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High-order harmonic generation (HHG) in solids opens new frontiers in ultrafast spectroscopy of carrier and field dynamics in condensed matter, picometer resolution structural lattice characterization and designing compact platforms for attosecond pulse sources. Nanoscale structuring of solid surfaces provides a powerful tool for controlling the spatial characteristics and efficiency of the harmonic emission. Here we study HHG in a prototypical phase-change material Ge2Sb2Te5 (GST). In this material the crystal phase can be reversibly changed between a crystalline and amorphous phase by light or electric current mediated methods. We show that optical phase-switching is fully reversible and allows for dynamic control of harmonic emission. This introduces GST as new addition to materials that enable flexible metasurfaces and photonic structures that can be integrated in devices and allow for ultrafast optical control.

This study demonstrates the ability to control the properties of TiO2–CuOx composite layers for photocatalytic applications by using a simple electrophoretic deposition method from isopropanol-based suspension. To obtain uniform layers with a controlled composition, the surfactant sodium lauryl sulfate was used, which influenced the electrophoretic mobility of the particles and the morphology of the deposited layers. The TiO2–CuOx composite layers with different CuOx contents (1.5, 5.5, and 11 wt.%) were obtained. It is shown that the optical band gap measured by UV–VIS–NIR diffuse reflectance spectra. When CuOx is added to TiO2, two absorption edges corresponding to TiO2 and CuOx are observed, indicating a broadening of the photosensitivity range of the material relative to pure TiO2. An open-circuit potential study shows that by changing the amount of CuOx in the composite material, one can control the ratio of free charge carriers (n and p) and, therefore, the catalytic properties of the material. As a result, the TiO2–CuOx composite layers have enhanced photocatalytic activity compared to the pure TiO2 layer: methanol yield grows with increasing CuOx content during CO2 photoreduction. [ABSTRACT FROM AUTHOR]

The influence of Bi doping on the thermal, electrical, and optical properties of [Ge.sub.2][Sb.sub.2][Te.sub.5] thin films was investigated. The existence of two Bi concentration ranges with different influence of dopant on the properties of thin films was established. At low concentrations (0.5-1.0 wt.% of Bi), anomalous deviations of physical properties from monotonous concentration dependences were observed. This effect is explained by the use of percolation theory, where formation of infinite clusters is accompanied by critical phenomena at critical concentrations. PACS Nos.: 65.60.+a, 73.61.Jc, 78.66.Jq. Nous etudions l'influence du dopage Bi sur les proprietes electriques, thermiques et optiques de films minces de [Ge.sub.2][Sb.sub.2][Te.sub.5]. Nous etablissons que deux domaines de concentration de Bi ont differentes influences dopantes sur ces films minces. A basse concentration (0.5 - 1.0%m de Bi), nous observons des deviations anomales des proprietes physiques s'eloignant d'une dependance monotone sur la concentration. Cet effet s'explique par la theorie de la percolation, oU la formation d'amas infinis est accompagnee de phenomenes critiques aux concentrations critiques. [Traduit par la Redaction], 1. Introduction Phase-change memory (PCM) materials are of great interest now because of their potential applications in micro- and optoelectronics. Switching and memory behavior in chalcogenide glassy semiconductors fundamental for [...]

Ge2Sb2Te5 (GST225) looks to be a promising material for rewritable memory devices due to its relatively easy processing and high optical and electrophysical contrast for the crystalline and amorphous phases. In the present work, we combined the possibilities of crystallization and anisotropic structures fabrication using femtosecond laser treatment at the 1250 nm wavelength of 200 nm thin amorphous GST225 films on silicon oxide/silicon substrates. A raster treatment mode and photoexcited surface plasmon polariton generation allowed us to produce mutually orthogonal periodic structures, such as scanline tracks (the period is 120 ± 10 μm) and laser-induced gratings (the period is 1100 ± 50 nm), respectively. Alternating crystalline and amorphous phases at the irradiated surfaces were revealed according to Raman spectroscopy and optical microscopy studies for both types of structures. Such periodic modulation leads to artificial optical and electrophysical anisotropy. Reflectance spectra in the near infrared range differ for various polarizations of probing light, and this mainly results from the presence of laser-induced periodic surface structures. On the other hand, the scanline tracks cause strong conductivity anisotropy for dc measurements in the temperature range of 200–400 K. The obtained results are promising for designing new GST225-based memory devices in which anisotropy may promote increasing the information recording density. [ABSTRACT FROM AUTHOR]

We have studied transmission spectra of a silicon nitride O-ring resonator with a Ge2Sb2Te5 (GST) thin-film cover. We have performed numerical simulations of the transmission, absorption, reflection, and scattering for the GST cells of various thicknesses and lengths and have also measured transmission spectra O-ring resonators for GST cells of various length and phase states. An analysis of the changes in the Q-factors has enabled us to identify the region of the GST cells where light scattering and absorption dominate and find the size dependence of amorphous and crystalline GST attenuation coefficients. The demonstrated results pave the way to high energy-efficient on-chip devices of a small footprint that can be switched either optically or electrically. [ABSTRACT FROM AUTHOR]

This article presents the results of irradiation treatment by the 180 fs laser pulses at the wavelength of 1030 nm of Ge2Sb2Te5 amorphous thin films. It is shown that high‐quality laser‐induced periodic surface structures (LIPSS) are formed at certain parameters of irradiation with a period corresponding to the laser wavelength. A distinctive feature of the observed LIPSS is the alternation of amorphous and crystalline regions corresponding to the ridges and valleys of the periodic structures which are perpendicular to the polarization of the femtosecond light beam. These surface crystalline–amorphous alternating structures are characterized by significantly less pronounced surface relief than for previously observed LIPSS formed by femtosecond laser pulses at the wavelength of 515 nm. Possible mechanisms are considered for the LIPSS formation including the formation of surface plasmon polariton and its subsequent interference with the incident beam resulting in spatial modulation of the temperature on the film surface. As the results indicate, the formation of alternating structure has a threshold nature with the threshold laser fluence for LIPSS formation depending logarithmically on the number of laser pulses, which is attributed to the transient nature of the crystallization process. [ABSTRACT FROM AUTHOR]

The influence of the adjacent layers (SiO2, Al, Ni, Ti, W, TiN, TiN/W) on the crystallization kinetics of Ge2Sb2Te5 thin films was investigated using the techniques based on the application of two different methods—differential scanning calorimetry and measurements of the resistivity temperature dependences. Thin films were deposited by the magnetron sputtering. The composition of the films was determined by the Auger spectroscopy and was close to Ge2Sb2Te5. X-ray diffraction was used to investigate the structure of thin films and showed that the as-deposited films were in an amorphous state, while heat treatment at 250 °C for 30 min led to the crystallization to the cubic (rock salt) phase. Effective activation energy of crystallization obtained by differential scanning calorimetry at the beginning of the crystallization was 1.8 eV and then slightly decreased to 1.7 eV at the end of the process. The values of the effective activation energy obtained from the measurements of the resistivity temperature dependences were in the range of 2.5–2.9 eV at the beginning and in the range of 2.2–3.5 eV at the end of the crystallization process. The difference in the effective activation energies of crystallization for the GST225 thin films deposited on the different sublayers is caused by the influence of the neighboring sublayers on the crystallization process. It was found that crystallization temperatures correlate with the effective activation energies and increase with their growth. [ABSTRACT FROM AUTHOR]