We report on a newly built laser-based tabletop setup which enables generation of femtosecond light pulses in the XUV range employing the process of high-order harmonic generation (HHG) in a gas medium. The spatial, spectral, and temporal characteristics of the XUV beam are presented. Monochromatization of XUV light with minimum temporal pulse distortion is the central issue of this work. Off-center reflection zone plates are shown to be advantageous when selection of a desired harmonic is carried out with the use of a single optical element. A cross correlation technique was applied to characterize the performance of the zone plates in the time domain. By using laser pulses of 25 fs length to pump the HHG process, a pulse duration of 45 fs for monochromatized harmonics was achieved in the present setup.
© 2014 Optical Society of America
Short light pulses in the extreme ultraviolet (XUV) range find a wide variety of applications in studies on electronic and structural dynamics of molecules and molecular complexes [1–3]. In particular, photoelectron spectroscopy with the use of XUV radiation is a powerful method to probe the electron density in a valence shell of a molecular system. In combination with a pump-probe technique, this method enables to reveal mechanisms of molecular processes, which typically occur on the subpicosecond or femtosecond time scale. Recently, XUV photoelectron spectroscopy was developed for studying molecular dynamics in the liquid phase, which represents a natural environment for many interesting processes in chemistry and biology [4, 5].
The modern femtosecond laser technology provides the possibility to develop XUV light sources with such an ultrashort pulse duration via upconverting the laser frequency in the process of high-order harmonic generation (HHG) induced in a gas medium. Nowadays the HHG technique represents an established method to generate XUV radiation and is used in a variety of different research areas such as photoelectron spectroscopy [6–10], transient absorption spectroscopy , diffractive imaging , material properties [13, 14], and microscopy . With this tabletop technique, it is possible to achieve a femtosecond XUV-pulse duration, which is by orders of magnitude shorter than the typical pulse duration of synchrotron radiation  and is comparable to the pulse length of a free-electron laser . The great advantage of using the HHG method in a pump-probe experiment is that the pump and the XUV-probe pulses are intrinsically synchronized, since typically the same laser system is used to generate both pules. In the present work we report on our newly built HHG setup designed for time-resolved studies on electronic and structural dynamics of molecular complexes in solutions and at interfaces.
The HHG process in an atomic gas is well understood on a fundamental level and is described in detail in literature [18–20]. It consists in a nonlinear response of atoms to the strong laser field, which involves absorption of several laser photons by a single atom and emission of one photon with the cumulated energy under relaxation of the atom to its initial ground state. In the range of higher harmonics, the envelope of the HHG energy spectrum exhibits a plateau extending up to a well-known cutoff energy . As an example, HHG spectra extending up to photon energies of 100 eV were achieved in Ref.  by pumping noble gases with femtosecond Ti:sapphire laser pulses of 1014 − 1015 W/cm2 peak intensity.
To conduct spectroscopic studies, a single harmonic of desired photon energy needs to be selected from the HHG spectrum. Monochromatization of XUV light represents a subject of particular interest in the present work. It becomes a challenging task when a short XUV pulse is required, since the use of dispersive optics required for energy resolution introduces temporal broadening. This distortion is defined by the total difference in the optical paths of the rays. In a grating monochromator it can easily reach several hundred femtoseconds or even a picosecond, e.g., when 104 grooves are illuminated by XUV light of 30 nm wavelength. This represents a dramatic value. The time dispersion can be significantly compensated with the use of a second grating [22–24]. The two-grating monochromator enabled to compress the XUV pulse duration down to 11 fs  and even to 8 fs . However, a setup with two gratings causes significant losses in the transmission efficiency and complicates the design and handling of the beam pass through the monochromator. An alternative approach to achieve a short XUV pulse duration was recently developed with the use of a single-grating monochromator by employing the off-plane mount (OPM) grating configuration [27–29]. While the OPM design enables to reach a pulse duration of a few tens of fs and possesses a higher transmission efficiency, it requires, nonetheless, additional mirrors to collimate the incident beam and to focus the diffracted beam onto a slit. Some other methods for the selection of harmonics which are based on using transmission zone plates  and narrow-band multilayer mirrors  were reported.
