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The recent development of x-ray free electron lasers providing coherent, femtosecond-long pulses of high brilliance and variable energy opens new areas of scientific research in a variety of disciplines such as physics, chemistry, and biology. Pump-probe experimental techniques which observe the temporal evolution of systems after optical or x-ray pulse excitation are one of the main experimental schemes currently in use for ultrafast studies. The key challenge in these experiments is to reliably achieve temporal and spatial overlap of the x-ray and optical pulses. Here we present measurements of the x-ray pulse induced transient change of optical reflectivity from a variety of materials covering the soft x-ray photon energy range from 500eV to 2000eV and outline the use of this technique to establish and characterize temporal synchronization of the optical-laser and FEL x-ray pulses.
©2012 Optical Society of America
1. Introduction
The Linac Coherent Light Source (LCLS) is an x-ray free-electron laser (XFEL) at the SLAC National Accelerator Laboratory. It is the first hard x-ray free-electron laser operating at the wavelength range of 2.6-0.12 nm [1]. The LCLS provides sub-picosecond pulses of coherent radiation with peak brightness nearly ten orders of magnitude beyond conventional third-generation synchrotron light sources. These properties hold promise for LCLS to have great impact on ultrafast dynamics research in a variety of disciplines such as atomic and solid state physics, plasma physics, materials science, chemistry and biology.
A majority of the experiments probing ultrafast dynamics are based on a pump-probe scheme, where the evolution of a system excited by an optical or x-ray pulse is investigated by probing its transient states at variable time delay after the excitation. Establishing temporal coincidence between the x-ray and optical pulses is an essential step in all pump-probe experiments. Therefore a reliable method for rapidly reaching temporal overlap on the femtosecond time scale is crucial.
There have been several demonstrations of temporal synchronization of a VUV-FEL and optical-laser pulses [4–8]. In particular, the ultra-violet induced change of optical reflectivity on the GaAs(100) surface was demonstrated at the free-electron laser in Hamburg (FLASH) at a photon energy of 40 eV [4]. It was suggested that this technique should work beyond the UV photon energy region (hν>100 eV). The concept of using shot-by-shot timing measurements for improving time resolution in optical pump x‐ray probe experiments was demonstrated by Cavalieri et al. [2] and subsequently used to measure the excited state potential energy surface of photoexcited bismuth by Fritz et al. [3]. At the LCLS, synchronization techniques using gas phase effects [9] as well as optical transmission [10] have been demonstrated recently, but the demonstration of a technique that works equally well across the whole soft X-ray range has been lacking.
In this work, we experimentally demonstrate a reliable and simple pulse synchronization technique that works in the soft x-ray region of 500-2000 eV by making use of the transient changes of the optical reflectivity of Si3N4, Sm0.9Y0.1S and GaAs semiconductors as well as metallic Au induced by intense FEL pulses. These results were obtained during the commissioning phase of the soft x-ray material science (SXR) instrument at the Linac Coherent Light Source [11–13].
2. Experiment
The experiments were performed with the zero order soft x-ray beam at discrete photon energies between 500 and 2000 eV. Single crystals of GaAs and Sm0.9Y0.1S and thin films of amorphous Si3N4 and polycrystalline gold were used for observing the transient change of the optical reflectivity induced by absorption of the x-ray pulses. GaAs was chosen as a reference material because the transient change of the optical reflectivity has been measured in the UV photon energy region [4]. Si3N4 thin films were chosen because they are more stable under intense X-ray irradiation [5], while Sm0.9Y0.1S is a mixed valent system with a semiconductor to metal transition involving a large isostructural volume collapse. Here the presence of soft x-ray absorption M edges may allow one to probe (or even affect) the Sm valence changes associated with the structural and electronic phase transition. Gold was measured for comparison between metallic and semiconducting surfaces. The semi-insulating Fe-doped high-resistance GaAs crystals were cut and polished in the (100) orientation. Sm0.9Y0.1S samples were grown by the Bridgman technique [14]. The 1 µm thick amorphous Si3N4 films were deposited by low pressure chemical vapor deposition (LPCVD) on the (100) surface of a 200 µm thick Si wafer. Finally, the thin film of gold with 1 µm thickness was grown on a 200 µm thick Si wafer by sputter deposition.
