Xiangchao Zhang

Low lattice thermal conductivity of stanene.

A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures.

Graphene Covered on Microfiber Exhibiting Polarization and Polarization-dependent Saturable Absorption

Graphene has attracted a lot of research interest, especially as a saturable absorber (SA). However, improvement on thermal damage threshold is critical for the SA. Here, graphene covered on the microfiber is developed for this purpose by use of the light-graphene interaction via the evanescent field of the guided mode in the microfiber. Such interaction is numerically studied by using the theory of the electromagnetic field. Theoretical and experimental results indicate that graphene covered on the upper surface of the microfiber can be used as a polarization-dependent SA as well as an optical polarizer. When the radius size of the microfiber is down to 0.8 μm, its polarization extinction ratio is up to ~27 dB. When the radius of microfiber is up to ~3 μm, a polarization-dependent SA can be obtained with high thermal damage threshold of ~975.82 MWcm-2 for p -polarization and ~1233.2 MWcm-2 for s-polarization, and its polarization-dependent modulation depth varies from ~10.25% to ~12.85%.

Fabrication and characterization of areal roughness specimens for applications in scanning probe microscopy

Reference structures are urgently required to characterize the performance of scanning probe microscopes for the quantitative measurement of micro-/nano-scale roughness. In this paper, preliminary areal roughness specimens are fabricated by electron beam lithography (EBL) and direct laser writing (DLW). The geometric topography and statistical quantities of the surface structures are specially designed. Precise registration between the design template and the measured data has been carried out to evaluate the fabrication quality. Two matching algorithms, the iterative closest point method and the Levenberg–Marquardt method, are compared. Results imply that the latter method prevails against the former one with improved accuracy and efficiency. It is demonstrated that the typical root-mean-square residuals of the EBL and DLW fabricated surfaces are approximately 43.5 and 195.3 nm, respectively. The spectral properties of the fabricated structures are also analyzed. These areal roughness specimens have a potential for validating scanning probe microscopes and other nano-measurement instruments for areal roughness measurements.

A minimax fitting algorithm for ultra-precision aspheric surfaces

Aspheric lenses show significant superiority over traditional spherical ones. The peak-to-valley form deviation is an important criterion for surface qualities of optical lenses. The peak-to-valley errors obtained using traditional methods are usually greater than the actual values, which in turn causing unnecessary rejections. In this paper the form errors of aspheric surfaces are evaluated in the sense of minimum zone, i.e. to directly minimize the peak-to-valley deviation of the data points with respect to the nominal surface. A powerful heuristic optimization algorithm, called differential evolution (DE) is adopted. The control parameters are obtained by meta-optimization. Normally the number of data points is very large, which makes the optimization program unacceptably slow. To improve the efficiency, alpha-shapes are employed to decrease the number of data points involved in the DE optimization. Finally numerical examples are presented to validate this minimum zone evaluation method and compare its results with other algorithms.

Abnormal behaviors of Goos–Hänchen shift in hyperbolic metamaterials made of aluminum zinc oxide materials

In this paper, the Goos–Hanchen shift (GHS) at the interface between air and the aluminum zinc oxide (AZO)/ZnO hyperbolic metamaterial (AZO-HMM) is theoretically examined. The results herein show that AZO-HMM can enhance the GHS at the interface to a value at 3 orders of the incident wavelength under special conditions, which is much larger than conventional GHS values. Moreover, the GHS can be tuned to be negative or positive by changing the incident wavelength or the volume fraction of AZO.

Evaluating the form errors of spheres, cylinders and cones using the primal–dual interior point method

In precision metrology, the form errors between the measured data and reference nominal surfaces are usually evaluated in four approaches: least squares elements, minimum zone elements, maximum inscribed elements and minimum circumscribed elements. The calculation of minimum zone element, maximum inscribed element and minimum circumscribed element is not smoothly differentiable, thus very difficult to be solved. In this article, a unified method is presented to evaluate the form errors of spheres, cylinders and cones in the sense of minimum zone element, maximum inscribed element and minimum circumscribed element. The primal–dual interior point method is adopted to solve this non-linearly constrained optimisation problem. The solution is recursively updated by arc search until the Karush–Kuhn–Tucker conditions are satisfied. Some benchmark data are employed to demonstrate the validity and superiority of this method. Numerical experiments show that this optimisation algorithm is computationally efficient and its global convergence can be guaranteed.

