Tuneable red, green, and blue single-mode lasing in heterogeneously coupled organic spherical microcavities

Tuneable microlasers that span the full visible spectrum, particularly red, green, and blue (RGB) colors, are of crucial importance for various optical devices. However, RGB microlasers usually operate in multimode because the mode selection strategy cannot be applied to the entire visible spectrum simultaneously, which has severely restricted their applications in on-chip optical processing and communication. Here, an approach for the generation of tuneable multicolor single-mode lasers in heterogeneously coupled microresonators composed of distinct spherical microcavities is proposed. With each microcavity serving as both a whispering-gallery-mode (WGM) resonator and a modulator for the other microcavities, a single-mode laser has been achieved. The colors of the single-mode lasers can be freely designed by changing the optical gain in coupled cavities owing to the flexibility of the organic materials. Benefiting from the excellent compatibility, distinct color-emissive microspheres can be integrated to form a heterogeneously coupled system, where tuneable RGB single-mode lasing is realized owing to the capability for optical coupling between multiple resonators. Our findings provide a comprehensive understanding of the lasing modulation that might lead to innovation in structure designs for photonic integration.


II. Morphological characterization of spherical microcavities
The spherical microcavities were synthesized through a controlled emulsion-solvent-evaporation method. Supplementary Fig. 1 displays the topand side-view of the microsphere, indicating that the self-assembled microcavities have a spherical shape and perfectly smooth surface, which are favorable for the Whispering-gallery-mode (WGM) resonance. The diameter of self-assembled WGM resonators can be finely tuned from 3 to 20 μm through increasing the concentration of polystyrene (PS), which is critical for the construction of optimized heterogeneously coupled cavity system. Bright-field optical microscopy images of organic microspheres with different sizes are shown in Supplementary Fig. 2. The size distributions of spherical microcavities were provided in Supplementary Fig. 3. With smooth surface and controllable size, self-assembled microspheres are ideal candidates for constructing the well-designed heterogeneously coupled cavity system. As shown in Supplementary Fig. 1a, the acquired structures have perfect circle boundary and ultra-smooth surface. The spherical structure is further confirmed by the side-view SEM image ( Supplementary Fig. 1b). Such a microsphere is favorable for the whispering-gallery-mode (WGM) resonance.

III. Synthesis procedure and luminescence properties of the model compounds.
Due to the strong π-π interactions between the phenyl groups of PS and π-conjugated dye molecules, the microspheres can be doped with various conjugated dyes to provide optical gains at different wavebands. Accordingly, three π-conjugated luminescent dyes, C153, CNDPASDB, and DCM, with photoluminescence (PL) emission across the visible region, were selected as gain medium. The CNDPASDB were synthesized with Knoevenagel condensation reactions ( Supplementary Fig. 4). The spectral data of the selected laser dyes are presented in Supplementary Fig. 5.
Supplementary Figure 4 | The synthetic route of compound CNDPASDB.
Step A home-built microphotoluminescence system was used to exam the optical properties of the heterogeneously coupled WGM resonators. A focused 400 nm pulse laser beam, which was generated from the second harmonic of the fundamental output of a regenerative amplifier (Solstice, Spectra-Physics, 800 nm, 100 fs, 1000 Hz), was used to pump the heterogeneously coupled system. An objective lens (50×, numerical aperture 0.8) was used to focus the pump beam.
After passing through the corresponding filters (400-nm long-pass), the collected emissions were dispersed with a grating (1200 G/mm) and the recorded using a thermal-electrically cooled CCD (Princeton Instruments, ProEm 1600B). could be ascribed to WGM resonance.

V. Construction and lasing performances of the heterogeneously coupled WGM resonators
The controllable fabrication process of the heterogeneously coupled microcavity system is shown in Supplementary Fig. 9 and Supplementary Fig. 10. The heterogeneously coupled microcavities with desired gap distances were constructed by alternately exerting an axial force to one of the microspheres in the micromanipulation process, as illustrated in Supplementary Fig. 11. The coupling effect of heterogeneously coupled WGM cavities was shown in We constructed the heterogeneously coupled microspheres with desired gap distances by alternately exerting an axial force to one of the microspheres in the micromanipulation process, as illustrated in Supplementary Fig. 10. First, we push two heterogeneous microspheres together by using a microprobe to fabricate the heterogeneously coupled structures with no/little space between two component microspheres ( Supplementary Fig. 10a). Next, one of the microspheres was pushed away from the axial direction by exerting a force to one of the microspheres (Supplementary Fig. 10b). Finally, the microsphere was move backwards by a force in the opposite direction, which would result in a gap with a desired distance ( Supplementary Fig. 10c). This axial force technique enables us to control the gap distance in the range of effective coupling distances. The mode selection effect has a low requirement on the gap distance in coupled cavity system. 5 Here, we studied the evolution of the laser spectrum by gradually changing the gap distance of the heterogeneously coupled microspheres. A CNDPASDB-doped microsphere was selected as the lasing cavity, while a C153-doped microsphere as the mode filter. As shown in Supplementary Fig. 12, the mode selection effect can be achieved in a range of gap distance ~0-~250 nm.
When the gap width reaches ~600nm, more modes began to emerge because of the decrease of optical coupling between two microspheres. When the gap width reaches ~1200nm, there is no obvious mode selection effect anymore. This result indicates that single-mode microlaser can be steadily outputted when the gap distance between coupled microcavities varied from 0 to 250 nm, manifesting that the mode selection effect in heterogeneously coupled system has a low requirement on the gap distance. As shown in Supplementary Fig. 13a, multicolor single-mode lasing actions were realized in the heterogeneously coupled WGM resonators. Supplementary Fig.   13b and Supplementary Fig. 13c depicted the PL intensity and the FWHM at 495nm and 572nm as a function of pump fluence, showing a threshold characteristic at ~2μJ cm -1 . Above the onset power, the peak intensity increased rapidly with the pump fluence, and the FWHM dramatically decreased down to ~0.9 nm, which revealed the multicolor single-mode lasing behavior in heterogeneously coupled resonators.