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The two-color operation of free electron laser (FEL) facilities allows the delivery of two FEL pulses with different energies, which opens new possibilities for user experiments. Measuring the arrival time of both FEL pulses relative to the external experimental laser and to each other improves the temporal resolution of the experiments using the two-color FEL beam and helps to monitor the performance of the machine itself. This work reports on the first simultaneous measurement of the arrival times of two hard X-ray FEL pulses with the THz streak camera. Measuring the arrival time of the two FEL pulses, the relative delay between them was calculated and compared to the set values. Furthermore, we present the first comparison of the THz streak camera method to the method of FEL induced transient transmission. The results indicate a good agreement between the two methods.
© 2017 Optical Society of America
1. Introduction
The advancement of free electron lasers (FELs) around the world [1–4] allows the performance of pump-probe measurements that study the atomic and molecular processes in the ultrafast time domain. The synchronous delivery of two X-ray pulses at FEL sources [5, 6] opens new possibilities for new discoveries. In this case, the sample can be both pumped and probed by the FEL pulses leading to a better understanding of the X-ray induced phenomena in different materials. The two-color mode has been used to study the solid-to-plasma transition process induced by an X-ray pulse [7] and to investigate the intense radiation damage thresholds of materials [8]. Other experiments study the non-equilibrium states of crystalline systems caused by X-ray heating [9] or the intramolecular dynamics during the X-ray absorption [10].
In case of the seeded FELs [3] a high stability can be achieved [11] for the delay between the two FEL pulses, while for the SASE FELs [1,2,4] this delay can jitter, since the FEL generation in this case is based on a stochastic process. Therefore, monitoring the temporal separation between the two SASE FEL pulses can provide a useful information for the mentioned user experiments, helping to better understand the evolution of the X-ray induced phenomena in time. Furthermore, comparing the measured separations to the relaxation times of the processes being studied will shed more light on the experiment results.
The most common technique to measure the relative arrival times is the temporal cross-correlation of the FEL pulses with the experimental laser by using the X-ray induced transient transmission or reflectivity change of a membrane [12–16]. Sub-femtosecond resolution was reported by [17] from the measurements of relative arrival time at LCLS [2]. The THz streak camera [18–21] is another method commonly utilized for measuring the relative timing between the FEL and the optical pulses. The first arrival time measurements with hard X-ray photon pulses using this method showed an rms accuracy of sub-10 fs [22]. Thus far, there are no reports of using any of these two techniques for the two-color operation of FEL machines. These two methods were also never compared to other measurement methods or to each other.
We report here on the first simultaneous measurements of two X-ray FEL pulses with different energies, performed at SACLA [4]. The measurements characterize the THz streak camera method for the relative arrival time application and open new possibilities for the FEL temporal diagnostics. The photon pulses were provided by the two-color operation mode of SACLA [6]. The relative delay between the two FEL pulses was changed during the measurements, and this change was registered by the THz streak camera as well as the individual arrival time of each FEL pulse with respect to the experimental laser. Such measurements allow the monitoring of the machine performance and help to improve the temporal resolution of the experiments using the two-color beam. In addition to the two-color operation, measurements were also performed using the standard operation mode of SACLA, where the THz streak camera was operated in tandem with the transient transmission setup using spatial decoding that is available at SACLA [16,23]. The measurements from these two detectors were compared on a pulse-to-pulse basis to verify the results from the two techniques. The measurements described in this work are the first of their kind to our knowledge.
