Moderate fabrication and characterization of the microcrystalline Sr2CuO3 glass films with effective nonlinearities

Since nonlinear optical materials used in the ultrafast all-optical switching is an important part for the modern optical technology, cuprates have been widely investigated for their specific Cu-O chain structure and intriguing optical properties. We present a new preparation method of microcrystalline Sr2CuO3 glass films on glass substrates combining spin-coating and co-sintering techniques. Then, the as-prepared samples were polished for different times to obtain microcrystalline Sr2CuO3 glass films with varying thickness. The influence of polishing time on the structure, the valence state and the nonlinear optical response were discussed, respectively. The purity of the Sr2CuO3 phase, surface morphology and the chemical composites of these synthesized glass films were given with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Importantly, optical absorption spectroscopy and Z-scan technique were used to measure linear absorption and third-order optical nonlinearity of the films. The experiments showed that third-order nonlinear susceptibility of the 140 min polished film sample with a thickness of 18 μm was up to 1.23 × 10−12 esu, indicating its potential application in the nonlinear field.

odd-charge transfer states. After that, M. Ohtani revealed the synthesis of desired orientations and high crystallinity for (Sr x Ca 1−x ) 2 CuO 3 thin films and the linear relation of x and the charge transfer gap 24 . Furthermore, A. Maeda had successfully grown Sr 2 CuO 3 crystals on the LaAlO 3 substrate, and then discussed its nonlinear property with Ca 2 CuO 3 to considering excitonic effect 25 . In a word, scholars prepare Sr 2 CuO 3 materials with the use of pulsed laser deposition (PLD), TFSZ and RF magnetron sputtering techniques, which are always labor-intensive and time-consuming with the instrument hard to manipulate. Furthermore, the research of the thickness dependence of structure and optical property for the microcrystalline Sr 2 CuO 3 films is seldom reported.
In this paper, microcrystalline Sr 2 CuO 3 glass films were obtained by combining co-sintering and spin-coating technique for the first time 26,27 . The Sr 2 CuO 3 particles were firmly embedded in the Pb-glass matrix on the surface of K 9 glass substrate. Subsequently, through mechanical polishing for different times, different thicknesses of film samples were acquired. Thickness dependence of structure and optical property for the microcrystalline Sr 2 CuO 3 glass films were investigated in detail. As a result, the final Z-scan experiment showed that the 140 min polished microcrystalline Sr 2 CuO 3 glass film with a thickness of 18 μm owned a fast nonlinear response, could be a suitable nonlinear optical candidate compared to some known materials 28 .

Results and Discussion
SEM and EDS analysis. To capture the transformation of the morphology and elements distribution in the microcrystalline Sr 2 CuO 3 glass film under the polishing process, SEM and EDS spectra analyses were conducted. Figure 1a-c provide the SEM images of the microcrystalline Sr 2 CuO 3 glass film at three polishing times: 60, 100 and 140 min under 500 °C sintering, as an example. The as-prepared films reveal that the gray microcrystalline Sr 2 CuO 3 particles were tightly embedded in the Pb-glass layer, which were fabricated by using co-sintering the original materials, and they were adhered to the surfaces in the K 9 glass substrates. With the increase of polishing times, the surfaces of microcrystalline glass film transformed from rough and holey morphology to relatively well-distributed and smooth surfaces. As though there are still some sporadic holes exist in the cross-sections of the various film sample, and the smooth skin layers illustrate the polishing process could effectively adjust the film thickness, as shown in Fig. 1d-f. Additionally, by measurement, the corresponding thicknesses of these film samples polished for 60 min, 100 min, 140 min were 45, 32 and 18 μm, respectively. The unique structure and steerable thickness of the as-synthesized film samples have a large impact on the valence state and the nonlinear optical response, which will be discussed below. Furthermore, the EDS spectra of these film samples are presented in Fig. 1g-i, revealing there are no impurities in these films except the Sr 2 CuO 3 particles and the Pb-glass matrix.

XRD analysis.
To assess the influence of the dependence of synthesis temperature and polishing time on the structure of microcrystalline Sr 2 CuO 3 glass film, the XRD patterns are presented in Fig. 2. The crystalline and purity of microcrystalline Sr 2 CuO 3 glass film sintered at 100 °C to 600 °C orderly may be depicted as in Fig. 2a. Obviously, the diffraction patterns correspond to the JCPDS 85-2487 (Sr 2 CuO 3 ) from 100 °C to 500 °C. No extra peaks are detected, which indicate that the Sr 2 CuO 3 has a single-phase structure during the whole heating process. Nevertheless, there are some extra peaks conforming to PbO and SrPbO 3 appear when the sinter temperature rise up to 600 °C, mainly attribute to the interaction between Sr 2 CuO 3 and the glass compositions. In addition, Fig. 2b shows the effect of different polishing times vary from 0 to 180 min on the particle size of microcrystalline film prepared at 500 °C, considering the better combination of glass substrate and Sr 2 CuO 3 particles and phase purity. We can see that as the polishing time increases, the intensity of the diffraction peak gradually decreases. From the XRD patterns of the microcrystalline Sr 2 CuO 3 glass film polished for 180 min, we could assume that the surface of the film sample is K 9 glass substrate. Similarly, all the XRD patterns exhibit a large amorphous peak with 2θ at approximately 28°~32° attributing to the diffraction from the glass matrix.

