A general transformation is proposed to design the compact waveguide coupler with homogeneous media. By dividing the coupling region into several triangle blocks and engaging the transformation, material inhomogeneity of the coupler can be eliminated, and thus the device only requires homogeneous and anisotropic media. Also, it is found that the electromagnetic field in the coupling region can be controlled artificially and the field in the two waveguides is little influenced. Thus, much freedom and flexibility is provided in the design of waveguide coupler. Besides, the general transformation can also be extended to design waveguide bender with homogeneous media. Furthermore, Full wave simulation based on finite element method is performed to verify the performance of the waveguide coupler.Highlights► In this paper, a general transformation is proposed to design the compact waveguide coupler with homogeneous media. ► As long as the coupling region is divided into several triangle blocks (two, three or four blocks) and general transformation is employed, material inhomogeneity of the device can be eliminated, and thus the device only requires homogeneous and anisotropic media. ► Numerical results indicate that the electromagnetic field in the coupling region can be controlled artificially and the field in the two waveguides is little influenced. ► As a consequence, much freedom and flexibility is provided in the design of waveguide coupler. ► Besides, our general transformation can also be extended to design waveguide bender with homogeneous media. ► Full wave simulation based on finite element method is performed to verify the performance of the waveguide coupler.

The doping-dependent photoconductive properties of individual GaAs nanowires have been studied by conductive atomic force microscopy. Linear responsivity against the bias voltage is observed for moderate n-doped GaAs wires with a Schottky contact under illumination, while that of the undoped ones exhibits a saturated response. The carrier lifetime of a single nanowire can be obtained by simulating the characteristic photoelectric behavior. Consistent with the photoluminescence results, the significant drop of minority hole lifetime, from several hundred to subpicoseconds induced by n-type doping, leads to the distinct photoconductive features. Moreover, by comparing with the photoelectric behavior of AlGaAs shelled nanowires, the equivalent recombination rate of carriers at the surface is assessed to be >1 × 1012 s–1 for 2 × 1017cm–3 n-doped bare nanowires, nearly 30 times higher than that of the doping-related bulk effects. This work suggests that intentional doping in nanowires could change the charge status of the surface states and impose significant impact on the electrical and photoelectrical performances of semiconductor nanostructures.Keywords: GaAs nanowire; intentional doping; minority carrier lifetime; photoconductive property; photoelectric device; surface states

The bare M5+13 and ligand-protected nanoparticles M25(SR)−18 and M13(PR)10Cl3+2 (M = Au, Ag, Cu) are investigated using the density functional theory. There are strong interactions between the metal core atoms and the ligands. It is found that the electronic structures agree well with the Jellium model for gold and copper nanoparticles. The superatoms's S and P orbitals are shown. However for silver ones, as the adding of the ligands, the S orbital of the nanoparticle disappears. The binding energy of these nanoparticles are also obtained by our calculation. The Au nanoparticles are most stable, the Cu ones take second place, and the Ag ones are the third stable. Our results could be essential for further understanding of the properties of ligand-protected isolated superatoms.

We investigate modes excitation with the input field of different positions in two-dimensional multimode photonic crystal waveguides. Odd modes can be selectively excited by the input field of odd symmetry. The input field with different positions can excite different modes due to the field intensity distribution of modes. When the input field locates at the position of the zero field, intensity of waveguide modes is zero and the modes are not excited. The finite-difference time-domain method is used to obtain the excited field distributions.Research highlights► We investigate modes excitation with the input field of different positions in two-dimensional multimode photonic crystal waveguides. ► Odd modes can be selectively excited by the input field of odd symmetry. ► The input field with different positions can excite different modes due to the field intensity distribution of modes.

A kind of transformation functions is proposed to realize the nonmagnetic invisibility cloak with minimized scattering on the basis of generalized transformation. By matching the impedance at the outer surface of the cloak, the transformations with two parameters are determined. The good performance of the cloak is indicated by the full wave simulation based on the finite element method. Furthermore, based on the calculation of total scattering cross section, it is shown that the scattering cross section is very sensitive to the different parameters even though the impedance at the exterior boundary matches perfectly with the free space. In addition, from the effective media theory, an alternating layered system composed of two isotropic materials is proposed to realize experimentally the cloak.

We have investigated the self-imaging phenomena based on the field distribution in two-dimensional multimode photonic crystal waveguides (PCWs) by the finite-difference time-domain (FDTD) method. The new self-imaging conditions are given for the mirrored images and the direct images. The self-imaging distributions in multimode PCWs can be explained by the new self-imaging conditions and the superposition of “sub-images”. The results of numerical simulation show a good agreement with the theoretical predictions.

First-principles calculations have been performed on the C-doped anatase TiO2. The doped TiO2 shows half-metallic properties and a 2.0 μB magnetic moment per cell. The magnetic coupling is closely related to the C–C distance. When the distance is shorter than a typical CC double bond (1.34Ǻ), the system is nonmagnetic and the dopants tend to form a cluster through a direct CC bonding interaction [Appl. Phys. Lett. 93 (2008) 132507]. When the distance is between 3.8 and 5.5Ǻ, there exists strong ferromagnetic or antiferromagnetic coupling between two C atoms. The ferromagnetic coupling is induced by the bent C–Ti–C unit. When the distance is further increased, the system becomes paramagnetic.

