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Abstract
We design and realize an arrival time diagnostic for ultrashort X-ray pulses achieving unprecedented high sensitivity in the soft X-ray regime via cross-correlation with a ≈1550 nm optical laser. An interferometric detection scheme is combined with a multi-layer sample design to greatly improve the sensitivity of the measurement. We achieve up to 275% of relative signal change when exposed to 1.6 mJ/cm2 of soft X-rays at 530 eV, more than a hundred-fold improvement in sensitivity as compared to previously reported techniques. The resolution of the arrival time measurement is estimated to around 2.8 fs (rms). The demonstrated X-ray arrival time monitor paves the way for sub-10 fs-level timing jitter at high repetition rate X-ray facilities.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Free-electron lasers, today$'$s brightest coherent X-ray generators, are gaining worldwide popularity due to high demands in ultrafast X-ray science for applications in solid-state physics, biology and medicine. The Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory has been in operation since 2009 and is running at a pulse repetition rate of 120 Hz [1]. In the pursuit to broaden the range of scientific experiments and the capabilities of FELs, higher repetition rate X-ray sources are being developed at various facilities around the world [2–4]. In case of the LCLS-II upgrade, which is currently under construction, the X-ray pulse repetition rate will increase to $\approx$1 MHz. While the average power of the X-ray beam will increase, the energy per X-ray pulse will decrease as a result of the significantly higher repetition rate. The large-scale nature of free-electron lasers often preclude control over various noise sources affecting the arrival time of the X-ray pulses. As the relative timing between X-ray and optical pump laser pulses plays a significant role in all pump-probe type experiments, an arrival time monitor (ATM) such as an X-ray/optical cross-correlator is usually implemented in which an X-ray induced material change is used to derive the arrival time of the X-rays [5–9]. These ATMs generally require X-ray fluence levels of >1 mJ/cm$^2$ to detect a discernible arrival time signal, and typically a few tens of mJ/cm$^2$ are required to achieve sub-10 fs-level timing jitter monitoring for soft X-rays [10]. Such fluences cannot be achieved with high repetition rate FELs or even at low repetition rates when few and sub-fs X-ray pulses are generated [11,12]. This is especially true when considering that only a fraction of the total X-ray energy is available for diagnostic purposes such as the arrival time measurement. Achieving sub-10 fs arrival time measurement uncertainty of such sources is of particular importance as the overall time resolution is a critical parameter that enables new science directions.
In this work, we demonstrate the ability to significantly relax the X-ray fluence requirements present in previous ATMs by designing a novel cross-correlator scheme that achieves unprecedented high sensitivity to soft X-rays. Moreover, in contrast to current ATM designs that use a portion of the Ti:Sapphire pump laser beam at 800 nm or white light in the visible spectrum [6,7,10,13], the ATM demonstrated here cross-correlates X-rays with an optical laser at a wavelength of 1550 nm. This wavelength was specifically chosen for compatibility with state-of-the-art optical timing distribution systems (TDS) [14]. Using a TDS as the common clock opens the path to tighter synchronization of X-rays and pump lasers with significantly reduced timing jitter.
2. Principle
The X-ray/optical cross-correlator is based on a time-to-space mapping geometry as shown in Fig. 1(a) in which an ultrafast X-ray induced change in the optical reflectivity of a sample material is used to determine the relative timing between probe laser and X-ray pulses. The X-ray pulses photo-excite carriers in the sample via X-ray absorption, which modify the reflectivity for the probe beam within the first 100 fs [9]. This X-ray induced effect decays on picosecond-level timescales and, as such, is applicable to applications in the MHz regime where energy dissipation prior to the arrival of the following pulse is critical.
Fig. 1. a) Principle of the time-to-space mapping geometry. An angle between X-rays and probe laser is used to measure the relative timing between the two beams. The sample is a multi-layer structure with a thin film of germanium as the top layer. Cam: Camera b) Lower panel: Reflectivity spectrum of the Ge/diamond/SiO$_2$/Si sample showing a node around the target wavelength together with a simulation (Sim.) of the optical properties using a transfer matrix calculation [15]. The index of refraction of the germanium thin-film was used as a free parameter in the simulation and is compatible with literature data on germanium thin-film [16]. The reflectivity data was collected at an angle of incidence of 14$^{\circ }$. Upper panel: Simulated relative reflectivity change of the sample assuming an X-ray induced index of refraction change of the germanium layer of $\Delta$n = 0.1.
The probe laser beam impinges under an angle, here chosen to be 45$^{\circ }$ with respect to the X-ray beam. Measuring the spatial properties of the probe laser beam then allows the reflectivity of the sample to be probed as a function of time. The relative timing between probe and X-ray beam can be adjusted in such a way that the first half of the probe beam arrives prior to the X-ray pulse and therefore experiences the equilibrium sample reflectivity, while the second half of the probe beam arrives after the X-ray pulse and therefore experiences the X-ray-modified reflectivity. Imaging the reflected beam onto a camera reveals the relative arrival time between probe and X-ray beam through the position of the onset of the reflectivity change.
