Introduction

Luminescent materials that convert excitation photons into prescribed emissions are at the core of many photonics technologies such as varicolored displays and programmable photoactivation1,2. Amongst various luminescence processes, photon upconversion characterized by high-energy emission upon excitation of lower-energy photons is of exceptional interest. Upconversion primarily takes advantage of lanthanide-doped materials, in which the stepwise excitation through the energy levels of the lanthanide activators results in visible and ultraviolet emissions by successive absorption of multiple near-infrared photons3,4,5,6,7. The unique upconversion process has enabled a diversity of applications ranging from bioimaging to solar energy conversion and optical storage8,9,10,11,12,13. In particular, upconversion is considered as a promising solution to generating short-wavelength lasing by pumping with longer-wavelength light sources that are more readily acquired14,15.

Frequency upconversion holds potential for cost-effective construction of miniaturized deep-ultraviolet (UV) emission devices that find enormous medical and industrial applications, such as microbial sterilization and biomedical instrumentation16,17,18,19. However, the implementation of such a technique has been constrained by the limited spectral tunability of upconversion, which occurs in special lanthanide ions comprising fixed sets of energy levels. For example, one important class of light sources are lasers operating in the telecommunication wavelengths (1260 to 1675 nm)20,21, which are extensively used in fiber-optic communication and photonic circuits because of minimal optical attenuation, ready accessibility in various forms, and low cost for device fabrication. In addition, the wavelengths fall in the second near-infrared window (NIR-II) that is favorable for high-resolution in vivo bioimaging owing to maximal tissue transparency and minimal autofluorescence22,23. However, only a small number of Er3+-sensitized materials are capable of upconverting excitation light in this wavelength range, which displays dominated Er3+ emissions across a limited spectrum24,25,26,27,28. It remains a daunting challenge to achieve deep-UV emission by excitation in the telecommunication wavelengths.

To expand the spectral tunability of upconversion, herein we propose a domino upconversion (DU) scheme, in which energy amassed in one upconversion course triggers another succeeding upconversion process (Fig. 1). By a tandem combination of Er3+- and Tm3+-based upconversion in a core–shell–shell nanostructure, deep-ultraviolet emission is realized by excitation at 1550 nm with an ultralarge anti-Stokes shift of up to 1260 nm. We systematically investigate the energy cascade processes in the core–shell–shell nanostructures and demonstrate deep-ultraviolet lasing at 289.2 nm through the DU scheme by excitation at the telecommunication wavelength.

Fig. 1: Comparison of the conventional energy transfer upconversion (ETU) and the proposed domino upconversion (DU) processes.
figure 1

a In an ETU process, the excitation energy is only amassed in one type of lanthanide upconverting ion. b In a DU process, the excitation energy amassed in one upconverting ion triggers energy amassment in a second upconverting ion, leading to an ultralarge anti-Stokes shift.

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Results

Synthesis and characterization

As a proof-of-concept experiment, we constructed a NaYF4:Yb/Tm@NaErF4:Ce @NaYF4 core−shell−shell nanoparticle with the Tm3+- and Er3+-based upconversion processes separately incorporated into the core and interlayer of the nanoparticle, respectively (Fig. 2a). The outermost shell of NaYF4 was designed to protect the nanoparticle against surface quenching29,30. The spatial separation of the dopant ions was intended to minimize the cross-talk between different upconversion processes31,32, which were independently optimized in their respective doping domains. To facilitate the DU process through interfacial energy transfer, we also employed high concentrations of Er3+ and Yb3+ dopants that are highly resistant to concentration quenching (Fig. 2a)25,33,34.

