Picosecond X-ray Sensor

A coplanar stripline detector has been developed for picosecond resolution of x-ray synchrotron pulses. When triggered by synchronized 100 fs laser pulses, this detector measures the x-ray pulse waveform with 1 ps resolution, a 100-fold improvement compared to standard pump-probe experiments at synchrotron sources. The detectors have been fabricated with CMOS compatible microfabrication processes. Time resolution is limited by photocarrier lifetimes, which is typically hundreds of picoseconds in semi-insulating GaAs. Standard implantation techniques, which add defects that serve as trapping sites to produce subpicosecond lifetimes, are insufficient because of the large x-ray absorption depths. Instead, 8 MeV protons that penetrate hundreds of microns are implanted. Strong signals have been measured using focused monochromatic x-ray beams at the Advanced Photon Source (Argonne, IL, USA). Laboratory excitation with 100 fs laser pulses, instead of x-rays, demonstrates the feasibility of this detector. The 1 ps target time resolution allows for x-ray studies in a crucial time regime where laser-induced electronic excitations transfer energy to the atomic lattice.