Related Links: Science Highlight SPPS Page Press Release TIP Article Stanford Report Apr 2005 Headlines SSRL Home Page SLAC Home Page Stanford Home Page

Friday, 29 April 2005 First SPPS Results: Electro-optic Sampling and Ultra-fast Melting summary written by Heather Woods, SLAC Communication Office 20 consecutive single shot electron bunch measurements. The bright band in each column is the electro-optic signal, its location indicates the time of arrival of the electron bunch with respect to the laser probe pulse, and its width corresponds to the electron bunch duration. The Sub-Picosecond Pulse Source (SPPS) collaboration has published data from the first experiments ever using a linear accelerator-based femtosecond x-ray source. SPPS makes the world's shortest bunches of electrons in the SLAC linear accelerator and turns them into very bright pulses of x-ray light 1,000 times shorter than those made in synchrotron rings like SPEAR3. SPPS has many similarities to future free electron lasers like the Linac Coherent Light Source (LCLS) currently being built at SLAC. Researchers used SPPS to develop and test a new timing technique, electro-optic sampling, which will be essential for many SPPS and LCLS experiments. To put data in order chronologically - important for seeing chemical or other reactions over time - researchers need to time-stamp the arrival of the laser pulse that starts a reaction, and the arrival of the x-ray pulse that observes the system. The strong electric field generated by each electron bunch alters the properties of an electro-optic crystal placed next to the beam, but only at the instant the electrons pass by. The characteristics of the laser light exiting the crystal reveal the electron bunch length and arrival time, and thus the x-ray pulse arrival time. In the other experiment, researchers shone laser light to melt a room-temperature crystal of semiconductor material, and sent x-ray pulses to probe the material. The scattered x-rays provided the first look at the first few femtoseconds in the transition from solid to liquid. In that time, the atom positions had on average the initial crystalline (regular, repeated) structure of the solid, yet the atoms had moved far from their starting positions, with the disordered structure of a liquid. The result is a very unusual, intermediate state of matter. Many authors contributed to this research. Full author lists for the associated papers can be found in the reference section in the technical science highlight