A study of laser-generated strain fields with X-ray microdiffraction

In its 324-bunch mode of operation, the Advanced Photon Source (APS) allows new femtosecond (fs) laser pump/X-ray probe experiments to be developed. In this mode of operation, if one uses the tightly focused low pulse energy (nJ), high-repetition-rate fs-laser Ti:Sapphire oscillator on beamline 7ID, every laser and X-ray pulse can be temporally delayed with respect to each other, as the frequency of the laser oscillator and the X-ray bunches are both 88 MHz. This can result in a high-repetition-rate pump-probe experiment which uses X-rays from every bunch. This presentation describes an example of how coherent X-ray imaging and microdiffraction experiments may be used to study laser-generated strain fields in semiconductors. With an oscillator beam focused to 7 μ m onto GaAs, we have observed coherent X-ray diffraction patterns with a high-resolution camera. We have also studied the strain fields with a focused X-ray beam generated by a Fresnel zone plate. Results from the two X-ray techniques will be compared. These experiments may help to develop techniques that will be used at the future free electron laser sources where coherent and pump-probe experiments can be done simultaneously.