The design principle for the realization of an RGB single-mode laser is schematically presented in Fig. 1. Isolated dye-doped microspheres can serve as WGM resonant cavities, with which multimode RGB microlasers were achieved through doping of corresponding laser dyes. Here, a strategy was proposed to achieve an RGB single-mode microlaser by building a heterogeneously coupled system composed of three interacting microresonators. RGB-emissive spherical resonators were integrated with a micromanipulator. The WGM resonators were arranged side by side due to the strong coupling of the optical field distributed along the cavity interface18, 22. The dye-doped microspheres therein steadily deliver multimode lasing, while coupled microcavities act as filters of the resonance modes. With each spherical cavity serving as a laser source and a modulator simultaneously, RGB single-mode laser output would be achieved in heterogeneously coupled microcavities. Moreover, a tuneable RGB single-mode laser might be obtained by varying the manner of the optical pumping.
The fabrication of organic spherical microcavities incorporating laser dyes is illustrated in Fig. 2a. Polystyrene (PS), due to its flexibility and processability, was selected as the matrix material to fabricate the microspheres. Well-mixed conjugated dye/PS/dichloromethane (CH2Cl2) solution was added into a cetyltrimethylammonium bromide (CTAB) aqueous solution, which formed an oil-in-water emulsion. After vigorous stirring, the mixed CH2Cl2 solution was encapsulated into the hydrophobic interior of CTAB. With the evaporation of CH2Cl2, spherical droplets consisting of PS molecules aggregated into microspheres with dye molecules dispersed inside. After the removal of CTAB, dye-doped spherical microcavities with uniform size were acquired.
The spherical geometry of the self-assembled microcavities was confirmed by top-view (Fig. 2b) and side-view (Supplementary Fig. 1) scanning electron microscopy (SEM) images. With perfect circular boundaries and ultrasmooth surfaces, the self-assembled microspheres minimize undesirable optical scattering, which is favorable for WGM resonance30. Based on the formation mechanism mentioned above, the diameter of self-assembled microcavities is directly proportional to the size of the micelles, which depends on the interfacial tension between water and the CH2Cl2 solution. The interfacial tension positively increases with the amount of PS, generating larger micelles with smaller specific surface areas to reduce the interfacial energy. Accordingly, the diameter of the self-assembled microsphere was finely tuned from 3 to 20 μm by changing the concentration of PS molecules, which is essential for constructing an optimized heterogeneously coupled cavity system (Supplementary Figs. 2 and 3). Due to the π–π interactions between the phenyl groups of PS and π-conjugated laser dyes, the self-assembled PS microspheres can be doped with various conjugated dyes to provide optical gains at different wavebands31. Three laser dyes were selected to serve as gain media: coumarin-153 (C153), 1, 4-bis(α-cyano-4-diphenylaminostyryl)-2, 5-diphenylbenzene (CNDPASDB; Supplementary Fig. 4), and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), which have photoluminescence (PL) emissions in blue, green, and red wavebands, respectively (Supplementary Fig. 5).
The self-assembled microcavities doped with C153, CNDPASDB, and DCM emitted uniform blue, green, and red fluorescence, respectively, under ultraviolet (UV) excitation (Fig. 2c–e). The introduction of various conjugated dyes caused very little surface damage to the microspheres due to the superior compatibility of organic materials (Supplementary Fig. 6), making them ideal candidates for lasing action. When each dye-doped microsphere was pumped with a pulsed laser beam (400 nm, ~200 fs) in a homebuilt microphotoluminescence system (Supplementary Fig. 7), multimode lasing action was observed (Fig. 2f). The linewidth of the individual lasing mode was ~0.5 nm. The quality factor of such spherical microcavities was determined to be ~1600, indicating the low optical loss of the microcavities32. The PL images of the dye-doped microspheres recorded above the thresholds exhibited bright ring-shaped patterns at the boundary (Fig. 2f, inset), which is a typical characteristic of WGM resonances33. Further investigation of the FSR showed that the mode spacing is inversely proportional to the diameter of the microspheres, verifying the WGM resonance (Supplementary Fig. 8)34.
