Creating a terahertz wave source through nonlinear difference frequency generation in ferroelectric crystals

This paper proposes that terahertz waves may be generated from higher frequency sources by exploiting the difference-frequency nonlinear effect in ferroelectric crystals. The nonlinear response of ferroelectric crystals together with the fact that they are resonant in the terahertz region is the main reason for proposing their use. The proposal is explored theoretically using Landau-Devonshire theory to calculate nonlinear susceptibility coefficients for difference frequency generation. Also it is shown how the slowly varying amplitude approximation can be used to solve the wave equation, thus describing the propagation of the generated waves in the crystal. Conditions on the length through which the waves travel that optimize the phase matching are a useful result of this part of the analysis. An example of why such an approach is useful is to consider that quantum cascade lasers can produce frequencies above the terahertz range at room temperature, but need to be cooled to low temperatures in order to operate in the terahertz range. However two room-temperature produced frequencies can be used to generate a terahertz-range output by arranging for the difference of the frequencies to fall into this range.