In the present work, we explore the possibility of using an off-axis reflection zone plate (ZP) for the purpose of monochromatization of ultrashort XUV pulses. The basic principles of this optical element were formulated in . Similar to a toroidal grating, an off-axis ZP diffracts different spectral components of the incident beam at different angles and focuses them into spatially separated spots. The zone structure can be designed to approach the Fourier-transform limit of the transmitted pulse duration, as it was implemented in [28, 29] for the single-grating OPM design. Recently, the application of ZPs has been developed as a novel method of handling synchrotron beams in the X-ray energy range. In particular, it has been successfully used at the femtosecond slicing beam line of BESSY II . The potential of using ZPs for the selection of high harmonics was recently discussed in . Below we describe our newly built HHG setup which implements off-center reflection zone plates for monochromatization of the femtosecond XUV pulses generated in argon. We will present the time-resolution characteristics of the monochromator measured using a cross-correlation technique. To our knowledge this is the first performance test of reflection zone plates in the time domain below 100 fs.
2. Experimental setup
2.1. Generation of XUV light
A femtosecond Ti:sapphire laser system operated at 5 kHz repetition rate was used to generate high harmonics in a gas cell filled with argon. The laser output of 2.5 mJ pulse energy had a pulse length of 25 fs at a central wavelength of 800 nm. Laser pulses were split into two beams by a beam splitter so that an energy of up to 1.5 mJ per pulse was used to pump the HHG process. The other split beam is dedicated for future pump-probe experiments. In the present work it was used in the cross-correlation experiment to characterize the XUV pulse duration.
A schematic view of the experimental setup is shown in Fig. 1. Laser pulses were focused with a lens of 600 mm focal length into a gas cell which was filled with argon and positioned in a vacuum chamber. An iris aperture and a λ/2 plate in front of the lens were used to control the intensity and the polarization axis of the pump beam, respectively. The lens was mounted on a translation stage in order to adjust the focus position in front of the gas cell. Without attenuation of the laser beam, a peak intensity of 6 × 1014 W/cm2 can be reached in the laser focus. The pulse energy was reduced with the use of an iris aperture to avoid saturation of ionization in the Ar gas. Varying the intensity in a range below saturation enabled us to tune the spectral bandwidth of XUV pulses as shown in section 3.2. The cell had a length of 16 mm and was sealed with an aluminum foil into which the pump laser made the entrance and the exit apertures by itself. The argon pressure in the cell was adjusted by using a dosing valve to maximize the XUV photon flux which was detected with a calibrated photodiode behind the ZP monochromator. A typical pressure in the cell was 20 mbar during operation.
The ZP monochromator was positioned at a distance of 1000 mm from the HHG source. It consists of three gold-coated zone structures made on a single silicon substrate. The structures were designed to select the 17th, 21st, and 25th harmonic of the pump beam, respectively. Details of their design will be given in a separate section below. The silicon substrate was mounted in a motorized stage that could be adjusted in three translational and three rotational directions with a precision of 0.1 μm and 2 μrad, respectively. A differential pumping stage enabled to maintain a low pressure of 10−8 mbar in the monochromator chamber during operation of the HHG source. A thin aluminum foil of 150 nm thickness was used in front of the ZPs to filter out the residual infrared (IR) pump beam. In order to prevent the foil from melting under the exposure of intense IR radiation, an aperture of 2 mm diameter was installed in front of the foil to block the main part of the pump beam. This aperture restricted the divergence of the XUV beam to 3.3 mrad in both transversal dimensions, resulting in a spot size of 3.3 mm of the incident beam at the ZP position.
In the monochromator, the harmonic of interest was deflected in the first diffraction order at an angle of 20° with respect to the incidence direction and was focused at a distance of 350 mm behind the ZP. Accordingly the 17th, 21st, and 25th harmonics, respectively, were focused into the same spot when the center of the corresponding ZP was illuminated by the XUV light. A slit with an appropriate width, positioned at this focal point, was used to transmit only the desired harmonic since the other harmonics were reflected and focused differently. The slit width and its longitudinal and transversal positions with respect to the diffracted beam direction could be adjusted with micrometer precision.