An oscilloscope with an input bandwidth of 13 GHz was used for observation of electrical pulses induced on an exposed inner conductor of a coaxial radio frequency (RF) cable with 25 GHz bandwidth by optical and x-ray pulses on a picosecond time scale. The measurements were performed in a vacuum chamber at the base pressure of 10−7 Torr.
The x-ray-pump optical-probe time resolved reflectivity measurements were performed by monitoring intensities of optical laser pulses reflected from the semiconductor and metal surfaces while being exposed to x-ray FEL pulses. The experimental geometry is sketched in Fig. 1(a) . The repetition rate of the FEL and the optical laser was set to 60 Hz and 120 Hz, respectively. With this measurement scheme the optical pulses detected alternate between the optical reference for the unpumped reflectivity and the optical signal of the pumped reflectivity. A femtosecond Ti:Sapphire laser amplifier system was used to generate ultrafast optical laser pulses with an 800 nm central wavelength and 100 fs full width at half maximum (FWHM) pulse duration. The laser is synchronized to the x-ray pulses by a control loop that adjusts the Ti:sapphire laser cavity length to match the laser repetition rate to the LCLS master RF signal. Residual shot to shot fluctuations are corrected to some extent by measuring the electron beam arrival time with respect to the RF signal [9]. The FEL x-ray pulse energy was 1-2 mJ and the pulse duration was in the range of 50-150 fs. The spot size of the unfocused x-ray beam was 800x700 µm2, giving x-ray fluences well below the materials damage threshold. All measurements were made at room temperature.
Fig. 1 (a) Schematic layout of the experimental geometry in the experiment for probing transient change of optical reflectivity induced by the X-ray pulses. Inset: YAG crystal used for spatially overlapping FEL X-ray and optical laser beams. (b) Two electrical pulses observed with the oscilloscope and induced by a photoemission process from a metal surface upon exposure to FEL X-ray (blue curve) and optical laser (red curve) pulses. The blue line shows the position at which the leading edges were matched. Inset: Tip of the RF coaxial cable.
3. Spatial overlap and coarse temporal synchronization
Spatial overlap of the optical laser and the x-ray FEL beams at the sample surface was accomplished by monitoring the x-ray induced fluorescence and the diffuse reflection of the optical laser beam on the surface of an unpolished Ce:YAG crystal (shown on the inset of Fig. 1(a)). Two independent cameras observe the beam coincidence location on a Ce:YAG crystal from different angles. With the coincidence location noted on each camera the sample can be steered to the marked position.
For the coarse temporal synchronization down to a few picoseconds accuracy, the arrival time overlap of the FEL (pump) and the optical laser (probe) pulses was determined by monitoring the electrical pulses induced in a RF cable upon impinging the optical and x-ray pulses on the exposed inner-conductor tip at the end of the cable (see Fig. 1(b)). Electrical pulses were generated via photoemission from the metallic surface of the RF cable tip. The time resolution in this experiment is limited by the transmission frequency of the RF cable and the input bandwidth of the oscilloscope used in the experiment. In the case of optical laser excitation at a wavelength of λ = 800 nm (hν = 1.55 eV), overcoming the work function (4.7 eV for the Cu cathode) requires multiphoton excitations with at least 3 optical photons, while single-photon absorption dominates the x-ray excitation. The highly nonlinear optical absorption has considerable efficiency for IR pump intensity on the order of 108 W/cm2. It is interesting to note that this is of the order of 105 times lower than that required to observe this effect in gases. It has been suggested that strong local enhancements of the IR field at the surface due to surface roughness and coupling of the photons into surface plasmons dramatically increases the number of photoemitted electrons [15,16].
