Intensity Correlations in Quantum Cascade Laser Harmonic Frequency Combs

A novel study on harmonic frequency combs emitted by quantum cascade lasers (QCLs) is presented here, demonstrating the presence of intensity correlations between twin modes characterizing the emission spectra. These originate from a four-wave mixing process driven by the active medium's third-order nonlinearity. The study of such correlations is essential for the engineering of a new generation of semiconductor devices with the potential of becoming integrated emitters of light with quantum properties, such as squeezing and entanglement. Starting from experimental results, the limits of state-of-the-art technology are discussed as well as the possible methodologies that could lead to the detection of nonclassical phenomena, or alternatively improve the design of QCLs, in the compelling perspective of generating quantum correlations in mid-infrared light.