The Linac Coherent Light Source (LCLS) is an X-ray free electron laser (FEL) at the SLAC National Accelerator Laboratory in Menlo Park, CA. It is a unique facility designed to reveal fundamental processes in materials, technologies and living things. It provides an ultrafast, coherent, X-ray probe with a pulse duration on the order of 1-10 fs, that can be used to track dynamics on their natural, femtosecond timescales with atomic-level resolution.

While the facility provides short pulses ideal for investigating fast processes, there is an uncorrectable inherent temporal jitter between the X-ray pulses and other tools used to study these phenomena. Unless compensated for, this jitter renders it impossible to obtain fine time-resolved mesaurements. I developed a measure-and-sort approach that time-stamps the arrival time of each LCLS pulse with respect to the optical laser pulse so that users can time sort their data in post processing, yielding time-dependent results with fine time resolution. This is done using a spectral encoding method.

A broadband super-continuum probe is chirped such that its different frequency components arrive at slightly different times, providing a mapping of wavelength to time. The probe is passed simultaneously with an X-ray pulse through an optically transparent sample, such as Si₃N₄ or Ce:YAG, during which the X-ray induces a change in the complex index of refraction of the material. This produces a feature in the transmitted spectrum, the location of which corresponds to the arrival time of the X-ray pulse. The spectral time-tool is now part of the standard configuration for all LCLS experiments. This means that nearly every time-resolved experiment performed at the LCLS uses this tool to temporally sort data. Time-stamps are now automatically calculated and saved into the data stream as experiments are being performed.

• Mina R. Bionta, H. T. Lemke, J. P. Cryan, et al., “Spectral encoding of x-ray/optical relative delay,” Optics Express 19(21), 21855 (2011).

• M. R. Bionta, N. Hartmann, M. Weaver, et al., “Spectral encoding method for measuring the relative arrival time between x-ray/optical pulses," Review of Scientific Instruments 85(8), 083116 (2014).

• M. Harmand, R. Coffee, M. R. Bionta, et al., “Achieving few-femtosecond time-sorting at hard X-ray free electron lasers,” Nature Photonics 7(3), 215 (2013).