Coarse synchronization of the FEL and optical lasers is performed by electronically adjusting the phase of the Ti:Sapphire oscillator with respect to the RF master clock of the linac [9]. The adjustment minimizes the time delay between leading edges of the x-ray and optically induced electrical pulses on the oscilloscope (shown by the blue line on Fig. 1(b)). The coarse timing with the oscilloscope is important to initially find the very short (~100 fs) x-ray and optical laser pulses having the low repetition rate (~100Hz), when one has to scan a very wide time range. The oscilloscope allows fast switching of the monitored time scales covering the time range from ms down to ps. With this method the temporal synchronization between the two pulses is reliably established within 5 ps.
4. Transient reflectivity and fine timing
On a sub-picosecond time scale, the ultrafast transient change of the optical reflectivity of semiconducting surfaces of Si3N4, GaAs, Sm0.9Y0.1S and the metallic surface of gold induced by femtosecond x-ray pulses has been studied. GaAs and Si3N4 are direct bandgap semiconductors with minima of the direct band gaps being located at the Γ points in the center of the Brillouin zone. In GaAs the gap is equal to 1.43 eV [17]. Crystalline Si3N4 is a large-band-gap material with the gap estimated to be 5.0–5.7 eV [17–20]. For the amorphous SiNx films slight composition variations lead to gaps ranging from 1.65 eV [21] to 4.74 eV [22]. SmS is known to have a very small gap (~0.1-0.2 eV) [23,24]. The gap size is a critical parameter for excitation or probing using optical laser photons, while for excitation with FEL x-ray pulses it plays less of a role due to the much higher energy of the x-ray photons. A typical transient change of the optical reflectivity as a function of time delay after x-ray pulse excitation of the Si3N4 film is represented in Fig. 2(a) . The leading edge of the optical reflectivity change (defined as (R-R0)/R0, where R0 is the equilibrium reflectivity) has ~180 fs fall time (defined as the time between the points of corresponding to the 10% and 90% intensity levels of the leading edge), which is composed of the cross-correlation of both pulse lengths, the intrinsic electronic response of the sample converting into the reflectivity change and the relative arrival time jitter of both sources during the measurements. The x-ray arrival time in the data has been corrected by the electron-bunch arrival time relative to the master clock of the linac through phase cavity measurements [9].
Fig. 2 (a) Transient optical reflectivity change of the Si3N4 thin film induced with x-rays of 540 eV photon energy. The leading edge width is about 180 fs (fall time corresponding to 10% - 90% intensity change of the leading edge). Inset: X-ray induced change of optical reflectivity for the Au thin film, at a photon energy of 1200 eV and pump fluence of 35 mJ/cm2. (b) The optical-optical and x-ray-optical pump-probe reflectivity signals measured on the surface of Sm0.9Y0.1S single crystals.
After the coarse synchronization, fine timing between the FEL and the optical laser on a sub-picosecond time scale is performed by finding conditions corresponding to the leading edge mid-point on the x-ray induced optical reflectivity curve. Therefore the optical reflectivity measurements serve as a tool for temporal synchronization of the FEL and the optical laser pulse arrival times on the femtosecond scale.
In Fig. 2(b) we compare the optical-pump optical-probe differential reflectivity of Sm0.9Y0.1S taken with an amplified Ti:sapphire laser at a repetition rate of 250 kHz. The pump wavelength was 400nm obtained by frequency doubling the fundamental (800 nm) on a beta-barium borate (BBO) crystal. The probe wavelength was 800 nm. Comparison between the optical-optical and the x-ray-optical pump-probe reflectivity signals shows the leading edge width of ~65 fs (fall time) for the 400 nm optical excitation, while it is measured to be ~175 fs (fall time) for the 540 eV x-ray FEL excitation. Assuming the electronic response to x-ray and optical excitations occur on similar time-scales, the relatively slow edge for the x-ray pumped signal is dominated by the inherent pulse-to-pulse arrival time fluctuations of the optical laser and the FEL x-ray pulses. This is a result of averaging over the jitter during the data collection period. Consequently this limits the possible time resolution of pump-probe experiments that use both the x-ray FEL and the optical laser pulses thus underscoring the need for a reliable single-shot arrival time diagnostic.
