Pioneering intense sub-terahertz-wave generation by nonlinear optical effects
Sub-terahertz (sub-THz) frequencies, below 1 THz, have seen remarkable development in next-generation (6G) wireless communications. On the other hand, an alternative attraction of sub-THz waves is their high material penetration, which makes them suitable for non-destructive testing applications. In these applications, there is a high demand for real-time imaging, which can be achieved by irradiating the test object with a wide spread of THz waves and using a two-dimensional focal plane array to measure at video rate. In addition, a variety of materials are candidates for nondestructive testing, including resin, paper, ceramics, wood, and carbon fiber, and their penetration varies on a logarithmic scale. To meet all these requirements in practical applications, an intense sub-THz source is needed. We have developed intense terahertz wave sources using nonlinear optical crystals based on the high-brightness performance of lasers. The sub-THz regime is the most challenging area to pioneer from the optical regime. In this talk, I will review our research efforts and results on how to efficiently generate sub-THz waves using laser photonics technology. In particular, given the photon energy of sub-THz waves, the wavelength difference between two near-infrared waves in optical parametric three-wave mixing is a few nanometers. In difference frequency generation (DFG), we have demonstrated a dual-wavelength fiber ring laser system operating around 1060 nm based on broadband chirped fiber Bragg gratings (CFBGs) and semiconductor optical amplifiers (SOAs). By using the pump laser and an organic nonlinear DAST crystal, the type-0 collinear phase matching of the DFG process could be fulfilled and a monochromatic THz wave in the range of 0.5 to 2 THz was successfully generated. In another way, we proposed a 1300 nm injection-seeded $\beta$-BaB$_2$O$_4$-OPG (is-BBO-OPG) without cavity structure to generate spectrally separated wavelengths with difference frequency down to the sub-THz range. The dual-wavelength pump source demonstrated broadly tunable THz wave generation from 0.3 to 4 THz in the DAST DFG process.
I will present our studies using organic and inorganic nonlinear optical crystals, and finally reached to the discovery of backward THz-wave parametric oscillation (BW-TPO), which would be a remarkable achievement in the development of intense sub-THz wave source.