Optofluidic Lasers with Monolayer Gain at the Liquid–Liquid Interface

We report optofluidic lasers with a monolayer gain material that self-assembles at the two-phase liquid–liquid interface. The self-assembly process deterministically introduces the gain at the surface of a microdroplet optical cavity, where the lasing mode has maximal interaction with the gain medium. A complete monolayer gain can be achieved in this surface-gain geometry, giving a surface density on the order of 10<sup>14</sup> cm<sup>–2</sup>, which proves to be difficult, if not impossible, to achieve in the monolayer gain created at the solid–liquid interface via the surface immobilization method. We demonstrated that the lasing characteristics are drastically different between the gain material that is confined to the liquid–liquid interface and that homogeneously distributed in the bulk liquid solution. Our study reveals the unique capabilities of the surface-gain geometry optofluidic laser, which can be developed into a novel sensing platform to study biophysical and biochemical processes at the molecular level and has vast applications in biomedical diagnostics.