2. Measurement method
The concept of the THz streak camera is to encode the temporal properties of the X-ray photon pulse in the energy spectrum of the photoelectrons produced by this pulse. This is done by exposing the photoelectrons to a THz/IR radiation, which is generated with a portion of the experimental optical laser, giving it an intrinsic synchronization with the laser pulses used by the experiment. Due to the interaction with the external electric field, the electrons experience a shift in their kinetic energies. Depending on the ionization time of the electrons, they interact with the external field at different phases, leading to different final kinetic energies. This fact allows the calculation of the X-ray pulse arrival time relative to the external THz pulse by measuring the central energy of the photoelectron spectrum. The theory of the THz streak camera is discussed in detail by [18,24,25]. It is shown that the central kinetic energy Kf of the photoelectrons in presence of the external electric field can be written as:
Here K0 is the kinetic energy of the photoelectron before the interaction with the external field, ωTHz is the frequency of the THz pulse, ti is the time of the ionization and is the ponderomotive potential, with E0THz being the electric field amplitude, while me and e are the mass and the charge of the electron, respectively. Equation 1 is written for a linearly polarized THz field, and the angle θ indicates the direction of the electron propagation with respect to the external electric field. When the electrons drift along the field direction or opposite to it, cos θ = ±1 can be used in Eq. (1). When the X-ray pulse arrives at the linear part of the THz vector potential, around the zero-crossing of the sine-function, the energy shift ΔK = Kf − K0 of the photoelectron spectrum depends linearly on the relative arrival time: where s = ∂ΔK/∂t is a linear coefficient showing the speed of the energy change. Once this coefficient is defined for a certain THz pulse, the arrival times of the photon pulses can be obtained from the energy shifts of the photoelectron spectra.During the two-color operation, when two FEL pulses of different photon energies are synchronously delivered, the linear coefficients for both photon pulses need to be calculated. The individual arrival times of the two pulses can be obtained using Eq. (2) by measuring the energy shifts of the electron spectra created by each of the two pulses. In the case of measurements with the two-color mode it is important that the linear part of the THz vector potential is long enough to accommodate the arrival times of both FEL pulses.
3. Experimental setup
The schematic overview of the measurement setup at SACLA beamline [26] is demonstrated in Fig. 1. The experimental laser beam of SACLA was split into two parts. One of the beams was used in the experimental hutch 1 (EH1) for the SACLA arrival time monitor, while the second part was used in the experimental hutch 2 (EH2) as the pump laser for THz generation. Before entering EH1 the FEL pulse propagates through a transmission grating that splits a small fraction of the incoming pulse as a 1st order branch. The X-ray beam of this branch is focused onto a gallium arsenide (GaAs) crystal causing a transient change in its transmission. This change is imprinted in the spatial profile of the Ti:sapphire experimental laser pulse, which is then detected by a CCD camera. More detailed description of the arrival time measurement system at SACLA is given in [16,23].
Fig. 1 Sketch of the measurement layout at SACLA beamline. The timing tool of SACLA and the THz streak camera are positioned in the experimental hutches 1 and 2, respectively.
The THz streak camera setup installed in EH2 consists of a vacuum chamber and a THz generation setup. The THz radiation is generated by optical rectification [27] with a tilted pulse front method, using a lithium niobate (LiNbO3) crystal [28–31]. The pumping laser pulse energy was about 10 mJ. The single-cycle THz pulse created during the experiment had a duration of about 1.7 ps (frequency of 0.6 THz) and was intrinsically synchronized with the SACLA experimental laser. The shape of the THz pulse obtained during the measurements is shown in Fig. 2. The THz pulse was reconstructed by measuring the energy shifts of the photoelectron spectra during a full delay scan between the THz and the FEL beams. The THz radiation enters the vacuum chamber from a side and is focused on the ionization region by a parabolic mirror with a focal length of 150 mm. The electric field strength of the THz beam at the focus was measured to be about 8 MV/m (Fig. 2) with a transverse spot size of about 1 mm. The parabolic mirror has a hole with 5 mm diameter at the upstream end and 3 mm at the downstream end, to let the FEL beam through, which propagates along the vacuum chamber and ionizes two pulsed gas jets of xenon (Xe) atoms. The gas pulses are injected from the top of the chamber and are synchronized with the FEL and THz pulses. During the measurements the electrons from the atomic shell of 2p3/2 of Xe were used with a binding energy of 4786 eV. When the Xe atoms are ionized by the FEL pulses, the photoelectrons are detected by electron time-of-flight spectrometers (eTOFs) that are positioned in the plane of FEL polarization. At the first interaction region there is no THz beam, and the eTOF positioned there measures the non-shifted energy spectrum (K0 in Eq. (1)). The second ionization region produces electrons in presence of the THz field and their energy spectra are shifted in accordance to Eq. (1). Two eTOFs are positioned on opposite sides of the vacuum chamber and point towards the interaction region to measure the energy spectra shifted in opposite directions. The acceptance angle of the eTOF from the interaction region is about θ = 3.8° (cos θ = 0.998). This means that the eTOFs measure only the electrons that propagate parallel to the THz field, and cos θ = ±1 can be applied in Eq. (1) when calculating the energy shifts of the photoelectrons. By comparing the central energies of the two shifted energy spectra to the non-shifted electron spectrum from the first interaction point, we obtain the energy shift caused by the THz radiation. The calibration coefficient s in Eq. (2) was obtained by scanning the arrival time of the THz pulse against the FEL pulse around its linear slope, and measuring the energy shift ΔK for each delay value. The arrival time of the THz pulse was changed using an optical delay line before the THz generation setup. After the calibration was done, about 10000 consecutive FEL pulses were simultaneously measured by the THz streak camera and the SACLA timing tool. The measurements were performed such that both of these monitors were measuring the arrival time of the same FEL pulse relative to the same experimental laser pulse. The photon energy of the FEL beam was 9 keV during the standard operation with a pulse energy of about 250 μJ.
