Figure 5
Two-dimensional plot of the coherence length Lc = π/Δk for HMQ-TMS gives a direct representation of the optical rectification phase matching mechanism (also known as velocity matching) for different pump wavelengths.
The calculations are restricted to frequencies below 12 THz, where the indexes of refraction have been measured. The velocity matching between the pump pulse and the generated THz frequencies depends strongly on the optical wavelengths. In particular, when the pump is shifted toward longer wavelengths large coherence length manifests for broader THz spectrum. In this conditions the velocity matching is achieved over the entire THz gap.
