In recent decades, the development of ultraviolet (UV) solid-state lasers has attracted great interest for their advantages of stable physical and chemical properties, narrow fluorspar spectral lines, strong absorption band, and high fluorescence quantum efficiency. UV lasers based on solid-state materials have many practical applications, such as medical procedures, semiconductor processing, optical communications and remote sensing, and so on. Although various kinds of lasers and tunable color center lasers have been investigated as UV sources, their applications are still restricted because many of them only operate at low temperatures and some of their materials deteriorate^[1,2].

Rare earth ion-doped solid-state materials are mainly considered for devices due to the abundant energy levels of rare earth ions and the super properties of some of their matrixes. The Ce3+ ion is one of the most important active ions applied to UV lasers because of the transition 5d→4f around 325 nm^[3]. The optical spectra and UV lasers based on Ce3+ ion-doped single crystals, glasses, and glass ceramics have already been studied^[3–7].

Besides the rare earth ion dopant, the host material also plays an important role in the doping concentration, luminescence efficiency, thermal stability, etc. In the reported Ce3+ ion-doped materials, fluoride single crystals such as LiYF4 were mostly mentioned due to their low phonon energy, good chemical and thermal stability, high transparency at UV wavelengths, and high luminous efficiency^[3,8].

NaYF4 crystals, such as the cubic α-NaYF4 and the hexagonal β-NaYF4, have been widely studied for their application as excellent host for up-conversion^[2] because of their low maximum phonon energy, which is about 360cm−1. More recently, α-NaYF4 bulk single crystals doped with Ho3+/Yb3+^[9], Tb3+/Eu3+^[10], and Tm3+^[11] were successfully grown in our laboratory by an improved flux Bridgman method. The transmittance in the UV wavelength and the Raman spectra of an α-NaYF4 single crystal have already recorded^[9–11]. Compared with LiYF4 single crystals, the α-NaYF4 has a lower-value maximum phonon energy, about 390cm−1, and a higher transparency in the UV range^[2,8]. The excellent properties of α-NaYF4 single crystals in thermal, mechanical, chemical, and optical behaviors are beneficial for its application in UV lasers to obtain a high luminous efficiency and high stability. However, there have been few reports for Ce3+ ion-doped α-NaYF4 bulk single crystals for UV lasers because of the difficulty of the crystal growth. In this Letter, the Ce3+ was incorporated in an α-NaYF4 single crystal. The concentration of Ce3+ in the α-NaYF4 single crystal was measured, and the optical performance of the obtained single crystal was explored to get a better-quality UV emission.

The Ce3+ doped α-NaYF4 single crystal was grown by a vertical Bridgman method using KF as the flux in a NaY-YF3 system. The raw materials were commercially available powders of high purity NaF (99.99%), KF (99.99%), YF3 (99.99%), and CeF3 (99.99%). The moisture and the some oxygen impurities in the fluoride powders could be removed completely using the high-temperature hydrofluorinating method, by which the powders were sintered with anhydrous HF at 750°C for 8 h. The molar ratio of the powders was NaF:KF:YF3:CeF3=30∶18∶48∶4. Then, the mixture of those materials was ground about an hour in a mortar. The growth orientation was along the a-axis by using an oriented seed crystal. The temperature gradient cross-solid-liquid interface was 70–90°C/cm; the growth process was carried on by lowering the crucible at a rate of ∼0.6mm/h. The detailed process of crystal growth is described elsewhere^[9,10]. The obtained crystal shows high transparency. However, a white region of several centimeters in length which is main KF, CeF3, and some deviating components excluded from the melt exists at the top of the boule, corresponding to the final portion of the melt to freeze. The as-grown crystal was cut into pieces and well polished. The inset of Fig. 1 shows a polished piece of crystal. An fxamination using a polarizing microscope showed that the single crystal part is free from macroscopic defects.

The Ce3+ concentration in the α-NaYF4 single crystal was measured by an inductive coupled plasma (ICP) atomic emission spectrometer (PerkinElmer Inc., Optima 3000) and was about 0.788×1020ions/cm3. The phase composition of the as-obtained crystals was recorded by x ray diffraction (XRD) using an XD-98X diffractometer (XD-3, Beijing). The scan was performed in the 2θ range from 10°–90° with a scanning step size of 0.02° and a 6 s swept time. A Cary 5000 UV/VIS/NIR spectrophotometer (Agilent Co., America) recorded the absorption spectra ranging from 200 to 600 nm. The optical properties of the α-NaYF4 single crystal doped with Ce3+ ions were characterized by excitation spectra and photoluminescence spectra measured using an F-4500 spectrophotometer (Hitachi high-technologies Co., Tokyo, Japan). All the above measurements were carried out at room temperature, and the samples were under the same conditions to get comparable results.

