  We observe intense pulses of far-infrared electromagnetic radiation emitted
from arrays of InAs nanowires. The THz radiation power efficiency of these
structures is about 15 times higher compared to a planar InAs substrate. This
is explained by the preferential orientation of coherent plasma motion to the
wire surface, which overcomes radiation trapping by total-internal reflection.
We present evidence that this radiation originates from a low-energy acoustic
surface plasmon mode of the nanowire. This is supported by independent
measurements of electronic transport on individual nanowires, ultrafast THz
spectroscopy and theoretical analysis. Our combined experiments and analysis
further indicate that these plasmon modes are specific to high aspect ratio
geometries.