In the next step we used the THz streak camera to synchronously characterize two FEL pulses of different photon energies and slightly different arrival times. This was the first simultaneous measurement of two hard X-ray FEL pulses with the THz streaking method. The two pulses with different photon energies are obtained by propagating the electron bunch through two different undulator sections, where it radiates at two different wavelengths (photon energies). The temporal separation between the pulses is controlled by delaying the electron bunch with a magnetic chicane positioned between the two undulator sections. More details about the two-color operation of SACLA are given in [6]. During the measurements the two X-ray pulses had photon energies of 9 keV and 8.8 keV with the higher energy pulse arriving earlier. The delay between the two pulses was changed from 0 fs up to 50 fs with steps of 10 fs. For each value of the delay the arrival times of both FEL pulses were measured for about 10000 consecutive shots, and the temporal separation between the two pulses was calculated.
The maximum delay of 50 fs between the two pulses applied during the measurements was limited by the geometry of the magnetic chicane at SACLA. Nevertheless, the THz streak camera method is capable of measuring larger separations, being limited only by the duration of the linear part of the THz pulse.
4. Results
During the data analysis of the measurements with the standard operation mode the central energies of the electrons were found by calculating the center of mass of the spectra measured by the eTOFs. Examples of such single-shot spectra are shown in Fig. 3(a). The solid red curve shows the spectrum measured at the first interaction region without the THz radiation. Meanwhile, the blue and green dashed curves correspond to the spectra shifted in the opposite directions due to the interaction of the electrons with the THz field. It is worth noting here that the spectra of the photoelectrons created in the presence of the THz field (streaked spectra) are the convolution of the non-streaked photoelectron spectra and the temporal profile of the FEL pulse that ionizes the electrons. When the photoelectron spectra are streaked in the opposite directions the temporal profile of the FEL pulse appears in the streaked spectra in different ways (the electrons created by the head or tail of the photon pulse will gain or lose energy depending on the direction of streaking). For this reason, the spectra streaked in the opposite directions may look different, as one can see in Fig. 3(a). Furthermore, due to the limited number of the photoelectrons, the spectra experience statistical fluctuations and the shapes of the spectra can be slightly different from one another.
Fig. 3 Normalized energy spectra of the photoelectrons measured by the three eTOFs (a). The dashed lines correspond to the shifted spectra in presence of the THz field, while the solid line shows the non-shifted spectrum. The measured energy shifts plotted against the arrival time readings of the SACLA monitor are shown in (b).