The WGM oscillation in the spherical microcavity would result in the optical field being confined along the cavity interface, which enables strong coupling in a side-by-side coupled structure18. Accordingly, a heterogeneously coupled WGM resonator composed of a C153-doped microsphere and a CNDPASDB-doped microsphere was designed to modulate the laser output from the spherical microcavities. Here, a micromanipulation technique (Supplementary Fig. 9) was applied to controllably fabricate these heterogeneously coupled microcavity systems with desired structural parameters, including the diameter of the coupled microcavity (Supplementary Fig. 10) and intercavity gap distance (Supplementary Fig. 11), which provides a reliable means of precisely constructing the heterogeneously coupled microstructure.
The morphology of the as-prepared heterogeneously coupled resonators was demonstrated in PL (Fig. 3a) and SEM (Fig. 3b) images. Under UV excitation, the heterogeneously coupled microcavities maintain uniform emission without evident scattering points. The result manifests that the micromanipulation technique introduced little surface damage to the microspheres, which is supported by the SEM image. Hence, the heterogeneously coupled resonators constructed with the micromanipulation process preserve the optical properties of the individual WGM microcavities. The distance between microspheres in each heterogeneously coupled system was controlled to the nanometer scale (inset of Fig. 3b), enabling effective optical interaction between the resonators (Supplementary Fig. 12)18.
The lasing action in the heterogeneously coupled system was investigated by comparing the lasing spectra of identical WGM resonators with and without the coupling of distinct microspheres. The lasing spectrum of an isolated C153-doped microsphere exhibited a series of sharp peaks (Fig. 3c, top). In contrast, when the same microsphere was heterogeneously coupled with a distinct microcavity, one of the lasing modes in the isolated resonator was selectively oscillated in the coupled resonator, enabling blue single-mode lasing (Fig. 3c, bottom). Such a phenomenon could also be found in the green-emissive microcavity, indicating that mode selection in the heterogeneously coupled system could be achieved in other color regions. As shown in Fig. 3d, one of the multiple laser modes in an isolated CNDPASDB-doped microsphere was selected when coupled with a C153-doped microsphere, and green single-mode lasing was generated. The single-mode microlasers can produce steady output when the gap distance between coupled microcavities is varied from 0 to 250 nm (Supplementary Fig. 13), indicating that the mode selection effect has a low requirement on the gap distance. The thresholds of the microspheres doped with C153 and CNDPASDB in the coupled system were ~2.61 and 2.55 μJ cm−2, respectively, slightly higher than those of the isolated resonators (~2.56 and 2.46 μJ cm−2) (Fig. 3e, f). The slight increase in the lasing threshold can be ascribed to the radiation loss introduced by the coupled structure35.
The mechanism behind the generation of a single-mode laser in the heterogeneously coupled microcavities is shown in Fig. 4a. In the coupled system, the generated light propagates around the circumference of the lasing cavity, which makes the guided waves accessible for coupling to the external cavity. When the emitted light is coupled to the external WGM resonator, a series of sharp dips are observed in the transmission spectrum18, which can be attributed to the transmission resonance of the external cavity. When the transmission dips overlap the resonant frequencies of the lasing cavity, the optical power near these resonant frequencies transfer to the external cavity, resulting in a filtering effect23. By contrast, the optical power near the least overlapped resonant frequency of the lasing cavity has the lowest leakage into the passive cavity. Because of the lowest radiation loss introduced by the filter cavity, single-mode lasing at this resonant frequency will be achieved in the lasing cavity (Fig. 4a, top)35, 36. Thus, the passive cavity serves as a filter of the lasing modes in the active cavity, which leads to a mode selection effect37. Such a mode selection strategy can also act on other wavebands because transmission dips of the filter cavity exist in other gain regions. When the green-emissive microsphere serves as the lasing cavity, single-mode lasing in the green waveband can be realized with the coupling of a filter cavity (Fig. 4a, bottom).
