  Collinear phase-matched optical rectification is studied in ZnGeP$_{2}$
pumped with near-infrared light. The pump-intensity dependence is presented for
three crystal lengths (0.3, 1.0 and 3.0 mm) to determine the effects of linear
optical absorption, nonlinear optical absorption and terahertz free-carrier
absorption on the generation. Critical parameters such as the coherence length
(for velocity matching), dispersion length (for linear pulse broadening) and
nonlinear length (for self-phase modulation) are determined for this material.
These parameters provide insight into the upper limit of pulse intensity and
crystal length required to generate intense terahertz pulse without detriment
to the pulse shape. It is found that for 1-mm thick ZnGeP$_{2}$(012), pumped at
1.28 micron with intensity of ~15 GW/cm2 will produce intense undistorted
pulses, whereas longer crystals or larger intensities modify the pulse shape to
varying degrees. Moreover, phase-matching dispersion maps are presented for the
terahertz generation over a large tuning range (1.1-2.4 micron) in longer (3
mm) crystal, demonstrating the phase-matching bandwidth and phase mismatch that
leads to fringing associated with multi-pulse interference. All observed
results are simulated numerically showing good qualitative agreement.