To obtain the central energies, the center-of-mass of the overall peak was calculated, including also any substructures that might be present in the peak. The waveforms that had a signal-to-noise ratio (snr) of less than 20 (about 30% of overall waveforms) were not included in the data analysis procedure as the evaluation of these spectra induces large uncertainties. By obtaining the energy shifts from the eTOFs for different arrival times of the FEL pulses measured by the timing tool of SACLA, the shape of the THz vector potential was reconstructed. This shape is shown in Fig. 3(b), where the red crosses correspond to the linear part of the vector potential, where the measurements are most sensitive. One can see from the figure that the linear part is 400 fs long. The calculations showed that the calibration coefficient s from Eq. (2) is 0.23 eV/fs along this linear area. It can be seen from Fig. 3(b) that the rms jitter of the FEL arrival time relative to the optical laser is about 250 fs rms, which is consistent with the results reported in [23]. The photon pulses arriving within the 400 fs temporal window of the linear area were about 25–30% of the overall pulses. By measuring the energy shifts of these photoelectron spectra and using the calibration constant s, the arrival times of the FEL pulses were calculated. The results obtained by the THz streak camera were compared to the measurements of the timing tool of SACLA on a single-shot basis. The results of the comparison between the two measurement techniques are shown in Fig. 4. Subfigure (a) shows single-shot measurements from the two methods compared for 100 consecutive pulses that were simultaneously measured by the two detectors. One can see that the results are consistent and follow the same trend. Figure 4(b) shows the correlation of the arrival time values measured by the two techniques for about 3100 FEL pulses that arrived at the linear part of the THz pulse (shown also in Fig. 3(b) by red crosses). The correlation coefficient between the two sets of data is 0.99. The difference between the results from these two techniques is shown in the histogram in Fig. 4(c). The standard deviation of this distribution gives the relative accuracy between the two methods in rms. It is calculated to be about 16.7 fs. The energy measurement accuracy for a single eTOF is σeTOF = 2.4 eV rms, meaning that the accuracy of the energy shift measured by two eTOFs is . Using the value of the linear coefficient s = 0.23 eV/fs, the accuracy of the arrival time measurement by the THz streak camera is obtained: σTHz = 14.8 fs. The rms accuracy of the arrival time monitor at SACLA is σSACLA = 7 fs as reported by [23]. Therefore, the relative uncertainty between the measurements from these two methods should be . The measured distribution of 16.7 fs is in a good agreement with the expected accuracy of 16.4 fs. We assume that the remaining contribution is caused by the relative arrival time jitter of the two optical laser pulses arriving in the EH1 and EH2, where, respectively, the SACLA timing tool and the THz streak camera are positioned. Even though these laser pulses are two parts of the same laser pulse, they propagate through different optical components in the two experimental hutches with different environmental conditions (temperature and humidity), which can cause a relative temporal jitter between them. The agreement within 16.7 fs between the two monitors allows to verify the arrival time measurements of the THz streak camera by another independent measurement technique.
Fig. 4 Comparison of the results from two different measurement methods. Subfigure (a) shows the measurements from the THz streak camera (red dashed) and the membrane transmission monitor (blue solid) for 100 FEL pulses. The correlation between the two sets of results is shown in (b), while (c) shows the distribution of the difference of the results from these two techniques.
A similar data analysis procedure was used for the measurements of the arrival times of the two FEL pulses in the two-color operation of SACLA. The central energy of the photoelectron spectra from each FEL pulse was found by calculating the center of mass of the spectral distribution. Examples of single-shot spectra measured by the three eTOFs in the two-color mode are shown in Fig. 5(a). One can clearly see two energy peaks in the spectra corresponding to the two FEL pulses with an energy separation of 200 eV. The red solid curve here shows the non-shifted energy spectrum, while the green and blue dashed lines correspond to the spectra measured in presence of the THz field by the two eTOFs position on the opposite sides of the chamber. Comparison of these spectra with the spectra from the standard operation shown in Fig. 3(a), shows that the spectra measured during the two-color operation were noisier. This is caused by the smaller number of photons per FEL pulse in the two-color mode and therefore, less photoelectrons registered by the eTOFs.
Fig. 5 Normalized energy spectra of the photoelectrons measured by the three eTOFs (a), with the dashed lines corresponding to the shifted spectra due to the THz field. Subfigure (b) shows the shape of the THz pulse reconstructed for the two FEL pulses, with a temporal separation of 30 fs between them. Linear fits applied to the linear part of the THz pulse (indicated by crosses) are shown along with the fitting constants.
