Design and simulation of strip loaded and rib waveguide with integration of 2D material

ObjectiveTo design Graphene-Silicon based rib waveguide and reduce the losses in the strip in order to meet the requirement for ultra-fast & ultrahigh optical bandwidth communication and computing in integrated optical devices. Method –Propagation losses and effective refractive index are the two key parameters. In order to meet the objective, the effects of Graphene for manufacturing passive devices/components in the field of Integrated Photonic like integrated optical waveguide have been analysed by measuring the changes in propagation losses and effective refractive index of the silicon photonics devices for operating at different wavelengths. FindingsWe have presented the design and simulation of SOI (Silicon-on-Insulator) platforms with 2D layer materials (graphene) which has been used along with their mode of propagation, effective refractive index (ne f f ), propagation losses (dB/cm) and varying wavelength range for optimum performance. In addition to this, we have also calculated the boundary limit for both the speed and bandwidth. We also reported the development of Silicon rib waveguide, Graphene-Silicon based rib waveguide and Ge on SOI with graphene later at the top of strip waveguide.Minimum loss of strip waveguide is 2.9 dB/cm which has been obtained for Mid-IR wavelength generally used for high power midIR sensing.


Introduction
Photonics bears a fundamental concept with various technologies for the transmitting and signal processing of light. In fact, it is an engineering of processing the light, which is suitable for system implementations (1)(2)(3) . The importance of computational photonics, nanostructure & physics, simulations of materials, and optoelectronics devices have arisen from the advancement of computational power (4) . An increasing demand of bandwidth in computing and signal processing, the vested limitations in metallic interconnections are gravely blusterous the future of conventional IC industry. Therefore, the photonics can provide a low-cost approach confirming the spout of high https://www.indjst.org/ data rate transmission by replacing the original electronic integrated circuit with the photonics integrated circuits (5) (6) .
Integrated photonic component and devices are used for processing of light in optical structure called optical waveguide. Optical waveguide is the basic and very important element in the field of Integrated Photonics Technology. The waveguides are indispensable for communication and also for computing application, as these are immune to electromagnetic interference and induced cross talk as well as differential counters (7) (8) . The channel waveguide is a guiding factor in the form of different structure with different dimensions. In the channel waveguide or 2D waveguide, the high refractive index is surrounded by other media in the core region (where radiation is concentrated) (9) .
The channel waveguide allows the light confinement in both direction and propagates in only one direction. Unlike in planar waveguide, light confinement is only in one direction i.e. perpendicular to the interfaces. In this way, the radiation travelling in the 2D waveguide can propagate without suffering diffraction, which would otherwise lead to a power loss (10) . Therefore, to perform functions like Modulation, Switching, amplification, etc., the 2D or channel waveguide is the right choice for manufacturing integrated optical devices. The 2D waveguide is made from rare earth doped dielectric materials which are used for high power waveguide laser and amplification. Pump light can be injected along with amplified beam, from the side or the top. Even at the conditions of strong heating, the waveguide can help stabilize single mode laser beams with higher beam quality (11) .
The analysis of two types of channel waveguide including rib and strip waveguide with integration of 2D layer materials has been presented in the current research article. The rib waveguide guiding layer basically consists of dielectric slab with a strip superimposed on it.The strip waveguide is the part of the waveguide core (12)(13)(14) . The strip waveguide is made of three dielectric layers such as a substrate, a planar and a ridge. The planar waveguide (without strip) already offers optical light confinement in the vertical direction (y-axis) and apart from this the strip can provide localized optical light confinement under the strip, because of the increase local effective refractive index (n e f f ) (15)(16)(17) .
Graphene, a crystalline of allotropy of carbon with two dimensions, can exhibit a large diversity of physical behaviours ranging from wide band insulator to narrow gap semiconductor to semi-metal or metal. It offers aggravating opportunities for multifarious photonics and optoelectronics function enabling new conceptual photonics devices based on conventional bulk materials (18) (19) .
Here, we have analysed the effects of Graphene on Integrated photonics devices to invent the passive devices/components like integrated optical waveguide by analysing & measuring the change in propagation loss and effective refractive index of the silicon photonics devices at the different wavelength range (20) .
The geometrical analysis of integrated optical waveguide, the classification of channel waveguide on the basis of their geometries namely as rib and strip loaded waveguide are illustrated. Analysis of SOI (Silicon-on-Insulator) platforms with 2D layer materials (graphene) used along with their mode of propagation, effective refractive index (n e f f ), propagation losses (dB/cm) and wavelength range is presented. In our design simulation of Ge -SOI with graphene layer at the top of strip waveguide, we have obtained a minimum loss of ∼ 2.9 dB/cm for Mid-IR wavelength which is used for high power mid-IR sensing and is much better in comparison with the propagation loss of 3.5 dB/cm reportedin graphene-silicon waveguide at the Mid-IR wavelength (21) .

