Flexible and Red-Emissive Organic Single-Crystal Microresonator for Efficient Active Waveguides.

We fabricated a flexible and red-emissive microcrystal resonator for highly efficient optical waveguiding. The microfiber crystals of diketopyrrolopyrrole (DPP) used in this work possess a high photoluminescence (PL) quantum efficiency (Φ PL = 0.45) and exhibit a micromechanical deformation shape in the curved state. The crystals show optical fringes in their PL spectra, suggesting the existence of a naturally formed Fabry-Pérot crystal resonator owing to its flat crystal surface. The group refractive index ( n g ) and Rabi splitting energy ( ℏ Ω) indicating the coupling strength between excitons and resonator photons are large ( n g = 3.7-6.0, ℏ Ω = 1.38 eV), suggesting the strong confinement of waveguiding photons. Spatially resolved PL measurements revealed that the PL in a crystal fiber is efficiently waveguided, even in a curved crystal with a very small curvature radius of 11 μm. Strong photon confinement inside a crystal resonator is a plausible origin of efficient optical waveguiding.