Femtosecond x-ray diffraction of structural changes in a semiconductor superlattice

Femtosecond excitation of spatially modulated electron hole plasmas leads to an ultrafast structural response within the unit cell of GaAs/AlGaAs superlattices. The transient rocking curve reflects the expansion of wells and compression of barriers

The direct measurement of the position of nuclei by ultrafast x-ray diffraction complements the information on the rapid response of a solid, obtained by monitoring the dynamics of the electronic system by optical spectroscopies, Laser-driven table-top plasma sources which generate femtosecond pulses of hard x-rays provide an important method to investigate fundamental microscopic mechanisms which underlie ultrafast structural changes e.g. of crystalline solids. Only recently the coherent lattice dynamics near the Lindemann stability limit were monitored with sub-picosecond resolution [1]. Fig. 1. a) Scheme of the ultrafast x-ray diffraction setup. The 1 kHz Ti:sapphire laser provides compressed pulses of 45 fs duration with 10 Watts average power. Focusing onto a liquid Ga-jet [2] or various solid band targets (Cu, Ti, Fe) generates femtosecond pulses of different hard x-ray characteristic emission lines up to 9.3 kV with more than 1010 photons/(sr · s). b) Diffraction from superlattice of Fig. 2 as recorded by the CCD. The ellipse indicates pumped regions of the sample.