The X-ray Free Electron Lasers (XFELs) are nowadays used to study the structure and dynamics in matter with unprecedented temporal and spatial resolutions. With the current X=x-ray methods, however, the access to attosecond domain, such as x-ray spectroscopy and x-ray diffraction, remains elusive. In this work we report on a new experimental approach to study sub-femtosecond processes in matter. Based on the x-ray chronoscopy concept, it explores the time distribution of ultra-short x-ray pulses before and after interaction with a sample. The pulse time structure can be measured using the state-of-the-art terahertz streaking cameras at XFELs arranged in the camera-sample-camera sequence.
In this study, we place a strong emphasis on understanding the ultrafast dynamics of carrier recombination pathways in p-type ZnO, especially in the midgap region. Synthesizing and controlling the properties of p-type ZnO remains a pivotal yet challenging task for numerous optoelectronic and spintronic applications due to intrinsic midgap (defect) states. Through an advanced sol-gel process, we have successfully produced ZnO quantum dots (QD), eliminating unreacted molecules that decrease the excitonic emission. This refined method supports the generation of ZnO with p-type characteristics, primarily attributed to zinc vacancies in oxygen-rich scenarios. Notably, our analysis across timescales from femtoseconds to microseconds unveiled carrier lifetimes at room temperature, and associated long-lasting carriers with zinc vacancy defects, corroborating the p-type nature of our synthesized ZnO QDs.
The stochastic nature of the self-amplified spontaneous emission (SASE) process in hard x-ray free electron laser (FEL) photon pulse generation often requires online spectral monitoring on a pulse-to-pulse basis. Experimenters and machine operators use the spectral information to better understand their data and guide the experiment, or to optimize the machine parameters through feedbacks to achieve a desired stability. Even experiments that use monochromators benefit from online spectral measurements as it allows the FEL operators to use machine feedbacks on the generated spectra so that the light would not jitter too far out of the monochromator’s acceptance bandwidth.
Online spectrometers have existed for several years in several facilities like LCLS, SACLA, and SwissFEL, and have mostly been used to characterize hard x-rays at photon energies typically above 4 keV. This abstract presents ongoing work on the development of the online tender x-ray spectrometer (TXS)], meant to cover the energy range between 2000 and 4000 eV and fill a gap for online spectral measurements at SwissFEL.
The Photon Arrival and Length Monitor (PALM), a THz streak camera device developed by PSI for non-destructive hard x-ray measurements of photon pulse length and arrival time versus a pump laser[1], was brought to the SACLA XFEL[2] in Japan in a cross-calibration temporal diagnostics campaign after an initial experiment where only the PALM was being used[3]. The device was used with 9 keV pink beam and a 9.0 and 8.8 keV two-color mode, successfully measuring the temporal ifnromation of the pulses for several different FEL operating conditions. The most interesting achievement is the PALM’s ability to measure two arrival times of the two colorors as tey are shifted against each other by the FEL, opening up new possibilities in temporal accuracy for two-color experiments. SwissFEL will employ two such devices at the end stations for use by both operators and experimenters to improve the operation of the FEL and to better interpret experimental data.
References
[1]P. N. Juranić et. al, Journal of Instrumentation (2014) 9.
[2]T. Ishikawa et. al., Nature Photonics (2012) 6(8).
[3] P. N. Juranić et. al., Optics Express (2014) 22.
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