XPS analysis.
With the purpose of evaluating the effect of the thermal treatment and polish procedure on the valence state and element type of the microcrystalline Sr 2 CuO 3 glass films, ulteriorly, the XPS was performed to analyze the as-prepared samples polished for 140 min and 180 min under sintering temperature of 500 °C. Figure 3a depicts the survey XPS spectra of these two glass films correspond to the red and blue curves for the samples polished for 140 min and 180 min, respectively. From the red curve, we can observe that the peaks of Pb, Zn, Si, Sr, Cu and O were presented in the as-synthesized glass film, agreeing with glass composition and Sr 2 CuO 3 .  From XPS and fitting curves, we can calculate that the atomic ratio of Sr and Cu was 2:1, which correspond to the theoretical value (Sr 2 CuO 3 particles). Meanwhile, the film sample polished for 180 min was also tested, mainly contain Zn, Si, and O from the blue curve, indicating that the film sample was in accordance with the K 9 glass substrate under excessive polishing procedure and no external element was introduced. Additionally, the high-resolution spectra for Cu 2p in these two film samples polished for 140 min and 180 min were showed in Fig. 3b. The red curve gives two peaks of Cu 2p 3/2 and Cu 2p 1/2 at approximately 931.9 eV and 951.16 eV, respectively, illustrating the valence state of Cu in the film sample is +2 or +1 28 . As for the other, the Cu signals don't appear on the blue curve. Obviously, the red curve in Fig. 3b shows a precisely characteristic peak of Cu 2+ at about 942.16 eV between the two strong peaks conforming to the XRD analysis of the 140 min polished microcrystalline Sr 2 CuO 3 glass film.
Third-order optical nonlinearity. Optical absorption spectroscopy and Z-scan technique were used to measure linear absorption and third-order optical nonlinearity of the microcrystalline Sr 2 CuO 3 glass films. Linear optical absorption of the film samples polished for 140 min and 180 min are described in Fig. 4a. In general, while for the sample with a lower transmittance (less than 30%), most of the testing energy are absorbed by the sample in the process, causing an increase of thermal effects on the sample. The linear optical absorption includes the linear loss. Figure 4b reveals that the transmittance of the glass film is about 40% at a wavelength of 787 nm, meeting the essential requirements of the Z-scan measurements. While the other glass films polished for shorter time would gather high powers on the surface under excitation laser irradiation, affecting the accuracy of measurement seriously for the lower transmittance.
The Z-scan is a single excitation beam technique to measure the nonlinear refractive index and nonlinear absorption coefficient of the materials 29 . As can be easily understood, Fig. 5 simply shows schematic diagram of the Open-aperture (OA) and closed-aperture (CA) Z-scan experiment. The nonlinear refraction and absorption of the microcrystalline Sr 2 CuO 3 glass films polished for 140 and 180 min were acquired by using OA and CA Z-scan technology. As shown in Fig. 6, the hollow triangles represent the measure results and the solid line show the theoretical fit of the normalized transmittance. Figure 6a and b give the OA and CA Z-scan results for the microcrystalline Sr 2 CuO 3 glass film polished for 140 min. The film sample polished for 180 min was contrasted for reference shown in Fig. 6c,d. We can see that this is the reverse saturable absorption in the microcrystalline Sr 2 CuO 3 glass film on account of the single symmetrical valley of peak relative to the focus (z = 0) of the solid line in Fig. 6a. Meanwhile, Fig. 6b displays the signature of valley of peak indicating the occurrence of self-focusing process and negative value for the nonlinear refractive index (γ < 0) in the film sample at CA measurements. As  well for the glass substrate in Fig. 6c,d, the signature is similar to the film sample polished for 140 min but the intensity is relatively weak.
According to a well-known theory 30 , from the OA and CA Z-scan curves, we can calculate the nonlinear absorption coefficient (β) and nonlinear refractive index (γ) of the film. Furthermore, we can calculate the real and imaginary parts of the third-order nonlinear susceptibility (χ (3) ) of the film samples by equations (1)  where n 0 , c, ε 0 and λ are linear refractive index, speed of light, permittivity of free space and the wavelength of the laser light of the film sample, respectively. The total third-order nonlinear susceptibility χ (3) can be calculated by equation (3).
As shown in Table 1, the parameters of third-order optical nonlinearity are listed. We introduced microcrystalline Sr 2 CuO 3 in the Pb-glass matrix on the surface of K 9 glass substrate, and the results were expected to lead to a large third-order nonlinearity of the film sample, whose nonlinear susceptibility could be as high as 1.23 × 10 −12 esu. Furthermore, we introduce the photon theory to explain the third-order nonlinear phenomenon more deeply. In the one-dimensional Cu-O composites, due to the occupied p-band of oxygen between the upper and lower Hubbard bands of the metal and the big field Coulomb interactions at the metal sites, the Mott-Hubbard gap opens in the d-band of copper 21 . The charge transfer between Cu and O is major determinant of the nonlinear optical response, while the Sr only serves as a bridge which concatenates the parallel Cu-O chains. In short, the absorption of photons and the dipole moment in the Cu-O composites result in the noticeably increase of third-order nonlinear susceptibility χ (3) principally.

