  High spectral purity frequency agile room temperature sources in the terahertz spectrum are foundational elements for imaging, sensing, metrology, and communications. Here we present a chip scale optical parametric oscillator based on an integrated nonlinear microresonator that provides broadly tunable single frequency and multi frequency oscillators in the terahertz regime. Through optical to terahertz down conversion using a plasmonic nanoantenna array, coherent terahertz radiation spanning 2.8 octaves is achieved from 330 GHz to 2.3 THz, with 20 GHz cavity mode limited frequency tuning step and 10 MHz intracavity mode continuous frequency tuning range at each step. By controlling the microresonator intracavity power and pump resonance detuning, tunable multi frequency terahertz oscillators are also realized. Furthermore, by stabilizing the microresonator pump power and wavelength, sub 100 Hz linewidth of the terahertz radiation with 10-15 residual frequency instability is demonstrated. The room temperature generation of both single frequency, frequency agile terahertz radiation and multi frequency terahertz oscillators in the chip scale platform offers unique capabilities in metrology, sensing, imaging and communications. 
Coherent terahertz radiation with 2.8-octave tunability through   chip-scale photomixed microresonator optical parametric oscillation