Fig. 2: Ultralarge anti-Stokes emission through DU in core–shell–shell nanoparticles.
figure 2

a Schematic design of a NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 core–shell–shell nanoparticle for DU (left panel) and proposed energy transfer mechanism in the nanoparticle. b HAADF-STEM image of the NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 nanoparticles highlighting the layered structure. c Digitally processed high-resolution TEM image of a NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 nanoparticle showing the single crystalline nature. d An enlarged view of the selected area in c, indicated by a white box, showing the hexagonal structure of the lattice in accord with the NaYF4 crystal (right panel). e Schematic illustration of the waveguide circuit for excitation of upconversion nanoparticles. Due to the convergence of the laser beam, the power density in the waveguide circuit (IW) was amplified relative to that in the incident fiber (IF). f Emission spectrum of the NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 nanoparticles by excitation of the waveguide circuit at 1550 nm with a high-power density of 2073 kW cm−2.

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The nanoparticles were synthesized by a layer-by-layer epitaxial growth protocol35, which involved the preparation of NaYF4:Yb/Tm core nanoparticle followed by the epitaxial growth of the NaErF4:Ce interlayer and the NaYF4 shell (Supplementary Fig. 1a). Figure 2b shows the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the sample, revealing the highly uniform size and morphology of nanoparticles, with a distinguished Z-contrast between the NaErF4:Ce (30%) interlayer and outermost NaYF4 layer. The high-resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction (XRD) measurements further confirmed the high crystallinity of the nanoparticles with a single hexagonal phase (Fig. 2c, d and Supplementary Figs. 1b, 2).

Ultralarge anti-Stokes emission through DU

We next measured the emission spectrum of the nanoparticles under 1550 nm excitation. The nanoparticles were deposited on the top of a waveguide structure that functioned as the excitation source (Fig. 2e). The waveguide circuit was semi-buried in a SiO2 substrate, with the top surface exposed to contact the nanoparticles (Supplementary Fig. 3a, b). Owing to its small dimension, the waveguide structure spatially confines the incident light and thus enhances the power density of the excitation field36. In a specific case, we estimated a high excitation power density of 2073 kW cm−2 at an input power of 311 mW (Supplementary Fig. 3c). The emission spectrum consists of characteristic emission peaks of Tm3+ that can be assigned to the 1I6 → 3H6 and 3F4 (290 and 347 nm), 1D2 → 3H6 and 3F4 (362 and 453 nm), 1G4 → 3H6 and 3F4 (478 and 649 nm), and 3H4 → 3H6 (803 nm) transitions, respectively (Fig. 2f). The observation of strong upconversion emission in the short-ultraviolet wavelength region suggests an efficient Tm3+ upconversion sensitized by Er3+. Note that the Yb/Tm-doped upconversion layer alone does not respond to the 1550 nm excitation (Supplementary Fig. 4).

It is worth noting that the inclusion of Ce3+ dopants in the NaErF4 layer is essential for achieving the DU process. Figure 3a compares the emission spectra of nanoparticles without and with Ce3+ dopants in the interlayer, which revealed substantial attenuation of Tm3+ emissions in the absence of Ce3+. The Ce3+ ions contributed to the DU by inhibiting high-order upconversion in Er3+ ions through cross-relaxation (Supplementary Fig. 5), which resulted in a preferential population of the 4I11/2 state28. A large 4I11/2 population facilitated energy transfer to Yb3+ ions and subsequent upconversion in the Tm3+ ions (Fig. 3b, c and Supplementary Fig. 6a). Without the Ce3+ dopants, the Er3+ ions were straightforwardly excited to the higher-lying excited states, followed by radiative transitions to the ground state that gave rise to the dominated emission of Er3+ ions (Supplementary Fig. 6b).

Fig. 3: Mechanistic investigation of Ce3+-induced cross-relaxation.
figure 3

a Emission spectra of the NaYF4:Yb/Tm@NaYF4:Ce/Er@NaYF4 nanoparticles under excitation of 1550 nm at 2073 kW cm−2 as a function of Ce3+ doping concentration in the interlayer. b, c Proposed energy transfer pathways in the NaYF4:Yb/Tm@NaYF4:(Ce/)Er@NaYF4 nanoparticles without and with Ce3+ dopants, respectively. d Emission spectra of the NaYF4@NaYF4:(Ce/)Er@NaYF4 nanoparticles without and with Ce3+ dopants under 1550 nm excitation at high (2073 kW cm−2) and low (5 kW cm−2) powers, respectively. e Emission intensity at 346 nm (Tm3+) as a function of excitation power density in the NaYF4:Yb/Tm@NaYF4:(Ce/)Er@NaYF4 nanoparticles without and with Ce3+ dopants, respectively. f Schematic illustrations of excitation power-dependent preferential population of energy levels in Er3+ ions through cross-relaxation with Ce3+ dopants. g Simulated populations of 4I11/2 energy levels as a function of excitation power density in NaYF4:(Ce/)Er without and with Ce3+ dopants, respectively. Mechanistic calculations by formulating the rate equations as in the Supplementary Methods.

