Counter-propagating frequency mixing with Terahertz waves in diamond
  Frequency conversion by means of Kerr-nonlinearity is one of the most common and exploited nonlinear optical processes in the UV, visible, IR and Mid-IR spectral regions. Here we show that wave mixing of an optical field and a Terahertz wave can be achieved in diamond, resulting in the frequency conversion of the THz radiation either by sum- or difference-frequency generation. In the latter case, we show that this process is phase-matched and most efficient in a counter-propagating geometry. 
Matteo Clerici, Lucia Caspani, Eleonora Rubino, Marco Peccianti, Marco   Cassataro, Alessandro Busacca, Tsuneyuki Ozaki, Dan...
physics.optics