Improved laser sources for the generation of terahertz radiation which comprise a semiconductor diode laser for generating optical pulses for integration into a THz pulse generation system wherein the pulse generator comprises an ultrabroadly tunable quantum dot (QD) based laser diode (191) which acts as a single or dual-mode optical pump source. The QD is deposited in layers which may be in a range of physical sizes, dot densities and layer spacings. The external cavity may comprise a beam splitter (187) and two diffraction gratings (197) for individual control of the two wavelengths emitted via the opposite face of the QD gain chip (191). THz radiation is generated at the frequency difference of these two laser wavelengths.