Development of Attosecond Light Sources @ Stanford PULSE Institute
Free Electron Laser based Attosecond Pulse Development
Through an ongoing collaboration, led by Agostino Marinelli, we have developed an electron bunch shaping method based on the enhanced self-amplified spontaneous emission (ESASE) technique. In the ESASE technique an energy modulation of the electron bunch is used to substantially increase the peak current of the electron bunch prior to lasing in the undulator. This increased peak current results in a reduction of the gain length for the SASE process and a modification of the structure of the output x-ray radiation. The energy modulation in the initial electron bunch is induced via the interaction with a laser pulse in a wiggler magnet. As a result of this process the x-ray pulse output is temporally linked to the laser radiation, which provides an opportunity for better synchronization. The setup for generating sub-femtosecond x-ray pulse is detailed below.
This technique is being implemented at LCLS through the XLEAP project. Recently, we conducted experiments to measure the duration of soft X-ray pulses generated using the ESASE method. In these experiments, we recorded a 2-dimensional projection of the 3-dimensional photoelectron momentum distribution produced in two-color photoionization of gas-phase targets. Depending on the phase of the infrared streaking field at the time when the X-ray pulses photoionizes the target, the photoelectrons will experience a momentum shift proportional to the magnitude of the streaking laser vector potential. We use this phase dependent impulse can be used to reconstruct the temporal profile (and phase) of the incident X-ray pulse.
High Harmonic Generation Based Attosecond Pulse Development
In our main laboratory space in the PULSE Institute, we have a laser source and beam line designed to produce high intensity XUV pulses with sub-femtosecond pulse durations. HHG from laser pulses containing multiple laser cycles produces attosecond bursts of VUV/XUV radiation every half cycle of the driving infrared laser pulse. In order to produce an IAP, one needs to filter out the attosecond burst from a single cycle of the driving laser pulse. In our laboratory, we have employed a spatio-temporal coupling (STC) scheme. In a spatio-temporal coupled laser field, spatial properties of the beam vary in time; for example, the direction of propagation can vary throughout the laser pulse duration, which is referred to as ultrafast wavefront rotation (WFR). An intense pulse with this particular STC will produce XUV bursts (or beamlets) every half laser cycle, but the beamlets will propagate in different angular directions. These beamlets can be filtered to isolate a single attosecond burst.