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An experimental demonstration of a wide-range narrowband multilayer mirror for selecting a single-order high-harmonic (HH) beam from multiple-order harmonics in the photon energy range between 40 eV and 70 eV was carried out. This extreme ultraviolet (XUV) mirror, based on a pair of Zr and Al0.7Si0.3 multilayers, has a reflectivity of 20–35% and contrast of more than 7 with respect to neighboring HHs at angles of incidence from 10 to 56.9 degrees, assuming HHs pumped at 1.55 eV. Thus, specific single-order harmonic beams can be arbitrarily selected from multiple-order harmonics in this photo energy range. In addition, the dispersion for input pulses of the order of 1 fs is negligible. This simple-to-align optical component is useful for the many various applications in physics, chemistry and biology that use ultrafast monochromatic HH beams.
© 2016 Optical Society of America
1. Introduction
High order harmonics (HH) provide coherent extreme ultraviolet (XUV) bursts of a few hundred attoseconds. In recent years, the pulse width of isolated attosecond pulses has progressed below the sub-100as region [1], and the pulse energies has progressed the order of a microjoule [2,3]. Monochromatic HH beams provide a powerful light source for many applications (see review Ref. 4), such as time-resolved and angle-resolved photoelectron spectroscopies [5–9], and coherent imaging [10–12] in the XUV region. A lithography mask inspection system with HH has also been demonstrated [13].
To obtain ultrafast monochromatic HH pulses, several techniques and optical elements have been proposed [14–20]. To select a single harmonic with a wide spectral range, a time-delay compensated monochromator (TDCM) [14–19] consisting of a pair of gratings that compensate for the tilt of the pulse-front has been proposed. However, one drawback of TDCMs is the low throughput, because of the low diffraction efficiency of the gratings. Moreover, TDCMs require, not only complex optical configurations, but also temporal characterization of the extracted HH beam in order to compensate for the tilt of the pulse-front. Furthermore, it is extremely difficult to completely compensate for pulse stretching by the second grating. Another monochromatization optical component that can be used is a narrowband multilayer mirror at normal incidence [20]. A molybdenum / silicon (Mo/Si) narrowband multilayer mirror has been demonstrated at a photon energy of 90 eV, achieving a full-width half-maximum (FWHM) bandwidth (E/ΔE) of 46 and reflectivity of 42%. Although the bandwidth can be controlled by changing the Mo:Si thickness ratio, this multilayer optic is difficult to use as a wide-range monochromator in this photon energy range because of the existence of a long reflectivity tail when the mirror is used with a large angle of incidence.
Previously, we carried out a theoretical examination of a multilayer mirror based monochromator designed to efficiently extract a single-order HH [21]. By considering the fabrication procedures for multilayer mirrors, a design rule was established for monochromatizing multiple order HH beams using multilayer mirrors. In this work, we performed an experimental demonstration of a wide-range narrowband multilayer mirror for separating a single order HH beam from multiple-order harmonics in the photon energy range of 40–70 eV. This zirconium/aluminum-silicon (Zr/AlxSi1-x) multilayer mirror has high reflectivity, negligible dispersion for single femtosecond pulses, and a high extinction ratio, for HHs driven by a 1.55 eV pump. This simple reflective component can provide wide photon energy tunable femtosecond XUV light for various applications in physics, chemistry and biology.
2. Multilayer design
In order to accommodate several photoelectron spectroscopy experiments and pump-probe experimental conditions [7,22,23], the multilayer mirror was designed with the following attributes; i) FWHM bandwidths at each angle of incidence larger than 2 eV, enabling a 1 fs pulse to be supported. ii) Contrast with neighboring HH peaks, defined as R(E)/R(E + 3.1 eV) and R(E)/R(E-3.1 eV), where R(E) is the reflectivity at photon energy E, larger than 7 for all angles of incidence. iii) Peak reflectivity at all angles of incidence higher than 20% and iv) reflectivity of any secondary diffraction peak much lower than that of the main peak.
