Stable High‑Q Bouncing Ball Modes inside a Fabry–Pérot Cavity

We discover that stable high-Q bouncing ball modes with tight lateral confinement and whole-body light–matter interaction can exist in an imperfectly aligned, wedged plano–plano FP (WFP) cavity by simply introducing a vertical dielectric interface (DI) between two media with different refractive indices. Compared to a pure WFP cavity without the DI, which is known to have a low-Q factor and a finesse of 10–300 due to the large diffraction and geometrical walk-off losses, our WFP-DI cavity exhibits a high-Q factor of ∼10⁵, a finesses of ∼3000, and a mode size (or diameter) down to 2 μm. Furthermore, optical properties of our WFP-DI cavity, including stability, loss, dependences of the Q-factor, and the effective mode area on the DI refractive index contrast and the mirror tilt angle are systematically investigated using numerical methods. Finally, we experimentally demonstrate WFP-DI dye lasers in the total internal reflection (TIR) and non-TIR cases. A lasing threshold of 1–2 μJ/mm² is achieved, which is 7–40× lower than the pure WFP laser without the DI and agrees well with the theoretical predications. Our results suggest that the WFP-DI cavity provides a promising technology platform for miniaturized photonic devices, optofluidic lasers, and microfluidics for biological/chemical analysis.