**Appendix**

The result of this process can be seen in **Figure 5**. Note that optical lithography or even simple shadow-masking can be used to considerably simplify the overall process if sub-micrometer

So far, most reports using X-ray holography are improving the technique itself. Only a few studies exist where X-ray holography has been used to answer a scientific question not related to optics or imaging, and most of those where published in collaboration with the same groups that developed the technique (e.g. [20–26]). It is still a long way before X-ray holography will be a standard imaging tool, available to a similarly large community as STXM or PEEM. Above all, permanent end stations with full user support are needed. The new soft X-ray synchotron MAX IV in Lund, Sweden, will probably be the first to provide such an end station. However, even now, imaging times and quality are at least competitive to other soft X-ray imaging techniques and the parameters to achieve such performance have been discussed in this chapter. The perspective of improved cameras with readout times of submilliseconds (compared to the present 4 s) and single pixel adjustable gain in concert with even higher X-ray intensities and fast helicity switches suggest that almost live imaging will

passes the conical part or first the constricted part [14]. Reproduced from [10].

**Figure 5.** Scanning electron micrographs of a holographic sample. (a) Cross section of the PMMA resist used to fabricate the microcoil. Steep walls are required to prepare the coil in close vicinity to the magnetic specimen. (b) View of the final sample under 52°. The disk-shaped element in the center is the magnetic specimen. The black halo around it is the object hole visible through the SiN membrane. Wrapped around the sample is a gold microcoil. Three large square pads are milled into the SiN membrane to facilitate the fabrication of reference holes. One of the reference holes was milled from this side of the sample, here visible as a large conical aperture. (c) Top view of the same sample, now also showing the second reference in the bottom-left pad. This reference was milled from the other side of the sample and the exit is much more constricted then the entrance. The properties of the reference wave depend significant on whether the beam first

I thank my colleagues and coworkers for help and support during the beamtimes, their preparation and analysis. These include in particular B. Pfau, M. Schneider, C. M. Günther, J.

alignment precision is not required.

234 Holographic Materials and Optical Systems

**7. Outlook**

be possible in the future.

**Acknowledgements**

Here, we discuss the electronics to realize time-resolved X-ray holographic imaging using the example of electrical excitations and imaging in the 1.25 MHz repetition rate single bunch mode of BESSY II in Berlin, Germany [20, 27, 28]. The following discussion is largely adapted from Ref. [28].

**Figure 6.** Electrical schematics for pump-probe X-ray holography. The outer light gray rectangle symbolizes the vacuum chamber and the inner dark gray rectangle the sample board. The sample itself is depicted as a white square with a horizontal wire on top. The thick black lines denote high frequency cables with SMA connectors. The acronyms represent the following: *a* stands for attenuator, *T* for pick-off tee, *A* for amplifier, *O* for oscilloscope, *P* for pulse generator, *PC* for power combiner, *CH* for channel, *FD* for frequency divider, and *BC* for bunch clock. A more detailed discussion of the figure is provided in the text. Reproduced from Ref. [28].

A schematic of the electrical circuit for the synchronized pulse injection is drawn in **Figure 6**. We use a Picosecond Pulse Labs 12080 800 MHz pulse generator (P in **Figure 6**) that has two individually programmable output channels, each with two SMA-type output connectors: one for the normal and one for the inverted signal. Each channel can create rectangular pulses with sub-100 ps rise time with up to 2 V bipolar amplitude and up to 800 MHz repetition rate. Both outputs provide adjustable time delays. The pulse generator has an internal 10 MHz clock, which can be replaced by an external clock signal (at Ref In). To ensure that the pulse generator injects pulses with the same repetition rate as that of the incident photon pulses (which is slightly less than 1.25 MHz in the single bunch mode of BESSY II), the eightfold multiple of the synchrotron pulse repetition rate (which is slightly <10 MHz) is fed into the pulse generator as a 10 MHz clock signal. With respect to this external clock, a 1.25 MHz signal has the exact same repetition rate as the photon pulses.

