Since Maiman’s first demonstration [1], lasers, distinguished by high brightness, monochromaticity, and coherence, have become indispensable across various fields [2–5]. As the trend toward miniaturization continues, whispering gallery mode (WGM) microcavities have emerged as an ideal platform for confining light through total internal reflection, enabling ultrahigh quality factors (Q) and small mode volume (V). The large Q/V results in extremely high energy density within the cavity, which enhances light–matter interaction and improves laser performance [6–9]. These optical properties facilitate not only low-threshold single-wavelength lasing but also multi-wavelength emission [10–13].

Multi-wavelength lasers, capable of simultaneous emission at multiple wavelengths from a single device, are promising for lighting [14], color imaging [15,16], laser display [17,18], and optical communications [19]. The realization of multi-wavelength lasing critically depends on gain materials with broad spectral response and high luminescence efficiency, as well as effective optical modulation. Lanthanide-doped upconversion nanoparticles (UCNPs) have served as viable candidates for multi-wavelength lasing due to their unique nonlinear optical properties and multiple energy level characteristics. UCNPs enable a large anti-Stokes shift, converting low-energy photons into high-energy photons via multiphoton absorption. By optimizing the doping of lanthanide ions and structure design, UCNPs can achieve efficient multi-wavelength emission. Moreover, the real intermediate energy states enhance population inversion for lasing compared to traditional luminophores [20–23]. Typically, UCNPs are applied directly or mixed with silicone resins and coated onto the surface of WGM microcavities, such as optical fibers [24,25], polystyrene microspheres [26–28], and on-chip microdisks [18,29,30]. Recent studies have demonstrated excellent performance of UCNP-based WGM multi-wavelength lasers under continuous-wave pumping at room temperature, spanning from the near-infrared to deep ultraviolet region [18,30–32].

Despite recent advancements, the performance of UCNP-based WGM lasers is still constrained by free-space coupling. The isotropic and weak radiation of cavity modes results in low radiative exchange efficiency between the microcavity and free space, with typical coupling efficiency below 20% [33,34]. Tapered fibers, however, provide a transformative approach by significantly increasing the localized energy density of the evanescent field. As light propagates, the progressively narrowing fiber core confines the optical mode to a smaller region, substantially intensifying the evanescent field compared with normal optical fiber. When the enhanced evanescent field overlaps with the evanescent field of the WGM microcavity, efficient coupling is achieved [35]. Under optimal conditions, the coupling reaches critical coupling with efficiency as high as 99.99% [36]. Furthermore, tapered fiber serves as an effective pathway for signal collection, offering great potential for integrated devices [37]. The efficient coupling facilitated by tapered fibers offers a robust strategy to improve the performance of UCNP-based WGM lasers, supporting the discovery of novel lasing emission and enabling the generation of extended multi-wavelength laser outputs.

In this study, we employed tapered fiber coupling with nanoparticle-coated microspheres to enhance the performance of WGM lasers. NaYF4@NaYbF4:1%Tm3+@NaYF4 core–shell–shell structure nanoparticles were synthesized and uniformly coated onto a silica microsphere (∼11μm). Through tapered fiber coupling, multi-wavelength upconversion lasing was simultaneously achieved across all emissions of Tm3+ (D21→F43, G41→H63, D21→H53, G41→F43, D21→F33, and H43→H63) in the visible and infrared regions under continuous-wave 980 nm laser pumping. These microlasers exhibited low threshold and narrow linewidth, along with good stability over 180 min. In addition, visible upconversion lasing was also achieved for Er3+- and Ho3+-activated microspheres.

