  An intense, short laser pulse propagating across a plasma boundary
ponderomotively drives THz radiation. Full format PIC simulations and
theoretical analysis are conducted to investigate the properties of this
radiation. Simulation results show the THz emission originates in regions of
varying density and covers a broad spectrum with maximum frequency close to the
maximum plasma frequency. In the case of a sharp vacuum-plasma boundary, the
radiation is generated symmetrically at the plasma entrance and exit, and its
properties are independent of plasma density when the density exceeds a
characteristic value determined by the product of the plasma frequency and the
laser pulse duration. For a diffuse vacuum-plasma boundary, the emission from
the plasma entrance and exit is asymmetric: increasing and decreasing density
ramps enhance and diminish the radiated energy respectively. Enhancements by
factors of 50 are found and simulations show that a 1.66 J, 50 fs driver pulse
can generate ~400 \mu J of THz radiation in a 1.2 mm increasing density ramp.
We present a model that attributes this effect to a plasma resonance process in
the density ramp. The results from the model match those of the simulations for
ramp lengths less than 600 \mu m. For longer ramps for which simulations are
too time consuming the model shows that the amount of radiation reaches a
maximum at a ramp length determined by collisional absorption.