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The DU process is strongly affected by the content of Ce3+ ions. By correlating the emission intensity with Ce3+ concentration in the interlayer, the optimal Ce3+ doping concentration was determined to be 30% (Supplementary Fig. 7b). The reduction of upconversion emission at substantially high Ce3+ concentration (>30%) is partially attributed to the large lattice mismatches between the core/shell components, which resulted in nonuniform epitaxial growth processes (Supplementary Fig. 7a)37,38,39.

To substantiate the role of Ce3+ ions in the selective quenching of Er3+ ions, we compared the visible and NIR emissions of NaYF4@NaErF4:Ce@NaYF4 nanoparticles with and without Ce3+ dopants (Supplementary Fig. 8). Yb3+ and Tm3+ ions were removed to avoid disturbance to the Er3+ emission. As anticipated, we observed enhancement of the NIR emission in the Er3+ ions at the expense of the visible emissions due to the inclusion of Ce3+ dopants (Fig. 3d). Furthermore, the decay times of the 4S3/2 and 4F9/2 states of the Er3+ ions were shortened by the Ce3+ dopants, indicating a nonradiative energy transfer from Er3+ to Ce3+ ions (Supplementary Fig. 9).

It is worth noting that a high excitation power density is also essential for achieving the DU process40,41. We observed that the Er3+ emission at around 980 nm (4I11/2 → 4I15/2) was quenched by Ce3+ dopants at low excitation powers (Fig. 3d, bottom panel). Correspondingly, DU emission in Tm3+ ions was also quenched by Ce3+ under low-power excitation (Fig. 3e and Supplementary Fig. 10). The results were ascribed to 4I11/2 → 4I13/2 cross-relaxation in Er3+ ions induced by Ce3+. A high excitation power promotes the 4I13/2 → 4I9/2 excitation process, which subsequently enhances population in the 4I11/2 state through the 4I9/2 → 4I11/2 cross-relaxation (Fig. 3f). The numerical simulations based on rate equations confirmed that Ce3+ dopants increase the population in the 4I11/2 state of Er3+ only at high excitation powers (Fig. 3g).

In a further set of experiments, we demonstrate the critical role of Yb3+ in mediating the energy transfer from the Er3+ to the Tm3+ ions across the core/shell interface. When the Yb3+ ions in the core level were replaced by optically inert Lu3+ ions (Supplementary Fig. 11), the Tm3+ emission was hardly detected even at a high excitation power density of 2073 kW cm−2 (Fig. 4a). The observation was ascribed to the large physical separation between the Er3+ and Tm3+ ions. Owing to the low dopant concentration of Tm3+ (1%), their average distance from the core/shell interface was too far for the energy transfer to proceed. The introduction of a high concentration (40%) of Yb3+ ions created an energy conduit to the Tm3+ ions by forming a network of the Yb3+ lattice, which permits fast energy migration over a long distance42,43.

Fig. 4: Mechanistic investigation of energy transfer in the DU process.
figure 4

a Emission spectra of NaYF4:Yb(Lu)/Tm@NaErF4:Ce@NaYF4 nanoparticles under excitation of 1550 nm at 2073 kW cm−2, demonstrating the necessity of Yb3+ ions for mediating energy transfer across the core/shell interface. b Emission spectra of NaYF4:Yb/Tm@@NaYF4(d)@NaErF4:Ce@NaYF4 nanoparticles by 1550 nm excitation, verifying involvement of interlayer energy transfer in the DU process. c Power-density-dependent emission spectra of NaYF4:Yb/Tm/Er/Ce@NaYF4 nanoparticles by 1550 nm excitation, necessitating the use of core–shell–shell design for obtaining high-efficiency DU. d Schematic of uncontrollable energy exchange interactions in the homogeneously doped nanoparticle.