Theoretical investigation of a broadband all-optical graphene-microfiber modulator

Graphene pasted on the surface of a nanostructure, e.g., microfiber, enables such graphene-based devices to have excellent photonic and electronic properties. In this paper, an all-optical modulator based on a polymer-supported bilayer graphene film half-wrapped on the surface of the microfiber has been theoretically presented and discussed to make the best use of its ultrawide bandwidth. According to the theoretical analyses on the graphene-microfiber modulator, 100 Gbit/s 8 channel synchronous modulation has been realized with ∼14.88 and ∼14.80  dB modulation depth, respectively, for two polarized modes. The channel wavelength of such modulator covers C, L, and U bands. To ensure the extinction ratio of 16 channel synchronous modulation to be larger than 5 dB, the bilayer graphene-microfiber interaction length of the modular should be greater than 93.06 μm and the diameter of the microfiber should be around 1.8 μm. Under this condition, it also has a tolerance on signal deterioration, as it could effectively decrease the noise of “0” signal.

Anisotropic ultrahigh hole mobility in two-dimensional penta-SiC2 by strain-engineering: electronic structure and chemical bonding analysis

Monolayer pentagonal silicon dicarbide is a 2D material composed entirely of pentagons, and it possesses novel electronic properties possibly leading to many potential applications. In this paper, using first-principles calculations, we have systematically investigated the electronic, mechanical and transport properties of monolayer penta-SiC2 by strain-engineering. By applying in-plane tensile or compressive strain, it is possible to modulate the physical properties of monolayer penta-SiC2, which subsequently changes the transport behaviour of the carriers. More interestingly, at room temperature, the uniaxial compressive strain of −8% along the a-direction can enhance the hole mobility of monolayer penta-SiC2 along the b-direction by almost three orders of magnitude up to 1.14 × 106 cm2 V−1 s−1, which is much larger than that of graphene, while similar strains have little influence on the electron mobility. The ultrahigh and strain-modulated carrier mobility in monolayer penta-SiC2 may lead to many novel applications in high-performance electronic and optoelectronic devices.

Bias in parameter estimation of form errors

The surface form qualities of precision components are critical to their functionalities. In precision instruments algebraic fitting is usually adopted and the form deviations are assessed in the z direction only, in which case the deviations at steep regions of curved surfaces will be over-weighted, making the fitted results biased and unstable. In this paper the orthogonal distance fitting is performed for curved surfaces and the form errors are measured along the normal vectors of the fitted ideal surfaces. The relative bias of the form error parameters between the vertical assessment and orthogonal assessment are analytically calculated and it is represented as functions of the surface slopes. The parameter bias caused by the non-uniformity of data points can be corrected by weighting, i.e. each data is weighted by the 3D area of the Voronoi cell around the projection point on the fitted surface. Finally numerical experiments are given to compare different fitting methods and definitions of the form error parameters. The proposed definition is demonstrated to show great superiority in terms of stability and unbiasedness.

Edge control in precision robotic polishing based on space-variant deconvolution

Abstract In the ultra-precision manufacturing of large optical surfaces, industrial robots with small tools have the potential to become an intelligent and economical choice of surface polishing. But one of the most challenging problems is the severe edge roll-off error caused by the small tools. In this paper, an effective method is proposed to reduce the edge error in the polishing of large mirrors. The convergence rate of the form quality can be improved by adjusting the polishing removal amount. A generic space-variant deconvolution algorithm is developed to precisely calculate the dwell time. Experiments are conducted using an industrial robotic polisher and the edge roll-off error is effectively suppressed. As a consequence the polishing accuracy and efficiency of the robotic polishing technology can be improved significantly.