The amplitude of the observed transient reflectivity change on the Si3N4 and the Sm0.9Y0.1S surfaces is about 30%-40% of the equilibrium value, while it is reaches only about 5% for the metallic surface of gold. (Inset in Fig. 2(a)). At the same time the width of the leading edges are the same in all cases. Therefore, transient reflectivity changes on semiconducting surfaces are simpler to use and are a more robust technique for temporal synchronization between the x-ray FEL and optical lasers.
Transient changes of optical reflectivity of semiconducting surfaces induced by x-ray excitation are related to the population of the conduction band and depopulation of the valence band. After the absorption of x-ray photons, Auger decay and auto-ionization processes are expected to lead to electron-hole pair generation near the band gap on a femtosecond time scale. The altered density of these low-energy charge carriers leads to the transient change of the optical properties of the materials upon x-ray excitation.
Figure 3 shows the x-ray induced transient change of the Si3N4 optical reflectivity as a function of the time delay between the pump and probe pulses for x-ray excitation energies of 540, 1100, 1200, and 2000 eV covering the soft x-ray region. ΔR is negative for all photon energies, while the amplitude of the leading edge of the differential reflectivity change is about 30% - 40% of the equilibrium value.
Fig. 3 The transient optical reflectivity curves for the Si3N4 film as a function of the x-ray photon energy and fluence. The curves were offset for clarity, with the signal level before the zero time delay corresponding to zero.
While the x-ray penetration depth varies considerably across the soft x-ray region reaching a few microns at 2000 eV, the penetration depth of the optical laser at 800 nm wavelength is hundreds of nm. Therefore the probed region always remains within the silicon nitride films, being smaller than the region which is excited.
The observed width of the leading edge of about 180 fs is independent of the excitation energies of the x-ray photons. Rather mild dependence of the x-ray induced transient reflectivity on the photon energy and the relatively high pump fluences used in these measurements indicate that the observed amplitudes of the transient reflectivity are close to the maximum values. These relatively large amplitudes of the transient reflectivity change observed over the full soft x-ray photon energy range is one of the key parameters which enables this technique to be used widely at XFEL sources for temporal cross-correlation of the optical laser and x-ray FEL pulses.
The reflectivity of Sm0.9Y0.1S measured using the lower pump fluences is more sensitive to the x-ray photon energy, requiring reduced pump fluence upon increasing the x-ray photon energy to obtain nearly the same magnitude of reflectivity change (Fig. 4 ). The changes of reflectivity are negative for all photon energies with amplitude of about 30% - 40% of the equilibrium value. The leading edge width of ~175 fs is very similar to the Si3N4 and gold data.
Fig. 4 The transient optical reflectivity curves for the Sm0.9Y0.1S film as a function of the x-ray photon energy and pump fluence. The curves corresponding to the 540 eV and 1100 eV excitation energies (blue and red) were offset for clarity.
The observed amplitudes depend on both the x-ray photon energy and the pump fluence. The effect of the x-ray photon energy can be understood as follows. The single photon ionization cross-section decreases by increasing the x-ray photon energy, except for photon energies near absorption edges. Nevertheless, increasing the x-ray photon energy results in a higher energy deposited per single absorbed photon. This leads to the larger number of electron-holes pairs created upon absorption of a higher energy x-ray photon.
Thus, in the electronically relaxed sample, the density of optically exited electron-hole pairs will depend on a variety of parameters: the X-ray photon energy, the single photon ionization cross-section, the pump fluence, absorption lengths and photoelectron mean free paths, etc. In turn, this leads to a similar effect on the optical constants.
