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<jats:p>Recent years, optically controlled phase-change memory draws intensive attention owing to some advanced applications including integrated all-optical nonvolatile memory, in-memory computing, and neuromorphic computing. The light-induced phase transition is the key for this technology. Traditional understanding on the role of light is the heating effect. Generally, the RESET operation of phase-change memory is believed to be a melt-quenching-amorphization process. However, some recent experimental and theoretical investigations have revealed that ultrafast laser can manipulate the structures of phase-change materials by non-thermal effects and induces unconventional phase transitions including solid-to-solid amorphization and order-to-order phase transitions. Compared with the conventional thermal amorphization, these transitions have potential superiors such as faster speed, better endurance, and low power consumption. This article summarizes some recent progress of experimental observations and theoretical analyses on these unconventional phase transitions. The discussions mainly focus on the physical mechanism at atomic scale to provide guidance to control the phase transitions for optical storage. Outlook on some possible applications of the non-thermal phase transition is also presented to develop new types of devices.</jats:p>

<jats:p>We design a novel all-normal flat near-zero dispersion chalcogenide photonic crystal fiber (PCF) for generating mid-infrared (MIR) supercontinuum (SC). The proposed PCF with a core made of As<jats:sub>2</jats:sub>Se<jats:sub>3</jats:sub> glass and uniform air holes in the cladding is selectively filled with As<jats:sub>2</jats:sub>S<jats:sub>5</jats:sub> glass. By carefully engineering the PCF with an all-normal flat near-zero dispersion profile, the anomalous-dispersion soliton effect is reduced, thus enabling broadband highly coherent SC to be generated. We also investigate the influence of the pulse parameters on the SC generation. Broadband SC covering <jats:inline-formula> <jats:tex-math> <?CDATA $1.4\,{\rm{\mu }}{\rm{m}}-10\,{\rm{\mu }}{\rm{m}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.4</mml:mn> <mml:mspace width="0.50em" /> <mml:mi mathvariant="normal">μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> <mml:mspace width="0.25em" /> <mml:mi mathvariant="normal">μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104204_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> with perfect coherence is achieved by pumping the proposed 3-cm-long PCF with 3-<jats:inline-formula> <jats:tex-math> <?CDATA ${\rm{\mu }}{\rm{m}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104204_ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> 100-fs pulses. The results provide a potential all-fiber realization of the broadband coherent MIR-SC.</jats:p>

<jats:p>Based on the theory of coherence, the model of multi-Gaussian Schell-model (MGSM) beams carrying an edge dislocation generated by the MGSM source is introduced. The analytical cross-spectral density of MGSM beams carrying an edge dislocation propagating in oceanic turbulence is derived, and used to study the evolution properties of the MGSM beams carrying an edge dislocation. The results indicate that the MGSM beam carrying an edge dislocation propagating in oceanic turbulence will evolve from the profile with two intensity peaks into a flat-topped beam caused by the MGSM source, and the beam will evolve into the Gaussian-like beam due to the influences of oceanic turbulence in the far field. As the propagation distance increases, the MGSM beam carrying an edge dislocation propagating in oceanic turbulence with the larger rate of dissipation of mean-squared temperature (<jats:inline-formula> <jats:tex-math> <?CDATA ${\chi }_{{\rm{T}}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>χ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">T</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_104207_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) and ratio of temperature to salinity contribution to the refractive index spectrum (<jats:italic>ς</jats:italic>) or the smaller rate of dissipation of kinetic energy per unit mass of fluid (<jats:italic>ε</jats:italic>) evolves into the flat-topped beam or a Gaussian beam faster.</jats:p>

<jats:p>By designing and fabricating a series of dual-interferometer coupled silicon microrings, the coupling condition of the pump, signal, and idler beams can be engineered independently and then we carried out both the continuous-wave and pulse pumped four-wave mixing experiments to verify the dependence of conversion efficiency on the coupling conditions of the four interacting beams, respectively. Under the continuous-wave pump, the four-wave mixing efficiency gets maximized when both the pump and signal/idler beams are closely operated at the critical coupling point, while for the pulse pump case, the efficiency can be enhanced greatly when the pump and converted idler beams are all overcoupled. These experiment results agree well with our theoretical calculations. Our design provides a platform for explicitly characterizing the four-wave mixing under different pumping conditions, and offers a method to optimize the four-wave mixing, which will facilitate the development of on-chip all-optical signal processing with a higher efficiency or reduced pump power.</jats:p>

<jats:p>Traditional geoacoustic inversions are generally solved by matched-field processing in combination with meta-heuristic global searching algorithms which usually need massive computations. This paper proposes a new physical framework for geoacoustic retrievals. A parabolic approximation of wave equation with non-local boundary condition is used as the forward propagation model. The expressions of the corresponding tangent linear model and the adjoint operator are derived, respectively, by variational method. The analytical expressions for the gradient of the cost function with respect to the control variables can be formulated by the adjoint operator, which in turn can be used for optimization by the gradient-based method.</jats:p>