The sample shown in Fig. 1(a) is made of a 170 nm thin film of germanium, sputtered on a 2 $\mu$m thick diamond layer grown by Chemical Vapour Deposition (CVD) onto a silicon dioxide layer on top of a silicon substrate. The diamond layer is introduced to enhance heat removal for high repetition rate operations. This multi-layer structure shows optical interference effects (etalon) in reflectivity, which are designed to exhibit a node in the reflection spectrum around 1550 nm as shown in Fig. 1(b). The choice of the sample materials, the thicknesses and the associated refraction indices can lead to a narrow node that exhibits strong reflectivity changes close to the node wavelength. The X-ray induced changes to the sample, mostly involving the top germanium layer, shift the center wavelength of this node (see Fig. 1(b)) leading to dramatic amplitude and phase changes of the reflected 1550 nm light, allowing to detect extremely weak X-rays. In addition, a very low absolute reflectivity at the center wavelength is important to achieve high relative reflectivity changes. Importantly, the choice of germanium as the top layer is critical to maximize the carrier-injection induced change of index of refraction around 1550 nm due to the proximity of the photon energy to the band gap [17].
The sample structure was designed to achieve a narrow reflection minimum at 1550 nm for normal incidence. However, the reflectivity measurements shown in Fig. 1(b) indicate the minimum to be close to 1585 nm [18]. The reason for this unintentional red shift lies in the somewhat special properties of germanium for sub-$\mu$m thin films [19]. Both, the index of refraction and the extinction coefficient of germanium are strongly dependent on the film thickness. A node centered exactly at 1550 nm can be easily achieved via slight adjustment of the germanium thickness, however here we focused on demonstrating the proof-of-principle of such an approach. The thickness of the top layer is a particularly critical parameter as will be discussed in Section 4.
The upper panel in Fig. 1(b) shows the simulated relative reflectivity change assuming an X-ray induced index of refraction change of the germanium layer of $\Delta$n = 0.1. The absorption of the X-rays effectively shift the node towards longer wavelengths in the spectral reflectivity curve which leads to a strong change in the relative reflection of up to 100% for a $\Delta$n = 0.1.
3. Experimental setup
The optical setup is shown in Fig. 2. We use a Ti:Sapphire laser pumped optical parametric amplifier (OPA) that is synchronized to the X-ray beam to generate ultrashort pulses covering a wavelength range from around 1550 to 1600 nm. The broad optical spectrum produced by the OPA is reduced with a bandpass filter (FWHM = 12 nm) to match the optical spectrum typically used in optical timing distribution systems which supports optical pulse durations of roughly 200 fs [14,20,21]. After a clean-up polarizer, the light first passes through a 50/50 polarization insensitive beamsplitter. The reflected light is guided onto a photodiode to serve as an intensity monitor for the output of the OPA. This improves the signal-to-noise ratio when a photodiode is used in place of the camera as described in Section 4. No intensity monitor is needed when using a laser with low amplitude fluctuations or for single-shot operation using a camera. After the beamsplitter, the light passes through a birefringent crystal made out of 4 mm thick $\alpha$-BBO. The birefringent crystal is set under 45$^{\circ }$ with respect to the input polarization to generate two pulses with orthogonal polarizations and a fixed time delay $\Delta$t = 1.54 ps. The light then passes through a quarter waveplate, generating circularly polarized pulses that impinge on the sample.
Fig. 2. Experimental setup of the X-ray/optical cross-correlator. Light around 1550 nm is split into two orthogonally polarized pulses with a time delay of $\Delta$t = 1.54 ps. The X-ray pumped sample causes a sharp variation of the amplitude and phase of the reflected pulses, ultimately resulting in a sharp vertical edge in the camera image. See text for details. Inset: Single-shot camera images at different relative time delays between X-rays and 1550 nm laser. The profile of a single pixel row in white shows the relative reflectivity change (right axis). Pol: polarizer, L: lens, PD: photodiode, BS: polarization insensitive beamsplitter, BC: birefringent crystal, QW: quarter waveplate, BF: bandpass filter, CAM: camera
The sample, whose reflectivity will be temporarily altered by incoming X-rays, is adjusted for normal incidence of the 1550 nm light and about 45$^{\circ }$ with respect to the X-ray beam. While the geometry is slightly different compared to Fig. 1(a), the principle is the same. After the light is reflected off the sample, it passes a second time through the quarter waveplate, which makes each pulse linearly polarized on the opposite axis of the birefringent crystal as compared to the first pass. The pulses now pass through the birefringent crystal for a second time, precisely removing the delay between them. Using only one crystal to introduce and remove the time delay between the orthogonally polarized laser pulses eliminates the need to carefully match the thickness of the two crystals or to have to account for environmental variations. The light then reflects off the beamsplitter and is guided through a second polarizer and onto a CCD camera. Destructive interference is achieved by adjusting the rotation and incidence angle of the birefringent crystal so that no light is detected by the camera in the absence of X-rays. Once X-rays are present and temporally overlapped with the 1550 nm light, the two reflected optical pulses experience both amplitude and phase changes at different points within their temporal envelopes, such that when they are recombined at the detector, the destructive interference condition is no longer met and a signal is detected at the camera [22,23]. This interferometric detection technique substantially increases the sensitivity by providing an effective background subtraction and an increase of the relative reflectively signal $\Delta$R/R.
