An ultra-fast photodiode for picosecond optical, UV, and x-ray detection

Fusion and high energy density physics facilities perform ultra-fast timing experiments that require good quantum efficiency in deep UV and x-ray regimes. As an example, beam timing measurements at free electron laser facilities monitor timing for electron bunch pulses with 100 kHz repetition rates in laser- driven shock, plasma, and optical/UV pump-probe experiments. These measurements require detector rise- fall times to be below 50 ps to monitor coarse timing. Device size is another important factor, particularly in the crowded and complex target chambers of fusion facilities. The need for robust, condensed packaging, and picosecond time resolution extends to fields of laser characterization, synchrotrons, and high-speed communication. There is not currently a fast photodiode available in the market with a UV/x-ray response time < 50 ps. Readout electroncis that can take advantage of the fast time response are nearly as rare. A photodiode based off of a metal semiconductor metal technology will be fabricated, tuned and tested to produce < 50 ps response times with high QE for ultra violet and x-ray wavelengths. To complement this technology, integrated readout electronics will be developed to complete the photodiode system. During Phase I, knowledge transfer for diode photo-lithoraphic fabrication was completed. Two iterations of prototype packaging were produced, EUV response was tested, and prototype diode response time with AlGaN thin film substrate was validated to be < 25 ps. Readout electronics research and customer discus- sions also led to the conclusion that developing low cost, fast electronics would be on the critical path to bringing this product to the market. The main objective of Phase II will be to develop and test integrated readout electroncics with the photodiode prototype developed in Phase I in target applications. This effort will include review of Phase I designs, development of analog front end electronics, and system integration with a high speed software interface. A research vein of the project will continue advancing diode fabrication. It will focus on studying alternative substrates to sapphire with a closer lattice constant to AlGaN and GaN, and study p-i-n diode structure performance instead of metal semiconductor metal which will have long term manufacturing benefits. A commercially supported fast photodiode with sub-50 ps resolution will enable scientists to conduct other- wise impossible timing measurements in pulse time monitoring and pump-probe experiments. Free electron laser facilities and pulsed soft x-ray source researchers will be able to measure pulse to pulse variations. Larger facilities observing dozens of ultra-violet and soft x-ray pulses simultaneously can take advantage of the integrated electronics, removing the need to allocate expensive oscilloscopes to a single application.