A gold-coated toroidal mirror was used to refocus the divergent XUV beam into the interaction chamber equipped with a time-of-flight (TOF) electron spectrometer. The spectrometer characteristics are given in . The refocussing mirror was mounted on a stage which, similar to the ZP stage, could be adjusted in three translational and three rotational dimensions. In order to minimize the focus aberrations, the focal length of the mirror was designed to refocus the beam with a magnification factor of 1 at the distance of 1200 mm, which is equal to the distance between the mirror and the slit. A plane gold mirror was inserted into the path of the refocused beam to monitor a replica of the XUV focus on a home-built position sensitive detector shown in Fig. 1. The detector is composed of a double stack of MCPs with a phosphor screen behind them and a CCD camera, which recorded light pulses from the phosphor screen at the detection positions of XUV photons.
2.2. Zone-plate monochromator
A simplified optical layout of the monochromator is shown in Fig. 2. Its central element possessing dispersion characteristics is the reflection zone plate, which represents a projection of the transmission Fresnel zone plate on the plane mirror surface . As shown in Fig. 2, not the full elliptical lens structure is illuminated but only an off-axis section marked by deep blue color [37, 38]. This off-axis section can be used as an ideal monochromator. Radiation incident onto this section is focused along the optical axis at high dispersion due to the high off-center mean line density. In addition, the specular reflex (zero-order reflection) can be easily separated and, most valuable for monochromatization, a slit in the plane perpendicular to the optical axis can be installed for the energy selection. The use of a mirror of high external reflectivity as a substrate for the ZP enables applications of this monochromator in a wide range of photon energies between 1 and 1500 eV, as it was first suggested in .
The energy dispersion ΔE in the ZP focal plane is defined by the relation :
According to Eqs. (1) and (2), the energy resolution of a ZP monochromator is determined by the values of β, d, and Δx′. Equation (1) indicates that the value of d sinβ is constant for a defined geometry of the HHG setup and a particular energy resolution E/ΔE desired in an experiment. A calculation procedure of the optimal structure period d is presented in . For the first diffraction order of the ZP it has the form:Fig. 2.
In the present setup, the three ZPs designed for selection of the 17th, 21st, and 25th harmonics, respectively, were fabricated on a single silicon substrate of 50 mm diameter by e-beam lithography and reactive ion etching. When a particular HHG photon energy has to be selected, the substrate can simply be moved along the x-axis in Fig. 2 to transfer the optical axis of the corresponding ZP into incident beam. This facilitates the alignment of the entire ZP assembly by monitoring the focus position of different harmonics while translating the assembly along the x-axis.
The geometrical parameters of the HHG setup and the chosen energy resolution are summarized in Table 1. Taking the HHG source size of ∼ 100 μm and the geometrical demagnification factor M = 2.86 into account, the slit size in the focal plane of ZPs should be larger than 35 μm. The specifications of the individual ZPs are presented in Table 2. The meridional structure period d in the geometrical center of the ZP’s operation area was determined for the three specific photon energies E by using Eq. (3) and the parameters given in Table 1. The ZP sections were manufactured with a length of 40 mm and a width of 4 mm. The optimal depth of the gold-coated structure profiles was calculated by using the program REFLEC . In the energy range of 25 – 40 eV the optimal depth is in the order of 58 nm.
The temporal broadening of the incident pulse can be estimated as Δt = λ2/Δλc. For the spectral resolving power of the monochromator E/ΔE = 167 (see Table 1), this estimation yields the value Δt ≃ 21 fs at the photon energy of the 21st harmonic. It corresponds to the XUV pulse duration of 25 fs assumed in the design of the ZP. One could approach closer the Fourier-transform limit of transmitted pulses by choosing a lower value of E/ΔE in the design. However, the lower energy resolution would lead to a lower contrast ratio of the selected harmonic.
3. XUV pulse characterization
Results presented below were obtained with the use of the ZP designed for selection of the 21st harmonic. Performance characteristics of the two other ZPs were found to be similar and therefore are not shown.
