1. Introduction

Continuous-wave (cw) ultraviolet (UV) lasers have found a number of promising applications such as: optical data storage, color displays, undersea communication, environmental monitoring, etc [1–3]. Upconversion (UC) pumping by means of the ladder-like energy levels in rare-earth ions can play a significant role for realizing such short-wavelength lasers, due to the ample availability of cost-effective high-power near infrared (NIR) diode lasers [1]. Efficient two-photon visible UC radiations have been extensively investigated in recent years [4–7], resulting in, e.g., the development of a room temperature cw green UC laser with an output power of 0.5 W [4]. However, high-order multi-photon UV UC radiations induced by NIR lasers have rarely been reported due to their inefficient generation mechanisms [8–12]. Switching of inefficient high-order multi-photon mechanisms into efficient two-photon mechanisms could have large impact in developing cw UV UC lasers and for extending UV UC emissions to even shorter wavelengths.

Decreasing the photon process can be realized by utilizing saturation effects induced via the intrinsic radiation characteristics of rare-earth ions in appropriate host lattices [13,14]. Usually, for a specific rare-earth ion, the higher the energy level is, the shorter the lifetime will be [15]. Long-lived intermediate excited states can thereby become energy reservoirs and function similarly to the ground state when populating higher energy levels. This merit can allow populating rare-earth ions’ excited energy levels through a series of saturation at the intermediate states. These consecutive saturations of intermediates can decrease the effective photon processes and transfer most of the absorbed energy to the highest energy level, resulting in a two-photon sensitization mechanism which populates the highest energy level. Such super saturation (SS) UC process can be utilized for achieving UV lasers, since it offers a novel way to allow efficient pumping via the use of cost-effective high-power NIR diode lasers. Although the sensitized SSUC effect has been theoretically discussed in Ref. 14, however, its importance for UV UC lasers and their experimental realization in near vacuum UV range has not been reported so far. In this letter, we report on SSUC processes for generating near vacuum UV UC radiations in Er³⁺ and Gd³⁺ ions under 974-nm diode laser excitation.

NaGdF₄ is known to be one of the most efficient host materials for UC emissions, since it has low nonradiative losses due to a low phonon cutoff energy of about 400 cm⁻¹ [16]. Trivalent Gd³⁺ ions have large energy gaps, e.g., of about 32000 cm⁻¹ between the first excited ⁶P_7/2 state and the ground state [10,17]. These characteristics determine the long lifetimes of the Gd³⁺ ions’ excited states, e.g., about 10 ms for the ⁶P_7/2(Gd) state in a single NaGdF₄ crystal [18]. Trivalent Er³⁺ ions have several energy levels that overlap with the Gd³⁺ ions, e.g., the ²P_3/2, ²K_13/2, and ²D_5/2 states have similar energy to the ⁶P_7/2(Gd) state [19,20]. The strong coupling between Er³⁺ and Gd³⁺ enables the generation of common long-lived states or the accommodation of large populations produced via the SSUC process in the long-lived reservoir states of high Gd³⁺ ions. Therefore, it is challenging to investigate near vacuum UV UC radiations in Yb³⁺/Er³⁺-codoped NaGdF₄ microcrystals.

2. Experimental

NaGdF₄ (NaYF₄) powders doped with 2 mol% Er³⁺ and 20 mol% Yb³⁺ ions were synthesized by a co-precipitation method [21], through annealing at 600 °C for 5 h in air. Details for the measurement of room temperature UC fluorescence have been presented in our previous work [22]. In the present work a Hamamatsu R7154 photomultiplier tube was employed to detect the UV UC radiations due to its sensitive wavelength range of 160-320 nm. X-ray diffraction experiments evidenced that the prepared NaGdF₄ (NaYF₄) powders have a hexagonal crystal structure, and transmission electron microscopy showed that the obtained powders had a size range of 200-500 nm.

