

Figure 5 shows the dependence of UCPL intensity at the blue line 489 nm on the pumping power. It can be observed that this dependence is almost linear despite the usual expected quadratic relation in two or higher absorption photons in upconversion process. It is worth to mention that the quadratic relation strongly depends on the strength of the pumping power and only valid at low power regime. In this regard, Pollnau et al. [23] and Suyver et al. [24] have theoretically and experimentally investigated the behavior of the power dependence of the upconversion in lanthanides. They found that, as the pumping power increased, the dependence of the upconversion luminescence changed from quadratic to linear, regardless the number of the absorption and/or the energy transfer steps in either the ESA and/or ETU processes. They also attributed this deviation, in the power dependence, mainly to the competition mechanisms between the nonlinear (upconversion) and the linear (downconversion) processes that occur for the depletion of the intermediate excited state. This likely explains our obtained linear power-dependent behavior as we are working at high power regime, which led to the start of saturation in the upconversion at the maximum peak power (1.7 MW). This saturation effect has been also reported and referred to the abovementioned strong competitions between the nonlinear and linear decays in a strong upconversion process [23, 25, 26].

Fig. 5
figure 5

Upconversion PL intensity excited by 1064 nm as a function of the laser power for the as-sintered Nd-doped Y2O3 ceramic sample

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Excitation with high power and nonresonant wavelength, 1064 nm, stimulate us to propose the ESA as the favorable mechanism occurred in our sample. This is because the ESA considers as a weak process, due to its small cross-section, and preferentially occurs at high power [2]. It is also proposed to be the dominated mechanism in ceramic materials, particularly, of low transparency. In this line, Suárez et al. [20] reported that the ESA mechanism is dominating in ceramic Nd-doped YAG sample compared to the single crystal counterpart. This was attributed to the polycrystalline nature of ceramics. Thus, the excited zone of the sample may re-excited again (favoring the ESA mechanism) by the unabsorbed pump photons subjected to the multi-reflections from the grain boundaries in ceramic materials.

On the other side, luminescence decay kinetics can promote useful information in this regard. The time-resolved photoluminescence of the IR emission bands (downconversion) and the visible emission bands (upconversion) have been measured (Fig. 6). For this purpose, pulsed laser excitations at 532 nm and 1064 nm lines of Nd:YAG laser with pulse duration 40 ns, respectively, were used. Both the decay curves have been fitted with a single exponential function:

Fig. 6
figure 6

Time resolved PL of the down conversion PL excited by 532 nm (a) and time resolved PL of the upconversion PL excited by 1064 nm (b) of the as-sintered Nd-doped Y2O3 ceramic sample

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\(I\left(t\right)={I}_{o }\mathrm{exp}\left({^{-\mathrm{t}}/_{{{\uptau}_f}}}\right)\), where I(t) is the intensity at time t and Io is the initial intensity and (\({\tau }_{\mathrm{f}})\) is the fluorescence lifetime. The fluorescence lifetime of the downconversion can be assigned to the transitions from 4F3/2 to (4I9/2, 4I11/2, and 4I13/2) and estimated around 75 µs. Although this value is shorter than the reported fluorescence lifetime of the same transitions in Nd-doped Y2O3 ceramic [9], it is still in the acceptable range when the discrepancy in the used excitation wavelength and the quality of ceramics are considered. It also infers to the influence of the structural defects and grain boundaries, which are responsible for inducing nonradiative pathways and eventually decreasing the fluorescence lifetime. On the second hand, the fluorescence lifetime of the upconversion emission falls in the nanosecond range, about 130 ns, and characterizes the upper emitting states, which are expected to be shorter than the metastable states in Nd3+ ions [27, 28]. The temporal nanosecond scale of the upconversion emission also suggests the ESA to be considered as dominating mechanism, unlike the ETU which characterizes by a much longer decay time [17].

4 Conclusion

Nd-doped Y2O3 nanocrystals have been prepared via co-precipitation method then transformed to bulk ceramic by sintering process using SPS technique. The structure and morphology, XRD and SEM, results revealed the crystallite phase and size. The DCPL showed the well-resolved NIR emission lines related to the electronic transitions from 4F3/2 to (4I9/2, 4I11/2, and 4I13/2) in Nd3+ ions. The UCPL excited by 1064 nm resulted in emission spectra in the visible region (blue, green, orange). The origin of the UCPL was assigned to (4G11/2, 2G9/2, 2K15/2–4I9/2), (4G11/2, 2G9/2, 2K15/2, 4G9/2, 4G7/2, 2K13/2–4I11/2, 4I9/2), and (2G9/2,2K15/2, 4G9/2,4G5/2, 2G7/2–4I13/2, 4I11/2, 4I9/2) transitions. The mechanism of the UCPL was also discussed in the light of the excitation power-dependent and TRPL measurements. It was found that the UC mechanism likely occurred via excited state absorption process (ESA).