3.1. Spatial intensity distribution
Before assembling the entire setup, the XUV detector was positioned at the focal plane of the ZPs instead of the slit. Figure 3 shows an image of the intensity distribution of XUV radiation recorded at this position. The ZP designed for selection of the 21st harmonic was used in this measurement. The image clearly demonstrates the high-dispersion performance of the ZP, which can be seen on the left hand side of the image where the sequence of spots reveals contributions of different harmonics in the first diffraction order. The smallest spot in the middle of this sequence represents the signal of the 21st harmonic. It is well separated from contributions of other harmonics and from the unfocused specular reflex on the right hand side of the image. The 21st harmonic gives rise to a relatively weak signal in the image. This is because of saturation of the detector caused by the high flux density of XUV photons tightly focused onto the MCP.
Figure 3 also demonstrates the focusing property of the ZP. While the 21st harmonic is focused at the detector position into a small spot, other harmonics form larger spots in the image because their focuses lie either in front or behind the detector plane. Since the energy dispersion in the focal plane (given by the ZP design) and the energy interval between neighboring harmonics are constant, the larger spots partially overlap. Figure 3 shows that a slit of a few tenths of a millimeter width can be used in the focal plane to transmit the 21st harmonic only. Similar results were obtained for the other two ZPs designed to select the 17th and the 25th harmonics, respectively.
As mentioned above, the high intensity of focused XUV light saturated the MCP detector. This prevented the measurement of the focus size by recording the CCD images. Instead, the spot size of the refocused XUV beam was measured in the experimental chamber by scanning a razor blade across the beam and recording the transmitted photon flux with the use of a photodiode. Such a measurement was performed at different positions along the propagation direction of the refocused beam. For the 21st harmonic, a focus size of 82 μm was obtained. Since the magnification factor of the refocusing mirror is 1, approximately the same focus size should be expected at the slit position of the monochromator.
3.2. Spectral characteristics of selected harmonics
The spectral bandwidth of XUV pulses was measured by recording kinetic energy spectra of photoelectrons generated in the process of ionization of argon gas. XUV light was focused in the experimental chamber in front of the skimmer of the TOF spectrometer shown in Fig. 1. The spectrometer has a magnetic-bottle configuration. Its characteristics, such as the energy resolution and the collection efficiency of electrons, are presented in detail in . The Ar pressure in the experimental chamber was maintained at 8×10−4 mbar during the data acquisition. Special efforts were made to verify that the recorded spectra are not affected by charge effects in the ionized medium. It involved acquisition of a number of spectra for different gas pressures in the range between 3 × 10−4 and 2 × 10−3 mbar. The spectral bandwidth was found to be constant in these measurements, while the photoelectron yield was proportional to the pressure value.
Figure 4 shows a kinetic energy distribution of photoelectrons obtained with the use of the 21st harmonic. A slit size of 100 μm was used in this experiment. For comparison, a spectrum recorded with an open slit is shown in the inset. It demonstrates how the contribution of the neighboring harmonics can be eliminated by reducing the slit size without affecting transmission of the selected harmonic. For the spectrum shown in Fig. 4, the intensity ratio of the selected harmonic and the admixture of neighbouring harmonics is in the order of 1 : 6 × 10−4.
Apart from the spectral bandwidth of the XUV pulse, the spectrometer resolution and the spin-orbit structure of the residual Ar+ ions contribute to the width of the recorded energy peak of photoelectrons. The spin-orbit splitting of the 2P state of Ar+ is 0.177 eV . This value is smaller than the energy resolution of the TOF spectrometer, which is in the order of 0.4 eV in the considered kinetic energy range . Therefore, the contributions of two ionization channels, associated with the generation of the residual ion in different spin-orbit states, are not resolved in the spectrum shown in Fig. 4. Considering the fine structure, the energy distribution was fit to a sum of two Gaussian profiles, lying at the fixed distance of 0.177 eV on the energy scale and having the same width which was a fit parameter. The fit yielded a value of 0.81 eV (FWHM) for the width. Taking the spectrometer resolution into account, we obtain that the spectral width of XUV radiation is 0.70 eV (FWHM). This spectral width was obtained with a pump intensity of 2.35 × 1014W/cm2. By reducing the latter to 1.5 × 1014W/cm2, the XUV bandwidth was decreased to 0.3 eV. This finding is in agreement with the recent results by He et al. in .
