THz -driven phonon upconversion

Here is a short movie showing data from our Nature Physics paper. Here strontium titanate (STO) is excited by a strong single cycle THz pulse and we observe the lattice motion with femtosecond x-ray pulses. You can see how the oxygen atoms (red) follow the THz field more or less linearly while the titanium atoms (blue) wiggle at a much faster rate. This is the indirect excitation of the 5 THz phonon mode.