Terahertz (THz) sources, ranging from 0.1 THz to 10 THz, between microwaves and infrared waves, have important applications in spectral detection, medical imaging, communication, etc. Difference frequency generation (DFG) is an effective method for generating terahertz with the characteristics of low cost, simple structure, widely tunable range, no threshold, and room-temperature operation. This paper reviews various optical terahertz sources of difference frequency generation based on nonlinear crystals, including DFG with inorganic crystals, DFG with organic crystals, DFG with quasi-phase-matching (QPM) crystals, DFG in waveguides, cavity-enhanced DFG, and cascaded DFG. Their recent advances, as well as their advantages and disadvantages, are fully present and discussed. This review is expected to provide a comprehensive reference for researchers in this field and a quick understanding of optical THz sources of difference frequency generation with nonlinear crystals.

We proposed a new optomechanical system (OMS) based on parallel suspended one-dimensional optomechanical crystal (1D-OMC) nanobeam cavities for optomechanical synchronization. The optomechanical oscillators (OMOs) were spaced apart by an air-slot gap and coupled through optical radiation fields. The numerical simulation showed that the evolution process of 1D-OMC nanobeam cavities to mechanical synchronization could be divided into three clear stages. The synchronization of two mechanical breathing modes at 5.8846 GHz was achieved by using a single laser source. Finally, we investigated the relationship between the threshold power and detuning of an input laser for self-sustaining and synchronization states. Such chip-based structures hold great potential for large-scale synchronized oscillator networks.