DESC0021468

Free-Electron Laser (FEL) x-ray sources are continuously being improved and upgraded, opening the doors for many scientific frontiers in biology, chemistry, and material science, such as structural dynamics, matter at extreme conditions and nanostructure imaging. However, the lack of suitable ultrafast sources and detectors covering X-rays to Vis-NIR has been preventing/limiting such studies in the past. The Hamamatsu G4176-03 ultrafast Metal-Semiconductor-Metal (MSM) photodetector, which is low temperature-GaAs-based, is a potential solution, but was recently discontinued. A replacement with equivalent or better performance is thus needed, as currently called for by the DOE. The need for ultrafast detectors is being addressed by developing a picosecond photodetector based on our in-house grown Cadmium Magnesium Telluride (Cd1-xMgxTe) single crystal. CdMgTe is sensitive to both optical and x-ray pulses for coarse timing in FEL applications. A CdMgTe-based photodetector has the potential of exhibiting better detector performance than the Hamamatsu G4176-03 ultra-fast MSM Photodetector, while having similar form factor/packaging. A CdMgTe photodetector with better performance than the Hamamatsu G4176-03 photodiode was demonstrated. The 2-3 ps photo-response has been shown via femto-second laser pump-probe measurement. The current (dark and light) levels of the CdMgTe device was much smaller, more linear and more stable compared to the Hamamatsu reference. A prototype device using an initial R&D electrode pattern (for signal collection) and an SMA connector (same connector used in the Hamamatsu G4174-03) was built and has been sent for further X-ray ultrafast response measurements. In Phase II we will fully demonstrate the advantageous features of our ultrafast CdMgTe photodetector using an improved device design with the interdigitated electrode pattern and same form factor/packaging as the Hamamatsu G4176-03 device. The proposed device will have the additional transmission mode capability via a 3-terminal device configuration (The Hamamatsu G4176-03 can only be operated in reflective mode with the 2-terminal device). We will demonstrate a prototype picosecond photodetector based on Cd1-xMgxTe that is sensitive to both optical and x-ray pulses for coarse timing in FEL applications. Applications that rely on ultrafast timing of FELs, such as femtosecond X-ray/optical cross-correlation diagnostics, will benefit, especially in biology, chemistry and material science. At this time, ultrafast Cd1-xMgxTe photodetector materials are not available commercially. There is a tremendous potential for widespread commercialization of the materials and devices that will be developed during this program, including various applications in the nuclear radiation detection field. We hope to develop a domestic source for this material as an alternative to the high cost, conventional cadmium zinc telluride (CZT) radiation detection material.