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Abstract: A novel two-dimensional nanoscale structure for sensing which consists of a metal–insulator–metal (MIM) waveguide with a defect, is designed and numerically simulated by using the finite element method (FEM). Both of the refractive index sensing characteristics and the temperature sensing characteristics of the structure are analyzed systematically by investigating the transmission spectrum. The numerically simulated results show that the dip positions of the transmission spectrum have linear relationships with the refractive index of the material under sensing and the ambient temperature, respectively. Based on the relationships, the refractive index of the material under sensing or the ambient temperature can be obtained from the dip wavelength shift detection. When the width and the height of the defect are set to be 50nm, 300nm, respectively, the refractive index sensitivity can be obtained as high as 1736nmRIU−1 and the temperature sensitivity is about 0.51nm/°C with a FOM of 9.79. This work has a certain significance for designing nanoscale refractive index sensors and temperature sensors. [Copyright &y& Elsevier]

Waveguide-integrated reconfigurable nanophotonic devices are of importance for on-chip all-optical connection and flexible photonic circuits. In this paper, we propose inversely designed reconfigurable TE0-to-TE1 and TE0-to-TE2 mode converters based on digital metamaterials consisting of nanoscale silicon and non-loss phase change material Sb 2 Se 3 pixels. The design strategy combines finite element method, density method and method of moving asymptotes. Only 73 and 56 optimization iterations are taken to obtain the optimal digital metamaterials for targeted mode conversions. The footprints of ultra-compact functional regions are 0.92 and 1.04 square center wavelengths, respectively. TE0-to-TE1 and TE0-to-TE2 mode conversions with high transmission efficiency and output mode purity are realized with crystalline Sb 2 Se 3 , while TE0 mode directly pass through with amorphous Sb 2 Se 3. Meanwhile, by cascading these two mode converters, TE1-to-TE2 mode conversion is achieved as well. Moreover, the robustness of these designs is demonstrated by introducing random wrong pixels. • Reconfigurable mode order converters are inversely designed with pixelated Sb 2 Se 3 phase change material. • Highly efficient inverse design scheme combines finite element method, density model and method of moving asymptotes. • Footprints of functional regions are as compact as 0.92 and 1.04 square center wavelengths. • Performances of the reconfigurable mode converters are comparable and satisfactory. [ABSTRACT FROM AUTHOR]

Abstract: Nano-torus particles are promising in nano-photonics applications. In this paper, we investigate the plasmonics resonant spectrum of torus and the corresponding horizontal and vertical dimers with the finite element method (FEM) based on strict electromagnetic theory. Compared with sphere, single torus and horizontal dimer under perpendicular polarization are characterized with narrower line width and wider tunability throughout visible and infrared frequencies. These properties make single torus and horizontal dimer suitable for high sensitivity sensing. Furthermore, Purcell factor and local field enhancement of horizontal dimer which are important to high efficient single photon devices are one order of magnitude higher than those of single torus respectively. However, vertical dimer under perpendicular polarization has no above advanced characteristics. [Copyright &y& Elsevier]

Abstract: We calculate the minimum Gibbs free energy of the InAs/InP quantum dot multilayer by combining the method of moving asymptotes and the finite element method. Based on the principle of the least energy, the transition between vertically aligned and anti-aligned quantum dot multilayers is studied. We investigate the influence of quantum dot base size and density on critical spacer thickness for the transition. The study results indicate that the critical thickness increases with the decrease in the density of quantum dots, while the base size of the quantum dot is linear to the critical thickness when the density is given. [Copyright &y& Elsevier]

The waveguide-integrated reconfigurable power splitting is of great importance for on-chip all-optical connection. Based on the combination of silicon and Sb-based phase change materials (PCMs), a series of ultra-compact reconfigurable power splitters is topologically designed. The footprints of the functional regions are kept less than 1.0λ2. The power splitting ratio between the output channels can be reconfigured through the amorphous and crystalline switching of Sb-based PCMs. We demonstrate Sb 2 Se 3 -based and Sb 2 S 3 -based digital metamaterials for the reconfiguration between power splitting ratio of 5:5 and power splitting ratios of 5:5, 6:4, 7:3, 8:2 and 9:1. The three-dimensional FDTD simulations show that the targeted power splitting ratio can be achieved with acceptable precision. Moreover, the robustness of design is validated by introducing pixel errors. • Reconfigurable power splitter is topologically designed with pixelated Sb-based phase change materials. • The reconfiguration between splitting ratio 5:5 and splitting ratios 5:5, 6:4, 7:3, 8:2 and 9:1 are achieved. • The topology design scheme combines finite element method, density model and MMA. • The footprint of the functional region is less than λ2. [ABSTRACT FROM AUTHOR]

