Advanced THz diagnostics
How to reach femtosecond sub-cycle timing resolution
The TELBE facility generates intense THz pulses with stable carrier envelope phase (CEP). This enables sub-cycle temporal resolution when probing the THz-induced dynamics in a sample. Since TELBE is based on an electron accelerator and we are using femtosecond lasers as optical probes, we are facing the problem of temporal jitter between the two types of sources. This jitter smears out any sufficiently fast experimental signals. Therefore a highly precise synchronization and potentially a data sorting algorithm are required to resolve dynamical processes on femtosecond timescales.
Schematic timing distribution at TELBE. The timing signal from the ELBE facility master oscillator ensures synchronization between TELBE THz source and femtosecond probe laser in the experiment.
The ELBE facility uses an optical synchronization system that distributes a timing signal generated by a master oscillator to all other sources. In case of TELBE, these sources are the photoinjector laser of the ELBE electron gun and the femtosecond probe laser in the TELBE user lab. The typical temporal jitter that remains in the TELBE lab is on the order of 100 fs (RMS). This allows for direct measurements of dynamics with oscillation periods below ∼2 THz using a lock-in amplifier. However, oscillating signals at higher frequencies are smeared out in time, so that a secondary sorting method has been implemented.
This scheme measures the exact arrival time of each THz pulse relative to the probe laser pulse. Based on the acquired timestamp, the experimental data is sorted accordingly, achieving a temporal precision of better than 10 fs. The optical scheme, which is based on so-called spectral decoding [1], can be seen in the image below. An FPGA-based data acquisition system analyses the timing and experimental data as it is measured, analysing and sorting typically 50000 datasets per second [2]. Therefore, the experimental data can be monitored live while it is being measured with highest precision.
Experimental setup used for time-domain measurements at the TELBE facility. A single-cycle THz pulse from the TELBE CDR is used as timing reference in the spectral decoding scheme. The superradiant undulator source is used for the ultrafast experiments. Figure from [2].
Demonstration experiment data from the FPGA-based data acquisition system at TELBE. The electro-optic sampling data was taken with an artificial jitter of 500 fs (RMS).
The results of this measure-and-sort algorithm can be seen in the figure on the left. Here, experimental data based on electro-optic sampling of a 2.1 THz pulse are compared when the sorting is implemented vs. unsorted data. The effect is particularly strong, as an artificial jitter of 500 fs (RMS) was imposed on the probe laser system. (a) Shows 10000 data points measured and sorted when the delay stage was static, corresponding to 200 ms of acquisition time.The waveform becomes visible only because of the large jitter that effectively acts as temporal delay. (b) Shows the measurement of this effective delay caused by the jitter betwen THz pulse and EOS probe. In (c) a complete pulse train is shown. The comparison between direct measurement (green) and FPGA-based measurement (blue) shows that the 500 fs jitter completely smears out the 2.1 THz signal.
Our work on THz (timing) diagnostics is embedded within the program topic accelerator-research and development: subtopic 3 ps - fs photon and electron beams (ARD - ST3). To this end (T)ELBE serves within ARD-ST3 as a test facilitiy for diagnostics on quasi-CW electron and photon beams.
Related publications
[1] S. Kovalev et al., Probing ultra-fast processes with high dynamic range at 4th-generation light sources: Arrival time and intensity binning at unprecedented repetition rates. Struct. Dyn. 4, 024301 (2017).
[2] A. N. Ponomaryov et al. FPGA-based measurements of the relative arrival time of a high-repetition rate, quasi-cw fourth generation light source. Rev. Sci. Instrum. 95, 103008 (2024).
M. Chen, et al., Terahertz-slicing — an all-optical synchronization for 4th generation light sources. Opt. Express 30, 26955 (2022).
M. Chen, et al., Pulse- and field-resolved THz-diagnostics at 4th generation lightsources. Opt. Express 27, 32360 (2019).
B. Green et al., "High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter", Sci. Rep. 6 (2016), 22256.
S.S. Dhillon et al., "The 2017 terahertz science and technology roadmap", Journ. Phys. D 50 (2017), 043001.
