Manipulation of polarization conversion and dual foci in a twisted caustic vector optical field in free space.

Manipulation of polarization states in a complex structured optical field during propagation has become an important topic due to its fundamental interest and potential applications. This work demonstrates the effect of the caustic and twisting phases on the polarization states of a vector beam experimentally and theoretically. The novel properties of polarization evolution, especially the conversions of different states of polarization (SoPs) in a twisted caustic vector beam, occur during propagation in free space because of the modulation of twisting and caustic phases. The orthogonal polarization components tend to appear on the beam centers of two foci, and the two focal distances are closely related to the caustic and twisting phases. The twisting and caustic phases can manipulate the conversions between linear and circular polarization components that occur during propagation. These results provide a new approach to more complex manipulations of a structured optical field, especially in tailoring the evolution of polarization states and two foci. They may find potential applications in the corresponding field.

Mueller transform matrix neural network for underwater polarimetric dehazing imaging.

Polarization dehazing imaging has been used to restore images degraded by scattering media, particularly in turbid water environments. While learning-based approaches have shown promise in improving the performance of underwater polarimetric dehazing, most current networks rely heavily on data-driven techniques without consideration of physics principles or real physical processes. This work proposes, what we believe to be, a novel Mueller transform matrix network (MTM-Net) for underwater polarimetric image recovery that considers the physical dehazing model adopting the Mueller matrix method, significantly improving the recovery performance. The network is trained with a loss function that combines content and pixel losses to facilitate detail recovery, and is sped up with the inverse residuals and channel attention structure without decreasing image recovery quality. A series of ablation experiment results and comparative tests confirm the performance of this method with a better recovery effect than other methods. These results provide deeper understanding of underwater polarimetric dehazing imaging and further expand the functionality of polarimetric dehazing method.

Deposition, Characterization, and Modeling of Scandium-Doped Aluminum Nitride Thin Film for Piezoelectric Devices.

In this work, we systematically studied the deposition, characterization, and crystal structure modeling of ScAlN thin film. Measurements of the piezoelectric device's relevant material properties, such as crystal structure, crystallographic orientation, and piezoelectric response, were performed to characterize the Sc0.29Al0.71N thin film grown using pulsed DC magnetron sputtering. Crystal structure modeling of the ScAlN thin film is proposed and validated, and the structure-property relations are discussed. The investigation results indicated that the sputtered thin film using seed layer technique had a good crystalline quality and a clear grain boundary. In addition, the effective piezoelectric coefficient d33 was up to 12.6 pC/N, and there was no wurtzite-to-rocksalt phase transition under high pressure. These good features demonstrated that the sputtered ScAlN is promising for application in high-coupling piezoelectric devices with high-pressure stability.

Vectorial effect on the evolution of fractional-order vector vortex beams in a strongly nonlocal nonlinear medium.

Propagation of a vector vortex optical field (VVOF) with both fractional order of polarization topological charge $m$m and fractional order of vortex topological charge $n$n with spatially variant states of polarization (SoP) in a strongly nonlocal nonlinear medium (SNNM) is studied. The optical field always evolves reciprocally with a cycle of stretch and shrink in a SNNM with dark stripes forming at $z=t\pi {z_p}$z=tπzp ($t$t denotes an integer number, and ${{z}_p}$zp is a parameter that depends on the initial power of the VVOF and the material constant associated with the response function), as a result from the coherent superposition of the vortices with different order of topological charges and weighting coefficients. In particular, the conversions between linear and circular polarization components occur during propagation, and the converted SoP distributions in different propagation distances depend closely on the topological charges and the initial powers. The evolutions of the Stokes parameters of the fractional-order VVOF (FO-VVOF) during propagation in a SNNM show that the spatial distributions of different polarization components are closely related to the topological charges, the initial powers and the propagation distances, implying that the FO-VVOF can be regarded as a superposition of two different fractional-order vortices with orthogonal circular polarization components. These results provide new strategies on tailoring polarization states in a structured optical field with fractional topological charges.

Energy Band Gap Modulation in Nd-Doped BiFeO3/SrRuO3 Heteroepitaxy for Visible Light Photoelectrochemical Activity.

The ability of band offsets at multiferroic/metal and multiferroic/electrolyte interfaces in controlling charge transfer and thus altering the photoactivity performance has sparked significant attention in solar energy conversion applications. Here, we demonstrate that the band offsets of the two interfaces play the key role in determining charge transport direction in a downward self-polarized BFO film. Electrons tend to move to BFO/electrolyte interface for water reduction. Our experimental and first-principle calculations reveal that the presence of neodymium (Nd) dopants in BFO enhances the photoelectrochemical performance by reduction of the local electron-hole pair recombination sites and modulation of the band gap to improve the visible light absorption. This opens a promising route to the heterostructure design by modulating the band gap to promote efficient charge transfer.

Correlation between polar surface area and bioferroelectricity in DNA and RNA nucleobases.

