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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]

• A new antenna-less unbiased terahertz photomixer with recessed contact electrodes is introduced. • A systematic design of the recessed contact electrodes' geometrical and physical parameters. • 3400 % enhancement in radiated THz power of the presented emitter compared with the conventional emitter. • High frequency tuning capability of the proposed emitter in a wide range of 0.1–5 THz. • Presented emitter can be as a promising candidate to different biomedical applications like endoscopic imaging. We propose an antenna-less unbiased CW terahertz photomixer comprising an array of recessed contact electrodes. Each unit of the recessed contact electrodes array embedded in the photoconductive layer includes two pairs of unsymmetrical metals making up heterogeneous Schottky contacts. As compared to the analogous array emitters of the same size with planar electrodes, by optimizing the geometrical and physical properties of the recessed contact electrodes in the proposed emitter to excite the appropriate resonant modes in the larger volume of the active region, it is possible to benefit from the having of high optical absorption and so terahertz photo-current, which in turn leads to enhance the obtainable THz power by more than three orders of magnitude. This is also more than an order of magnitude larger THz power than that is radiated from the similar antenna-coupled emitter with planar contact electrodes. Furthermore, the calculation results show the proposed emitter has a frequency tuning capability in a broad range of 0.1–5 THz. [ABSTRACT FROM AUTHOR]