Fig. 2. (IMAGE) Forschungsverbund Berlin Caption THz emission of the TO phonons with (red) and without (black) excitation by a second pulse. The electric field is plotted as a function of time. The second optical pulse leads to a shortening of the oscillation period, corresponding to an increase of the frequency from 8 to 8.1 THz. Credit MBI Usage Restrictions The image may only be used with appropriate caption and credit. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.