Analysis and Simulations of Optical Rectification as a Source of Terahertz Radiation

The second order nonlinearity present in many crystals can be utilized to convert optical radiation into THz radiation via the optical rectification mechanism. This process becomes efficient if a phase matching condition is satisfied. The short pulses used for optical rectification can be more intense than the longer pulses used for difference frequency generation because of the pulse length dependence of the crystals damage threshold. However, optical rectification is more complicated than difference frequency generation because of the fact that the THz is broadband and group velocity dispersion cannot be neglected. Simulations show that conversion efficiencies of one percent can be obtained from optical rectification in a Gallium Selenide crystal, provided a means of coupling the radiation into and out of the crystal can be found. The saturation of the THz signal is due to frequency shifts in the laser pulse, which change the group velocity and spoil the phase matching. As part of the process, the laser pulse is dramatically compressed.