There has been an increasing interest in the application of terahertz (THz) waves to broadband wireless communications. In particular, use of frequencies above 275 GHz is one of the strong concerns among radio scientists and engineers, because these frequency bands have not yet been allocated at specific active services, and there is a possibility to employ extremely large bandwidths for ultra-broadband wireless communications. Introduction of photonics technologies for signal generation, modulation and detection is effective not only to enhance the bandwidth and/or the data rate, but also to combine fiber-optic (wired) and wireless networks. This paper reviews recent progress in THz wireless communications using telecom-based photonics technologies towards 100 Gbit/s.

Terahertz integrated platforms with high efficiency are crucial in a broad range of applications including terahertz communications, radar, imaging and sensing. One key enabling technology is wideband interconnection. This work proposes substrate-less all-dielectric waveguides defined by an effective medium with a subwavelength hole array. These self-supporting structures are built solely into a single silicon wafer to minimize significant absorption in metals and dielectrics at terahertz frequencies. In a stark contrast to photonic crystal waveguides, the guiding mechanism is not based on a photonic bandgap but total internal reflections. The waveguides are discussed in the context of terahertz communications that imposes stringent demands on performance. Experimental results show that the realized waveguides can cover the entire 260-400 GHz with single dominant modes in both orthogonal polarizations and an average measured attenuation around 0.05 dB/cm. We further demonstrate the transmission of uncompressed 4K-resolution video across this waveguide. This waveguide platform promises integration of diverse active and passive components. Thus, we can foresee it as a potential candidate for the future terahertz integrated circuits, in analogy to photonic integrated circuits at optical frequencies. The proposed concept can potentially benefit integrated optics at large.

The terahertz (THz, 0.1–10 THz) region has been attracting tremendous research interest owing to its potential in practical applications such as biomedical, material inspection, and nondestructive imaging. Those applications require enhancing the spatial resolution at a specific frequency of interest. A variety of resolution-enhancement techniques have been proposed, such as near-field scanning probes, surface plasmons, and aspheric lenses. Here, we demonstrate for the first time that a mesoscale dielectric cube can be exploited as a novel resolution enhancer by simply placing it at the focused imaging point of a continuous wave THz imaging system. The operating principle of this enhancer is based on the generation—by the dielectric cuboid—of the so-called terajet, a photonic jet in the THz region. A subwavelength hotspot is obtained by placing a Teflon cube, with a 1.46 refractive index, at the imaging point of the imaging system, regardless of the numerical aperture (NA). The generated terajet at 125 GHz is experimentally characterized, using our unique THz-wave visualization system. The full width at half maximum (FWHM) of the hotspot obtained by placing the enhancer at the focal point of a mirror with a measured NA of 0.55 is approximately 0.55λ, which is even better than the FWHM obtained by a conventional focusing device with the ideal maximum numerical aperture (NA = 1) in air. Nondestructive subwavelength-resolution imaging demonstrations of a Suica integrated circuit card, which is used as a common fare card for trains in Japan, and an aluminum plate with 0.63λ trenches are presented. The amplitude and phase images obtained with the enhancer at 125 GHz can clearly resolve both the air-trenches on the aluminum plate and the card’s inner electronic circuitry, whereas the images obtained without the enhancer are blurred because of insufficient resolution. An increase of the image contrast by a factor of 4.4 was also obtained using the enhancer.