Practical aspects of THz generation

As described in Chapter 4 the used titled pulse front set-up consist of a femtosecond pump laser, an optical grating, an imaging telescope and the LiNbO3 crystal. In order to align and optimize the THz generation setup, most of the components are mounted on translation and/or rotation stages. The alignment of the generation setup is a lengthy procedure as all the parameters are coupled to each other. The setup conditions, the crystal properties and NIR pulse properties will affect the properties of the generated THz radiation. In particular, the temporal shape of the THz pulses is strongly dependent on to the tilt angle of the pulse front, the input amplitude, and the temporal and spatial properties of the NIR pump pulse.

In addition, the transport and the focusing of the THz radiation will also influence the temporal shape. Moreover, the generated THz pulse will show spatial chirp due to the way it is generated and also temporal chirp is not excluded [113].

Problems may arise from the fact, that only optimizing for energy by observing the THz output with a pyro-detector may not result in the optimum THz properties. For example, different combination of the telescope position, (the telescope images the pump laser on the grating into the crystal and compensates for the spatial separation of the different frequencies) and the LN crystal positions can be found with similar THz conversion efficiencies, but with different THz pulse properties. In order to compare different setup settings EOS measurements have to be performed. However, already small changes in the THz setup may result in big differences in the spatial beam properties of the generated THz radiation, which in turn changes the focusing of the THz.

The exact NIR pump pulse properties also influence the actual alignment of the generation setup. Ideally a transform limited pulse should be used. Usually, small temporal chirp in the pump beam does not influence the conversion efficiency, as the grating angle or the imaging optics position can be slightly changed to compensate for the chirp. However, it might influence the properties of the pulse produced.

The pump-probe laser at FLASH has to be transported several meters to the actual experiment, where the streaking measurement is performed. In order to avoid self-phase

81 modulation the NIR pulse is not entirely compressed after the amplifier in the laser hutch.

Only, after the beam transport to the experimental end-station, the NIR pulses are passed once through a pair of parallel transmission gratings to compress the pulse temporally. In this single pass geometry, the output beam acquires spatial chirp. In Ref. 114 it could be demonstrated that simultaneous spatial and temporal chirp (= dispersive medium) yields pulse front tilt. This changes the alignment conditions of the pulse front tilt set-up and phase matching conditions may not be optimal.

THz pulse shape for different pump pulse conditions

In Figure 5.12 in the left panel there different EO-sampling measurements are depicted.

The THz streaking pulse that was used for the measurements presented here is shown in blue color. The orange and the green dashed EO traces have been recorded at a different beamtimes at FLASH. Although, the THz radiation was generated with the same THz generation setup and the same pump-probe laser at FLASH, the resulting temporal shape of the three pulses is quite different. Firstly, the main peak in the green waveform is substantially longer than the peak of the two other EO traces. This leads to an increase of the rise time of the streaking ramp and consequently to a reduction of the temporal resolution.

Secondly, the blue trace shows one main positive peak and two much smaller peaks in the negative direction, while for the other two curves the ratio of the positive amplitude and the negative amplitudes increases. For the THz waveform plotted in green, the peak in the positive direction and the peak in the negative are almost equal. The difference in these curves could not be explained by the Gouy phase shift. The differences might result from different NIR pulse properties, including linear and higher orders of chirp. On the right panel, a Ne streaking spectrogram recorded with ~258 eV FEL pulses and the green transient as the streaking pulse is shown. For this measurement the THz beam was transported and focused into the interaction region with a telescope consisting of a 300 mm collimating Teflon lens and a 150 mm focal length off axis parabolic mirror, resulting in half of the streaking power as compared to the streaking measurements presented in the main part of this chapter (however The THz energy was the same). The rise time of the streaking ramp is much slower than the rise time of the spectrogram in Figure 5.7, were the blue curve was used as the streaking pulse. Furthermore, the photoelectron spectra are mainly shifted to lower energies, while the shift to higher kinetic energies is much lower. The streaked spectra at the zero crossing that are used to analyze the temporal properties of the FEL pulse are barely broadened due to the low streaking strength.

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Streaking versus EOS for streaking pulse parameter recovery

In the measurements presented here, we constructed the transformation map from energy to time from the THz pulse characterized with EOS. However, wavelength-dependent absorption and dispersion in the ZnTe detection crystal may result in distortions of the measured electric THz waveform. In addition, impurities and saturation effects in the detection crystal may also lead to significant deterioration of the detectable field strength. In the streaking measurement, instead, a flat and dispersion free response is expected, and the averaged streaking spectrogram should provide a more precise transformation map.

However, the presented measurements were hampered by the large temporal jitter Figure 5.12: Comparison of the temporal waveform of the generated THz. a) Three different EO-sampling measurements are depicted. For better comparison the maximal EO-signal was scaled. The blue curve corresponds to the THz streaking pulse that was used for the measurements presented in this chapter. The orange and the green dashed EO traces have been recorded at different beamtimes at FLASH, though the same THz generation setup and similar NIR pump pulse properties were used.

Clearly, the depicted electric field transients are quite different. Firstly, the main peak in the green waveform is substantially longer than the peak of the two other EO traces. This is important, as the duration of this peak (at its zero crossing) corresponds to the rise time of the streaking ramp.

Secondly, the blue trace shows one main positive peak and two much smaller peaks in the negative direction, while for the other two curves the ratio of the positive amplitude and the negative amplitudes is more similar. The curve in green is almost symmetric. In b) a Ne streaking spectrogram recorded with ~258 eV FEL pulses and the green transient as the streaking pulse is depicted. For this measurement the THz beam was transported and focused into the interaction region with a telescope consisting of a 300 mm collimating Teflon lens and a 150 mm focal length off axis parabolic mirror, resulting in half of the streaking power as compared to the streaking measurements presented in the main part of this chapter (THz pulse energy was roughly the same in both measurements). The rise time of the streaking ramp is ~900 fs.

83 between the FEL and the THz pulses. If the arrival time jitter is much smaller than the rise time of the streaking ramp it is expected that the precise shape of the THz vector potential can be determined from the spectrogram itself.

Moreover, the frequency components of the broadband pulse are focused to focal plane, which means that the strength of the THz electric field is distributed over a certain volume.

It might be that the sampled volume is different in the EO measurement and the streaking measurement, which could lead to different estimations of the absolute electric field strength and may explain the discrepancy in the field-strength in the EO-measurement and streaking measurement.