Cherenkov radiation of terahertz surface plasmon polaritons from a superluminal optical spot

We propose to use a cylindrically focused ultrashort laser pulse obliquely incident on a semiconductor surface to launch surface plasmon polaritons with frequencies in the terahertz range. The laser pulse creates a spot of nonlinear polarization moving along the semiconductor surface with superluminal velocity. We show theoretically that the spot will emit the surface and free-space waves via Cherenkov mechanism. We develop a theory and calculate the radiated fields and frequency distribution of the radiated energy. The efficiency of the optical-to-terahertz conversion is estimated for the case of nonlinear optical rectification in GaAs. We show that one can control the radiation pattern and spectrum of the surface waves by varying the incident angle of the optical beam and crystalline orientation of the semiconductor. This technique opens new ways to build terahertz sources and perform surface spectroscopy.