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We report on focusing of terahertz (THz) plasmons in two-dimensional electron gas (2DEG) at III-N heterostructures and in graphene by using planar circular grating lenses. Propagation of a broadband pulse of EM waves in 0.5-10 THz is studied by using finite difference time-domain (FDTD) approach. The results show that plasmonic modes excited by incident THz radiation are concentrated into λ/180 area localized under the central disc. Electric field intensity under the central point is found to be orders of magnitude larger than the outer grating area. Optimal geometries for enhanced radiation coupling and plasmon focusing are investigated. Plasmonic lens modes supported by system has the advantage of tunability by an applied voltage to gratings. The large field enhancement by plasmonic confinement presents the potential for sub-wavelength imaging.