Multiphoton upconversion (UC) emissions from the high-energy states (4 G 9/2, 4 G 7/2, 2 K 13/2, and 2 P 3/2) of Er3+ ions were observed under 980 nm excitation. These high-energy excited states were populated by a five-photon or a four-photon UC process conditionally, which depended on the near-infrared (NIR) pump density. Experiments exhibited that the power dependence originated from the varied populating routes of intermediated 4 S 3/2 and 4 F 9/2 of Er3+ under different NIR pump power. A mechanism of the power density-dependent multiphoton UC processes was proposed based on experimental data and analysis.
©2010 Optical Society of America
Upconversion luminescence (UCL) is an effective physical mechanism for achieving short-wavelength laser radiation. In recent years, RE ions (e.g., Er3+, Tm3+, Ho3+, Nd3+, Pr3+, and Gd3+) doped UCL materials have become a research focus in the material field owing to their unique applications in solid-state lasers, color displays, optical storage, biosensors, and medical diagnostics, etc [1–4]. Especially, the study around IR-to-UV UC compact solid-state lasers is extremely attractive not only because IR laser diodes are compact, power-rich, and inexpensive, but also because UV solid-state lasers have unique potential application in electronic industry [5,6]. Consequently, it has been a great challenge for researchers today to synthesize high-efficiency UV UCL materials and elucidate the mechanism in them.
Erbium, as a famous lanthanide, has been extensively used in optical communication, biolabeling, biosensor, and luminescent materials, etc [7,8]. Er3+ has equally spaced long-lived excited states, which is favorable for the frequency upconversion. However, unlike Tm3+ in some Yb3+−Tm3+ codoped systems  [9,10], high-order UC processes of Er3+ have rarely been detected. The main reason, unfortunately, also comes from the ladder-like medial excited-state manifolds. Closest intermediate excited states along the energy-ladder form numerous dissipative tunnels and lead to excited Er3+ ions losing energy radiatively or nonradiatively before they reach to higher states. In addition, the inevitable back energy transfer from excited Er3+ to adjacent Yb3+ ions also oppress the progress of high-order UC [11,12]. Thus, it is attractive to explore the high-order UC emissions of Er3+ ions for building the short-wavelength compact solid-state laser. In 2008, we reported a 318 nm UC emission of Er3+ ions in YF3 nanocrystals under 980 nm excitation and assigned it a four-photon UC process . The UV UC emission from the 2 P 3/2 → 4 I 15/2 transition of Er3+ in YF3 nanocrystals was not very strong. However, we surmised that a high-efficiency UC matrix, such as β−NaYF4, would improve the high-order UCL of Er3+ ions greatly.
In this letter, we presented an observation of UV UC emissions from Er3+ ions in Yb3+−Er3+ codoped β−NaYF4 microcrystals. Upon excitation with NIR radiation of 980 nm, 276, 289, 304, and 317 nm UC emissions of Er3+ were recorded. In the microcrystals, Yb3+ ions, served as sensitizers, absorbed NIR photons and transferred the energy to Er3+ ions consecutively to populate their high-energy states. These high-energy states of Er3+ ions were populated by a five-photon UC process or by a four-photon UC process conditionally, which depended on the NIR pump density.
Hexagonal microprisms of Yb3+−Er3+ codoped β−NaYF4 were synthesized via the ethylene diamine tetraacetic acid (EDTA)-assisted hydrothermal method according to the procedure described in Ref  and annealed in an argon atmosphere at 400°C for 90 minutes. The crystal structure was analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using a nickel-filtered Cu-Ka radiation (λ = 1.5406 Å) in the range of 20°≤2θ≤80°. The size and morphology were investigated by scanning electron microscope (SEM, Hitachi TM-1000). A 2 W 980 nm continuous wave (CW) diode laser was used as the excitation source to study the UCL of the sample. Upconversion and downconversion luminescence spectra were recorded by a spectrophotometer (Hitachi F-4500) equipped with a hamamatsu R928 photomultiplier (PMT). All spectral measurements were carried out at room temperature with the same instrument parameters (2.5 nm slit width and 400V PMT voltage).
