  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.
