Gold implanted germanium photoswitch for cavity dumping of a free-electron laser

We present a plasma switch based on gold implanted germanium (Ge:Au) as a potential candidate for efficient cavity dumping of a free-electron laser (FEL). Ge:Au has a sub-nanosecond carrier lifetime – much shorter than the FEL pulsing period of 77 ns – and demonstrates a high photoinduced reflectivity in a broad range of infrared wavelengths from 6 to 90 µm. The Ge:Au plasma switch exhibits negligible absorption of the FEL radiation in the ʻoff ʼ state and requires only moderate thermoelectric cooling for incident FEL power of several Watts. A reflectivity level of more than 50 % in the ‘on’ state is achieved over the entire spectral range of this study. The corresponding optical pump fluence exhibits a linear relationship with the FEL frequency. This scaling is corroborated by our simulations highlighting the role of a finite sub-µm thickness of the photoinduced reflecting plasma layer. The demonstrated device is promising for the realization of the FEL cavity dumping for experiments that simultaneously require higher pulse energy and lower average power.