<jats:p>We build an experiment system based on total reflection (TR) method to observe the evolution of real contact area of polymethyl methacrylate (PMMA) in the continual stick-slip movement. The bilateral friction is adopted to overcome the bending moment in the lateral friction movement. Besides some classical phenomena of stick-slip movement such as periodical slow increase of frictional force in sticking phase and a sudden drop when slipping, a special phenomenon that the contact area increases with the tangential force is observed, which was called junction growth by Tabor in 1959. Image processing methods are developed to observe the variation of the junction area. The results show that the center of the strongest contact region will keep sticking under the tangential force until the whole slipping, the strongest point undergoes three stages in one cycle, which are named as sticking stage, fretting stage, and cracking stage, respectively. The combined analysis reveals a physical process of stick-slip movement: the tangential force causes the increase of the real contact area, which reduces the pressure between the contact spots and finally leads to the slipping. Once slipping occurs, the real contact area drops to the original level resulting in the pressure increase to the original level, which makes the sticking happen again.</jats:p>

<jats:p>We derive in this paper a time stable seventh-order dissipative compact finite difference scheme with simultaneous approximation terms (SATs) for solving two-dimensional Euler equations. To stabilize the scheme, the choice of penalty coefficients for SATs is studied in detail. It is demonstrated that the derived scheme is quite suitable for multi-block problems with different spacial steps. The implementation of the scheme for the case with curvilinear grids is also discussed. Numerical experiments show that the proposed scheme is stable and achieves the design seventh-order convergence rate.</jats:p>

<jats:p>Based on the volume of fluid (VOF) method, a numerical model of bubbles splitting in a microfluidic device with T-junction is developed and solved numerically. Various flow patterns are distinguished and the effects of bubble length, capillary number, and diameter ratio between the mother channel and branch are discussed. The break-up mechanism is explored in particular. The results indicate that the behaviors of the bubbles can be classified into two categories: break-up and non-break. Under the condition of slug flowing, the branches are obstructed by the bubbles that the pressure difference drives the bubbles into break-up state, while the bubbles that retain non-break state flow into an arbitrary branch under bubbling flow condition. The break-up of the short bubbles only occurs when the viscous force from the continuous phase overcomes the interfacial tension. The behavior of the bubbles transits from non-break to break-up with the increase of capillary number. In addition, the increasing of the diameter ratio is beneficial to the symmetrical break-up of the bubbles.</jats:p>

<jats:p>Behaviors of C or O in bcc Fe and interactions of C–O and oxygen–carbon–vacancy (O–C–<jats:inline-formula> <jats:tex-math><?CDATA $\square $?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>□</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_106102_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) are investigated by first principles calculations. Octahedral interstitial site is the most stable position for an O atom in bcc Fe. The migration energy of an O atom in bcc Fe is 0.46 eV. The strength of O–Fe (1nn) bond (0.32) is slightly greater than that of Fe–Fe metallic bond (0.26). Repulsive interactions of C–C, O–O, and C–O exist in bcc Fe. When the concentration of FIA (FIA refers to C or O) is relatively high, a vacancy can attract four FIAs and form stable FIAs–<jats:inline-formula> <jats:tex-math><?CDATA $\square $?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>□</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_106102_ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> complex.</jats:p>

<jats:p>Transition-metal chalcogenide nanowires (TMCN) as a viable candidate for nanoscale applications have been attracting much attention for the last few decades. Starting from the rigid building block of <jats:italic>M</jats:italic> <jats:sub>6</jats:sub> octahedra (<jats:italic>M</jats:italic> = transition metal), depending on the way of connection between <jats:italic>M</jats:italic> <jats:sub>6</jats:sub> and decoration by chalcogenide atoms, multiple types of extended TMCN nanowires can be constructed based on some basic rules of backbone construction proposed here. Note that the well-known Chevrel-phase based <jats:inline-formula> <jats:tex-math><?CDATA ${M}_{6}{X}_{6}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>X</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_106103_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math><?CDATA ${M}_{6}{X}_{9}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>X</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_28_10_106103_ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> (<jats:italic>X</jats:italic> = chalcogenide atom) nanowires, which are among our proposed structures, have been successfully synthesized by experiment and well studied. More interestingly, based on the construction principles, we predict three new structural phases (the cap, edge, and C&amp;E phases) of Mo<jats:sub>5</jats:sub>S<jats:sub>4</jats:sub>, one of which (the edge phase) has been obtained by top-down electron beam lithography on two-dimensional MoS<jats:sub>2</jats:sub>, and the C&amp;E phase is yet to be synthesized but appears more stable than the edge phase. The stability of the new phases of Mo<jats:sub>5</jats:sub>S<jats:sub>4</jats:sub> is further substantiated by crystal orbital overlapping population (COOP), phonon dispersion relation, and thermodynamic calculation. The barrier of the structural transition between different phases of Mo<jats:sub>5</jats:sub>S<jats:sub>4</jats:sub> shows that it is very likely to realize an conversion from the experimentally achieved structure to the most stable C&amp;E phase. The calculated electronic structure shows an interesting band nesting between valence and conduction bands of the C&amp;E Mo<jats:sub>5</jats:sub>S<jats:sub>4</jats:sub> phase, suggesting that such a nanowire structure can be well suitable for optoelectronic sensor applications.</jats:p>