The optical properties of amorphous group III–V compound semiconductors were investigated through the first principles calculations. The imaginary parts (ε2ε2) of dielectric function for amorphous GaAs, InAs, and InSb are given, respectively. There is a single broad peak found in the ε2ε2 spectrum. By comparing with the available experimental data of a-GaAs, it is found that the maximum of the ε2ε2 spectrum is sensitive to the topological local structures of amorphous materials. By comparison of the ε2ε2 spectrum for amorphous sample to that of the crystal, the dependence of the E1 and E2 peaks of the crystal on the local structures of amorphous sample becomes evident. The calculated results are in agreement with the available experimental data. The corresponding results should be generalized to cover the amorphous group III–V semiconductors.

The structural and optical properties of amorphous semiconductor mercury cadmium telluride (a-MCT) are obtained by the first principles calculations. The total pair distribution functions and the density of states show that the a-MCT has the semiconductor characteristic. The calculated results of dielectric function show that E2 peak of the imaginary of dielectric function for the crystal mercury cadmium telluride abruptly disappears in the amorphous case due to the long-range disorders. And the imaginary of dielectric function of a-MCT shows a large broad peak, which is in agreement with the available results of other amorphous semiconductors. From the linear extrapolation of the curve ħωɛ2(ω)1/2 versus ħω, it can be obtained that the optical energy gap of amorphous semiconductor Hg0.5Cd0.5Te is 0.51±0.05 eV.

We present a systemic theoretical study of the electronic properties of the quantum dots inserted in quantum dot infrared photodetectors (QDIPs). The strain distribution of three different shaped quantum dots (QDs) with a same ratio of the base to the vertical aspect is calculated by using the short-range valence-force-field (VFF) approach. The calculated results show that the hydrostatic strain ɛHvaries little with change of the shape, while the biaxial strain ɛBchanges a lot for different shapes of QDs. The recursion method is used to calculate the energy levels of the bound states in QDs. Compared with the strain, the shape plays a key role in the difference of electronic bound energy levels. The numerical results show that the deference of bound energy levels of lenslike InAs QD matches well with the experimental results. Moreover, the pyramid-shaped QD has the greatest difference from the measured experimental data.

Piezomodulated reflectance (PzR) spectroscopy has been used to study the InAs/In0.15Ga0.85As quantum dots-in-a-well (DWELL) samples in the temperature range of 70–300 K. The optical transitions from ground state to excited states in InAs DWELL were observed. The heavy- and light-hole transitions from the hybrid quantum well (HQW) were identified by combining the one-dimensional effective mass approximation (EMA) with the selected modulation effect in PzR spectrum. A remarkable red shift of the ground state has been observed in the InAs DWELL with optimized growth of In0.15Ga0.85As quantum well layers. Cooling both DWELL samples produces an abnormal broadening of the QD transitions in PzR spectra.

The understanding of the microstructures of the arsenic tetramer (Ast), dimer (Asd), and singlet (Ass) of HgCdTe is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to pp-type behavior. The stable configurations were obtained from the first-principles calculations for the arsenic cluster defects [Asn (n=1n=1, 2, and 4)] in as-grown HgCdTe. According to the defect formation energies calculated under Te-rich conditions, the most probable configurations of Ast, Asd, and Ass have been established. For the optimized Ast and Asd the energy is favorable to combine in a nearest neighboring mercury vacancy (VHg), and the corresponding configurations can be used to explain the self-compensated nn-type characteristics in as-grown materials. Ast is likely to be more abundant than Asd in as-grown materials, but arsenic atoms are more strongly bounded in Ast than in Asd, thus more substantial activation energy is needed for Ast than that for Asd. The atomic relaxations as well as the structural stability of the arsenic defects have also been investigated.

The understanding of the configurations of the arsenic tetramer (Ast), dimer (Asd), and singlet (Ass) is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to p-type behavior. In this work, the possible configurations were optimized from density functional calculations for arsenic defects Asn (n = 1, 2, and 4) in as-grown HgTe. According to the dominant electronic contribution to the defect formation energies, which was calculated under Te-rich conditions, the most probable configurations for Ast, Asd, and Ass have been established. The above discussion applies to the neutral arsenic defects. A further study is necessary to consider the entropy contribution to the defect formation energy.

The bare M5+13 and ligand-protected nanoparticles M25(SR)−18 and M13(PR)10Cl3+2 (M = Au, Ag, Cu) are investigated using the density functional theory. There are strong interactions between the metal core atoms and the ligands. It is found that the electronic structures agree well with the Jellium model for gold and copper nanoparticles. The superatoms's S and P orbitals are shown. However for silver ones, as the adding of the ligands, the S orbital of the nanoparticle disappears. The binding energy of these nanoparticles are also obtained by our calculation. The Au nanoparticles are most stable, the Cu ones take second place, and the Ag ones are the third stable. Our results could be essential for further understanding of the properties of ligand-protected isolated superatoms.