3. Results and discussions

3.1. Near vacuum UV UC radiations of Er³⁺ and Gd³⁺ ions

Figure 1 displays UV UC radiation recordings (with a relatively high resolution of 1 nm) of (a) NaGdF₄ and (b) NaYF₄ powders under 974 nm diode laser excitation of 37 W/cm². Figure 1(a) shows four bands centered at 245.8, 252.3, 275.8, and 311.1 nm corresponding to transitions from ⁶D_5/2,7/2, ⁶D_9/2, ⁶I_J, and ⁶P_J to the ground ⁸S_7/2 state of Gd³⁺ ions, respectively [23]. The sharp peaks in these bands have been assigned to the J sub levels of Gd³⁺ ions. Figure 1(b) displays the seven measured UV UC bands centered at 242.9, 254.6, 274.4, 284.2, 289.2, 303.3, and 316.1 nm, which correspond in order to the Er³⁺ ions’ transitions from ²L_17/2, ⁴D_7/2, ²H(2)_9/2, ²D_5/2, ⁴G_7/2, ²K_13/2, and ²P_3/2 to the ground ⁴I_15/2 state, respectively [20,24]. Comparison between Figs. 1(a) and (b) illustrates that the peaks marked by asterisks at 242.9, 254.6, and 316.1 nm in Fig. 1(a) arise from Er³⁺ ions’ UV UC emissions, and that the UV UC radiations in NaGdF₄:Yb³⁺/Er³⁺ powders are about two orders of magnitude higher than those of the NaYF₄:Yb³⁺/Er³⁺ powders. The inset in Fig. 1 is an extension of Fig. 1(a) towards shorter wavelengths. As can be seen, the shortest fluorescent bands that upconverted from the cw NIR light were observed at 191 and 202 nm, which correspond to ⁶G_13/2→⁸S_7/2 and ⁶G_J→⁸S_7/2 (J = 3/2, 5/2, 7/2, 9/2, 11/2) in the Gd³⁺ ions, respectively.

 

Fig. 1 Recorded UV UC radiation spectrum of (a) NaGdF₄ and (b) NaYF₄ microcrystals doped with 20 mol%Yb³⁺, 2 mol% Er³⁺ ions in the wavelength range 230-330 nm. The inset is an extended recording of Fig. 1(a) in the spectroscopic range of 185-230 nm.

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3.2. Mechanisms for near vacuum UV UC radiations of Er³⁺and Gd³⁺ ions

The energy level diagrams of Er³⁺, Yb³⁺, and Gd³⁺ ions as well as the proposed UC mechanisms are depicted in Fig. 2 . Regarding NaYF₄:Yb³⁺/Er³⁺, the ⁴G_11/2 and ²H_9/2 states emitting 390 and 410 nm radiations are populated by three energy transfers (ETs) from excited Yb³⁺ ions, employing two alternative UC pathways [25,26]. Subsequently, two other ETs from Yb³⁺ ions can excite Er³⁺ ions from the ⁴G_11/2 and ²H_9/2 to the ²H(2)_9/2 and ²D_5/2 states (with an energy mismatch of about 310 cm⁻¹ and 211 cm⁻¹), generating radiations at 274.4 and 284.2 nm, respectively. Cascading nonradiative relaxations can then populate in order the ⁴G_7/2, ²K_13/2, and ²P_3/2 states where 289.2, 303.3, and 316.1 nm UV UC radiations arise, respectively. Additionally, Er³⁺ ions in the ²P_3/2 state can be excited to the ²L_17/2 state by another ET from the ²F_5/2(Yb) state (with an energy mismatch of about 254 cm⁻¹), followed by nonradiative relaxations to the ⁴D_7/2 state. UV UC emissions at 242.9 and 254.6 nm were then generated by transitions from the ²L_17/2 and ⁴D_7/2 to the ground state, respectively. It should be noted that four-photon UC processes are involved to populate the ²H(2)_9/2, ²D_5/2, ⁴G_7/2, ²K_13/2, and ²P_3/2 states, while five-photon UC processes are involved to populate the ²L_17/2 and ⁴D_7/2 states.

 

Fig. 2 Energy level diagrams of Er³⁺, Yb³⁺, and Gd³⁺ ions as well as the proposed mechanisms to produce UV UC radiations.

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As for the NaGdF₄:Yb³⁺/Er³⁺ powders, ⁶I_J(Gd) states can be populated by an efficient ET 2 process from the ²H(2)_9/2 state of Er³⁺ ions with an energy mismatch of about 122 cm⁻¹, whereas ⁶P_J(Gd) states can be populated by an efficient ET 1 process from the ⁴D_5/2, ⁴G_7/2, and ²K_13/2 states of Er³⁺ ions (The energy mismatch is 1673 cm⁻¹, 859 cm⁻¹, and 196 cm⁻¹, respectively). Nonradiative relaxations from the ⁶I_J(Gd) state can also populate the ⁶P_J(Gd) state with the energy of about 2560 cm⁻¹ lower [10]. Decays from the ⁶P_J(Gd) and ⁶I_J(Gd) states to the ground state can then generate UV UC radiations at 311.1 and 275.8 nm, respectively. Subsequently, two ETs from Yb³⁺ ions can promote Gd³⁺ ions in the ⁶P_J state consecutively to the ⁶D_J and ⁶G_J states, which generate UV UC radiations at 245.8 and 200 nm, respectively. It should be noted that, due to the dominated resonant ET 1 and ET 2 processes, the populations in ⁶P_J and ⁶I_J of Gd³⁺ ions should be proportional to the populations in the ²K_13/2 and ²D_5/2 of Er³⁺ ions, respectively. Thus, the ⁶P_J(Gd) and ⁶I_J(Gd) states involve four-photon UC mechanisms, while the ⁶D_J(Gd) and ⁶G_J(Gd) states involve five- and six-photon UC processes, respectively.

