Lanthanide-doped upconversion nanocrystals (UCNCs) have tremendous applications in multicolor three-dimensional displays, photovoltaic devices, solar cell technology, nanoscale thermometry, data storage, and bioimaging because of their unique upconversion luminescence properties ^[1–5]. Recently, UCNCs have been used as a light converter to enable near-infrared (NIR) light-controlled release of payloads from ultraviolet-sensitive (UV-sensitive) polymer hydrogels or nanocarriers ^[6–8]. By charging UCNCs within UV-sensitive systems, the nanocrystals act as an inside UV light source and emitted UV photons under NIR excitation can be absorbed by the photosensitive polymer, leading to the polymer carrier’s disintegration and activating the photochemical reaction. A photosensitizer can convert light into heat that melts and softens drug-loaded hydrogel-based nanostructures. On the other hand, the high-power UV laser light emitted from UCNCs can also expect to kill the localized cancer cells and thus to achieve the biotherapy application ^[9]. This NIR excitation takes the place of UV light excitation, which is of great importance for light-controlled biomedical applications because there is less damage to healthy cells and enhanced tissue penetration. In this specific application, small-sized UCNCs are highly desirable because their encapsulation by UV-sensitive polymer nanocarriers (e.g., micelles and vesicles) may be easier than the generally used large particles. Therefore, the development of UV lasers emitted from small UCNCs that display efficient upconversion emission under NIR excitation is highly desirable.

The phenomenon that forms a random optical path through scattering in the grain boundaries of the random media is called the random lasing phenomenon, and it leads to the random distribution of lasing modes compared to defined lasing modes in conventional lasers ^[10]. In random lasers, the laser gain media are dependent strongly on the light interaction with the unordered amplifying medium and the scattering strength ^[11–13]. Recently, random lasers have attracted a lot of interest due to simple device structures, easy fabrication, small size, low cost, and their potential application in various fields, in particular, water purification and speckle-free imaging ^[14–18]. Random lasers have been realized in different systems, including conjugated polymer films, organic dye-doped gel films, silver nanoparticles, ZnO films, and TiO2 films ^[19–23]. However, NaYF4-based random lasers have rarely been demonstrated.

To address these issues, uniform NaYF4:20%Yb,2%Tm NCs were synthesized by a solvothermal method ^[24,25] and their room temperature upconversion emission properties were systematically investigated. Efficient upconversion luminescence from NaYF4:20%Yb,2%Tm NCs pumped by a continuous-wave (CW) 980 nm laser was obtained. Furthermore, the NaYF4:20%Yb,2%Tm NCs film was used as a random laser gain medium to demonstrate random lasing by planar microcavities, which hold strong optical confinement with lower cavity losses and a high quality factor ^[26]. The nanosecond pulsed laser with high peak intensity was used to achieve high gain from the NaYF4:20%Yb,2%Tm NCs. As a result, this work can facilitate the use of NaYF4:20%Yb,2%Tm NCs in biologic therapy.

In a typical solvothermal method for the synthesis of high-quality NaYF4:20%Yb,2%Tm NCs, YCl3, YbCl3, and TmCl3 were added to a 100 mL three-neck round-bottom flask containing 3 mL oleic acid (OA) and 15 mL 1-octadecene (ODE) ^[25]. The solution was stirred and heated to 160°C for 30 min to form homogeneous lanthanide oleate complexes, and then cooled down to 50°C. Afterward, a solution of NaOH and NH4F dissolved in 10 mL methanol was added into the flask and stirred quickly for 30 min at 50°C. Subsequently, the solution was heated to 120°C for 30 min to evaporate methanol from the reaction mixture completely, then heated to 300°C in an argon atmosphere for 60 min and then cooled to room temperature naturally. The resulting nanocrystals were precipitated from the solution by the addition of ethanol, and collected by centrifugation. Then the precipitates were washed with an ethanol and water (1∶1 volume ratio) mixture three times and finally redispersed in cyclohexane for further experiments.

