Terahertz (THz) science and technology is a typical overlapping frontier science. Its frequency is in the range of 0.1-10 THz, between microwave and infrared light waves. It is the transition zone from macroscopic classical theory to microscopic quantum theory and also the region from electronics to photonics. Because THz waves have efficient background emission noise suppression, good temporal and spatial coherence, and strong penetrability to non-polar substances, their energy is much smaller than X-rays, so humans can safely contact them, making the THz science and technology have a wide range of applications in communications, radar, imaging, medical diagnosis, material analysis and testing, environmental testing, and other fields.
In recent years, high-resistance silicon has become an effective integrated THz waveguide material due to its advantages of low loss, low dispersion, high refractive index in the THz band, and its fabrication is compatible with semiconductor technology (planar processing). However, in the THz bands, it is relatively difficult to detect and transmit terahertz waves on the chip. It is hard for the THz functional chip based on the silicon waveguides to work independently, and it needs to be connected to the traditional THz rectangular metal waveguides or antennas. Therefore, it is necessary to develop high-efficiency THz wave couplers to provide an effective solution for signal input and output of future on-chip terahertz systems.
Recently, associate professor Xie Jingya .et al from Shanghai University of Science and Technology combined a grating and a compact spot-size converter to realize a THz on-chip silicon waveguide and metal antenna coupler, as shown in Figure 1. Relevant research results were published in Chinese Optics Letters, Vol. 20, Issue 2, 2022 (Hongxiang Zhang, Jingya Xie, et al. Terahertz out-of-plane coupler based on compact spot-size converter).
Fig. 1 Schematic diagram of the structure of the terahertz out-of-plane coupler
Silicon photonics platforms have been developed over the past few decades because of their obvious advantages in cost and volume. The cross section size of optical silicon waveguide is much smaller than that of standard single-mode fiber, which easily causes the mode field mismatch and leads to low coupling efficiency. Therefore, the problem of chip coupling has been studied and discussed. Its coupling methods can be roughly divided into two categories: horizontal coupling and out-of-plane coupling. Out-of-plane coupling can be implemented at any position of the chip to improve the flexibility of the system. The device is simple to design and does not require high precision fabrication and polishing. However, the wavelength of signals in the low frequency range of THz is long (submillimeter/millimeter magnitude). Although the refractive index of silicon materials is large, directly applying the grating coupling method of traditional silicon photonics to the THz band will make the device size very large and even difficult to integrate on silicon wafers.
This work, using a compact spot-size converter to complete the conversion of the grating-coupled spot to a single-mode silicon waveguide, greatly decrease the size of the device. The device is only 2.9 cm in length, and can be used to 194 GHz THz signal coupling. It provides THz silicon waveguide wafer testing an efficient method and also a higher degree of freedom for on-chip integration.
