Active control of the amplitude and frequency of terahertz sources is an essential prerequisite forexploiting a myriad of terahertz applications in imaging, spectroscopy, and communications. Herewe present a optoelectronic, external modulation technique applied to a terahertz quantum cascadelaser which holds the promise of addressing a number of important challenges in this research area.A hybrid metamaterial/graphene device is implemented into an external cavity set-up allowingfor optoelectronic tuning of feedback into a quantum cascade laser. We demonstrate powerful,all-electronic, control over the amplitude and frequency of the laser output. Full laser switching isperformed by electrostatic gating of the metamaterial/graphene device, demonstrating a modulationdepth of 100%. External control of the emission spectrum is also achieved, highlighting the flexibility ofthis feedback method. By taking advantage of the frequency dispersive reflectivity of the metamaterialarray, different modes of the QCL output are selectively suppressed using lithographic tuning and singlemode operation of the multi-mode laser is enforced. Side mode suppression is electrically modulatedfrom ~6 dB to ~21 dB, demonstrating active, optoelectronic modulation of the laser frequency contentbetween multi-mode and single mode operation.
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