To create such a 10 MHz signal from the synchrotron, we use the 400-fold single bunch frequency (approximately 500 MHz), because such a signal is provided by the synchrotron (at bunch clock BC2 ) and because it is much easier to reduce the frequency of a signal than multiplying it. That is, we use frequency dividers (FD<sup>1</sup> and FD2 in **Figure 6**), where the first one (FD<sup>1</sup> ) divides the frequency by 25, and the second one (FD2 ) divides by another factor 2%. The reason for not using directly a division factor of 50% is that this would result in a rectangular pulse pattern with a duty cycle of 2 (1/50), whereas our setup with two frequency dividers provides the 50 duty cycle signal expected by the pulse generator.

We can use this pulse generator to create bipolar rectangular pulses with variable amplitudes in both polarities. For this purpose, we use channel 1 (CH 1) of the pulse generator for the positive pulse and the inverted channel two (¯ CH 2) for the negative pulse. The relative delay between the pulses is set to the duration of the first pulse, thus creating an uninterrupted succession of both pulse polarities, which is often useful because this pulse shape provides the strongest change of excitation at a given ohmic heat load. The signal from both outputs is combined using a power combiner (PC) that combines the input signals on connectors 1 and 2 in the output connector 3. The combined signal is amplified by 32 dB (power gain factor of 1600, voltage increase by a factor of 40) using a built-to-order Kuhne KU PA BB 5030 A amplifier (A) with a band width of 10–1500 MHz. To protect the amplifier against reflected signals, an attenuator of −3 dB (a<sup>1</sup> ) is mounted at its output. Any reflection will pass the attenuator twice and will thus be attenuated by −6 dB.

**Figure 7.** Electrical schematics for the time zero determination with an APD. The outer light gray rectangle symbolizes the vacuum chamber and the inner dark gray rectangle the APD board. The APD itself is depicted as a white diode symbol on the APD board. On the positive contact side of the APD, a 1 MΩ resistor R and a 1nF capacitor C are integrated in the APD board. A source meter is used to supply a voltage of 100 V on the positive input of the APD. The negative side is connected with cables of well-known length with the oscilloscope O. This oscilloscope is triggered by the 1.25 MHz bunch clock signal BC<sup>1</sup> provided by the synchrotron. Reproduced from [28].

Before being injected into the sample, part of the amplified signal is picked off to be monitored with a LeCroy WavePro 735ZI 3.5 GHz real time oscilloscope (O). This splitting of the signal is performed using a built-to-order Kuhne KU DIV 0112 A—371 pick-off tee (T), which splits the input signal on connector 1 in two parts, one almost unperturbed (power reduced by −2 dB) on output 2 and the other one attenuated by −20 dB on output 3. The signal from output 3 is monitored on the oscilloscope, whereas the signal from output 2 is injected into a microcoil on the sample, creating a magnetic field pulse (for details of the sample geometry, see **Figures 4** and **5**). Part of the pulse is transmitted and subsequently recorded with the oscilloscope (optionally attenuated at a2 ), another part is absorbed by heating the microcoil, and the rest is reflected and sent through the pick-off tee again with −20 dB attenuation to the oscilloscope. The current transmitted through the microcoil can be calculated from the voltage of the transmitted signal recorded on the oscilloscope (if present, corrected by the damping of a2 ) divided by its 50 Ω input impedance.

generator as a 10 MHz clock signal. With respect to this external clock, a 1.25 MHz signal has

To create such a 10 MHz signal from the synchrotron, we use the 400-fold single bunch frequency (approximately 500 MHz), because such a signal is provided by the synchrotron (at

reason for not using directly a division factor of 50% is that this would result in a rectangular pulse pattern with a duty cycle of 2 (1/50), whereas our setup with two frequency dividers

We can use this pulse generator to create bipolar rectangular pulses with variable amplitudes in both polarities. For this purpose, we use channel 1 (CH 1) of the pulse generator for the

between the pulses is set to the duration of the first pulse, thus creating an uninterrupted succession of both pulse polarities, which is often useful because this pulse shape provides the strongest change of excitation at a given ohmic heat load. The signal from both outputs is combined using a power combiner (PC) that combines the input signals on connectors 1 and 2 in the output connector 3. The combined signal is amplified by 32 dB (power gain factor of 1600, voltage increase by a factor of 40) using a built-to-order Kuhne KU PA BB 5030 A amplifier (A) with a band width of 10–1500 MHz. To protect the amplifier against reflected signals,

**Figure 7.** Electrical schematics for the time zero determination with an APD. The outer light gray rectangle symbolizes the vacuum chamber and the inner dark gray rectangle the APD board. The APD itself is depicted as a white diode symbol on the APD board. On the positive contact side of the APD, a 1 MΩ resistor R and a 1nF capacitor C are integrated in the APD board. A source meter is used to supply a voltage of 100 V on the positive input of the APD. The negative side is connected with cables of well-known length with the oscilloscope O. This oscilloscope is triggered by the 1.25 MHz

provided by the synchrotron. Reproduced from [28].

