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We propose a new generation of unbiased (biasfree) antenna-less CW THz photomixer emitters array made of asymmetric MSM gratings with subwavelength pitch, operating in optical nearfield regime. We take advantage of the size effects in near-field optics and electrostatics to demonstrate the possibility of enhancing the THz power by four orders of magnitude, as compared with a similar array of the same size that operates in the far-field regime. We show that with an appropriate choice of grating parameters in such THz sources the first plasmonic resonant cavity mode in nano-slit between two adjacent MSM can enhance the optical near-field absorption and hence the photo carriers' generation under the slit in the active medium. These photo carriers, on the other hand, are accelerated by the large built-in electric field sustained under the nano-slits by two dissimilar Schottky barriers to create the desired large THz power that is mainly radiated downward. A hybrid numerical simulation method is used to model and analyze these phenomena. [ABSTRACT FROM AUTHOR]