The materials chosen for the multilayer mirror in the photon energy range of 40–70 eV were Zr and AlxSi1-x. Al based multilayers for this photon energy region are well known and there have been several demonstrations of and much discussion about these highly reflective mirrors [24–27]. Following these discussions, the low-weight materials AlxSi1-x were selected. The addition of Si decreases the inter-diffusion between Al and Zr and crystallization of Al [26,27]. For the multilayer designs, the optimum amount of Si was estimated to be 30% [28]. Surface oxidation was taken into account in this estimate. Figure 1 shows the reflectivity profiles of the designed Zr/Al0.7Si0.3 multilayer mirror assuming an s-polarized input beam. A 60 period multilayer stack of Zr (1.6 nm)/Al0.7Si0.3 (14.4 nm) with the roughness estimated to be 0.5 nm was used in the calculation. Surface oxidation was also estimated to 1.6 nm of top Zr layer and 2.4 nm of second Al0.7Si0.3 layer.
Fig. 1 (a) Designed multilayer mirror. Calculated reflectivity profiles (solid line) and phase (dot line) at incidence angles of (b) 10°, (c) 20°, (d) 30°, (e) 40°, (f) 50° and (g) 60°.
The pulse durations were estimated using the reflectivity profiles and phases of the multilayer mirror. Figure 2(a) shows a 1 fs Fourier transform limited (FTL) Gaussian input pulse and the calculated output pulse profile at an angle of incidence of 45 degrees, and Fig. 2(b) shows the calculated dependency of the pulse duration on the angle of incidence. As shown in these figures, the duration of the pulse from this multilayer mirror is almost constant and will support a pulse of the order of 1 fs over the energy range from 40 eV to 70 eV. On the other hand, the calculated pulse duration increases immediately beyond 70 eV. This is due to the decrease in the reflective bandwidth near the Al K-edge (72 eV). In this design, the photon energy of the secondary peak is larger than that of this absorption edge, so the reflectivity of the second diffraction peak is much smaller than that of the main peak, as shown in Fig. 1(a). Note that the multilayer mirror was designed based on Henke’s optical constants [29].
Fig. 2 (a) 1 fs (FWHM) FTL input pulse profile (dashed line) and calculated output pulse profile (solid line) at an angle of incidence of 45 deg. The output pulse duration is 1.07 fs. (b) Incident angle dependence of the calculated pulse duration.
By rotating this multilayer mirror as shown in Fig. 3, the desired single order corresponding to the angle of incidence to the mirror can be easily selected from a multiple-order HH. The mirror was designed to select ultrafast pulses in an isolated HH from a multiple-order HH driven by a 1.55 eV pump. Of course by using a multilayer mirror pair arranged in parallel, the isolated single-order beam can be obtained without changing the beam direction, the same as the traditional setup for a multilayer monochromator for synchrotron applications [30].
3. Fabrication and characterization
The multilayer was deposited using DC magnetron sputtering [31]. The controlled deposition system can deposit each material with a thickness error of less than 1%. The XUV reflectivity of the fabricated multilayer mirror was measured on Beamline 6.3.2 of the Advanced Light Source [32]. The measured reflectivity at several angles of incidence are shown in Fig. 4. In addition, the bandwidth and contrast are compared with the calculated values in Fig. 5 (a) and 5(b), respectively. The measured results show good agreement with the calculated values. The reflectivity profiles cover the photon energy range between 40 and 70 eV, and the average reflectivity is 30%. Note that the input EUV light from ALS BL6.3.2 includes 10% p-polarized light, and this should be considered for particular characterizations.
Fig. 4 Measured reflectivity of fabricated multilayer mirror (blue symbols) compared with the calculated results (rad line). The angles of incidence are 56.9 °, 54.2 °, 51.5 °, 48.7 °, 45.5 °, 41.8 °, 37.4 °, 32.0 °, 24.7 ° and 12.5 ° from the highest to the lowest peak, respectively.
Fig. 5 Comparison between the measured and calculated (a) bandwidth (FWHM) and (b) contrast defined as R(E)/R(E + 3.1 eV) (triangle) and R(E)/R(E-3.1 eV) (diamond) of the fabricated multilayer mirror.
4. Conclusions
We have experimentally demonstrated a wide-range narrowband multilayer mirror for selecting a single-order HH beam from multiple-order harmonics at photon energies between 40 eV and 70 eV. The measured peak reflectivity and contrast of the Zr/Al0.7Si0.3 multilayer mirror are 20–35% and more than 7, respectively, at incident angles of 10–56.9 degrees. In addition, 1 fs order input pulses are not significantly stretched by the reflection. This simple-to-align optical component is useful for the many various applications in physics, chemistry and biology that use ultrafast monochromatic HH beams.
Acknowledgments
The authors would like to thank Dr. Eric M. Gullikson of the Lawrence Berkeley National Laboratory for the multilayer reflectivity measurements. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231.
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