As described in Section 2, the angle between X-rays and optical laser leads to a spatial encoding of the timing jitter in one dimension of the camera image. The single-shot time window, determined by the X-ray spot size, is about 1 ps for the setup described here but can easily be increased to multi-ps if desired. The arrival time of the X-rays is encoded in the position of a sharp edge that appears in the camera image, shown in the inset of Fig. 2 for different relative time delays between X-rays and 1550 nm laser pulses. The profile over one pixel row shows the rising edge and represents the relative reflectivity change. As only one dimension of the camera carries the relevant information, fast one-dimensional InGaAs line cameras can be used for single-shot operation at repetition rates of currently up to 200 kHz [24]. Further upgrades may enable repetition rates close to the MHz range in the near future [25].
4. Experimental results
The X-ray/optical cross-correlator described above has been tested during an experiment on the LCLS soft X-ray beamline in the Soft X-ray Research (SXR) hutch at an X-ray photon energy of 530 eV and a pulse repetition rate of 120 Hz [26]. The X-ray pulse duration was on the order of 130 fs and the X-ray spot size was set to about 800 x 900 $\mu$m which was measured by imaging the X-ray induced fluorescence signal on a cerium-doped yttrium aluminum garnet (Ce:YAG) crystal. The X-ray pulse energy was varied over a large range leading to X-ray fluences between 20 $\mu$J/cm$^2$ and 1.6 mJ/cm$^2$.
Although a 1D line camera is sufficient for this X-ray/optical cross-correlator, we used a 2D InGaAs camera (Allied Vision, Goldeye CL-008) in this experiment since the lower frame rate was sufficient and a 2D detector allows for X-ray beam profile analysis as described below. In addition, a 2D detector simplifies the optical alignment.
As described in Section 2, the X-rays induce a reflectivity change in the sample which is translated into a sharp rising edge of the reflectivity as a function of time followed by a picosecond-scale decay. Due to the relatively narrow minimum in the sample reflectivity (Fig. 1(b)), the magnitude of the reflectivity change shows a strong dependency of the center wavelength. To explore this dependency, we measured the signal response by replacing the camera in Fig. 2 with a photodiode in front of which a series of optical bandpass filters at 1580, 1590 and 1600 nm were placed. For each filter, the delay between the X-rays and the laser was varied to extract the wavelength dependence shown in Fig. 3(a). The data was taken when the sample is exposed to an X-ray fluence of 1.1 mJ/cm$^2$ and without using interferometric detection. This data is in good agreement with the simulated relative reflectivity change as shown in the inset. Here, we adjusted the simulation in Fig. 1(b) for normal incidence and a variation of index of refraction $\Delta$n = 0.04. Moreover, the thickness in the simulation was adjusted from 170 nm to 175 nm, likely accounting for the variability of the film thickness from the center to the side of the 10x10mm target. In fact, as mentioned above, the peak wavelength can be tailored by changing the layer thicknesses of the sample.
Fig. 3. a) Sample reflectivity response to X-rays at a fluence of 1.1 mJ/cm$^2$ showing a strong wavelength dependence. Inset: Simulated maximum reflectivity change together with the measurements for the three wavelengths. b) X-ray fluence dependence of the relative reflectivity change. Exploring etalon effects in the multi-layer sample increases the sensitivity by an order of magnitude, while the interferometric approach increased the sensitivity by about another order of magnitude, resulting in a $\approx$100-fold increase with respect to standard ATM schemes that use simple SiN samples [5–7]. The shaded areas indicate the range of variability for the signal strength for different measurements. The variability is mostly due to thickness variations across the sample.
