Magnetic skyrmions are particle-like, topologically protected magnetization entities that are promising candidates for information carriers in racetrack-memory schemes. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Recently, we demonstrated experimentally that chiral skyrmions in Cu2OSeO3 can be effectively manipulated by a magnetic field gradient, leading to a collective rotation of the skyrmion lattice with well-defined dynamics in a radial field gradient. Here, we employ a skyrmion particle model to numerically study the effects of resultant shear forces on the structure of the skyrmion lattice. We demonstrate that anisotropic peak broadening in experimentally observed diffraction patterns can be attributed to extended linear regions in the magnetic field profile. We show that topological (5-7) defects emerge to protect the six-fold symmetry of the lattice under the application of local shear forces, further enhancing the stability of proposed magnetic field driven devices.

Optical reflectance (OR) and electro-reflectance (ER) spectroscopies were employed to study the resonant optical properties of GaAs/AlGaAs multiple quantum wells based Resonant Bragg Structures (RBS) at excited states. The RBS samples have 60 periods of GaAs/AlGaAs quantum well/barrier grown on semi-insulating GaAs substrates by molecular beam epitaxy with slightly different thicknesses of well/barrier. We observed enhanced OR and ER features when exciton energy coincides with the energy of the Bragg reflection peak, a double resonance condition. Bragg peak can significantly be tuned by changing the angle of incidence of the light. Exciton energies can be tuned by changing the temperature, external electric field and the thickness of the quantum wells. By tuning the Bragg peak for double resonance in the RBS samples of different thicknesses, we observed the electro-reflectance features related to the transitions of x(e2-hh2), x(e2-hh1), x(e2-lh1), x(e2-hh3) and x(e1-hh3) excitons along with the sharp features of x(e1-hh1) and x(e1-lh1) ground state exciton transitions from the ER experiments. The excitonic transitions x(e2-hh1), x(e2-lh1) and x(e2-hh3) which are prohibited at zero electric field, were also observed due to the increased overlap of the electron and hole wave functions caused by the electric field; built-in electric field or applied DC bias.

One-dimensional hybrid Distributed Bragg Reflector (DBR) is constructed using Tris (8-hydroxy) quinoline aluminum (Alq3) molecules and Titanium dioxide (TiO2) nanoparticles via spin coating process. Light emission from thin films of low molecular weight organic semiconductor of Alq3 is dominated by excitons. This material has been widely used as a superior emitter for organic light emitting diodes. Titanium dioxide (TiO2) is an inorganic semiconductor with a high band gap. Photoluminescence (PL) of thin films of Alq3 showed a broad PL peak at 530 nm. In DBR structures, PL quenching is observed but there is no shift in the PL peak of the Alq3. The PL quenching is tentatively attributed to energy transfer via sensitization to wide band gap TiO2 layers. A simple excitonic model is suggested to explain the observation. Fabrication process and optical properties of the structure are presented.

Metal–insulator–metal (MIM) resonant absorbers comprise a conducting ground plane, a dielectric of thickness t, and thin separated metal top-surface structures of dimension l. The fundamental resonance wavelength is predicted by an analytic standing-wave model based on t, l, and the dielectric refractive index spectrum. For the dielectrics SiO2, AlN, and TiO2, values for l of a few microns give fundamental resonances in the 8-12 μm long-wave infrared (LWIR) wavelength region. Agreement with theory is better for t/l exceeding 0.1. Harmonics at shorter wavelengths were already known, but we show that there are additional resonances in the far-infrared 20 - 50 μm wavelength range in MIM structures designed to have LWIR fundamental resonances. These new resonances are consistent with the model if far-IR dispersion features in the index spectrum are considered. LWIR fundamental absorptions are experimentally shown to be optimized for a ratio t/l of 0.1 to 0.3 for SiO2- and AlN-based MIM absorbers, respectively, with TiO2-based MIM optimized at an intermediate ratio.

We report on structural, magnetic, optical, and dielectric properties of rare earth (Er and Yb)-doped ZnO nanoparticles with Na-co-doping for charge compensation by sol-gel process. The effect of doping concentration on the structural and dielectric properties of ZnO has been studied under magnetic field and UV excitation. The magnetodielectric and photodielectric effects at room temperature of doped ZnO are discussed in the light of magnetic and optical properties of Er3+ and Yb3+ ions.

We propose a method for precision control of the temporal pulse shape in 808 nm emission from Er-doped fluoroindate glasses. Previously, authors have reported the model based controller design, in which the controller varies and controls the pump rate in real time through the pump power. In model-based design, the performance of the resultant controller depends on the accuracy of the mathematical model used to represent the device in the design process. In this paper a more robust control scheme using model-free approach is presented. Specifically, the controller design is independent of the mathematical model and hence any modeling error has no effect on the device performance. This robustness against modeling error is critical for control purposes in optical materials where various up-conversion parameters are unknown or hard to determine with certainty.

