  The effect of terahertz (THz) pulse generation has revolutionized broadband
coherent spectroscopy and imaging at THz frequencies. However, THz pulses
typically lack spatial structure, whereas structured beams are becoming
essential for advanced spectroscopy applications. Non-linear optical
metasurfaces with nanoscale THz emitters can provide a solution by defining the
beam structure at the generation stage. We develop a non-linear InAs
metasurface consisting of nanoscale optical resonators for simultaneous
generation and structuring of THz beams. We find that THz pulse generation in
the resonators is governed by optical rectification. It is more efficient than
in ZnTe crystals, and it allows us to control the pulse polarity and amplitude,
offering a platform for realizing binary-phase THz metasurfaces. To illustrate
this capability, we demonstrate an InAs metalens, which simultaneously
generates and focuses THz pulses. The control of spatiotemporal structure using
nanoscale emitters opens doors for THz beam engineering and advanced
spectroscopy and imaging applications.