Design of the silicon photonic rib waveguide
The schematic structure of the silicon photonic Rib waveguide is shown in Figure 1 (a).
Guiding layer of the rib waveguide is basically consists of dielectric slab with strip superimposed on it.Thereby exhibits a similar structure as of the strip waveguide as part of the wave guiding core. Here the optical waveguide has been designed with different dimensions such as height, width, and thickness of the core and cladding. This photonic rib waveguide has been designed in 3D rectangular structure with a width of 700 nm and height 220 nm (ridge height of 70 nm). Figure 1(b) depicts the schematic design of silicon Rib waveguide. It consists of silicon (Si) based core & cladding on a silica (SiO 2 ) based substrate. The simulation results will be obtained by applying the selected parameters as given in Table 1 below. The perspective view of Silicon rib waveguide is presented in Figure 1(C). https://www.indjst.org/

Simulation of rib waveguide with width variations
Simulations have been performed in the state of Finite Difference Time Domain (FDTD) Method (22) , based on Lumerical Device Suit using MODE waveguide simulator. Simulated results obtained are shown in Figure 2 (a). The varation in effective refractive index (n e f f ) of the photonic Rib waveguide with the width ranging from 200 nm to 700 nm. Each mode propagates through the waveguide with a phase velocity of c/n e f f , where c is the speed of light in vaccum and n e f f is the effective refractive index felt by that mode. The Effective Refractive Index depends upon the waveguide cross-section and waveguide materials (23) . Figure 2(b) exhibits the guided mode with 700 nm width and 220 nm height of silicon photonic rib waveguide at a wavelength of 1.55 µm. Figure 2(c)shows the mode loss for silicon rib photonic waveguide with a cross-section of 200 x 220 nm at the wavelength of 1.55µm.

Design of the Graphene Silicon based rib waveguide
The schematic structure of the Graphene-Silicon based photonic rib waveguide is shown in Figure 5 (a). The optical Graphene-Silicon based Rib waveguide has been designed in 3D and 2D rectangular structure with a width of 550 nm, height 220 nm (having ridge height of 70 nm) and 0.03 nm thickness of graphene layer. The schematic design of silicon Rib waveguide is presented in Figure 5  The design has been made with dimensions of Silicon Photonic rib waveguide having width -550 nm, height -220 nm (with ridge height (r) =70 nm) and 0.3 nm thickness of the graphene layer. A silicon photonic rib waveguide superimpose with graphene layer is presented for obtaining the afore improvement in the execution of high sensing for sensor. The performance will be analysed using 3D FDTD method maxwel solver for desiging, eplication & optimization of integrated photonics device's process and material implicating wavelength scale framework.

