External cavity terahertz quantum cascade laser with a metamaterial/graphene optoelectronic mirror

Photonic engineering of the terahertz emission from a quantum cascade laser (QCL) is fundamental for the exploitation of this unique source in a myriad of applications where it can be implemented, such as spectroscopy, imaging, and sensing. Active control of the frequency, power, polarization, and beam profile has been achieved through a variety of approaches. In particular, the active control of the emitted frequency, which is difficult to determine a priori, has been achieved through the integration of a photonic structure and/or by using external cavity arrangements. In this work, an external cavity arrangement, which implements a metamaterial/graphene optoelectronic mirror as an external feedback element, is proposed and demonstrated. The reflectivity and dispersion properties of the external active mirror were tuned via electrostatically gating graphene. It was possible to electronically reproduce the mode-switch occurring in a QCL emitting ∼2.8 THz by mechanically changing the external cavity length formed by an Au mirror. The external cavity arrangement was investigated and described in the framework of the self-mixing theory. These results open a way for the all-electronic engineering of the QCL emission by the use of a fast reconfigurable external mirror. This approach can uniquely address both power and frequency control, with ∼100 MHz reconfiguration speeds, using an integrated external element. Furthermore, the metamaterial/graphene mirror's strong dispersive properties might be implemented for the active mode locking of THz QCLs. Finally, this approach offers a unique opportunity to study the laser dynamics and mode competition in THz QCLs in the self-mixing feedback regime.