The optical properties of composition-spread (SrxCa1−x)2CuO3(0⩽x⩽1) films showing high third-order optical nonlinearity are systematically investigated. Desired orientations and high crystallinity are obtained for the entire composition x by employing a composition-spread [(Sr2TiO4)x(LaSrAlO4)1−x] buffer layer and a temperature-gradient method in order to compensate the change in lattice mismatch and growth temperature depending on x. Linear absorption spectra and third-harmonic intensity with an exciting wavelength of 1.56 μm are measured as a function of x. As the Cu–O bond length increases with x, the charge-transfer gap decreases and the third-harmonic intensity increases, and both the charge-transfer gap and third-harmonic intensity change almost linearly with x between Ca2CuO3 and Sr2CuO3.

Recently, much attention has been paid to 1D correlated electron systems because some of the compounds, such as Sr 2 CuO 3 and Ca 2 CuO 3 , exhibit large and ultrafast nonlinear optical responses. [1,2] These properties are indispensable for all-optical switching devices to be used in next-generation, high-speed fiber-optic networks. In these compounds, a new concept in physics, called spin-charge separation, has been revealed as an inherent property. [3] These compounds have the crystal structure shown in Figure 1a ( been reported to exist in a 213 structure, and they were only synthesized in polycrystalline form. Therefore, exploring new materials systems with a 213 structure in a single-crystalline form is of great importance.For creating naturally unstable phases, epitaxial stabilization using a suitable template substrate is effective. In fact, we made a single-crystalline thin film of Sr 2 CoO 4 that had never been synthesized even in polycrystalline form using epitaxial stabilization.[6] The Sr 2 CoO 4 has a K 2 NiF 4 structure (Fig. 1a, center), referred to as the 214 structure hereafter, where 2D octahedral TMO 6 sheets are stacked in the direction of the c-axis with a half-phase (1/2, 1/2, 0) displacement along both the a-and b-axes. In the case of a-axis-oriented thin films with a 214 structure grown on the bc surface of an isostructural substrate, the half-phase displacement locks the crystalline structure and the in-plane orientation of the films as b film ʈb substrate and c film ʈc substrate , which is important for growing high-quality single-crystalline films and is known as "atomic graphoepitaxy". [7] The 213 structure can be considered to be a derivative of the 214 structure; it can be constructed by removing alternate oxygen ions in the 2D planes along the a-axis of the 214 structure. Therefore, if the films of a 213 structure are to be grown on the bc surface of a 214 crystal, as shown in Figure 1a, TM-O chains must be aligned to compensate for the half-phase displacement of the substrate surface, resulting in a-axis-oriented films. We have already demonstrated that this technique can be applied to the epitaxial growth of existing 213 compounds such as Sr 2 CuO 3 and Ca 2 CuO 3 on a LaSrAlO 4 substrate with a 214 structure. [8] In order to avoid a multidomain structure consisting of a-and b-axis orientations, the selection of a proper buffer layer (Sr 2 TiO 4 , b-axis lattice constant b 213film ∼ b buffer ) was shown to be essential. Here we report on the synthesis and characterization of single-crystalline thin films of novel Sr 2 NiO 3 and Sr 2 CoO 3 phases with a 1D TM-O chain structure that has never been synthesized even in polycrystalline form. To optimize the film growth conditions, we employed a combinatorial approach. [9] A 1D electronic structure and a sizable nonlinear optical susceptibility were revealed for Sr 2 NiO 3 by polarized absorption spectroscopy and third-harmonic-generation spectroscopy, respectively.Thin films were fabricated on a LaSrAlO 4 (100) substrate by a pulsed-l...

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