Frequency up-shifter for THz light via relativistic Doppler reflection

We realize the relativistic Doppler reflection of THz light from a counter-propagating plasma mirror in silicon (Si) with a practically simple geometry, which allows us to easily apply the frequency up-shift to the THz optics. The average frequency of the reflected THz light is 1.4 times higher than that of the input THz light. In view of application, the important facts are that the frequency up-shift can be achieved by a low pump energy density (1.3 μJ mm^-2), and that shifted frequency is almost constant in the pump energy range above this threshold.