The following specification particularly describes the invention and manner in which it is to be performed
DESCRIPTION
FIELD OF INVENTION
[0001] Integrated Optics, MEMS, MOEMS, Materials in optical sensing applications, Optical Sensor, Photonics
RELATED ART
[0002] The major challenge in the development of an optical sensor at visible wavelengths is finding an active material that enables a large modulation depth with negligible loss. One such promising material is graphene, a Single layer of carbon atoms in a closely packed honeycomb two-dimensional lattice structure, shown to have tremendous Potential in optical sensing applications Its conductivity can be electrostatically tuned over the entire wavelength ränge by the application of a gate potential with its optical and electrical properties being closely related, graphene is found to have a wide ränge of dynamic conductivity in the visible spectrum. In recent times, graphene has been explored for use in bio-sensing applications including glucose monitoring, photonic molecular sensing, and biochemical sensing applications including label-free optical sensing. The design of integrated optical waveguides with gap between them has shown great promise for sensing applications, including refractive-index- and absorbance-based sensing.
SUMMARY
[0003] In this Work, a systematic approach towards the design and analysis of a graphene-based SOI optical waveguide sensor is presented by incorporating graphene as a cover layer on top of a pure SOI waveguide, in the green wavelength region of the visible spectrum. Single mode Operation is achieved at a wavelength of 540 nm for a Channel height of 200 nm and a length of 2 um, with an effective refractive index of 3.773. In comparison to a pure SOI waveguide, the effective refractive index of the graphene-based SOI waveguide is increased by 24.94%, group velocity is decreased by 16.95%, and sensitivity is increased by 12.635% with a reduction of 18.57% in the optical power loss due to the absorption for a total propagation length of 5.5um and a wavelength of 540 nm. Thus, graphene was demonstrated as a suitable material for use as a cover layer over a Standard SOI structure, to be subsequently used for sensing applications in the visible wavelength ränge. Single mode graphene-based SOI optical sensors, if fabricated, can be effectively used for integrated Lab-on-a-Chip applications in the visible wavelength ränge.
[0004] Several aspects are described below, with reference to diagrams. It should be
understood that numerous specific details, relationships, and methods are set forth to
provide a füll understanding of the present disclosure. One who skilled in the relevant_ar.t,
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present disclosure.
BRIEF DESCRIPTION OF DRAWINGS
[0005] FIG.l Fig.l Schematic of an integrated optical sensor: (a) SOI waveguide. (b) Graphene-based SOI
[0006] FIG.2 Fig. 2 Waveguide geometry: (a) SOI waveguide.(b) Graphene-based SOI waveguide
[0007] FIG.3 Fig. 3 Mode Confinement Plots: (a) TE0 mode confinement in a SOI waveguide.(b)TM0 mode confinement in a SOI waveguide. (c)TE0 mode confinement in the graphene-based SOI waveguide. (d)TM0 mode confinement in the graphene-based SOI waveguide.
[0008] FIG.4 Fig. 4 TM0mode distribution in (a) Pure SOI (b) Pure SOI (3D View) (c) Graphene-based SOI (d) Graphene-based SOI (3D View
[0009] FIG.5 Fig. 5 (a) Optical power loss as a function of wavelength in a SOI waveguide (b) Optical power loss as a function of wavelength in a graphene-based SOI waveguide
[0010] FIG.6 Fig. 6 (a) Effective index as a function of wavelength in a SOI waveguide (b) Effective index as a function of wavelength in a graphene-based SOI waveguide.
[0011] FIG.7 Fig. 7 Optical field propagation: (a) SOI waveguide (b) Graphene-based SOI waveguide
[0012] FIG.8 Fig.8 Sensitivity as a function of Gap distance for Graphene based SOI and Pure SOI optical waveguides.