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Yb3+ ions facilitated the energy extraction from the interlayer also due to their relatively large absorption cross-sections (~10−20 cm2) and resonant energy level with Er3+ donors44, which resulted in a long critical distance of energy transfer. Our control experiments revealed that the Yb3+ mediated energy transfer can still proceed when the Er/Ce shell was isolated from the Yb/Tm core by a NaYF4 spacing layer of 2.5 nm (Fig. 4b and Supplementary Fig. 12). The energy transfer distance is appreciably larger than that observed for other ionic systems such as Gd3+ and Tb3+ (~1.1 nm)45.

The core−shell−shell structure is also essential for achieving the DU process. As we homogeneously doped all the lanthanide ions in the core layer of a NaYF4:Yb/Tm/Er/Ce@ NaYF4 core−shell nanoparticle (Fig. 4c, d and Supplementary Fig. 13), the overall emission was rather weak and the Tm3+ emission can hardly be detected. The result was ascribed to extensive and uncontrollable energy exchange interactions among the Yb3+, Tm3+, Ce3+, and Er3+ ions, which resulted in significant dissipation of excited energy. Note that the quenching processes in the quadruply-doped system were too strong to be alleviated by high-power excitation in our experiments (Fig. 4c).

Deep-UV lasing through DU

The laser characteristics of the NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 nanoparticles were examined under free-space excitation of a 1550 nm pulse laser with 6 ns frequency duration and 10 Hz repetition rate. The as-synthesized nanoparticles were incorporated into a toroidal microresonator as the laser cavity (Fig. 5a), which supports whispering gallery mode at the internal boundaries of the nanoparticle-doped microtoroidal resonator (Fig. 5b)46,47. Partly owing to the high processability and small size of the upconversion nanoparticles, the composite microcavity displayed a uniform size and smooth surface (Fig. 5c). Correspondingly, a high-quality factor (Q-factor) of about 2 × 105 was determined by assessing the transmission characteristic of a 1550 nm band diode laser (Santec, TSL-710) that was coupled to the microresonator through a tapered fiber (Fig. 5d).

Fig. 5: Deep-UV lasing in microresonator incorporated DU nanoparticles.
figure 5

a Schematic setup of the microtoroidal resonator platform for upconversion lasing. b Simulation of the excited WGMs (given in 2D cross-sectional geometry) on the surface of the microcavity with a diameter of 4 µm and ring width of 0.2 µm, respectively. c SEM image of a typical UCNPs-doped microresonator. d The transmission spectrum of a UCNPs-doped microresonator, revealing a Q-factor of around 2 × 105. Inset: schematic of the measurement setup (left) and top-view photograph of the system under measurement (right). e Emission spectra of a microresonator with Dm = 17 μm at different excitation powers. f Logarithmic plot of output intensity versus excitation power for the microresonator. g Lasing spectra of microresonators with different Dm. h Plots of measured mode spacing (Δλ) and threshold pump power (Pth) of the microresonator as a function of Dm. Data points for threshold pump powers represent mean ± standard deviation (SD, n = 3). Error bars indicate SD.

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To examine the lasing action, the emission spectra of a typical microresonator with a 17-μm diameter were recorded as a function of pump power. As shown in Fig. 5e, a sharp emission peak (linewidth < 0.05 nm) centered at 289.2 nm ascended from the emission spectrum as the excitation power increased above the threshold pump power (Pth, around 0.28 J cm−2). Moreover, the dependence of output intensity on the excitation power exhibited an “S” shape with three distinct regions (Fig. 5f), representing the transition from spontaneous emission through amplified spontaneous emission to gain saturation48,49. These results together confirm the onset of single-mode upconversion lasing.