The mode selection mechanism mentioned above provides us with a strategy to achieve single-mode laser emission in different wavebands, which is supported by the simulated electric field distributions. As shown in Fig. 4b (top), the lasing mode (λ1 = 486.3 nm) is well confined in the left WGM cavity because of the low transmission loss introduced by the filter cavity at λ1, resulting in blue single-mode lasing action35, 37. In the same coupled system, when the right resonator serves as the lasing cavity (Fig. 4b, bottom), another lasing mode (λ2 = 568.1 nm) dominates the right WGM cavity, and single-mode lasing in another gain region can be realized. With the two WGM resonators in the heterogeneously coupled system providing different optical gains, both of them can be applied as laser cavities. Modulated by the right WGM cavity, single-mode lasing can be achieved in the left microcavity, and vice versa. This result indicates that such a mode selection mechanism can act on the distinct gain regions in an identical heterogeneously coupled system, which has great potential for generating multicolor single-mode lasing.
This predicted result was confirmed by experimental measurements. In a heterogeneously coupled system constructed with a C153-doped microsphere and a CNDPASDB-doped microsphere, blue and green single-mode lasers, respectively, can be emitted from the two resonators (Fig. 4c, top and middle). A blue single-mode laser is obtained when the CNDPASDB-doped microsphere acts as a modulator for the C153-doped lasing cavity, whereas the C153-doped microcavity serves as a modulator for the generation of a green single-mode laser. These single-mode lasing behaviours indicate mutual mode selection, which would enable multicolor single-mode lasing when the heterogeneously coupled resonators serve as lasing cavities and mode filters simultaneously. As shown in Fig. 4c (bottom), a dual-color single-mode laser was achieved by pumping the entire heterogeneously coupled system. The pump power-dependent PL spectra of the dual-color single-mode lasing and plots of the pump power-dependent full-width at half-maximum are shown in Supplementary Fig. 14, verifying the multicolor single-mode lasing in the heterogeneously coupled resonators.
The colors of dual-wavelength single-mode lasers might be freely designed by varying the gain medium in the lasing cavities. As shown in Fig. 4d (top and middle), a DCM-doped microsphere and a CNDPASDB-doped microsphere were selected as lasing cavities in another coupled system because of their ability to realize red and green microlasers, respectively (Supplementary Fig. 15). When these two microcavities were heterogeneously coupled with each other, red and green single-mode lasing was realized in the coupled microcavities (Fig. 4d, bottom). This result shows that by building a coupled system with distinct microcavities, a single-mode laser covering all visible colors can be achieved based on the mode selection mechanism. Meanwhile, benefiting from the isotropic emission of the WGM resonator, the spherical microcavity could permit optical coupling with multiple microcavities simultaneously, which enables us to construct a coupled system composed of more resonators38. Such heterogeneously coupled systems may provide a general strategy for the generation of single-mode lasers covering a wider wavelength region.
The outstanding compatibility and isotropic emission of the spherical microcavities permitted us to design a three-component coupled system capable of simultaneously achieving RGB microlasers with optical coupling between them. The red-, green-, and blue-emissive microcavities were arranged into an angular-shaped chain structure, which not only enabled the interaction between the microcavities but also allowed us to simultaneously pump any two of the resonators, as shown in Fig. 5a. In such a heterogeneously coupled system, RGB single-mode lasers might be obtained in distinct microcavities, which is supported by the numerically simulated electric field distributions of the lasing modes (Fig. 5b–d). The green-emissive microcavity serves as a filter for the blue- and red-emissive microcavities, which leads to blue (λ1 = 483.6 nm) and red (λ3 = 610.2 nm) single-mode lasing. Meanwhile, the green-emissive microcavity is synchronously modulated by the other two resonators, and the lasing mode (λ2 = 554.3 nm) is mainly located inside the WGM resonator, which results in green single-mode lasing.
Indeed, tuneable RGB single-mode lasing was experimentally observed in such a three-component heterogeneously coupled system. The coupled cavity is composed of a DCM-doped microsphere, a CNDPASDB-doped microsphere, and a C153-doped microsphere. As shown in Fig. 5e, when each individual microsphere cavity was pumped above the threshold, single-mode lasing was achieved at the corresponding wavelength. When two of the coupled microcavities were pumped above their thresholds, any light combination comprising two of the RGB single-mode lasers could be generated. Tuneable multicolor single-mode lasers (B + G, G + R, and B + R) were obtained by adjusting the manner of the optical pumping, and an RGB single-mode laser was achieved when all three microspheres were integrally pumped. Such tuneable RGB single-mode laser output from the coupled system is desirable for full utilization of the advantages of RGB microlasers, which would greatly contribute to ultracompact photonic devices39-41.