Fig. 1: Experimental setup and microlaser mode splitting.

a, Schematic of the experimental setup. CCD, charge-coupled device; Obj., objective; PC, polarization controller paddles; PZT, piezoelectric stage; WDM, wavelength-division multiplexer; FBS, fibre-based beamsplitter; FPD, fast photodetector; OSA, optical spectrum analyser; DAQ, data acquisition board. b, A PE-WGM microlaser is pumped by an external laser beam coupled via a TF, initially exciting the clockwise (CW) mode. The presence of plasmonic gold NRs—acting as plasmon enhancers for single-molecule detection—introduces scattering between the CW and counterclockwise (CCW) modes, resulting in the formation of SWMs with non-degenerate eigenfrequencies. This mode splitting gives rise to a beatnote signal in the laser output. When small molecules interact with the gold NRs, they disturb the splitting frequency of the SWMs, enabling sensitive molecular detection. c, Representative optical spectrum of a dual-mode microlaser, showing lasing peaks around 1,040 nm and 1,070 nm. d, Corresponding beatnote spectrum of the modes shown in c, with beat frequencies observed near 30 MHz and 34 MHz.