Because of the higher level of noise for the two-color mode, the analysis procedure considered only those shots, where the spectra of the photoelectrons from both FEL pulses were measured with a signal-to-noise ratio of higher than 8. The shots that corresponded to the FEL arrival outside of the linear part of THz pulse were also excluded from the analysis. After this filtering about 20% of the overall pulses were calculated. The shape of the THz pulse reconstructed for the two FEL pulses for a 30 fs delay between them is shown in Fig. 5(b). The blue dots in the figure show the energy shifts corresponding to the first FEL pulse arriving 30 fs before the second pulse shown by red dots in the figure. The linear area of the THz pulse, where the arrival time of each FEL pulse was measured, is shown by yellow and magenta crosses. By measuring the energy shifts of the photoelectron spectra along the linear part of the vector potential, the calibration coefficient s from Eq. (2) was calculated and values of 0.25 eV/fs and 0.26 eV/fs were found for the two pulses. Using the calibration constants and measuring the photoelectron energy shifts for each of the FEL pulses, the individual arrival times were calculated. The difference between the arrival times of the two pulses gives the temporal separation between them. Different delay values set by the magnetic chicane and measured by the THz streak camera are shown in Fig. 6. One can see from the figure that the measured delay values are consistent with the values set by the magnetic chicane with a correlation coefficient of about 0.99. A fraction of the observed offset of about 4 fs could be attributed to a possible difference of the two-color lasing positions along the electron bunch. The vertical bars in Fig. 6 show the standard deviations from about 2000 measurements in average. The distributions of these delay measurements for different values set by the magnetic chicane are shown in Fig. 7. The rms spread of these distributions is about 31 fs in average. The main contribution in this overall jitter comes from the photoelectron energy measurement accuracies. The energy shift accuracy measured by the two eTOFs is about 4.3 eV for the first FEL pulse and about 4.5 eV for the second one. Using these accuracies and the corresponding values of the linear coefficients obtained earlier results in arrival time accuracies of 17.0 fs and 17.3 fs for the first and the second pulses, respectively. These accuracies are worse compared to the accuracy of 14.8 fs obtained for the standard operation as the measured spectra were noisier during the two-color mode. The accuracy of the delay measurement is given by the quadratic sum of the arrival time measurement accuracies for the two pulses: . The remaining uncertainty is obtained by subtracting this value quadratically from the overall jitter and is about 19 fs. This uncertainty includes the contribution of the relative arrival time jitter between the two FEL pulses and can be used to estimate the upper limit of this jitter. This result suggests that the relative jitter between the two FEL pulses in the two-color mode is much smaller than their jitter with respect to the experimental laser (250–300 fs rms). Such a high temporal stability (better than 19 fs) is a big advantage for the FEL-pump FEL-probe experiments.
Fig. 6 Delays between the two FEL pulses measured by the THz streak camera against the values set by the magnetic chicane. The red circles show the measured values, while the blue line is the linear fit to the data.
Fig. 7 Distributions of the measured delay values changing from 0 fs to 50 fs set by the magnetic chicane with steps of 10 fs.
The arrival time measurement reported in this paper were obtained from the accuracies of the photoelectron central energy measurements (about 2.4 eV and 3.1 eV for the standard and two-color operations, respectively). The resolution of the eTOF spectrometers is about 1.2 eV rms. The remaining uncertainty is caused by the jitter of the FEL spectra and the statistical fluctuations of the spectra of the photoelectrons due to their finite number. The fluctuations of the THz streaking field strength were small during the measurements, and the uncertainty caused by this effect was negligible compared to the electron energy spectra measurement uncertainties.
The geometry of the discussed experimental setup can still be improved to ensure a larger number of measured photoelectrons and, therefore, smaller statistical fluctuations of the electron spectra. The measurement accuracy can also be improved by delivering a stronger THz field at the interaction region. This can be done by applying a stronger focusing of the THz beam or by improving the THz generation efficiency. An average accuracy of about 7 fs rms has already been demonstrated in [22] for the arrival time measurements of hard X-ray FEL pulses using the THz streak camera technique. If a single FEL pulse is measured with such accuracy during the two-color operation, the relative delay between the two pulses can be obtained with an rms accuracy of better than 10 fs. This value does not indicate the accuracy limit of the THz streak camera method, and even better accuracies are possible to achieve by optimizing the geometry of the setup and improving the generation of the THz beam.
5. Conclusion
This work has presented the first synchronous measurement of two hard X-ray FEL pulses of different energies using the THz streak camera technique. The results show that the setup can resolve the 10 fs steps of the delay between the two FEL pulses induced by the magnetic chicane. Furthermore, the performance of the measurement setup was verified by comparing it to the X-ray induced transient transmission method during the standard operation of SACLA. The comparison indicates that the two methods agree with a relative accuracy of 16.7 fs, which is consistent with the individual accuracies of the two measurement methods. The obtained results show that the THz streaking method can be used for the arrival time measurements of the single FEL pulses as well as for the two-color FEL beam. The results from such measurements are able to improve the resolution of the pump-probe experiments carried out at the FEL facilities.
Funding
SBFI-COST (C13.0116).
Acknowledgments
The authors would like to express their gratitude to the technical staff at PSI and SACLA for the great support during the preparation and the execution of the experiment, with special thanks to Beat Rippstein and Arturo Alarcon. We thank Jia Liu and Rosen Ivanov for fruitful discussions during the experiment. The measurements were performed with the approval of JASRI of the proposal no. 2015B8002.
References and links
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