High-resolution transmission electron microscope (HR-TEM) images of the NaYF4:Yb/Tm NCs were characterized by a JEOL JEM-2100F with an acceleration voltage of 300 kV. The phase identification was performed by a Rigaku SmartLab Intelligent X-ray diffractometer (XRD) with filtered Cu-Kα radiation (λ=0.15406nm, operating at 45 kV and 200 mA). The step scan covered the angular range from 10° to 80° in steps of 0.04°. Upconversion emission spectra were obtained with an Ocean Optics Maya2000 Pro spectrometer by a power-adjustable CW 980 nm laser diode pumping. The lasing characteristics of NaYF4:Yb/Tm NCs were investigated by third harmonic generation from a YAG pulsed laser (355 nm, 10 Hz) with an optical parameter oscillator to expand the YAG laser to different excitation wavelengths. The lasing spectra were recorded by a Horiba iHR 320 spectrometer. The laser beam was focused on the sample by a 50 mm focal length optical lens and 800 μm laser spot diameters. All of the measurements were conducted at room temperature.

The detailed structure of the NaYF4:Yb,Er NCs was recorded by using TEM and HR-TEM, shown in Fig. 1(a). The TEM images reveal that the NaYF4:Yb,Er NCs are in a nearly spherical shape and uniformly distributed. These regular NCs, which display high-quality, uniform morphology, are likely to be self-assembled on the TEM grid because of the interaction of their surface hydrophobic surfactants (i.e., OA). From the HR-TEM image, we can clearly distinguish lattice fringes on the individual crystals, which indicates that the NCs are highly crystalline in nature and have structural uniformity. The interplanar distance of the NCs was measured to be about 0.5 nm, which matched to a (100) lattice plane of the hexagonal NaYF4 structure (0.515 nm, JCPDS # 16-0334). The selected area electron diffraction (SAED) and size distribution of NCs are given in Figs. 1(b) and 1(c), respectively. The SAED pattern of the NCs further demonstrates a perfect hexagonal crystal structure, which is in good agreement with the XRD results presented in Fig. 1(d). The NaYF4:Yb,Tm NCs with size distribution between 14 and 30 nm and average size about 22 nm without aggregation were observed. The crystal structures and phase purity of the samples were examined by XRD, which demonstrates that the sample was highly crystalline in nature. The peak positions and intensities of the sample pattern well correspond to the reported and calculated patterns for 100, 110, 101, 200, 111, 201, 210, 002, 300, 211, 102, 112, 220, 202, 310, 311, 312, and 302 reflections of hexagonal phase structure β-NaYF4 (JCPDS#16-0334) ^[27]. No cubic phase diffraction peaks or other impurities were detected, which revealed that pure β-NaYF4 had been fabricated. It can be seen that the diffraction peaks of the β-NaYF4 samples are very strong and sharp, which indicates that synthesized products with high crystallinity have been obtained at the high temperature treatment (300°C). Higher crystallinity is very important for phosphors generally due to less traps and stronger luminescence.

The energy transfer (ET) and excited state absorption (ESA) are efficient UC mechanisms in RE3+-doped UC materials. Because Yb3+ ions have a larger absorption cross section at 980 nm, the ET from Yb3+ to Tm3+ plays a key role in UC processes in Yb3+-sensitized Tm3+-doped materials. The schematic diagram of energy levels and transitions of Yb3+ and Tm3+ ions by 980 nm pumping is shown in Fig. 2(a). According to the schematic diagram of upconversion processes, the D21 level of Tm3+ ions cannot be directly populated by an ET from excited Yb3+ to the Tm3+ ions in the G41 level due to the large energy mismatch (∼3500cm−1). Hence, the cross-relaxation processes of F2,33+H34→H63+D12 between Tm3+ ions may alternatively play an important role in populating the D21 level. The upconversion luminescence spectra of NaYF4:Yb,Tm NCs with different pump power under CW 980 nm excitation at room temperature are presented in Fig. 2(b). The nanocrystals dispersed in cyclohexane were held by a quartz cuvette for the photoluminescence (PL) experiment. According to the energy level diagram shown in Fig. 2(a), there were four dominant emission peaks centered at 345, 360, 450, and 474 nm that correspond to the transitions between energy levels I61→F43, D21→H63, D21→F43, and G41→H63 of Tm3+ ions, respectively. It is observed that the upconversion emission intensity increases with an increase in the excitation power at 980 nm. The inset of the Fig. 2(b) gives the dependence of peak intensity ratios, I345/I364, I346/I450, and I450/I474 versus excitation power, Ip. It can be seen that the value of I346/I450 remains insensitive to Ip, and both I346 and I450 simultaneously saturate with increasing of Ip. This can be expected because the optical gain of I346 and I450 consumes the same pool of upconversion population (i.e., D21 excited state). However, their emission intensity is different owing to a difference in differential gain (i.e., stimulated emission cross section).