3. Results and discussion
3.1. Structure and morphology of NaYF4:Yb3+(20%)/Er3+(1.5%) microcrystals
The XRD pattern of the sample is presented in Fig. 1(a) . All the diffraction peaks can be readily indexed to those of the pure hexagonal NaYF4 (space group: p63/m) with a = 5.95105 Å and c = 3.5217 Å, which is in good agreement with the standard values for the β−NaYF4 (JCPDS No.16−0334). No other impurity peaks can be detected, which indicates that the microcrystals are single-phased. Figure 1(b) shows the corresponding SEM image of the sample. The microcrystals are hexagonal prisms with uniform size distribution. Their diameters are 2−3 μm and lengths 5 μm on average.
3.2. UV upconversion emissions of Er3+
Under 980 nm excitation, the annealed NaYF4:Yb3+(20%)/Er3+(1.5%) microcrystals emitted UV UC fluorescence, as shown in Fig. 2(a) . Besides the 2 P 3/2 → 4 I 15/2 transition peaked at 317 nm, there are other three weak emissions centered at 276, 289, and 304 nm, corresponding to the 4 G 9/2 → 4 I 15/2, 4 G 7/2 → 4 I 15/2, and 2 K 13/2 → 4 I 15/2 transitions of Er3+, respectively. Figure 2(b) shows the UC spectrum in the range of 330−750 nm, the characteristic emissions of Er3+ from 4 G 11/2, 2 H 9/2, 2 H 11/2, 4 S 3/2, 4 F 9/2 states are shown there clearly.
3.3. Power switched UC mechanisms for blue, red, and UV UC emissions of Er3+
In order to understand the UC mechanism well, the pumping power density dependence of UC fluorescence intensity was investigated. For an unsaturated UC process, the emission intensity is proportional to the n-th power of the excitation intensity, and the integer n is the number of the laser photons absorbed per upconverted photon emitted. Figure 4(a) shows the double logarithmic plots of the emission intensity as a function of excitation power density. The n value can be easily calculated from the slope of the linear fit. They are 4.66, 4.41, 4.46, and 4.39 for the emissions centered at 276, 289, 304, and 317 nm, respectively, demonstrating that these emissions came from five-photon UC processes. Additionally, the ns for 380 and 408 nm are 2.43 and 2.46 (unpublished data), respectively, corresponding to three-photon processes. However, in our previous report about the 2 P 3/2 → 4 I 15/2 transition of Er3+ ions in Y0.83Yb0.15Er0.02F3 nanocrystals, we concluded it to be a four-photon UC process (n = 3.855) . We noted that the different n values for the same transition were obtained in different ranges of pumping power density.
As we known, several factors, such as particle size, Yb3+ concentration, and pump power density, have effect on the population of green levels of Er3+ in Yb3+−Er3+ codoped UC materials [14–16]. Figure 4(b) shows the log-log graph of emission intensity versus excitation power density for 4 S 3/2 → 4 I 15/2 (green) and 4 F 9/2 → 4 I 15/2 (red) transitions of Er3+ in NaYF4:Yb3+(20%)/Er3+(1.5%). When pump power density is lower than 20 W/cm2, n values are 1.46 both for green and red levels; when the power density increases into the range of 20−80 W/cm2, the log-log graph yield slopes of 2.61 and 2.23 for green and red emissions, respectively. The changed n values indicate that populating the green and red levels need different numbers of 980 nm photon under different pump density. Therefore, we can deduce that there exist two different routes, i.e. two- and three-photon processes, in populating the green and red levels under the NIR excitation. The two- and three-photon populating modes work simultaneously in both of the power density ranges and dominate under low and high pump intensities, respectively. However, two-photon mode is the main UC mechanism in populating the red level at any range of power density, although three-photon process has been included partly and the obtained n is slightly larger than 2 at high power density. Similarly, the slope n for the green emission is found to be 1.46 under the lower power density and 2.61 under higher pump density, indicating that three-photon (or two-photon) process is the dominant way in populating the 4 S 3/2 state at high (or low) power density. This special mechanism for the UC process of the 4 S 3/2 state has been addressed well in literature , but three-photon red UC emission (4 F 9/2 → 4 I 15/2) of Er3+ in Yb3+−Er3+ codoped UC materials has not been mentioned at room-temperature, although J.F. Suyver et al have observed the similar result at low-temperature .
