Currently, terahertz (THz) optical systems are based on bulky free-space optics. This is due to the lack of a common
platform onto which different THz components, e.g., source, waveguide, modulator and detector, can be monolithically
integrated. With the development of THz quantum cascade laser (QCL), it has been realized that the QCL chip may be
such a platform for integrated THz photonics. Here, we report our recent works where the THz QCL is integrated with
passive or optoelectronic components. They are: 1) integrated graphene modulator with THz QCL achieving 100%
modulation depth and fast speed; 2) phase-locked THz QCL with integrated plasmonic waveguide and subwavelength
antennas realizing dynamically widely tunable polarizations.