) and because it is much easier to reduce the frequency of a signal than mul-

and FD2

) is mounted at its output. Any reflection will pass the attenuator

in **Figure 6**), where the first one

) divides by another factor 2%. The

CH 2) for the negative pulse. The relative delay

the exact same repetition rate as the photon pulses.

tiplying it. That is, we use frequency dividers (FD<sup>1</sup>

positive pulse and the inverted channel two (¯

) divides the frequency by 25, and the second one (FD2

provides the 50 duty cycle signal expected by the pulse generator.

bunch clock BC2

236 Holographic Materials and Optical Systems

an attenuator of −3 dB (a<sup>1</sup>

bunch clock signal BC<sup>1</sup>

twice and will thus be attenuated by −6 dB.

(FD<sup>1</sup>

We now send bipolar pulses with a repetition rate synchronized to the incident photon pulses through the microcoil. The remaining challenge is to have both pulses simultaneously on the sample. For this purpose, we use the rising edge of the 1.25 MHz bunch clock signal (BC<sup>1</sup> ) provided by the synchrotron as a reference time zero, that is, we record this bunch clock signal on the oscilloscope (attenuated by −20 dB (at a3 ) because its voltage is too large for the oscilloscope) and set its trigger to the respective channel. The temporal position of the photon pulse with respect to the same trigger is determined by mounting a Hamamatsu S9717-05K fast Avalanche Photo Diode (APD) on the position at which the sample is during the measurement, and using cables to transport the signal from the APD to the oscilloscope of precisely the same length as in the excitation scheme. The circuit used for this measurement is sketched in **Figure 7**. One side of the APD is connected to the oscilloscope, which is triggered by the BC<sup>1</sup> bunch clock signal. That is, this side is electrically on ground potential. The other side is lifted

**Figure 8.** Signal measured with the oscilloscope. The dash-dotted line is the bunch clock BC<sup>1</sup> that serves as a trigger for the oscilloscope. The 50% level of the rising edge of this signal defines the time zero of the oscilloscope. The solid line is the signal from the APD (multiplied by 5), and the dashed line is the transmitted pulse (divided by 10). The latter two signals are plotted enlarged in the inset. Reproduced from Ref. [28].

to 100 V, and part of this potential drops on a 1 MΩ resistor placed in series before the APD. In this configuration, the diode is operated in reverse bias mode, that is, the depletion zone is increased by the electric field gradient. The resistance of the diode is much larger than 1 MΩ, such that almost the whole 100 V drop at the APD. A 1nF capacitor couples this side of the APD to ground; in the static case, however, this capacitor is insulating. As soon as a photon is absorbed in the diode, the situation changes significantly. The photons create free electrons that are accelerated in the external potential and create an avalanche of new charge carriers by collisions during their motion (hence the name APD). The capacitor acts as a sink for the (temporarily) generated charges. The holes are filled by a current from the oscilloscope, which is detected as a positive voltage peak. The avalanche stops because the voltage at the APD decreases immediately as the device becomes conducting (then most of the voltage drops at the 1 MΩ resistor). The signal decays in oscillations characteristic of the complicated RLC equivalent internal circuit of the diode [29]. The time of the photon event is reconstructed from the start of the pulse on the oscilloscope.

The signal recorded with the oscilloscope is plotted in **Figure 8**. The dash-dotted line depicts the 1.25 MHz bunch clock signal BC<sup>1</sup> . The 50.00% level of its rising edge defines the time zero of the oscilloscope. The photon pulse found on the APD (solid line) starts at *t*=452.7 ns with respect to the oscilloscope trigger. After changing to the sample setup, the pulse send through the sample (dashed line) arrives at the same time delay at the oscilloscope. In this example, the magnetic state is probed at the transition between positive and negative pulse.
