Quantum Dots-Engineered Microlasers with Bidirectional Wavelength Tuning for Single-Cell Biosensing

While microlasers have revolutionized on-chip photonics, their spectral tunability remains constrained by conventional bandgap engineering and cavity reconstruction, limiting the dynamic range. Here, we introduce fluorescence resonance energy transfer (FRET) as a transformative mechanism for bidirectional, programmable wavelength tuning in whispering gallery mode (WGM) microlasers. Through strategic engineering of quantum dot-based donor-acceptor systems at the cavity interface, we demonstrate (1) forward FRET-WGM enabling blue shift via intra-to-extra-cavity energy transfer and (2) reverse FRET-WGM achieving red shift through extra-to-intracavity coupling. A semiclassical rate equation model quantitatively predicts tuning trajectories, revealing FRET-driven gain competition as the dominant spectral selector. When deployed as intracellular biosensors in living cancer cells, these FRET-WGM microlasers achieve 19.8 pM detection sensitivity for carbon quantum dots, a 5000-fold improvement over conventional confocal microscopy, while maintaining wavelength stability despite photobleaching. This study establishes RET-WGM as a versatile platform for quantitative single-cell analysis and enables future developments in reconfigurable nanophotonics and precision biomedicine.