Design and simulation of type-I graphene/Si quantum dot superlattice for intermediate-band solar cell applications.

Recent experiments suggest graphene-based materials as candidates for use in future electronic and optoelectronic devices. In this study, we propose a new multilayer quantum dot (QD) superlattice (SL) structure with graphene as the core and silicon (Si) as the shell of QD. The Slater-Koster tight-binding method based on Bloch theory is exploited to investigate the band structure and energy states of the graphene/Si QD. Results reveal that the graphene/Si QD is a type-I QD and the ground state is 0.6 eV above the valance band. The results also suggest that the graphene/Si QD can be potentially used to create a sub-bandgap in all Si-based intermediate-band solar cells (IBSC). The energy level hybridization in a SL of graphene/Si QDs is investigated and it is observed that the mini-band formation is under the influence of inter-dot spacing among QDs. To evaluate the impact of the graphene/Si QD SL on the performance of Si-based solar cells, we design an IBSC based on the graphene/Si QD (QDIBSC) and calculate its short-circuit current density (Jsc) and carrier generation rate (G) using the 2D finite difference time domain (FDTD) method. In comparison with the standard Si-based solar cell which records Jsc = 16.9067 mA/cm2 and G = 1.48943 × 1028 m-3⋅s-1, the graphene/Si QD IBSC with 2 layers of QDs presents Jsc = 36.4193 mA/cm2 and G = 7.94192 × 1028 m-3⋅s-1, offering considerable improvement. Finally, the effects of the number of QD layers (L) and the height of QD (H) on the performance of the graphene/Si QD IBSC are discussed.

All-optical graphene-on-silicon slot waveguide modulator based on graphene's Kerr effect.

All-optical graphene-based optical modulators have recently attracted much attention because of their ultrafast and broadband response characteristics (bandwidth larger than 100 GHz) in comparison with the previous graphene-based optical modulators, which are electrically tuned via the graphene Fermi level. Silicon photonics has some benefits such as low cost and high compatibility with CMOS design and manufacturing technology. On the other hand, graphene has a unique large nonlinear Kerr coefficient, which we calculate using graphene's tight-binding model based on the semiconductor Bloch equations. Its real and imaginary parts are negative at the wavelength of 1.55 µm and EF=0.1eV. To simultaneously use the benefits mentioned above, we present an all-optical, CMOS-compatible, and graphene-on-silicon slot (GOSS) waveguide extinction and phase modulator that consists of two different geometries. The first one consists of a one-stage GOSS waveguide with a single layer of graphene. To increase the light-graphene interaction and consequently enhance the modulation efficiency (ME), another stage of the GOSS waveguide is placed over the first one. This two-stage configuration is called a graphene-on-silicon double-slot (GOSDS) waveguide. The ME, insertion loss (IL), and modulation depth (MD) for a 12.5 µm GOSDS waveguide modulator with a double layer of graphene can reach 0.241 dB/µm, 1.31 dB, and 77%, respectively, at optical pump intensities about 9MWcm-2. Our design has a smaller waveguide length (17.6 times) than the previous all-optical graphene-on-silicon ribbon waveguide extinction modulator and high MD (about 2 times) in comparison with a graphene-clad microfiber all-optical extinction modulator. Compared with an all-optical Mach-Zehnder interferometer phase modulator, our design has short graphene coated waveguide length (≈0.1 times) and low local optical intensities (≈0.043 times) needed for π phase shift. This study may promote the design and realization of high-performance, wideband, compact, and all-optical control on a single chip with a reasonable contrast level.

Polymer multimode waveguide bend based on a multilayered Eaton lens.

Reducing the bending radius of low-index-contrast waveguides is essential in reducing the size of the integrated optical components. A polymeric multimode waveguide bend is presented based on the Eaton lens. Ray-tracing calculations are utilized to truncate the Eaton lens in order to improve the performance of the bend. The truncation of the lens decreases the footprint of the bend as well. A designed waveguide bend with a radius of 18.4 µm is implemented by a concentric cylindrical multilayer structure. Average bend losses of 0.69 and 0.87 dB are achieved for the TM0 and TM1 modes, respectively, in the C-band of optical communication. The bend loss is lower than 1 dB in a bandwidth of 1520-1675 nm for both modes.

High bandwidth and responsivity mid-infrared graphene photodetector based on a modified metal-dielectric-graphene architecture.

In this paper, a novel graphene-based mid-infrared photoconductive photodetector is designed that is composed of nanophotodetectors in series and parallel forming an array. The whole structure utilizes a single "unpatterned" graphene layer in a modified metal-dielectric-graphene (MDG) architecture that is composed of a conventional MDG structure combined with additional nanoelectrodes for photocurrent guiding and absorption enhancement. Finite difference time domain and finite element methods are utilized to obtain optical and electrostatic characteristics of the photodetector. Specifically, responsivity, quantum efficiency, dark current, bandwidth, noise equivalent power (NEP), and specific detectivity (D*) are extracted by employing realistic graphene as well as graphene-metal characteristics. For an optimized device, maximum absorption efficiency is as much as 70% at a wavelength of λ=6.77 µm; however, the peak absorption wavelength can be effectively tuned between λ=6-7 µm. It is shown that increasing the drain-source voltage enhances the responsivity and bandwidth with a side effect of increasing the dark current. Responsivity of the 2×2 photodetector is RA=0.63 AW-1 for V DS =0.5 V with I Dark =350 µA, NEP=16.91 pW/√Hz, and D*=1.53×105 Jones.

Multimode waveguide crossing based on a square Maxwell's fisheye lens.

Mode-division multiplexing (MDM) is an emerging large-capacity data communication technology utilizing orthogonal guiding modes as independent data streams. One of the challenges of multimode waveguide routing in MDM systems is decreasing the mode leakage of waveguide crossings. In this article, a square Maxwell's fish-eye lens as a waveguide crossing medium based on quasiconformal transformation optics is designed and implemented on a silicon-on-insulator platform. Two approaches were taken to realize the designed lens: graded photonic crystal and varying the thickness of the silicon slab waveguide. Three-dimensional numerical simulations show that the designed multimode waveguide crossing has an ultrawide bandwidth from 1260 to 1675 nm with a compact footprint of only 3.77×3.77 µm2. For the first three transverse electric modes (TE0, TE1, and TE2), the designed waveguide crossing exhibits an average insertion loss of 0.24, 0.55, and 0.45 dB; a crosstalk of less than -72, -61, and -27 dB; and a maximum return loss of 54, 53, and 30 dB, respectively. The designed waveguide crossing supports low-distortion pulse transmission with a high fidelity factor of 0.9857. Furthermore, the proposed method can be expanded to design waveguide crossings with an even higher number of supporting modes by increasing the size of the lens.