The hexagonal phase NaYF4@NaYbF4:1%Tm3+@NaYF4 core–shell–shell nanoparticles were synthesized by a modified co-precipitation method (see Appendix A) [38]. The transmission electron microscopy (TEM) image of the synthesized nanoparticles reveals uniform spherical morphology and monodisperse distribution, with an average size of ∼45nm [Fig. 1(a)]. The nanoparticles were strategically designed based on the principle of energy migration confinement [25]. The concentration of Tm3+ was set at 1% mole fraction to achieve large anti-Stokes emissions [29]. Yb3+ is commonly used as a sensitizer for Tm3+ due to its large absorption cross-section at 980 nm, and the efficient energy transfer between Yb3+ and Tm3+ promotes multiphoton absorption for Tm3+. Consequently, high concentration of Yb3+ is typically required [39]. However, increasing the Yb3+ concentration leads to non-radiative relaxation of the excited ions, significantly reducing the F25/2 state lifetime of Yb3+. To address this challenge, an inert core NaYF4 layer with a diameter of 26 nm and a 6 nm intermediate NaYbF4 layer were designed [Fig. 1(a)]. The configuration effectively reduces the number of defects accessible to the Yb sublattice, thereby minimizing energy loss in the host lattice and allowing high concentration of Yb3+ doping [25]. Additionally, to prevent surface quenching caused by the nanoscale size, an inert shell was employed as the outermost layer. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image clearly reveals the sandwich structure of the nanoparticles, further confirmed by elemental mapping showing distinct distributions of Y, Yb, and Tm [Fig. 1(b)]. The structure was also characterized by elemental line scanning obtained from energy-dispersive X-ray spectroscopy (EDS) on a single nanoparticle [Fig. 1(e)], demonstrating the localization of Y in both the core and outermost layers, while Yb is in the intermediate layer. The high-resolution transmission electron microscopy (HRTEM) image displays a clear lattice fringe with the d-spacing of 0.51 nm [Fig. 1(c)], corresponding to the (100) lattice planes. The selected area electron diffraction (SAED) pattern shows distinct diffraction rings [Fig. 1(d)], indicating a good crystalline property. The X-ray diffraction (XRD) pattern confirms that the nanoparticles exhibit a pure-phase hexagonal NaYF4 structure [Fig. 1(f)]. Figure 2(b) displays the upconversion emission spectrum of the nanoparticles dispersed in cyclohexane under continuous-wave 980 nm laser excitation. The spectrum exhibits characteristic Tm3+ emissions at 452 nm, 476 nm, 510 nm, 648 nm, 743 nm, and 803 nm, corresponding to the transitions D21→F43, G41→H63, D21→H53, G41→F43, D21→F33, and H43→H63 [see Fig. 2(a)]. The time decay curves are shown in Fig. 2(c). The millisecond-level lifetimes facilitate population inversion, making the promising realization of multi-wavelength laser output feasible.

The nanoparticle-coated microspheres were prepared by immersing silica microspheres in nanoparticle-dispersed cyclohexane solution, as shown in the schematic in Fig. 3(a). The silica microspheres were fabricated by stepwise CO2 laser processing of the Hi1060 optical fiber (Corning Incorporated, Corning, New York, USA), utilizing the surface tension of glass (see Appendix A) [40]. To enhance the resonant properties, the diameter of the connecting stem was kept as small as possible. The smoothly surfaced microspheres prepared by this method exhibit extremely high Q-factors [41]. To ensure effective deposition onto the microsphere surface, the nanoparticles were rendered hydrophobic through the oleic acid ligands introduced during their synthesis. Partial swelling of the microsphere in a polar solvent mixture promoted van der Waals interactions between the hydrophobic nanoparticle surface and the outer region of the microsphere [42]. Additionally, the surface charge of the nanoparticles caused mutual repulsion during deposition, preventing aggregation and leading to the formation of a uniform monolayer coating on the microsphere [27]. The coating effect was controlled by adjusting the immersion time of the microsphere in the nanoparticle-dispersed cyclohexane solution (0.2 mol/L) after ultrasonic treatment. The scanning electron microscopy (SEM) images of the microspheres subjected to different immersion times were examined to evaluate the coating quality [Figs. 3(b)–3(g)]. Initially, the nanoparticles formed an incomplete monolayer on the microsphere surface. As immersion time increased, the coating became uniform and dense, resulting in a relatively smooth surface with only minimal uncovered areas. However, prolonged immersion disrupted the monolayer structure, leading to significant nanoparticle aggregation and an increase in surface roughness.

The schematic of the experimental setup for obtaining microsphere lasing is illustrated in Fig. 4(a). According to the free spectral range (FSR), the larger microcavity has a smaller FSR in the visible range, resulting in densely packed modes and suboptimal spectra due to the spectrometer’s limited resolution of 0.04 nm. Conversely, a microsphere with a diameter below 10 μm suffers significant Q-factor degradation from radiation loss [43]. Thus, an optimal diameter of approximately 11 μm was chosen for the experiments. A continuous-wave 980 nm semiconductor laser with a linewidth of 2 nm was employed as the pump source. In terms of coupling, free-space coupling suffers from low efficiency due to phase matching challenges imposed by WGM resonance characteristics and optical alignment limitations [34]. The isotropic radiation of the microcavity further reduces signal collection efficiency, while free-space losses compound the signal intensity degradation. In contrast, tapered fiber coupling utilizes evanescent wave interaction—when the microcavity and fiber evanescent fields overlap, efficient pump coupling occurs while simultaneously enabling in-fiber signal collection through reciprocal evanescent coupling [35]. This dual functionality critically enables multi-wavelength lasing. To minimize pump power loss, Hi1060 fibers were tapered to approximately 1.2 μm waist diameter with a hydrogen–oxygen flame for coupling (see Appendix A). Additionally, we treated the microsphere and tapered fiber as an integrated device, using the energy input into the tapered fiber as the pump power. A small portion of the pump light was split using a 9:1 coupler and directed to a power meter to monitor the energy input into the tapered fiber.