We also demonstrated that the lasing features such as mode spacing, mode numbers, and threshold power can be precisely controlled by tuning the size of the microresonator (Supplementary Fig. S14). As the diameter of the microresonator increased, the number of lasing modes increased due to a decrease in the mode spacing (Fig. 5g, h). The observed mode spacing was well correlated with the parameters of the microresonators according to the following equation49,50:

$$\varDelta \lambda ={{\lambda }_{0}}^{2}/{n}_{{{{{{\mathrm{eff}}}}}}}L$$
(1)

where Δλ is the mode spacing, λ0 is the center peak wavelength, neff (=1.52) is the effective refractive index and L is the perimeter length of the microresonators. All of the above results confirm that, upon 1550 nm pumping, the toroidal microresonators were sufficient to create population inversion of a higher-lying excited state of Tm3+ ions through DU for ultraviolet lasing. It is worth mentioning that multi-wavelength lasing action can be recorded at different emission peaks of the Tm3+ and Er3+ dopants (Supplementary Fig. S15). The remarkable tunability of lasing emission in the judiciously designed NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 nanoparticles certainly expands the possibility in future studies.

The 289 nm lasing from UCNPs-doped microresonator with an ultralarge anti-Stokes shift is susceptible to the Q-factor of the cavity, which enables sensitive detection of small biological species by monitoring the Pth shift. As a proof of principle, we used a polystyrene (PS, 300 nm in diameter) sphere as the simulant of cancer cell secretion to conduct the sensing measurement. As anticipated, Pth values of the 290 nm lasing increased considerably from 0.13 to 2.34 J cm−2 by attaching a single PS sphere to the microresonator, due to the reduction of Q-factor from 2 × 105 to around 4 × 104 (Supplementary Fig. S16). The results demonstrate that our device integrating upconversion gain medium with high-Q microresonator structure is promising for designing high-quality sensing platforms.

Discussion

In summary, we have established a DU scheme that allows upconverting excitation light in the telecommunication wavelength into deep-ultraviolet emissions with an ultralarge anti-Stokes shift of 1260 nm (~3.5 eV). The DU was realized by the cooperation of two distinct upconversion processes that are integrated into a single NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 nanoparticle through a Yb-mediated energy transfer at the core/shell interface. By using the DU nanoparticles as gain media, we further developed a novel toroid microcavity laser that manifested single-mode lasing at 289.2 nm. Our findings initiate an effective tactic to obtain upconversion lasers operating in the deep-ultraviolet regime by excitation at the telecommunication wavelength, which minimizes optical attenuation in SiO2-based photonic circuits. Besides, the study of tandem combining different upconversion processes through heavy lanthanide doping also raises new possibilities of constructing upconversion nanocrystals with highly tunable excitation and emission spectra for advanced biological and photonic applications.

Methods

Nanoparticle synthesis

The multilayered NaYF4:Yb/Tm@NaErF4:Ce@NaYF4 nanoparticles were synthesized according to the method in ref. 35. Additional experimental details are provided in the Supplementary Information.

Fabrication of toroid microcavity comprising upconversion nanoparticles

A sol-gel silica film doped with upconversion nanoparticles was first made by using an acid-catalyzed hydrolysis-condensation reaction approach. Next, toroid microcavities were prepared from the as-synthesized upconversion sol-gel silica using a sequence of photolithography, etching, and laser-induced reflow. Additional experimental details are provided in the Supplementary Information.

Theoretical modeling

The electrical field in the waveguide structure was simulated by the three-dimensional finite-difference time-domain (3D-FDTD) method. The upconversion process in the Er3+-Ce3+ system was simulated by the rate equations of direct excitation and interionic cross-relaxation.

Physical measurement

HAADF-STEM images and HR-TEM images were measured with FEI Tecnai G2 F30 at 300 kV. The upconversion emission spectra were recorded with Ocean Optics USB 2000 and Maya 2000 PRO spectrometers. The lasing emission was measured by a monochromator (iHR-320) coupled with a photomultiplier tube. All measurements were performed at room temperature.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.