Letter of Intent for Ultra-short Xray Pulse Measurements at LCLS

Most of the experiments planned for LCLS will require accurate measurements of xray pulse temporal history, including pulse shape and relative timing with optical lasers. These requirements are particularly daunting given the recent demands for shorter pulse-widths (< 50 fs), some of which are discussed at the LCLS workshop[1]. This LOI addresses a novel approach to these measurements. There are two main elements: a new xray detection technology which converts the xray temporal history into an amplitude modulated optical probe beam and a high-speed recording technique that records this modulated optical probe beam, in a single-transient format. Conversion of the xray temporal history into the optical domain is accomplished in a semiconductor (e.g., GaAs). The xrays produce a modulation of the material’s dielectric function which is then sensed by the phase modulation of the optical probe beam. Through interferometry this phase modulation is converted into an amplitude modulation of the optical probe beam. With the xray information in the optical domain, both pulse shape information and cross-timing with other lasers is more readily accomplished. Our approach to the optical recording element is to use “temporal imaging” to “stretch” the temporal duration of the short optical pulse, in a very high-fidelity manner. The resulting optical pulse is “magnified” in time, resulting in a slower pulse which can then be measured and recorded with more conventional means optical techniques, such as optical streak cameras.