In the last couple of decades, terahertz (THz) technologies, which lie in the frequency gap between the infrared and microwaves, have been greatly enhanced and investigated due to possible opportunities in a plethora of THz applications, such as imaging, security, and wireless communications. Photonics has led the way to the generation, modulation, and detection of THz waves such as the photomixing technique. In tandem with these investigations, researchers have been exploring ways to use silicon photonics technologies for THz applications to leverage the cost-effective large-scale fabrication and integration opportunities that it would enable. Although silicon photonics has enabled the implementation of a large number of optical components for practical use, for THz integrated systems, we still face several challenges associated with high-quality hybrid silicon lasers, conversion efficiency, device integration, and fabrication. This paper provides an overview of recent progress in THz technologies based on silicon photonics or hybrid silicon photonics, including THz generation, detection, phase modulation, intensity modulation, and passive components. As silicon-based electronic and photonic circuits are further approaching THz frequencies, one single chip with electronics, photonics, and THz functions seems inevitable, resulting in the ultimate dream of a THz electronic–photonic integrated circuit.

We investigate the resonance property of a self-coupled optical waveguide (SCOW) with four coupling points. Under a special condition that the two outer couplers are in weak coupling and the two inner couplers are in strong coupling, an implicit standing-wave resonance mode is formed inside the SCOW structure. As the resonance is fed back via an S-shape bus waveguide, electromagnetically induced transparency (EIT)-like resonance features are observed in the output transmission spectrum. The SCOW resonator is analyzed using the coupled mode theory and FDTD simulation. The device is experimentally demonstrated on the silicon-on-insulator platform. The measurement results agree well with the theoretical calculation. The EIT-like peak can be tuned by a thermo-optic phase shifter made of a NIN type silicon resistor integrated in the feedback waveguide. The results point to new ways of creating on-chip optical analog of EIT phenomenon that can be utilized for various optical signal processing applications.

Attenuated total reflection (ATR) geometry is a suitable choice for in vivo measurements of human skin due to the deep penetration of the field into the sample and since it makes it easy to measure the reference spectrum. On the other hand, there are several critical factors that may affect the terahertz (THz) response in these kinds of experiments. Here, we analyse in detail the influence of the following factors: the contact positions between the thumb and the prism, the contact pressure, the contact duration, and the materials of the prism. Furthermore, we use the THz-ATR technology to evaluate different types of handcream and also establish the theoretical model to investigate the reflectivity after interacting with the skin. The results agree well with experimental ones. Our analysis makes it clear the importance of controlling the above factors during measurements to enable reliable THz response and results which, in turn, may be used to monitor water motion in human skin and to predict possible diseases.

Continuous wave (CW) photomixing is a widely utilized method for terahertz (THz) radiation generation, with diverse applications in sensing, spectroscopy, and wireless communication. However, most existing systems are dependent on discrete, bulky components, highlighting the demand for integrated solutions that can enhance energy efficiency, flexibility, and stability. Here, we exploit the advantages of silicon photonics within the THz domain utilizing a hybrid-integrated dual III-V/Si₃N₄ narrow-linewidth laser module to generate widely tunable THz waves, whose frequency is determined by the frequency difference of the lasers. To enhance the frequency stability of the produced THz signal, we synchronize thermal noise by integrating two external laser cavities on the same chip. Further, synchronization of electrical noise is accomplished by electrically connecting the two gain sections in series using a low-noise current source. The external cavity lasers incorporating low-loss Si₃N₄ microring resonator (MRR) filters, deliver optical power up to 13 dBm, exhibit a broad wavelength tuning range of approximately 55 nm, and maintain a narrow optical intrinsic linewidth below 0.77 kHz. By adjusting the laser frequency interval in the heterodyne synthesis setup, we achieved CW THz generation over a wide tuning range from 95.2 GHz to 1.012 THz. The 3-dB THz electrical linewidth is estimated to be less than 31 kHz. As far as we know, this represents the narrowest linewidth for THz signals generated by heterodyne synthesis with free-running integrated sources over such a wide tuning range. The hybrid-integrated narrow-linewidth compact dual laser module possesses a long-term THz frequency drift of 65 MHz, measured for 10 hours. Our study therefore highlights the huge potential of silicon photonics technology in the THz domain.

Low-loss dielectric terahertz (THz) chips are efficient platforms for diverse THz applications. One of the key elements in the chip is the coupler. Most of the available THz couplers are in-plane and couple the THz wave from the metal waveguide to the dielectric waveguide. However, out-of-plane couplers are more suitable for wafer-scale testing and tolerant of alignment variation. In this work, we propose an out-of-plane THz coupler for coupling the antenna to the dielectric waveguide. The device is constructed using a grating and a compact spot-size converter. As the conventional optical spot-size converters that apply directly to THz chips are too large, we have designed a compact spot-size converter based on a tapered waveguide with a lens. The total device is 2.9 cm long and can couple a 7 mm diameter THz beam to a 500 µm wide waveguide. The device can scan the THz beam, radiate the input rectangular waveguide mode to free space, and drive the rotation angle of the fan beam through the scanning frequency. We fabricated the device using a single lithography step on a silicon wafer. The out-of-plane coupling efficiency was found to be ∼5 dB at 194 GHz. The fan-beam steering range was found to be around 40° in the frequency range of 170–220 GHz. The proposed out-of-plane coupling technique may provide an effective way for THz wafer-scale testing with a higher degree of freedom for on-chip integration. Also, the proposed technique being non-mechanical, beam steering using it, may therefore find applications in THz radar, communication, and sensing.