High-efficiency and frequency-controllable terahertz pulses driven by two-color relativistic laser pulses

The two-color laser filamentation technique serves as a high-quality, laser-based strong-field terahertz source. However, the terahertz energy obtained from this method tends to saturate at high laser intensity, limiting its ability to fully exploit the advantage of the extremely strong field offered by relativistic lasers. Here, we proposed a novel approach based on photon deceleration in plasma to obtain controllable terahertz radiations with two-color relativistic femtosecond laser pulses. In our method, an 800-nm laser drives the plasma wave and controls the plasma density distribution, while the other 10.6-μm laser experiences a strong frequency downshift in the plasma wave and converts to a multi-cycle terahertz pulse, whose central frequency is adjustable by altering the parameters of laser and plasma. The conversion efficiency from the 10.6-μm laser to terahertz radiation can be over 12%, potentially unlocking novel applications in the terahertz domain.