Simulation results of Graphene -Silicon based rib waveguide with Chemical Potential variations
The Effective Mode Index (EMI) variations for TE mode under chemical potentials has been presented in Figure 6 (a). Propagation variations under different chemical potentials and the graphene's chemical potential varies from 0.1 to 0.6 eV as shown in Figure 6(b). Effective mode area of graphene-silicon rib waveguide with a cross-section of 550 x 220 nm at the wavelength of 1.55 µm has been presented in Figure 6(c). The best obtained values of effective refractive index is 2.3 at a propagation loss of 1.45 dB/cm operating at the wavelength of 1550 nm. All these simulated results are based on the incident light having wavelengh, λ = 1550nm and T = 300 K.

Design of Graphene-on-Germanium SOI strip waveguide
The schematic structure of the Graphene-on-Germanium-SOI based photonic strip waveguide is shown in Figure 7 (a) below.
This strip waveguide consists of 3 dielectric layers namely substrate, planar and ridge. Whereas, the planar (without strip) already offers optical light confinement in the vertical direction (y-axis). Apart from this, it also provides localized optical light confinement due to the increased local n e f f . The optical Graphene-on-Germanium-SOI strip waveguide has been designed in 3D and 2D rectangular structure with a width of 500 nm, height of 220 nm (with ridge height of 75 nm) and thickness of Graphene layer as 0.03 nm. Figure 7(b) shows the schematic design of Graphene-on-Germanium-SOI based photonic strip waveguide. It contains silicon (Si) for core and cladding, silica ( SiO 2 ) for Substate, germanium (Ge) for strip and then applies a Graphene atomic layer, which is placed on the top of the Strip waveguide in order to cover the fundamental mode area of the Germanium strip. Parameters selected for simulation are given in Tables 5 and 6.  The dimensions of Germanium-on-Silicon based strip waveguide with a 2D structure layer having a width of 500 nm, height of 220 nm and thickness of graphene as 0.3 nm. We have applied graphene sheet as an atomic layer on the top of the waveguide to cover the fundamental mode area of Gemanium Strip. The schematic design of Graphene-on-Germanium-SOI based photonic Strip waveguide is shown in Figure 7(c). It contains silicon (Si) for core and cladding, Silica ( SiO 2 ) for substate, Germanium (Ge) for strip and then applies a Graphene atomic layer which is placed on the top of the strip waveguide. It shows the variation in effective refractive index (n e f f ) of the graphene-on-germanium-on-SOI strip waveguide operating at a wavelength from 1550 nm to 2050 nm with an interval of 50 nm has been presented in Figure 8 (a). Propagation variations and losses at different wavelength from 1550 nm to 2050 are shown in Figure 8(b). The effective mode area of graphene-germaniumhttps://www.indjst.org/ on-silicon strip waveguide with cross-section 500 x 220 nm at the wavelength 2.05 µm has been presented in Figure 8(c). The parameters chosen for simulations are shown in Table 7. The couple of plots i.e. 2D plot and 3D plot have been obtained. 3D Far field plot shows the far field intensity pattern with a typical donut type shape as shown in Figures. Both the plots are obtained at the same frequency.

Conclusion
Integrated optical devices are designed to meet and foresee the futuristic requirements for ultra-fast & ultra-high optical bandwidth communication and computing. Graphene has been extensively used for designing various types of photonics and optoelectronic devices, and has been found to be operating at an enormously broad spectral enlarging from the ultraviolet, visible & Near-IR, Mid-IR, Far-IR range. It also covers the terahertz and microwave regions. The classification of channel waveguide on the basis of their geometries namely rib waveguide and strip waveguide have been illustrated in the current research. Analysis of SOI (Silicon-on-Insulator) platforms with 2D layer materials (graphene) used along with their mode of propagation, effective refractive index (n e f f ), propagation losses (dB/cm) and wavelength variation has been presented. In addition to this we have also calculated the optimum performance limit both for speed and bandwidth. A minimum loss of strip waveguide achieved is ∼ 2.9 dB/cm in our Ge-on-SOI with graphene layer at the top of the core for Mid-IR wavelength. Thereby we have obtained an improvement of approx. 22% from the earlier reported loss on 3.5 dB/cm.