(color online) (a) Dual-mode lasing spectrum at 50 mA for a dual-mode square microcavity laser with a side length of 20 μm. Left inset: spectrum after amplifier and filter. Right inset: combined spectrum of the dual-mode and a TLS. (b) Measured beating signal of the combined optical spectrum. RBW: resolution bandwidth. (c) Zoomed-in electrical spectra. Reproduced from Ref. [61].