The use of photo-mixing techniques for THz emission offers attractive performances such as tunability and modulation bandwidth, that are suitable for bio-medical sensing and imaging, communications, or security. We will present the state-of-the-art performances of a vertical-external-cavity surface-emitting laser that operates on two transverse modes to ensure a stable continuous-wave and coherent (longitudinal, transverse and polarization) dual-frequency operation. THz emission is subsequently obtained by excitation of an uni-traveling-carrier photodiode (UTC-PD). The stability of the dual-frequency operation is achieved thanks to different types of functionalized surfaces involving the micro-fabrication of integrated III-V absorbing metallic masks or metamaterial phase masks by e-beam lithography. These functionalized surfaces allow to shape the optical and THz performances in terms of power, tunability and coherence. The latter will be specifically detailed in terms of longitudinal coherence, showing a THz frequency noise that is orders of magnitude lower than the optical one thanks to a significant correlation of technical noise. Tunable emission will be demonstrated from 50 GHz up to few THz with a linewidth of 150 kHz (during 3-ms), for a power of 1 W at 260 GHz that is limited by the UTC-PD for an optical excitation at 1064 nm at room temperature. We will discuss on the possibility to improve such a power significantly by taking advantage of the involved high-order transverse mode, offering possible intrinsically coherent networks of photo-emitters, thus paving the way to compact and agile coherent THz sources offering an output power over few mWs at frequencies of 100s of GHz.