Dielectric waveguide lasers, microfluidic protein sensing, and raman spectroscopy

The work will be carried out within the ERC Advanced Grant “Optical Ultra-Sensor” (OPUS). This project will exploit the ultra-narrow linewidths, ultra-high quality factors (Q-factors), and ultra-high internal photon densities achievable in dual-wavelength, distributed-feedback (DFB), dielectric-waveguide lasers to perform on-chip-integrated, intra-laser-cavity optical sensing with unprecedented sensitivity. For our rare-earth-ion-activated Al₂O₃ lasers on silicon microchips, we will develop novel channel-waveguide and Bragg-grating geometries to decrease propagation losses, increase passive-cavity Q-factors, and enhance evanescent-field overlap with the measurands.

Together with frequency stabilization, these advances will improve the current few-kHz laser linewidths (Q-factors of ~10¹¹) to few-Hz linewidths (Q-factors of ~10¹⁴), thereby achieving intra-cavity laser powers of hundreds of Watts and light-measurand interaction lengths exceeding the circumference of the earth. Simple electrical read-out of the change in laser frequency and linewidth induced by the measurand is realized by photo-detection of the GHz beat-frequency signal produced between the perturbed and reference resonance of the dual-wavelength laser. Besides Al₂O₃:Yb³⁺ lasers emitting at 1-µm wavelength, we will develop Al₂O₃:Tm³⁺ and Al₂O₃:Ti³⁺ lasers on silicon chips operating at 2 µm and 700-1000 nm, respectively, thereby addressing new sensing applications. We will combine single and multiple dual-wavelength DFB lasers with microfluidic channels and arrayed-waveguide gratings in Al₂O₃, and place protein-antibody layers on top of the gratings or differently rare-earth-ion-doped nm-thin layers inside the active waveguide. This will enable intra-laser-cavity evanescent-field trapping and sensing of nano-sized bio-particles in optofluidic chips, first-ever demonstrations of on-chip intra-laser-cavity trace-gas detection and Raman spectroscopy, as well as intra-laser-cavity rare-earth-ion and energy-transfer spectroscopy approaching the single-ion level.