3.4. Analysis on comparison results of near vacuum UV UC in NaYF₄:Yb³⁺/Er³⁺ and NaGdF₄:Yb³⁺/Er³⁺ powders

The observed two orders higher UV UC radiation efficiency of the NaGdF₄ compared to NaYF₄ might arise from the strong coupling between the Er³⁺ and Gd³⁺ ions bridged over by ETs 1 and 2. It is found that the Er³⁺ ions have similar visible and NIR radiation parameters in NaGdF₄ and NaYF₄ lattices due to their similar properties (unpublished data), leading expectedly to similar magnitudes of the UV UC radiations. This contradiction might derive from the fact that the previous dynamic equilibrium in Yb³⁺/Er³⁺-codoped NaYF₄ is destroyed by ETs 1 and 2 which transfer nearly all the UV populations from lower excited Er³⁺ to higher excited Gd³⁺ ions. To establish a new dynamic equilibrium in the Er³⁺ ions, a large population from the lower energy levels needs to be transferred up by excitation, which seems as if the Yb³⁺/Er³⁺ system is being exerted to an ‘upconverting force’ by the Gd³⁺ ions. Similarly, this can induce a series of similar results in the intermediate states and thus increase their UC rates, leading together with their low losses to a series of saturations in these intermediate states. This conclusion has been verified by direct observations of a reduction of slope n values at the green, red, 390-nm and 409-nm UC radiations in NaGdF₄:Yb³⁺/Er³⁺ powders as compared to that in NaYF₄:Yb³⁺/Er³⁺ powders.

Efficient ETs 1 and 2 from lower Er³⁺ to much higher Gd³⁺ ions can cause the extinction of UV UC radiations from the ²H(2)_9/2, ⁴D_5/2, ⁴G_7/2, ²K_13/2 levels of Er³⁺ ions, as displayed in Fig. 1(a). The observed UV UC radiation from the ²P_3/2(Er) state in NaGdF₄ arises from the energy back transfer (EBT 1) from the ⁶P_J(Gd) to ²P_3/2(Er) state that has 500 cm⁻¹ lower energy [17]. The weaker UC fluorescent intensity of the ²P_3/2(Er) than ⁶P_J(Gd) state derives from the fact that Er³⁺ ions are about 40 times lower than Gd³⁺ ions and both states have the same radiation rate due to their strong coupling (unpublished data). Population of the ²P_3/2(Er) state by EBT 1 can lead to the population of the ²L_17/2(Er) and ⁴D_7/2(Er) states, which generates the 242.9 and 254.6 nm UV UC radiations marked by asterisks in Fig. 1(a). The EBT 2 process from the ⁶D_J(Gd) states may also play a role for populating the ⁴D_7/2(Er) state.

3.5. Demonstration of SSUC mechanism in NaGdF₄:Yb³⁺/Er³⁺ powders

Figure 3 shows the pump power dependences of all radiations in the NaGdF₄:Yb³⁺/Er³⁺ powders. The number of photons required to populate the upper emitting state can be obtained by the relation ${\text{I}}_{\text{f}}\propto {\text{P}}^{\text{n}}$, where I_f is the fluorescent intensity, P is the pump laser power, and n is the number of the laser photons required [9]. As illustrated in Fig. 3, n ≈2 is observed for 276- and 311-nm four-photon, and 252-nm five-photon UV UC radiations at such low pumping powers as 14-42 W/cm². This demonstrates the occurrence of a SSUC phenomenon for generation of the UV radiations. It is also noted from Fig. 3 that all the intermediate states (emitting red, green, 409- and 390-nm radiations) have the same value of n = 1 as the ²F_5/2(Yb) state (emitting 1000-nm radiation). This suggests that the SSUC mechanism is induced by consecutive saturations at the intermediate states, illustrating clearly that the UC rate in any intermediate state is much larger than its corresponding decay rates [14]. Therefore, it can be perceived that all the intermediate states function as a series of “railway stations” for “upconverted population trains”, finally leading to the occurrence of the SSUC phenomenon. The almost zero value of n = 0.3 arises from the saturation of the ⁴I_13/2(Er) state [27,28].

 

Fig. 3 Pump power dependences of all fluorescent radiations (>250 nm) in the NaGdF₄:Yb³⁺/Er³⁺ powders.

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4.Conclusions

In summary, we have demonstrated the occurrence of a SSUC mechanism for generating near vacuum UV emissions in Gd³⁺ and Er³⁺ ions under 974 nm cw excitation. A series of ETs from the Yb³⁺ ions, inducing consecutive saturations in Er³⁺ ion’s intermediate states, lead to UV UC radiations in Er³⁺ ions, while two resonant ETs from Er³⁺ to Gd³⁺ ions evoke UV UC radiations of Gd³⁺ ions. It is found that the SSUC can switch inefficient four- and five-photon processes into efficient two-photon process, offering a novel efficient way to pump cw UV UC lasers with NIR light.