We present the design and modeling of novel electro-optic modulators and switches that have large extinction ratios, such that these electro-optic modulators and switches operate at the optical communication wavelength range (around 1550 nm). Firstly, we describe the design of an electro-optic modulator based on a tunable slotted ring resonator, having two pairs of partially overlapping graphene layers above and below of the slotted ring (in some portion of the circumference). We demonstrate that the transmission of light through the through port can be modulated by the application of voltage across the graphene layers. Secondly, we discuss the design of electro-optic switches using phase change materials either in a micro disk resonator or in a photonic crystal slab waveguide. These devices are based on the shift in the resonant frequency of a micro disk resonator and on the shift in the photonic bandgap of the photonic crystal slab waveguide, respectively, when its refractive index changes upon the application of voltage across the phase change material. A three dimensional finite-difference time-domain modeling software (Lumerical FDTD) was used for optical modeling and a commercial device modeling software (Lumerical DEVICE) was for the electrical modeling. The proposed electro-optic modulators and electro-optic switches can be used in optoelectronics, as well in the telecom wavelength range.
In this paper, we present a new design for an electro-optic modulator ⎯ operating at the telecomm wavelength of 1550 nm and having a very high extinction ratio ⎯ based on photonic crystal (PhC) slab waveguide and phase change material Germanium Selenide (GeSe) embedded in core silicon layer. The device is based on the shifting of the photonic bandgap of the PhC slab waveguide when the refractive index of the GeSe layer changes on application of electric field. Since GeSe changes from its phase crystalline to amorphous on application of an electric field, its refractive index also changes when this phase transition occurs. As a result of a large refractive index contrast between the two phases, the change in the effective refractive index in the PhC slab waveguide is also very high. With two self-sustainable states, the hybrid modulator shows broadband switching capability and an On/Off extinction ratio > 37 dB around a wavelength of 1550 nm.
We present Indium-rich InGaN thin-film solar cells containing plasmonic and dielectric nanostructures such as Ag and ITO nanopillars. Finite-difference time-domain (FDTD) simulations were carried out for solar cells containing these nanostructures on the back side and on the front side of the solar cells, and an improvement in the performance of the solar cells was compared for the different geometries and sizes of these nanostructures. In order to develop highefficiency InGaN solar cells, the indium content in the InGaN active layer needs to be increased in order to cover the large solar spectral range. Recently, several reports have demonstrated the growth of single-crystalline Indium-rich InGaN alloys without phase separation by controlling the growth temperature and the pressure. Our FDTD simulation results demonstrate that the Ag nanostructures on the back side of the solar cell lead to an enhanced surface plasmonbased scattering mostly for longer wavelengths of light including band edge of active material, while the ITO nanostructures on the front side lead to enhanced scattering of a middle wavelength range from 450 nm to 700 nm. Hence, a combination of Ag and ITO nanostructures leads to a significant broadband absorption enhancement in the active-medium of the solar cells which in turn leads to a significant enhancement (~ 25 %) in the short circuit current density (J<sub>sc</sub>) of these solar cells.
We propose and design long-range hybrid plasmonic waveguides (HPW) consisting of a combination of plasmonic thin film and nano-scale structures of a high refractive index material (such as silicon), with a low refractive index material (such as silica) surrounding the nano-scale structures and the plasmonic thin film. The effective refractive index and the corresponding propagation length obtained for these plasmonic waveguides, obtained using a full-vector finite difference eigen mode (FDE) solver, demonstrates the viability of these hybrid plasmonic waveguides in applications that demands long propagation range with reasonable field confinement. These waveguides not only have high propagation lengths ⎯ even greater than 1 mm for certain geometrical parameters of the plasmonic waveguides ⎯ but can also have tight mode confinement (low effective mode area). Moreover, the proposed hybrid plasmonic waveguides can also be easily fabricated using the conventional nanolithography processes. Moreover, we study the effect of the variation of different waveguide parameters on the propagation length and effective mode area.