In this paper, we present a general design scheme for waveguide-integrated demultiplexing devices based on intelligent topology search method combining density model and gradient-based method of moving asymptotes. 2D finite element method is adopted to simulate and evaluate the performance of demultiplexers with quasi-3D configuration by setting equivalent refractive index of silicon. The obtained pixelated digital metamaterials individually fulfill functionalities including 1550nm/1310nm and 1500nm/1600nm wavelength demultiplexing, TE0/TE1 mode demultiplexing at 1550 nm, and TE0/TM0 polarization demultiplexing at 1550 nm. The footprint of the functional region is as small as 1.55 μm × 1.55 μm. To validate our designs with SOI configuration, 3D models are simulated by finite difference time domain. Comparable and satisfactory performances of the target functionalities are demonstrated. For all four demultiplexers, the worst transmission efficiency is 44.5% of TE0 in polarization demultiplexing, the worst contrast ratio is 14.1 dB of TE1 in mode demultiplexing and the worst crosstalk is −15.1 dB of TE0 in mode demultiplexing. Within an ultra-compact footprint, this design scheme can efficiently enable the device to separate or route photons based on specific dimension, such as wavelength, mode and polarization. • Digital metamaterials are designed for wavelength, mode and polarization demultiplexing. • The inverse design scheme combines DM and MMA method, which needs only 200 iterations to search for a design. • The functional region of the demultiplexer occupies only λ2. • The performances of the four ultra-compact demultiplexers are comparable and satisfactory. [ABSTRACT FROM AUTHOR]

Abstract: A finite element model is proposed to quantitatively predict the critical condition for a nitride quantum dot to bend threading dislocation to interfacial misfit dislocation. The energy balance approach adopted is based on the elastic residual strain energy and non-elastic dislocation core energy. We find that the critical misfit, beyond which the inclination of threading dislocation is energetically favorable, depends strongly on the shape, size and aspect ratio. The results provide guidelines for the design of quantum dot dislocation filter. [Copyright &y& Elsevier]

Abstract: We present a finite element method to simulate the strain field in InSb/GaSb quantum dots in the framework of anisotropic elasticity, before and after the onset of plastic relaxation. Taking the interaction between dislocation and free surface into consideration, the model directly calculates the residual strain in the dislocated system. Based on the energy balance between the two isolated states, the equilibrium critical size is determined. In the criterion, only the non-elastic dislocation core energy should be considered separately. The results are in satisfactory agreement with experimental data. [Copyright &y& Elsevier]

• Equilibrium structures of Janus MoSSe quantum dots are theoretically investigated. • Two distinct curling types are observed in MD simulations. • The curling behavior is mainly governed by the size of QD. • The predominant driving force is the relaxation of intrinsic strain induced by atomic asymmetry in Janus structure. Motivated by the fascinating properties of both two-dimensional transition metal dichalcogenide quantum dots (TMD QDs) and Janus TMD monolayers, we theoretically explore the equilibrium structures of free-standing Janus MoSSe QDs in which atomic asymmetry of chalcogen is introduced. Two distinct types of spontaneous curling are observed by molecular dynamics simulations, and the curling behavior depends on the size of QD. The bowl-like (tube-like) curling occurs in relatively small (large) MoSSe QDs with different shapes (hexagon and triangle) and edge types (zigzag and armchair). The transition between these two curling types occurs at the sizes of around 10 nm and 13 nm for hexagonal and triangular shapes, respectively. By applying equivalent misfit strains into two adjacent sublayers, finite element analysis reproduces similar curling behavior. This confirms the relaxation of intrinsic strain in Janus structure acting as the predominant driving force of spontaneous curling. In addition, the curvatures of Janus TMD QDs increase from MoSSe to MoSeTe to MoSTe, indicating the positive correlation between the curling and misfit. [ABSTRACT FROM AUTHOR]