Search

Generating a coherent optical frequency comb at an arbitrary wavelength is important for fields such as spectroscopy and optical communications. Dark solitons which are coherent states of optical frequency combs in normal dispersion microresonators can extend the operating wavelength and be excited via intermodal coupling. They have been investigated over the last decade due to their high conversion efficiency and deterministic excitation with no need for dispersion engineering. While the existence and dynamics of dark solitons has been examined extensively, requirements on the modal interaction for accessing the soliton state in the presence of a strong Raman interaction at near-IR wavelengths has been less explored. Here, analysis on the parametric and Raman gain in a silica microresonator is performed, revealing that parametric gain can be created by an additional frequency due to modal interaction and exceed the Raman gain. More complex interaction dynamics of the parametric and stimulated Raman scattering process is studied using numerical simulations based on the Lugiato-Lefever equation. It is found that exciting a dark soliton requires not only appropriate modal coupling but also a range of pump powers. The existence range of the dark soliton is analyzed as a function of pump power and detuning for given modal coupling conditions. We anticipate these results will benefit fields requiring optical frequency combs with high efficiency and selectable wavelength in a material with a strong Raman gain.

The ability to detect and identify molecules at high sensitivity without the use of labels or capture agents is important for medical diagnostics, threat identification, environmental monitoring, and basic science. Microtoroid optical resonators, when combined with noise reduction techniques, have been shown capable of label-free single molecule detection, however, they still require a capture agent and prior knowledge of the target molecule. Optical frequency combs can potentially provide high precision spectroscopic information on molecules within the evanescent field of the microresonator; however, this has not yet been demonstrated in air or aqueous biological sensing. For aqueous solutions in particular, impediments include coupling and thermal instabilities, reduced Q factor, and changes to the mode spectrum. Here we overcome a key challenge toward single-molecule spectroscopy using optical microresonators: the generation of a frequency comb at visible to near-IR wavelengths when immersed in either air or aqueous solution. The required dispersion is achieved via intermodal coupling, which we show is attainable using larger microtoroids, but with the same shape and material that has previously been shown ideal for ultra-high sensitivity biosensing. We believe that the continuous evolution of this platform will allow us in the future to simultaneously detect and identify single molecules in both gas and liquid at any wavelength without the use of labels.