To measure the reflectivity change as a function of the X-ray fluence, we used the setup shown in Fig. 2 to produce single-shot camera images from which we extract the reflectivity change of the sample over a wide range of X-ray fluences. The setup can switch between non-interferometric detection to interferometric detection by rotating the polarizer in front of the camera. Previous X-ray arrival time monitors commonly used a simple silicon nitride (SiN) or YAG target instead of the multi-layer sample described in Fig. 1(a) [5–7]. The narrow reflection node caused by etalon effects in the multi-layer sample leads to a significantly stronger reflection change compared to the simple SiN sample as can be seen in Fig. 3(b). Moreover, due to the vicinity of the bandgap, the index of refraction change for germanium around 1550 nm is much larger than wide-bandgap materials, such as SiN and YAG [17]. By exploiting these two sample design aspects, the reflectivity response is increased by about one order of magnitude. Furthermore, combining this enhanced signal with the interferometric detection scheme leads to another factor of 10 increase in reflectivity change, for a total improvement of about two orders of magnitude (100x) over the commonly used arrival time monitors. The shaded bands in Fig. 3(b) indicate different experimental settings such as slight differences in sample position, X-ray/optical laser beam overlap and interferometric nulling. Based on our simulation, the largest contribution is due to the germanium film thickness variations across the sample. The observed $\approx$30$\%$ bands in the reflectivity change (Fig. 3(b)) are compatible with the $\approx$2 nm variability of the germanium thickness. Thickness variations of the other sample layers, however, are less critical. Such variation can be suppressed by constructing the multi-layer structure on a larger wafer and subsequently dicing it to the appropriate size for an ATM target, thereby minimizing the variation in the sample characteristics over the sample area.
To determine the resolution of the X-ray/optical cross-correlator, we split off a portion of the Ti:Sapphire laser beam at 800 nm before the OPA that produces the 1550 nm light and guided it into the beamline to replace the X-rays. Here, the optical pump fluence is comparable to the $\approx$1 mJ/cm$^2$ level for X-rays. This configuration eliminates the X-ray arrive time jitter by cross-correlating the optical laser beam with itself. The resulting arrival time jitter is therefore caused only by optical path length variations in the laser setup from, for example, vibrations of optical components in the 800 nm and 1550 nm beams. Due to the optical imaging setup and the time-to-space mapping on the camera CCD, vibrations of the camera can directly translate into arrival time measurement errors. The 800 nm laser pulse arrival time as measured against the 1550 nm laser is shown in Fig. 4(a) for a randomly selected 0.4 s time window of the experiment. A periodic modulation is apparent and a Fourier transform of the arrival time reveals a modulation frequency of approximately 30 Hz. In post-experiment analysis and inspection of the beamline, we recognized this effect as originating from a rotary pump connected to the vacuum chamber that was not entirely mechanically isolated from the experiment.
Fig. 4. a) Laser pulse arrival time when measured against itself. Vibrations of the optical setup from a rotary pump caused a periodic modulation of about 30 Hz. b) Histogram of the laser pulse arrival time. The periodic modulation causes the arrival time to oscillate between $\pm$8 fs. After the correction, the arrival time uncertainly is 2.8 fs (rms).
Due to the periodic modulation, the arrival time oscillates between $\pm$8 fs as can be seen in the uncorrected histogram in Fig. 4(b). The arrival time uncertainty including the periodic modulation is 7.5 fs (rms). To take into account such a modulation, we fit a sine wave to the measurement data and calculated the residuals between data and sine fit. The result is shown in the corrected histogram in Fig. 4(b) and the uncertainty in the arrival time is 2.8 fs (rms). Such oscillation could have been eliminated by better mechanical isolation of the chamber and optical setup from the rotary pump, and the fact that the arrival time monitor allowed detection of this $\mu$m-level vibration is a testament to its inherent sub-10 fs precision.
5. Conclusion
We designed and characterized a novel X-ray/optical cross-correlator that is capable of measuring the relative timing between X-ray and optical laser pulses at around 1550 nm. This paves the path to the determination of the X-ray arrival time with respect to state-of-the-art pulsed timing distribution systems used in FEL and X-ray facilities around the world. A target made of a thin-film multi-layer structure together with an interferometric detection scheme leads to an $\approx$100-fold increase in the detection sensitivity allowing the measurement of the arrival time of weak soft X-rays generated from high repetition rate or sub-fs free-electron lasers. The measurement uncertainty of the X-ray arrival time is estimated to 2.8 fs (rms) which makes this cross-correlator well suited for sub-10 fs (rms) timing jitter operations at LCLS and other FELs. This low uncertainty was achieved despite using rather long (130 fs) X-ray pulses while shorter pulses are expected to improved the uncertainty. Further developments are needed to explore the possibility of improving the resolution to sub-fs by combining a camera with smaller pixel size, shorter X-ray and optical probe pulses, and different sample materials. The scheme presented here is compatible with a wide range of optical and X-ray wavelengths provided that a suitable sample material and thin-film structure is employed.
Funding
Office of Science (DE-AC02-76SF00515).
Acknowledgments
Use of the Linac Coherent Light Source, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-76SF00515.
Disclosures
The authors declare no conflicts of interest.
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