We have performed computational molecular modelling to study the polarization switching and hysteresis loop behaviours of DNA and RNA nucleobases using the PM3 semi-empirical quantum mechanical approaches. All the nucleobases: adenine (A), thymine (T), guanine (G), cytosine (C), and uracil (U) were modelled. Our study indicates that all the nucleobases exhibit a zero-field polarization due to the presence of polar atoms or molecules such as amidogen and carbonyl. The shape of polarization P versus an applied electric field E hysteresis loop is square, implying typical ferroelectrics behaviour. The total energy U as a function of an applied electric field E exhibits a butterfly-like loop. The presence of zero-field polarization and ferroelectrics hysteresis loop behaviours in nucleobases may support the hypothesis of the existence of bioferroelectricity in DNA and RNA. We also found an interesting relationship between the minimum electric field required for switching [Formula: see text] and the ratio of the topological polar surface area (TPSA) to the total surface area (TSA) of a nucleobase. In particular, the [Formula: see text] of a nucleobase is inversely proportional to the TPSA/TSA ratio. This work may provide useful information for understanding the possible existence of ferroelectricity in biomaterials.

First-principles study on the atomistic corrosion processes of iron.

The corrosion of iron presents an important scientific problem and a serious economic issue. It is also one of the most important subjects in materials science because it is basically an electrochemical process and closely related to other topics such as the electrocatalysis of the oxygen reduction reaction. So far, many studies have been conducted to address the corrosion of iron, a very complicated process that occurs when iron is exposed to oxygen and water. An important question is, at which site of the iron surface the corrosion starts and how it results in the final stage of the corrosion. In the present study, as an example of superficial defects, Fe dimers sticking out of Fe(100) surfaces are considered in order to understand the iron corrosion process from first-principles using density functional theory. We found that the Fe dimers spontaneously react with O2 and H2O to form Fe2(OH)4 + 4OH-. Here, it is interesting to note that the Fe dimer plays the role of a water splitting catalyst, because the space above it is always vacant and can accept oxygen molecules many times for reacting with the surrounding water molecules. Then, if the Fe2(OH)4 molecules are detached from the surface, they react with O2 to form Fe2O(OH)4 without an activation barrier, and, in turn, the Fe2O(OH)4 and H2O molecules react to form Fe2(OH)6 complexes with an activation energy of 0.653 eV. If these complexes further dissociate into Fe(OH)3 molecules, they react with each other to form Fe2O3·2H2O with an activation energy of 0.377 eV. This work may provide useful information on possible iron corrosion processes by water in the air.

Vectorial effect of hybrid polarization states on the collapse dynamics of a structured optical field.

The collapse dynamics of a structured optical field with a distribution of spatially-variant states of polarization (SoP) and a spiral phase in the field cross section is studied using the two-dimensional coupled nonlinear SchrÓ§dinger equations. The self-focusing of a structured optical field with an inhomogeneous SoP distribution can give rise to new phenomena of collapse dynamics that is completely different from a scalar field. The collapse patterns are closely related to the topological charges of the vortexas well as the polarization, the initial power, and the SoP distribution in the field cross section. A single on-axis collapse or multiple off-axis partial collapses may occur due to the self-focusing effects of linearly, elliptically and circularly polarized components located at different positions of the field cross-section. The polarization in the core of the collapsing beam is always linearly polarized. The structured collapsing beams, which are driven by the vortex, propagate along a spiral trajectory in a saturated medium.

Structured caustic vector vortex optical field: manipulating optical angular momentum flux and polarization rotation.

A caustic vector vortex optical field is experimentally generated and demonstrated by a caustic-based approach. The desired caustic with arbitrary acceleration trajectories, as well as the structured states of polarization (SoP) and vortex orders located in different positions in the field cross-section, is generated by imposing the corresponding spatial phase function in a vector vortex optical field. Our study reveals that different spin and orbital angular momentum flux distributions (including opposite directions) in different positions in the cross-section of a caustic vector vortex optical field can be dynamically managed during propagation by intentionally choosing the initial polarization and vortex topological charges, as a result of the modulation of the caustic phase. We find that the SoP in the field cross-section rotates during propagation due to the existence of the vortex. The unique structured feature of the caustic vector vortex optical field opens the possibility of multi-manipulation of optical angular momentum fluxes and SoP, leading to more complex manipulation of the optical field scenarios. Thus this approach further expands the functionality of an optical system.

Dynamic control of collapse in a vortex Airy beam.

Here we study systematically the self-focusing dynamics and collapse of vortex Airy optical beams in a Kerr medium. The collapse is suppressed compared to a non-vortex Airy beam in a Kerr medium due to the existence of vortex fields. The locations of collapse depend sensitively on the initial power, vortex order, and modulation parameters. The collapse may occur in a position where the initial field is nearly zero, while no collapse appears in the region where the initial field is mainly distributed. Compared with a non-vortex Airy beam, the collapse of a vortex Airy beam can occur at a position away from the area of the initial field distribution. Our study shows the possibility of controlling and manipulating the collapse, especially the precise position of collapse, by purposely choosing appropriate initial power, vortex order or modulation parameters of a vortex Airy beam.