3.4. Demonstration of power switched UC mechanisms of Er3+ in NaYF4:Yb3+/Er3+ microcrystals
To corroborate the deduction about the three-photon red emission of Er3+, two downconversion spectra were recorded with the sample under 408 and 520 nm excitation, respectively. The photoluminescence (PL) spectrum of 408 nm excitation, as shown in Fig. 3(b) , exhibits green and red emissions of Er3+ ions. Upon excitation with blue radiation (408 nm), Er3+ ions in ground state 4 I 15/2 can be excited to the 2 H 9/2 state directly, from which the excited Er3+ ions will relax nonradiatively to the 2 H 11/2 and 4 S 3/2 levels and subsequently to the 4 F 9/2 state. This result indicates that the populations in high-energy states (e.g. 2 H 9/2, 2 H 11/2, and 4 S 3/2) of Er3+ ions can be transformed into the populations in low-energy states (e.g. 4 F 9/2), which confirms that three-photon processes can be include in the red UC emission of Er3+ ions. The PL spectrum in Fig. 3(c) further confirmed that red and green levels could be populate in the same mode, which creates another powerful evidence for the three-photon red emission of Er3+. On the other hand, due to the 4 S 3/2 and 4 F 9/2 levels are the intermediate states in populating the 4 G 9/2, 4 G 7/2, 2 K 13/2, and 2P3/2 levels, these high-energy states of Er3+ also can be populated via two different routes.
To further recognize the UC mechanisms in the sample, possible UC processes are schematically given in the energy level diagrams of Yb3+ and Er3+, as shown in Fig. 5 . In a Yb3+−Er3+ codoped system, high-energy levels of Er3+ ions can be populated by the energy transfer (ET) from excited Yb3+ ions and nonradiative relaxation (NR) from higher levels of Er3+ ions by two different routes. Route 1: 4 I 15/2 4 I 11/2 4 F 7/2 2 H 11/2, 4 S 3/2 2 G 7/2 4 G 11/2 4 G 9/2 4 G 7/2, 2 K 13/2, 2 P 3/2. In this case, two photons are needed in populating the 4 S 3/2 states, and therefore four photons are needed to populate 4 G 9/2, 4 G 7/2, 2 K 13/2, and 2P3/2 levels. When the power density is lower than 20W/cm2, route 1 is the dominant way to populate the 4 S 3/2 and the n value is smaller than 2, as shown in Fig. 4(b). Route 2 can be described as: 4 I 15/2 4 I 11/2 4 I 13/2 4 F 9/2 2 H 9/2 2 H 11/2, 4 S 3/2 4 F 9/2. The population processes from 4 S 3/2 to 4 G 9/2, 4 G 7/2, 2 K 13/2, and 2P3/2 levels are the same as in route 1. Three-photon process of green and red levels can be explained by NR5 and NR6, which is confirmed by Fig. 3(b) and (c). Therefore, under high pump power density, it needs five photons to populate the high-energy excited states of 4 G 9/2, 4 G 7/2, 2 K 13/2, and 2P3/2 and the radiative transitions from 4 G 9/2, 4 G 7/2, 2 K 13/2, and 2P3/2 to 4 I 15/2 exhibited as five-photon processes. With increasing the power density up to 20W/cm2, this population mode becomes dominant, which is in accord with log-log graphs in Fig. 4(a).
In conclusion, Yb3+/Er3+ codoped β-NaYF4 microcrystals were synthesized through a facile EDTA-assisted hydrothermal method. The annealed microcrystals presented the characteristic emissions of Er3+ ions under 980 nm excitation. High-order UV UC emissions of Er3+ were observed and these UV emissions come from four- or five-photon UC processes conditionally, which depended on the pump power density of 980 nm NIR light.
This work was supported by the National High Technology Research and Development Program of China (863 Program:2009AA03Z309) and the National Natural Science Foundation of China (NNSFC) (grants 10874058 and 50672030).
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