On the time scale of about 100 fs after the excitation with the soft x-ray pulse, one can see that Si3N4 and GaAs initially show an ultrafast drop and rise in the optical reflectivity (on a few picoseconds time scale). These changes in reflectivity are followed by a slower relaxation towards the equilibrium value on the time scale of hundreds of ps (Fig. 5 ). The transient reflectivity of the Si3N4 film recovers monotonically, approaching its equilibrium value from the low reflectivity side. Interestingly, the initial prominent drop of ΔR of the GaAs sample observed on a few picoseconds time scale is followed by an increase as the transient reflectivity overshoots the equilibrium value before starting to approach the equilibrium from the high reflectivity side on the longer time scale.
Fig. 5 Evolution of the transient optical reflectivity of GaAs and Si3N4 after the x-ray excitation pulse. The photon energy is 800 eV with a pump fluence of 35 mJ/cm2 for Si3N4. The curves were offset for clarity, the signal before the zero time delay corresponds to the zero level. Inset: Pump fluence dependence for the GaAs single crystal.
This peculiar difference in behavior of the transient reflectivity cannot be understood within the simple Drude model of ultrafast population of the conduction band of semiconductors via direct optical excitations of electrons from the core levels using the rigid electronic band models. Rather, it requires consideration of several physical processes taking place upon excitation: starting from variations of the carrier concentration leading to variations of the refractive index and consequently to ΔR, via the Kramers-Kronig relations and Fresnel equations. This also includes effects such as impurities in the samples leading to the presence of gap states, modification of the band positions due to screening, many-body quasiparticle excitations, etc., [25,26] as has been observed in other works [4,27]. As the 800nm probe laser is nearly resonant with the band gap in GaAs, slight modifications of the band structure are expected to have a large effect on the optical constants. No such effects are expected for the non-resonant probing in Si3N4.
In all the studied materials, the amplitude of the initial drop in the transient change of the optical reflectivity for constant x-ray energy scales with the x-ray pump fluence, as illustrated for example on the GaAs surface, shown in the inset of Fig. 5, in line with what has been shown before [2]. It is clear that this amplitude is generally defined by several physical factors and experimentally depends on the x-ray pulse energy, the size and overlap of the FEL and optical laser beam spots on the sample surface, the absorption length of the x-ray photons and the probing depth of the optical pulses.
The next step in developing the x-ray induced transient optical reflectivity measurement technique for temporal synchronization of the x-ray FEL and optical laser pulses is the implementation of the single-shot optical reflectivity measurements [28] that is currently under active development at the SXR instrument. This technique allows the relative arrival time between x-ray and optical laser pulses to be measured on a shot-by-shot basis [29].
5. Summary
In conclusion, we present results of time-resolved pump-probe measurements of the transient change of optical reflectivity from the semiconducting surfaces of Si3N4, Sm0.9Y0.1S, GaAs and metallic gold induced with femtosecond x-ray pulses in the soft x-ray range of 500-2000 eV. The signals were found to be influenced by the details of the electronic structure of the materials, as well as by combined effects of the x-ray (pump) fluence and the x-ray photon energy. Amplitude changes of the transient reflectivity on the order of 30% - 40% were observed, while the leading edge width was on the order of a couple hundred femtoseconds for the excitation with the FEL x-ray pulses. Comparison with optical pump-probe data performed on Sm0.9Y0.1S clearly demonstrated that the edge width is limited by pulse-to-pulse jitter of the optical and the x-ray pulse arrival times. The method presented here enables application of this technique for synchronization of the FEL x-ray and the optical lasers on a femtosecond time scale in the soft x-ray energy region.
Acknowledgments
This work was carried out on the SXR Instrument at the Linac Coherent Light Source (LCLS), a division of SLAC National Accelerator Laboratory and an Office of Science user facility operated by Stanford University for the U.S. Department of Energy. The SXR Instrument is funded by a consortium whose membership includes the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES), Lawrence Berkeley National Laboratory (LBNL), University of Hamburg through the BMBF priority program FSP 301, and the Center for Free Electron Laser Science (CFEL). J. Lüning acknowledges support by the CNRS through the PEPS SASELEX. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.
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