The D21→F43 emission spectra of microspheres (∼11μm) with different immersion times were measured using the same tapered optical fiber for coupling [Figs. 4(b)–4(d)]. At a pump power of 2 mW, the brightness of the microsphere increased progressively with longer immersion time. Among the samples, only the microsphere with a uniform nanoparticle coating exhibited high spectral contrast in the laser output, displaying a characteristic blue WGM resonance ring with the divergence attributed to surface scattering. Multiple whispering gallery modes were identified through finite-element-based analysis of the intracavity modal field distributions. In contrast, for the microsphere with a 20 s immersion time, the coating was incomplete, leading to inadequate gain and preventing lasing. Although the microsphere immersed for 120 s exhibits high brightness under pumping, it fails to maintain sufficient spectral contrast due to mode splitting induced by the aggregated nanoparticles on its surface [44]. Moreover, the aggregated nanoparticles have a significant impact on the resonant properties. Excessive surface roughness, even with a relatively small number of deposited nanoparticles, can inhibit lasing and result in only spontaneous emission [27]. Excessive immersion leads to disordered deposition of nanoparticles, significantly increasing the surface roughness. Therefore, achieving a uniform and compact monolayer-coating structure is essential for realizing low-threshold multi-wavelength lasing.

After achieving a uniform monolayer nanoparticle coating on the microsphere, it was coupled to a tapered fiber, which was aligned using a three-dimensional adjustment stage. Figure 5 illustrates the output of typical upconversion WGM lasers at six emissions corresponding to Tm3+ transitions (D21→F43, G41→H63, D21→H53, G41→F43, D21→F33, and H43→H63), with the laser spectra showing sufficient spectral contrast. All upconversion emissions from the nanoparticles, spanning both visible and near-infrared regions, were successfully converted into laser output. To the best of our knowledge, this is probably the first demonstration of WGM laser emissions at D21→H53, G41→F43, and D21→F33 transitions at room temperature. The observed laser outputs from these transitions mainly result from the optimized coupling method, where the tapered fiber’s near-field coupling enables efficient pumping and weak-signal detection. Due to the relatively small FSRs in these emissions, the microsphere exhibited a dense set of resonance modes, resulting in multimode laser output. As the wavelength decreased, the FSR reduced, causing the mode spacing to decrease and leading to a more compact spectrum. The Q-factor can be roughly estimated using the laser mode through the relation Q=λ0/Δλ, where λ0 is the center wavelength and Δλ is the linewidth. For the 450 nm mode, the linewidth was measured to be 0.22 nm, resulting in an estimated Q-factor of approximately 2046.

The performance of each laser emission was further characterized. At low pump power, the intensity of spontaneous emission increased gradually. Upon reaching the lasing threshold, the intensity rose sharply in a nonlinear fashion, indicating the onset of lasing. Simultaneously, the linewidth of the modes narrowed rapidly and continued to decrease with increasing pump power [Figs. 6(a)–6(f)]. This narrowing can be attributed to the increased coherence of the stimulated emission, as the number of coherent photons circulating within the cavity grows with higher pump powers. The lasing thresholds were determined to be 15.4 μW, 20.6 μW, 36.3 μW, 19.4 μW, 20.3 μW, and 0.61 μW for the modes at 451 nm, 479 nm, 508 nm, 647 nm, 746 nm, and 801 nm, with the linewidths eventually converged at 0.22 nm, 0.22 nm, 0.34 nm, 0.19 nm, 0.47 nm, and 0.39 nm, respectively. These laser thresholds were attributed to the high gain of the nanoparticles [45] and the efficient coupling enabled by the tapered fiber, while the narrow linewidth is primarily due to the low loss of the uniformly coated microspheres. The lower threshold laser emission observed at H43→H63 was attributed to strong cross-relaxation. At high Tm3+ doping concentrations, the reduced distance between Tm3+ ions strengthened energy transfer, increasing the population of the intermediate state of H43 [see Fig. 2(a)] [46] and, thereby, facilitating population inversion. Additionally, enhanced by the two-photon process, the nanoparticles exhibit much stronger emission at H43→H63 compared to other emissions, resulting in a sub-μW-level lasing threshold. Although promising laser performance was achieved, further improvement is still possible due to limitations imposed by non-resonant pumping and coupling losses [35–37]. Further reduction of the lasing threshold is likely achievable with optimized conditions.