Conclusions
In a word, we report the synthesis of microcrystalline Sr 2 CuO 3 glass films, which were fabricated by co-sintering and spin-coating technique. Next, through continuous polishing, we acquire microcrystalline films with various thicknesses. The changes of microcrystalline Sr 2 CuO 3 glass films during the polishing process were monitored and discussed individually. From the analysis results of the SEM and EDS spectra, the Sr 2 CuO 3 crystalline phase were well-embedded in the glass matrix. Additionally, the XRD patterns of the polished film samples exactly verified that the existence of Sr 2 CuO 3 microcrystals in the glass matrices. At the same time, the composition and binding energy of Sr, Cu, and O were determined using the results of XPS curves. From XPS and calculations, we can concluded that the atomic ratio of Sr and Cu was 2:1, which correspond to the theoretical value (Sr 2 CuO 3 particles). Benefiting from the results, third-order optical nonlinearities of the microcrystalline Sr 2 CuO 3 glass films were measured through the Z-scan technique. The Z-scan experiment indicated that the microcrystalline Sr 2 CuO 3 glass film has a terrific third-order optical nonlinearity with the χ (3) as high as 1.23 × 10 −12 esu, exhibiting its potential application in the nonlinear field.

Methods
Samples preparation. The Sr 2 CuO 3 powder was prepared to use a traditional solid state method. SrO (99.99%) and CuO (99.99%) in stoichiometric amount were mixed evenly for 60 min, and the mixture was sintered at 980 °C for 20 h in the crucibles under an ambient atmosphere. After a short time, the subsequent acquired intermediates were again further heated at 980 °C for 10 h, with fully grindings. Subsequently, the Sr 2 CuO 3 powder in grayish was obtained. It is pointed out that the synthesized Sr 2 CuO 3 powder is hygroscopic, which should be stored in the dryer to avoid contact with moist air and carbon dioxide. After that, according to the ratio of 1:3 (mass fraction), the Sr 2 CuO 3 and Pb-glass powder (PbO 2 -ZnO-B 2 O 3 -SiO 2 ) were mixed homogeneously for 2 h. Then, their mixtures were blended with the naphtha evenly. The viscous composite was individually disposed on the surfaces of K 9 glass substrate through spin-coating technology with the rate of 2000 rpm for 30 s. Subsequently, we put the composites of Sr 2 CuO 3 and glass powder on the K 9 glass substrates into the muffle furnace, and adjust the sintering temperature from 100 °C to 600 °C. Last, we acquired film samples, which were named microcrystalline Sr 2 CuO 3 glass films by us, and the as-prepared samples were polished for different times to obtain microcrystalline Sr 2 CuO 3 glass films with varying thickness.
Samples tests. All of the film samples, no matter what they prepared in any conditions, were subjected to structural and phase characterization with the X-ray diffraction (XRD, D8 Focus, Bruker, Germany) in the range of 20-80 o (2θ). In addition, the morphology and elements distribution in the microcrystalline Sr 2 CuO 3 glass film were analyzed by field-emission scanning electron microscope (FE-SEM, Auriga, Carl Zeiss) with the measuring equipment of energy dispersive X-ray spectroscopy. The chemical compositions and the valence state of ions in the thin film surface were characterized with the X-ray photoelectron spectroscopy (XPS, SHI-MADZU) Furthermore, by using the UV/VIS Spectrophotometer (Mode V-570, JASCO), the linear absorption and transmittance spectra were acquired. Measurements of the third-order optical nonlinearities of the microcrystalline Sr 2 CuO 3 glass film were determined by Z-scan method at 787 nm, and the pulse width, single-pulse energy and repetition rate were 397 fs, 0.1 μJ and 1000 Hz, respectively. All of the above tests of the film samples were conducted at room temperature. Data availability. The datasets generated during the current study are available from the corresponding author on reasonable request.