Abstract
Third order non-linearities are important because they allow control over
light pulses in ubiquitous high quality centro-symmetric materials like silicon
and silica. Degenerate four-wave mixing provides a direct measure of the third
order non-linear sheet susceptibility (3)L (where L represents the material thick-
ness) as well as technological possibilities such as optically gated detection and
emission of photons. Using picosecond pulses from a free electron laser, we show
that silicon doped with P or Bi, has a value of (3)L in the THz domain that
is higher than that reported for any other material in any wavelength band.
The immediate implication of our results is the ecient generation of intense
coherent THz light via up-conversion (also a (3) process), and they open the
door to exploitation of non-degenerate mixing and optical nonlinearities beyond
the perturbative regime.