Optical microcavities are inherently non-Hermitian in nature, offering a fertile ground to test recent emerging development in non-Hermitian physics. Among them, representative examples include coherent perfect absorption (CPA)/critical coupling, electromagnetically induced transparency (EIT), parity-time symmetric coupled resonators, and exceptional points (EP). These works conducted in microcavities by manipulating additional factors like gain and loss reveal rich physics beyond the Hermitian regime. However, realizations of these experiments have been mainly limited to the spatial domain, for example, using two coupled microcavities to achieve PT symmetry. In this talk, we shall discuss our group’s efforts in non-Hermitian optics in a single microcavity by extending an additional dimension such as frequency through internal nonlinear optical processes. Within a single whispering-gallery-mode type microcavity, we demonstrate a new mechanism of optically induced transparency in a micro-cavity by introducing a four-wave mixing gain to nonlinearly couple two separated resonances of the micro-cavity in an ambient environment. On the PT-symmetry problem, we theoretically and experimentally demonstrate anti-PT symmetry in a synthetic spectral dimension induced by nonlinear Brillouin scattering in a single optical microcavity, where electromagnetically induced transparency or absorption in two spectral resonances provides the optical gain and loss to observe a phase transition between two symmetry regimes. Moreover, we will also show our studies on nonlinear bistable behaviors of such microcavity-based SBS near its EP, revealing a surprising memory effect depending not only on the perturbation variations and Hermiticity of the systems.