With the use of a stronger pump beam, the XUV photon flux was higher accordingly. For the 21st harmonic pumped with the intensity of 2.35 × 1014W/cm2, a flux of 106 photons per pulse was measured with a photodiode behind the monochromator.
3.3. Transmission efficiency of the monochromator
In view of the demand in many HHG applications to achieve the highest possible XUV flux, the transmission efficiency of monochromator represents an important characteristic and it deserves a special consideration. For a given harmonic, this quantity is defined by the ratio of the harmonic intensity before and after monochromatization. Since the intensity distribution over different harmonic orders generated in the gas cell could not be directly measured in the present setup, an experimental determination of the transmission coefficient was not possible. Therefore, in the evaluation of the transmission efficiency we rely on the software REFLEC  developed for optical engineering in the XUV/X-ray energy range. The design power of this software was demonstrated in , where the efficiency calculated for a ZP with the design energy of 640 eV reproduced well the experimental value obtained with the use of synchrotron radiation at BESSY II.
The transmission efficiency in the first diffraction order obtained with the use of REFLEC for the geometry of the present setup and for the depth of 58 nm of the zone-structure profiles is shown in Fig. 5 as a function of photon energy. One should note that the chosen depth is optimal for the photon energy of the 21st harmonic, yielding a transmission efficiency of 28% for the zone plate designed to select this harmonic. Profile depths of 69.5 and 47 nm would be needed for the optimum transmission of the 17th and the 25th harmonics, respectively. This would increase their transmission by a few percent, as compared to the values shown in Fig. 5, reaching approximately the same efficiency of 28%. Recent technology allows manufacturing of several structures with different profile depths on a single substrate, though it was not implemented in the present work.
3.4. Cross-correlation measurement of the XUV pulse duration
The pulse duration of XUV light was measured by means of a cross-correlation technique. It consisted in recording kinetic energy spectra of electrons generated in the process of IR-assisted ionization of Ar gas by XUV photons [26, 44, 45]. When the target atom is exposed to IR and XUV radiation at the same time, the ionization process can undergo a multiphoton transition where absorption of one XUV photon is combined with absorption or emission of several IR photons. The kinetic energy of the photoelectron can thus be expressed asEq. (4) should be positive.
The IR field needs to be sufficiently strong to initiate multiphoton transitions in the continuum spectrum of the parent atom. A criterion for the field strength was derived by Delone et al. from a semiclassical analysis of dipole matrix elements for atomic continuum states . According to this analysis, the continuum-continuum transitions are efficient if the laser intensity exceeds the value of (in atomic units). Using this criterion, we obtain that for the Ti:sapphire laser frequency the critical intensity value is 2.5 × 1012 W/cm2. On the other hand, the applied IR field should not deplete the population of Ar atoms in the interaction region due to the strong-field ionization process. The latter condition imposes an upper limit onto the intensity value. In previous cross-correlation experiments with the use of Ti:sapphire laser pulses of femtosecond duration, the peak IR intensity was restricted to a value below 1013 W/cm2 [17, 47]. At such intensities, the process of above-threshold ionization (ATI) of Ar in the IR field has a negligible yield at high electron kinetic energies and, thus, an overlap of the ATI spectrum with the cross-correlation spectrum is avoided as well.
In the present setup, the IR pulse energy was controlled with the use of an iris aperture positioned in front of a lens of 400 mm focal length which focused the IR beam into the region of overlap with the XUV beam (see Fig. 1). The IR peak intensity in the interaction region is estimated to be in the order of 1.7 × 1012 W/cm2. The time delay between the XUV and the IR pulses was varied with the use of an optical delay stage installed in the IR beam path. The delay could be controlled with a precision of 0.5 fs. Since the IR beam was passing through the beam splitter used to split the laser output and the HHG pump beam was the the reflected part, a plane glass window of an appropriate thickness was used in the path of the HHG pump beam to compensate for dispersion in the splitter. The laser compressor was tuned for the shortest temporal width of the cross correlation signal. The actual IR pulse duration of 25 fs in the interaction region was measured by using a SPIDER device.
