Continuous-Wave Operation of Microcavity Quantum Cascade Lasers in Whispering-Gallery Mode

Microresonator-based lasers are key light sources in photonic integrated circuits for future optical communication. Recently developed on-chip sensing makes microcavity lasers at mid-infrared wavelengths very appealing owing to the abundant absorption fingerprints in this spectral range (3-12 mu m). As leading semiconductor mid-infrared laser sources, quantum cascade lasers are ideal candidates for on-chip sensing, and significant achievements have been made using deformed cavities for unidirectional emission. However, the much higher operating voltage and thermal load from the cascade scheme and the sidewall current leakage due to the thermally activated surface states make the continuous-wave operation of microcavity quantum cascade lasers rather challenging. Here, we designed a selective electrical isolation scheme for the specific region of the cavity and combined it with surface passivation. We experimentally demonstrated the first continuous-wave operation of microcavity quantum cascade lasers up to a temperature of 5 degrees C at an emission wavelength of 8 mu m. This design significantly reduces the injection current while ensuring a low optical loss for the whispering-gallery mode with a high quality factor, thereby reducing the thermal dissipation power. It is worth noting that this design is applicable to a variety of whispering-gallery-mode resonators, especially large cavities, which greatly develops the potential of microcavity quantum cascade lasers as continuous-wave mid-infrared light sources for photonic integrated circuits and microcavity physics.