High-intensity high-order harmonics have been investigated intensively in recent years. In the development of a beam
line for the high-intensity high-order harmonics, however, utilizing a conventional beam splitter (BS) (Si or SiC) that
absorbs the fundamental waves has caused serious problems such as its thermal distortion. To solve these problems, we
proposed and investigated a novel BS with transparent materials that transmitted the fundamental waves and then
reflected the high-order harmonics. In BS for the high-order harmonics, reflection of the fundamental waves should be
minimized by entering the p-polarized fundamental waves at the Brewster's angle, which could improve the separation
between the fundamental waves and the high-order harmonics at the same Brewster's angle.
We have already investigated and fabricated WO3/TiO2 bilayers on c-plane sapphire substrates by controlled growth with
sequential surface chemical reactions (SSCR) using sequentially fast pressurized pulses of the vapor sources. Our
previous experimental results revealed that WO3 (221) and rutile TiO2 (200) thin films could be grown epitaxially on c-plane
sapphire substrates by SSCRs. Then, in this study, we proposed a WO3/TiO2 bilayer grown on c-plane sapphire
substrates, which could be utilized as a BS for the high-order harmonics. Reflectance characteristics were also
investigated at the same Brewster's angle using monochromatized synchrotron radiation (SR) located at Ultraviolet
Synchrotron Radiation Facility (UVSOR), Institute for Molecular Science, Okazaki, Japan.
The development of high-reflection mirrors with amorphous metal-oxide multilayers in the
"water-window"(λ=2.332nm-4.368nm) is desired for soft x-ray coherent optics. One of the authors has
already studied and fabricated amorphous Al2O3/TiO2 multilayer for the "water-window" wavelengths by
controlled growth with atomic layer deposition (ALD), and then acquired the reflectance of 33.4 % at
2.73nm and at the incidence angle of 18.2° from the normal incidence. In this study, we proposed
Al2O3/TiO2/Al2O3/ZnO multilayer mirrors. Al2O3 layers grown as amorphous layers were inserted
between TiO2 and ZnO layers. The Al2O3, ZnO and TiO2 thin films were grown on Al2O3 (0001) substrate
by controlled growth with atomic layer deposition (ALD) methods at 450°C. Experimental results
indicated that the growth of crystalline rutile TiO2 (100) and wurtzite ZnO (0001) were prevented. Thus,
inserting amorphous Al2O3 layers, the results indicated that the crystalline growth was prevented.
Moreover, we succeeded fabrication of amorphous TiO2/ZnO mirrors by ALD.
A novel TiO2/ZnO multilayer deposited by atomic layer epitaxy technique has been fabricated to achieve a high
reflective mirror and an attosecond chirped mirror in soft-x-ray "water-window" (λ=2.332-4.368 nm) wavelengths
region. The technique in this study is able to satisfy reguirements for atomic layer control through epitaxial growth using
sequential surface reaction and self-limiting nature. In preliminary experimental studies, both rutile TiO2 (200) and
wurtzite ZnO (0001) thin films were grown epitaxially on the same sapphire (0001) substrates at 450°C and moreover a
high reflectivity of 29.8% was obtained at around 2.734 nm and a grazing angle of 2θ=10°.
The authors conducted the ALE experiment of TiO2/ZnO multilayer using a ZnO buffer layer. As a result, the multilayer
using a buffer layer was able to be grown epitaxially on not only sapphire (0001) but also Si (100). In addition,
reflectivity of multilayer remained to be 24.6% even on Si (100) in contrast with that about 27.5% on sapphire (0001)
at grazing angle of 2θ
= 8°. Thus, the ZnO buffer layer becomes the key layer to fabricate the TiO2/ZnO multilayer on
various substrates.
In the presentation, ALE of TiO2/ZnO multilayer mirrors using buffer layer will be shown in detail.
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