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X-ray Photoemission Electron Microscopy

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X-ray streak cameras

Ultrafast spin dynamics

Lawrence Berkeley National Laboratory

Advanced Light Source
Rajesh Chopdekar
last update: 03/30/2020

Tutorial - X-ray streak cameras

A streak camera is a time-resolved detector, which works similar to a TV tube. An (optical) signal is converted into an electron pulse on a photocathode, accelerated by a strong electric field and deflected by a fast voltage ramp on two parallel deflector plates. Electrons arriving early see a low electric field and are deflected less than electrons arriving later, which are deflected more. Time information is thereby encoded by a spatial coordinate. A temporally extended signal is converted into a streak on the electron detector. The magnetic lens focuses the emitted electrons.
A GaAs photoconductive switch produces the voltage ramp on the deflection plates. The short laser pulse that triggers the switch is produced by a titanium sapphire femtosecond laser. The infrared (IR) pulse generates carriers in the semiinsulating substrate and closes the switch between two Gold electrodes, launching a ~500 V pulse. The pulse has a rise time on the order of ~100 ps, broadened by dispersion of the cables, and a slower trailing edge.
Streak camera are used for time-resolved experiment in order surpass the time resolution given by the length of the probe pulse. In x-ray experiments at synchrotron electron storage rings the time-resolution of stroboscopic experiments is given by the length of the x-ray pulse, which is typically between 30 ps and 100 ps long. A streak camera can resolve the dynamics of the sample within the x-ray pulse. In a pump-probe style experiment a first laser pulse from a Ti:sapphire amplified laser generates a dynamic response of the sample. A symchronized x-ray pulse probes the state of the sample, here by being partially absorbed in the sample, and is detected by the streak camera. The deflectors of the streak camera are ramped by a pulse that is derived from the same laser pulse that was used for pumping. Two more laser pulses are split off and converted to UV to monitor the temporal resolution of the streak camera. Using x-rays a resolution of 1-2 picoseconds has been achieved. For UV pulses a much higher time resolution of a few 100 femtoseconds is achievable because the energy bandwidth of emitted electrons is much reduced compared to that of x-ray generated electrons.
Synchronization to the storage ring requires a phase-locked laser ocillator which fires laser pulses, here at 62.5 MHz, synchronuously with the 500 MHz repetition rate of the storage ring. The laser amplifier picks and amplifies oscillator pulses at a much lower frequency of 5 KHz, synchronously with the round trip bunch marker of the storage ring to ascertain that always the same electron bunch is chosen, usually a cam-shaft or single/dual bunch pulse. The low repetition rate of the laser is necessary to provide enough laser power to trigger the photoconductive switch and to start the dyanmics in a millimeter size area of the sample. This scheme has the downside that the majority of the x-ray pulses have to be discarded and cannot be used to sample the dynamic response of the sample. Suppression of these pulse is achieved by a DC bias on the sweep plates and by using a gated channelplate detector.Laser pump - x ray probe experiments are extremely challenging because of the very low usable x-ray flux and the complex timing requirements.
A streaked x-ray pulse of about 70 ps length is shown together with two UV fiducials. The time resolution of this measurement was about 4 ps. To obtain good statistics, several 10000 streaks were averaged on the streak camera CCD detector at a 5 KHz repetition rate.