Real-time electron clustering in an event-driven hybrid pixel detector

Event-driven hybrid pixel detectors with nanosecond time resolution have opened up novel pathways in modern ultrafast electron microscopy, for example in hyperspectral electron-energy loss spectroscopy or free-electron quantum optics. However, the impinging electrons typically excite more than one pixel of the device, and an efficient algorithm is therefore needed to convert the measured pixel hits to real single-electron events. Here we present a robust clustering algorithm that is fast enough to find clusters in a continuous stream of raw data in real time. Each tuple of position and arrival time from the detector is continuously compared to a buffer of previous hits until the probability of a merger with an old event becomes irrelevant. In this way, the computation time becomes independent of the density of electron arrival and the algorithm does not break the operation chain. We showcase the performance of the algorithm with a 'timepix' camera in two regimes of electron microscopy, in continuous beam emission and laser-triggered femtosecond mode.

Jitter-free terahertz pulses from LiNbO3

Intense terahertz pulses are indispensable for modern science and technology, but time-critical applications require ultimate stability of the field cycles with respect to a reference clock. Here we report the nonlinear optical generation of terahertz single-cycle fields by femtosecond laser pulses under passive compensation of timing jitter. The converter is based on optical rectification in a LiNbO₃ slab with two silicon prisms for extracting and combining the emitted Cherenkov radiation from both sides into a single beam. In this way, we achieve suppression of timing jitter to <200 as/µm of beam displacement, a factor of >70 better than in conventional non-collinear geometries.