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We propose a new spiral-like dipole antenna-coupled and unbiased terahertz (THz) photomixer with plasmonic unsymmetrical interdigitated electrodes (UIEs). Each electrode pair is made of subwavelength metal–semiconductor–metal (MSM) structures constituting heterogeneous Schottky contacts. From the optical view and analysis of the electrical transport in the semiconductor, by applying a systematic procedure to achieve optimum design, it is possible to benefit from advantages of having high THz photocurrent derived from the plasmonic enhanced optical absorption and, hence, photogeneration as well as severe built-in Schottky field in the photoconductor. Furthermore, from the electromagnetic view, by introducing an output resonant THz antenna having great impedance matching with the photomixer device, the efficiency of free-space coupling has significantly been enhanced. By exploiting all aspects of design, it has been provided to achieve the THz power of 409 µW from the spiral-like dipole antenna coupled to the proposed plasmonic UIEs with 10×10µm^2 active area and pitch of Λ=395nm. This is more than 1 order of magnitude larger THz power than the highest THz power radiated from the same area of a conventional dipole antenna-coupled unbiased array of the near-field emitters. Also, this improvement is more than 3 orders of magnitude higher THz power as compared with the power radiated from a non-plasmonic far-field operating antenna-less THz chip of the same size.

We present a new bias-free antenna-coupled CW terahertz photomixer with interdigitated electrodes. Each finger pair is made of metal/semiconductor/metal (MSM) electrodes with dissimilar Schottky barriers. The two dissimilar metals in each MSM have a barrier height of difference (Δϕ_B) and a finite lateral spacing (s). In the proposed teraheretz emitter, not only is the optical absorption and hence photogeneration enhanced by the surface resonant modes, but also the high built-in field increases the terahertz current. Furthermore, by having a large radiation resistance of the antenna integrated to the array electrodes, based on our simulations, it is possible to achieve the maximum power of 35 μW from a dipole antenna coupled to the proposed electrode array with 10 μm ×10  μm active area and pitch of Λ=800  nm. This is more than 196 times larger terahertz power than the highest terahertz power radiated from the array emitters of the same area that contains the bias-free antennaless array of far-field emitters with double pitch size. We have also investigated the limits of thermal breakdown and efficiency degradation of the proposed emitter that play an important role due to the reduction of the active area. Such terahertz sources can pave the way to various biomedical applications such as endoscopic imaging without a need for hazardous external circuitry for biasing, reducing patient health risk.

A new generation unbiased antennaless CW terahertz (THz) photomixer emitters array made of asymmetric metal–semiconductor–metal (MSM) gratings with a subwavelength pitch, operating in the optical near-field regime, is proposed. We take advantage of size effects in near-field optics and electrostatics to demonstrate the possibility of enhancing the THz power by 4 orders of magnitude, compared to a similar unbiased antennaless array of the same size that operates in the far-field regime. We show that, with the appropriate choice of grating parameters in such THz sources, the first plasmonic resonant cavity mode in the nanoslit between two adjacent MSMs can enhance the optical near-field absorption and, hence, the generation of photocarriers under the slit in the active medium. These photocarriers, on the other hand, are accelerated by the large built-in electric field sustained under the nanoslits by two dissimilar Schottky barriers to create the desired large THz power that is mainly radiated downward. The proposed structure can be tuned in a broadband frequency range of 0.1–3 THz, with output power increasing with frequency.