The phase compositions of the as-prepared sample were analyzed by powder XRD. Figure 1(a) shows the XRD patterns of the α-NaYF4:Ce3+ sample. Figure 1(b) shows the XRD patterns of card JCPD 77-2042^[10,11]. It confirms that the transparent crystal obtained is an α-NaYF4 single crystal and that KF was excluded from the final part of the boule. Notably, compared with card JCPD 77-2042, the peak positions and intensity pattern matched well. It also confirms that the current Ce3+ ion doping level and introduction of K+ ions do not bring any meaningful changes to the single crystal structure. As is well known, an α-NaYF4 single crystal has a fluorite structure (CaF2), in which Na+ and Y3+ ions are randomly distributed in the cation sites, which are situated in the cubic center, and its lattice constants could be calculated from the XRD pattern data as the following lattice parameters^[12]: aL=bL=cL=0.5510nm for the lower part of the single crystal corresponding to the initial stage of crystal growth, and aU=bU=cU=0.5562nm for the upper part of the single crystal corresponding to the final stage of crystal growth, which are close to the standard parameters as shown in Fig. 1 (a=b=c=0.5470nm).

The absorption spectra of Ce3+-doped α-NaYF4 single crystals are shown in Fig. 2. It is obvious that the Ce3+:α-NaYF4 single crystals have only one intense absorption peak located at 300 nm. The absorption peak represents the absorption transition of Ce3+ ion in an α-NaYF4 single crystal from the ground state to the excitation state, viz, the 4d→5f electronic transition. In contrast, the single crystal doped with Ce3+ grown by the Czochralski method shows the absorption peak at 297 nm^[3]. They are almost similar. Figures 3(a) and 3(b) display the excitation spectrum of the Ce3+-doped α-NaYF4 single crystal by measuring the emission at 332 nm and the emission spectrum for the Ce3+-doped α-NaYF4 single crystal under excitation at 300 nm wavelengths. It can be noted from Figs. 3(a) and 3(b) that there exists an excitation band at ∼300nm and an strong, sharp UV emission band at the wavelength of 332 nm.

It is known that the stable state of the rare-earth Ce3+ ion has a 4f1 configuration and the excited state has a 5d1 configuration^[13–15]. The 4f configuration is in the 5s configuration and 5p configuration. Thus, interference from the surrounding plasma electric field is small, and its energy level splitting obeys the spin orbit. The 4f1 configuration splits into two levels with F7/22 and F5/22 for the spin-orbit (LS) coupling. The first excited state of Ce3+ ions is the 5d configuration; because the 5d configuration is more sensitive to the environment, the strong electro-acoustic coupling divides the 5d configuration into five configurations at room temperature. Because the 5f→4d electronic transition is allowed, the wavelength of 332 nm belongs to the electric dipole transition of the 5f→4d electronic transition and it can be seen as consisting of two broadbands. All luminescence spectra can be fitted by two Gaussian functions corresponding to the luminescence transitions from the lowest 5d state to the F5/22 and F7/22 levels of the 4f state^[3]. Figure 4 shows fitted results of the luminescence of Ce3+:α-NaYF4 by two Gaussian functions corresponding to the emission spectrum of the excitation wavelength of 300 nm. The fitted results indicate that the centers of the 5d→F5/22 and 5d→F7/22 luminescence transitions are at 330 and 350 nm, respectively. The splitting energy of about 2000cm−1 between the F7/22 and F5/22 levels can be reflected from the two emission peaks. Table 1 shows the fitted results of the luminescence spectra of the Ce3+:NaYF4 single crystal.

In conclusion, the Ce3+ ion can be incorporated into an α-NaYF4 single crystal by a modified flux Bridgman method. When excited by the wavelength of 300 nm light, the emission spectra of the α-NaYF4 single crystals doped with Ce3+ ions can emit an intense beam of UV at the wavelength of 332 nm, which is composed of 330 and 335 nm broadbands and which is due to the electric-dipole transition of the 5f→4d electronic transition. The strong emission band at UV wavelengths and the super chemical-physical properties indicate that the Ce3+ ion-doped α-NaYF4 single crystal has potential applications in UV lasers.