  Recently, two-dimensional terahertz spectroscopy (2DTS) has attracted
increasing attention for studying complex solids. A number of recent studies
have applied 2DTS either with long pulses or away from any material resonances,
situations that yield unconventional 2DTS spectra that are often difficult to
interpret. Here, we clarify the generic origins of observed spectral features
by examining 2DTS spectra of ZnTe, a model system with a featureless optical
susceptibility at low terahertz frequencies. These results also reveal possible
artifacts that may arise from electro-optic sampling in collinear 2DTS
experiments, including the observation of spurious rectified or second harmonic
signals.
