Microelectromechanical control of the state of quantum cascade laser frequency combs

Chip-scale frequency combs such as those based on quantum cascade lasers (QCLs) or microresonators are attracting tremendous attention because of their potential to solve key challenges in sensing and metrology. Though nonlinearity and proper dispersion engineering can create a comb—light whose lines are perfectly evenly spaced—these devices can enter into different states depending on their history, a critical problem that can necessitate slow and manual intervention. Moreover, their large repetition rates are problematic for applications such as dual comb molecular spectroscopy, requiring gapless tuning of the offset. Here, we show that by blending midinfrared QCL combs with microelectromechanical comb drives, one can directly manipulate the dynamics of the comb and identify new physical effects. Not only do the resulting devices remain on a chip-scale and are able to stably tune over large frequency ranges, but they can also switch between different comb states at extremely high speeds. We use these devices to probe hysteresis in comb formation and develop a protocol for achieving a particular comb state regardless of its initial state.