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We study a terahertz near-field enhancement in two-dimensional electron systems supporting two-dimensional plasmons. We consider a periodically gated two-dimensional electron system, with a central gate finger biased independently from the rest of the grating. Such structure allows creating a tunable plasmonic cavity in a planar plasmonic crystal. We demonstrate that at certain conditions plasmons resonantly excited in such plasmonic crystal can strongly pump the cavity plasmon modes leading to a deep sub-wavelength energy concentration with terahertz electric field enhancement factor up to 10⁴

terahertz, two-dimensional plasmons, near-field