The random lasing action from NaYF4:Yb,Tm NCs was studied using 980 nm nanosecond laser (6 ns, 10 Hz) excitation. We designed planar microcavities that sandwich the NaYF4:Yb,Tm NCs film with 300 μm thickness between a quartz plate and an aluminum (Al) mirror (Al-coated glass substrate). The mirrors are used to improve the longitudinal confinement of light and achieve optical feedback along the laser microcavities. The pump laser beam is focused on an 800 μm diameter spot by a 50 mm focal lens. The small beam size promotes the lateral confinement of the emission light so that a planer microcavity can be formed ^[26]. Laser emission is detected from the side of the quartz mirror. The spectral evolution with increasing pumping power density and the emission peak intensity as a function of the pumping power are shown in Fig. 3. Figures 3(a) and 3(b) depict the emission spectra of the NaYF4:Yb,Tm NCs laser around 345 and 474 nm. Only the weak spontaneous emission band centered at ∼345nm can be detected for the film of the NCs when the excitation power is well below the excitation threshold value of ∼450kW/cm2, namely, the kink of the light–light curve, as shown in Figs. 3(a) and 3(c). Similarly, as shown in Figs. 3(b) and 3(d), the weak spontaneous emission band centered at ∼474nm is obtained when the excitation power is below the excitation threshold value of ∼62kW/cm2. To further confirm the random lasing action, the full width at half maximum (FWHM) at different pump power densities is also shown in Figs. 3(c) and 3(d). The FWHM acutely decreases from 10 to 5 nm at 345 nm and 13 to 6 nm at 474 nm, respectively, when the pump power increases. In addition, more sharp peaks further emerge from the emission spectra when the pump power increases. However, once the random lasing action is achieved, the lasing wavelength and lasing intensity are determined by the resonance of the microcavity, making the formation of many narrow peaks ^[28]. The NaYF4:Yb,Tm NCs laser spectra show several sharp peaks with line widths less than 0.1 nm when the excitation power is larger than the threshold value. It also can be observed from Figs. 3(a) and 3(b) that the lasing modes are distributed randomly in the lasing spectra because the NaYF4:Yb,Tm NCs are aggregated with each other in the gain film after solvent evaporation. The aggregation results in light scattering in the gain medium ^[29]. As shown in Figs. 3(a) and 3(b), the lasing spectra do not reveal the presence of Fabry–Perot modes as the mode spacing is distributed nonuniformly over the emission spectrum. This phenomenon can also be verified by different lasing spectra obtained from different detection angles, as shown in Figs. 3(e) and 3(f). It is shown clearly that the recorded emission spectra are varied at different angles because the NCs are randomly distributed between the sandwich structure so that the corresponding scattering strength is strongly dependent on the detection angle ^[29]. The left inset of Fig. 3(e) is the optical microscope image of the NaYF4:20%Yb,2%Tm NCs, which verifies that the random light scattering occurs in the NaYF4:Yb,Tm NCs gain medium film. As results, it is verified that NaYF4:Yb,Tm NCs film supports coherent random lasing action.

We have demonstrated UV and blue light upconversion random lasing from NaYF4:Yb,Tm NCs. It is noted that lasing emission with a peak wavelength of ∼345nm and ∼474nm under 980 nm nanosecond excitation is obtained from the NaYF4:20%Yb,2%Tm NCs film sandwiched between an Al mirror and a quartz mirror. This longitudinal optical confinement is achieved via the optical feedback between the two interfaces. Hence, the formation of a low-loss planar microcavity can support the random lasing action at room temperature. The discrete sharp peaks show the formation of a closed light loop path, with a linewidth ∼0.1nm achieved from the emission spectra. The variation of the emission spectra with different detection angles verifies the support of random lasing action. As a result, our proposed NaYF4:20%Yb,2%Tm NCs, for which the realization of upconversion random lasing has been verified unambiguously, are potential optical gain media suitable for optical applications.