The dependence of lasing performance on the microcavity diameter was further investigated. As the microcavity diameter increases from 11 to 25 μm, FSR decreases significantly, resulting in densely packed resonant modes [Fig. 7(a)]. The enhancement of the microcavity’s resonant capability, attributed to reduced radiation loss, results in a Q-factor increase from 2046 to 2258, consequently lowering the lasing threshold for D21→F43 from 15.4 to 9.6 μW [Fig. 7(b)]. It should be noted that the improvement in resonant capability remains relatively modest due to the scattering loss limitation.

Compared to pulsed laser pumping, continuous-wave laser pumping is more prone to thermal effects due to the continuous energy input and faster heat accumulation [13]. To evaluate the stability of the upconversion laser, we measured the intensity and wavelength variations of a uniformly coated microsphere under 1 mW excitation over a period of 180 min (Fig. 8). The laser intensity exhibited fluctuations, with the maximum reduction not exceeding 5% across all emissions. Additionally, the maximum wavelength shift was within 0.5 nm. Despite these minor fluctuations, the laser demonstrated consistent performance throughout the measurement period, indicating its suitability for practical applications.

In addition to Tm3+, Er3+ and Ho3+ are also efficient activators for upconversion emission. Er3+/Yb3+ and Ho3+/Yb3+ co-doped upconversion nanoparticles were synthesized, and microspheres with a diameter of approximately 11 μm were coated with these nanoparticles using a similar immersion method. For the Er3+/Yb3+ co-doped nanoparticle-coated microsphere, multi-wavelength laser output is observed under a 200 μW pump [Fig. 9(a)]. The dominant outputs are green and red emissions, with lasing thresholds of 39.8 μW at 543 nm (S3/24→I15/24) and 12.4 μW at 658 nm (F9/24→I15/24), respectively. As the pump power increases, the linewidth gradually narrows, converging to 0.29 and 0.28 nm [Figs. 9(d) and 9(e)]. Additionally, weaker transitions are observed at 409 nm (H9/22→I15/24) and 521 nm (H11/22→I15/24), with thresholds of 180.7 μW and 153.8 μW, respectively, and linewidths narrowing to 0.27 nm and 0.26 nm [Figs. 9(b) and 9(c)]. The Ho3+/Yb3+ co-doped nanoparticle-coated microsphere exhibited distinct red WGM resonance ring under laser pumping [Fig. 10(a)]. The lasing threshold for the green emission at 542 nm (S25→I85) was found to be 73.7 μW, with the linewidth narrowing to 0.31 nm [Fig. 10(b)]. For the red emission at 643 nm (F55→I85), the lasing threshold was 7.2 μW, with the linewidth narrowing to 0.13 nm [Fig. 10(c)].

In this work, we have demonstrated multi-wavelength upconversion laser output through the uniform coating of nanoparticles on silica microspheres, coupled using a tapered fiber. The nanoparticle coating significantly influences the laser performance, with uniform coating ensuring sufficient gain and reasonable surface roughness. Thanks to the efficient coupling facilitated by the tapered fiber, we achieved laser emission from all upconversion emissions (D21→F43, G41→H63, D21→H53, G41→F43, D21→F33, and H43→H63) of NaYF4@NaYbF4:1%Tm3+@NaYF4 nanoparticles, spanning from the near-infrared to the visible region. The lasers exhibited low threshold and narrow linewidth, with the H43→H63 transition showing a sub-μW threshold of 0.61 μW. To the best of our knowledge, this is the first demonstration of WGM laser emission at D21→H53, G41→F43, and D21→F33 transitions at room temperature. Furthermore, the multi-wavelength lasers showed great stability over 180 min. Additionally, multi-wavelength laser output can also be achieved with Er3+ and Ho3+. These results highlight the potential of tapered fibers in enhancing the performance of upconversion nanoparticle microlasers, offering a promising universal path toward achieving superior upconversion laser performance with nanoparticles.