  Terahertz time domain spectroscopy employing free-space radiation has
frequently been used to probe the elementary excitations of low-dimensional
systems. The diffraction limit blocks its use for the in-plane study of
individual laterally defined nanostructures, however. Here, we demonstrate a
planar terahertz-frequency plasmonic circuit in which photoconductive material
is monolithically integrated with a two-dimensional electron system. Plasmons
with a broad spectral range (up to ~400 GHz) are excited by injecting
picosecond-duration pulses, generated and detected by a photoconductive
semiconductor, into a high mobility two-dimensional electron system. Using
voltage modulation of a Schottky gate overlying the two-dimensional electron
system, we form a tuneable plasmonic cavity, and observe electrostatic
manipulation of the plasmon resonances. Our technique offers a direct route to
access the picosecond dynamics of confined transport in a broad range of
lateral nanostructures.