Sol-gel blends are created using a combination of a high refractive index (n∼2.4) TiO2 based sol-gel and a low refractive index (n∼1.5) SiO2 based sol-gel. The blends are prepared with different ratios of sol-gels and films are created using the spin coating method on silicon and ITO-on-glass substrates. The film thickness, refractive index, and dielectric constants of the resulting films are measured using profilometry, prism coupling, and LCR measurements, respectively. Results show that including more SiO2 based sol-gel in the initial mixture creates thicker films ranging from 1-7 μm, but results in lower refractive index and lower dielectric constants. This is consistent with expectations due to SiO2 having a lower refractive index and dielectric constant than titania over a range of wavelengths andfrequencies. The ability to fine tune the properties is explored.

Sparsely distributed and self-organized gold-nanocones are fabricated by broad argon ion beam sputtering on the gold surface with grazing incident angle. The rotation of the sample with respect to the vertical axis has found to influence the morphology of the obtained nanostructures. Ion beam irradiation of the sample leads to formation of nanoripples when the sample is held stationary, otherwise nanocones are formed if the sample is rotated during irradiation. A hybrid gold-nanocone/graphene/gold-nanohole based surface-enhanced Raman scattering (SERS) sensor is proposed and shown to exhibit an enhancement factor of 109 via finite-difference time-domain (FDTD) simulations.

Amorphous Si (a-Si) is used for fabrication of commercial low-cost flat panel image detectors for radiographic applications such as computed tomography (CT) imaging. a-Si photodiodes are known to exhibit a rapid decrease in quantum efficiency near 750nm. While crystalline Si does not suffer from such an early decline, the large-area and low-cost constraints of medical imagers make it challenging and costly to use crystalline Si for such devices. In this work, we report on the development of a sensitive layer for upconversion from 785 nm to green region of the spectrum, which nearly matches the peak quantum efficiency of a-Si detectors. Various host materials have been extensively studied in literature with rare earth ions such as Er3+(emission: green+red), Tm3+(emission: blue), Ho3+(emission: red+green) along with Yb3+ as a sensitizer for upconversion to the visible regime at high incident optical power (∼100 mW) for colloidal solutions. We carried out a thermal decomposition synthesis of NaYF4:Yb(18%),Er(2%),Gd(15%) at moderate temperature (∼320°C), resulting in a nearly pure hexagonal phase material. This is confirmed by powder X-ray diffraction (PXRD) of the unannealed sample with a lattice constant (∼5.17 Å). High-resolution transmission electron microscopy (HRTEM) measurements reveal the formation of nearly spherical nanoparticles. The observed plane ([100]) inferred from lattice fringes in TEM data with a visibly estimated interplanar distance (4.4±1.6 Å) is in reasonable agreement with standard data (∼5.17 Å) for comparable NaYF4-based materials. Excitation (785 nm) of the deposited thin films of Gd-doped unannealed material at relatively low incident power (∼0.4 mW) exhibits a PL response in green (539 nm) and red (665 nm) region of the spectrum. Gd-based upconversion material based thin films are thus a feasible photonic material for potential effective extension of high quantum efficiency range in a-Si for flat panel image detectors.

Transdimensional photonics has emerged as a new field of science and engineering that explores the optical properties of materials and nanostructures in the translational regime between two and three dimensions. In the present work, we study an example of such transdimensional lattice consisting of nanoparticle array, and we aim at a direct comparison of lattice resonances excited in the periodic lattices of either plasmonic (gold) or silicon nanoparticles of the same size and interparticle spacing. We numerically analyze extinction cross-sections and reflection from the array, and we include electric and magnetic dipoles and electric quadrupoles into consideration. Lattice resonances are excited at the wavelength close to Rayleigh anomaly which is defined by the array periodicity, and different multipoles respond to one or another period of rectangular array depending on incident light polarization. We show that lattice resonances originating from dipole moments are extended to the larger spectral range than electric-quadrupole lattice resonances. Overlap of resonances causes a decrease in reflection (generalized Kerker effect) and, in the case of electric quadrupole and magnetic dipole moments, the coupling of the multipoles is enabled by the lattice.

The effect of surface-enhanced Raman spectroscopy (SERS) was investigated in N719 dye thin films deposited on silicon wafer with a thin film of silver nanoparticles (Ag-NPs) fabricated by laser ablation in an aqueous solution, using a NdYAG laser (λ = 1064nm). Optical absorption spectroscopy of the Ag-NPs colloidal solution shows an absorption peak at λ = 400nm, associated with a localized surface plasmon resonance in the Ag-NPs. Scanning electron microscopy (SEM) reveals that these NPs have an approximately spherical shape, with their diameter being tunable by laser power intensity. Raman spectroscopy measurements were performed using low laser power to avoid damage to the N719 dye films. Thus, a small Raman signal is obtained. The Raman intensity was greatly increased when the N719 film was deposited on a substrate with a thin film of Ag-NPs due to the SERS effect. The process was also used in Rhodamine-B to clearly demonstrate the SERS effect obtained by the use of these NPs produced by laser ablation.

Lead sulphide thin films have been successfully deposited by spin coating lead hexadecyl and octadecyl xanthate onto glass substrates and annealed at moderate temperatures. The thin films were characterized using powder x-ray diffractometer (p-XRD) and were found to be face centred cubic (FCC) with the (200) being the most preferred orientation. The optical band gaps obtained were 0.93 and 1.00 eV respectively for PbS from complexes (1) and (2) annealed at 350 °C. They were all blue shifted from the bulk value of 0.41 eV.