Figure 6 shows a series of kinetic energy spectra obtained with the use of the 21st harmonic at different time delays between the XUV and the IR pulses. These results were obtained with the reduced HHG pump intensity of 1.5 × 1014W/cm2. Each spectrum was recorded with the same acquisition time and at fixed experimental parameters such as the laser intensity and the gas pressure. The appearance of several sidebands (SB) at small delays and at both sides from the central peak can be seen in the figure. Their amplitudes and temporal widths monotonically decrease with the increase of the SB number. For a quantitative analysis of the cross correlation signal, the electron yield was integrated over the energy peak of each SB. The dependency of the integrated yield as a function of the time delay is shown in Fig. 7.
For reduced IR intensities applied in the present experiment, one can use the pertubation theory to describe the cross correlation signal. Considering a multiphoton process that involves absorption of one XUV photon and absorption or emission of number |N| of IR photons, the ionization rate is proportional to the product , where IXUV and IIR are intensities of XUV and IR radiation, respectively. Thus, the ionization yield SN(τ) in the Nth SB at a given time delay τ is proportional to the integralEq. (5) we obtain that the cross-correlation signal has the Gaussian shape Equation (7) demonstrates that the temporal width τN of the cross correlation signal is decreasing with the increase of the SB number and converges to the value of the XUV pulse duration in the limit of large positive or negative N.
The temporal dependencies of the cross-correlation signal in the three SBs shown in the inset of Fig. 7 were fitted to the Gaussian profile described in Eq. (6) with τN as a fit parameter. The obtained values of τN were used to calculate the full widths at half maximum of the cross correlation temporal profiles as . This yielded values of 54 fs, 46 fs, and 45 fs for the first, the second, and the third sideband, respectively. The value of converges to approximately 45 fs for an increasing SB number, which represents the XUV pulse duration achieved in the present setup.
The measured pulse duration is given by a convolution of the XUV pulse length before monochromatization with the temporal broadening caused by the ZP monochromator. Assuming the generated XUV pulse duration is in the order of the pump pulse length, we obtain that the time distortion of the ZP is approximately 37 fs. This value is comparable to the initially calculated pulse dispersion of 21 fs.
We presented results of the first practical implementation of a ZP-based monochromator to select femtosecond XUV light pulses generated in the HHG process. In the present setup, a single harmonic is selected with a pulse duration of 45 fs, corresponding to a temporal broadening of approximately 37 fs due to the ZP’s optical dispersion. This value can be minimized further by reduction of the aperture size of the incident HHG beam.
One should emphasize that the ZP monochromator consists of a single optical element that combines reflection, focusing and dispersion properties together. This results in a high transmission efficiency (in the order of 28% at 32.55 eV) and simplifies handling of the XUV beam.
The ZPs of the monochromator are designed for selection of a desired harmonic with an optimal combination of the energy and the time resolution. This combination is chosen according to the criterion that considers the Fourier transform of generated pulses. Namely, in the present setup an XUV pulse duration of 25 fs was taken into account while choosing the spectral resolving power of 167, giving rise to the temporal broadening of transmitted pulses which is in the order of the pulse duration itself. With this design, a high contrast ratio of 1 : 6×10−4 of the transmitted harmonic was achieved. The temporal distortion can be reduced by using an area on the ZP with lower density of the structure elements, which would also lead to a lower resolving power and, consequently, to a lower contrast ratio. The balance between the energy and the time resolution can be varied within a single ZP while using its different parts with different structure densities. This provides flexibility in the application of a ZP-based monochromator.
In the present work we focused on the performance characteristics of a monochromator consisting of a single zone plate. We did not consider the XUV pulse compression with the use of an additional dispersive element. Implementation of a second ZP to achieve a transform-limited duration of a HHG pulse represents an interesting task for further development of the setup.
This work is funded by the European Research Council, Grant No. 279344 (E.F.A.), and by the Helmholtz-Gemeinschaft via the VH-NG-635 Grant (E.F.A.). The authors acknowledge support by the BMBF, Project 05K12CB4 ”Next generation instrumentation for ultrafast X-ray science at accelerator-driven photon sources” and a Marie Curie FP7-Reintegration-Grants within the 7th European Community Framework Program (project No. PCIG10-GA-2011-297905). Discussions with Prof. Bernd Abel and his co-workers are greatly appreciated.
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