Mentions: The pronounced anisotropic azimuthal distribution of the emission does not change with pump power and is the fingerprint of a cavity eigenmode, which helped us to identify the exact lasing mode (section S5). Using a three-dimensional full wave simulation, we found a cavity eigenmode that matches the lasing mode observed in the experiment well, as shown in Fig. 3 (A to C). We can see that both the simulated and experimental patterns have 23 distinguishable emission beams, marked by white dashed lines, and most of these emission beams match each other in emission directions. Furthermore, on the top middle of the patterns, dislocations of a few emission beams match each other in the simulated pattern and in the experimental one. We also observe interference patterns in both spatial space and momentum space images, which match the simulated results (sections S6 and S7). Although the simulated and experimental patterns share good similarity, they are not perfectly matched because we are not able to perfectly rebuild such a large and irregular cavity in the simulation. The matched mode is a plasmonic mode, with the field strongly confined in the metal-insulator-semiconductor interface (Fig. 3D). The mode volume is about 0.017 μm3, and the cavity quality factor is about 22, limited by metal loss (see Materials and Methods). In a spaser, the excited electron-hole pairs in the gain medium recombine and then radiate directly into surface plasmons because of their high emission rate, which is accelerated by the Purcell factor (22, 23). This excitation-relaxation process of surface plasmon generation does not need the external laser and sophisticated setup required for the indirect generation of surface plasmons by phase match, which is an intrinsic property of spasers as a surface plasmon amplifier, according to its original definition (10, 33).
Spasers can serve as a pure surface plasmon generator with a coupling efficiency to plasmonic modes approaching 100%.
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