  Scattering induced mode splitting in active microcavities is demonstrated.
Below the lasing threshold, quality factor enhancement by optical gain allows
resolving, in the wavelength-scanning transmission spectrum, the resonance dips
of the split modes which otherwise would not be detected in a passive
resonator. In the lasing regime, mode splitting manifests itself as two lasing
modes with extremely narrow linewidths. Mixing of these laser modes in a
detector leads to a heterodyne beat signal whose frequency corresponds to the
amount of splitting. Lasing regime not only allows ultrahigh sensitivity for
mode-splitting measurements but also provides an easily accessible scheme by
eliminating the need for wavelength scanning around resonant modes. Mode
splitting in active microcavities has immediate impact in enhancing the
sensitivity of sub-wavelength scatterer detection and in studying light-matter
interactions in strong coupling regime.