DETAILED DESCRIPTION
[0013] In this study, the design and analysis of Silicon on insulator (SOI)-based integrated optical sensing System, which uses graphene as a cover layer over the SOI waveguide, has been investigated. The optical sensor is designed for Single mode Operation in the green wavelength region of the visible spectrum. The effect of the use of graphene on the total losses, effective refractive index, mode spot size and optical power coupling has been analyzed and compared to that of a Standard SOI structure over a ränge of visible wavelengths for sensing applications. The analysis is performed using an Eigen-mode solver. A schematic of the proposed integrated optical sensor structure is shown in Fig. 1. Fig. l(a) shows a pure SOI waveguide sensor and Fig. l(b) shows the SOI waveguide sensor using graphene as a cover layer over the SOI waveguide sensor. Figs. 2(a) and (b) show the geometrical details of a pure and a graphene-based SOI waveguide (Graphene of 1 nm thickness as a cover layer over a Standard SOI structure), respectively^_Ihe_use-oLggafihene——-
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and Silicon in a Standard SOI waveguide. Modal analysis of the waveguides provides information about the propagation of light through the waveguide by a set of guided

electromagnetic waves. The proposed integrated optical sensor is simulated and analyzed for Single mode Operation at the green wavelength of 540 nm. The material dimensions used in the waveguide design and Simulation are described in Table 1.
[0016] In this work, comparative analysis between pure SOI, and graphene based SOI waveguide sensor for optical sensing applications has been discussed. Following parameters have been analyzed namely, effective refractive index, modal properties, absorption losses, group velocity. Finally the effect of gap distance on Sensitivity of the sensor has been presented.Figs. 3(a) and (b) present the mode confinement in a pure SOI waveguide for TEO and TMO mode. It is evident that the optical confinement of light is pronounced at the core of the waveguide (red color) when compared to the optical power leakage into the cladding (yellowish green color). Figs. 3(c) and (d) show mode confinement in a graphene-based SOI waveguide for TEO and TMO modes. In this case, optical confinement at the core is weaker than it is for the pure SOI, while the power leakage into the cladding is higher due to the mode spreading into the cladding. Fig. 3 displays the field distribution of TMO mode for both the pure and graphene-based SOI waveguides. Figs. 4(a) and (b) show the 2D and 3D plots of field propagation in a pure SOI waveguide, respectively. Figs. 4(c) and (d) present the 2D and 3D plots of field propagation in the graphene-based SOI waveguide, respectively. A significant pulse broadening of the evanescent wave into the cladding region in the graphene-based SOI waveguide, as opposed to that in the pure SOI waveguide (as shown in green color plot in Figs. 4(a) and (b) can be observed. The presence of a graphene layer on top of the SOI waveguide reduces the high refractive index contrast between the air and the Silicon layer, resulting in a higher mode spread in the cladding region. A higher mode spread indicates an increase in the Mode FieldrDjame^r^^gg)^^e^TO;paqa1äB-aim^e^^^
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purposes. Figs.5 (a) and (b) depict the optical power loss as a function of wavelength, in a pure and graphene-based SOI structure, respectively. The wavelengths chosen occur in the

green region (from 530 nm to 540 nm) of the visible spectrum. It is apparent that a reduction in the optical loss occurs in a graphene-based SOI sensor over the chosen wavelength ränge, indicating that this sensor will operate optimally in the green region of the visible spectrum. Figs.6 (a) and (b) exhibit the effective index as a function of wavelength, for pure and graphene-based SOI waveguides, respectively. At a wavelength of 540 nm, the effective index of the SOI waveguide is found to be 3.02 and that of the graphene-based SOI waveguide is 3.78. This indicates that the use of graphene as a cover layer increases the net refractive index of the whole structure. Thus, the use of graphene as a cover layer above the SOI structure not only increases the net effective refractive index but also reduces the group velocity of the propagating mode significantly.
[0017] The Power coupling between the input and Output waveguides with the presence of gap for a total transmission length of 5.5um (length of input waveguide is 2 um, gap distance of 1.5 um, length of Output waveguide 2 um) is shown in Fig 7. Figs. 7(a) and(b) display the 3D plots of optical field propagation in the pure and Graphene-based SOI waveguides, respectively. The field propagation is simulated for the proposed Single mode integrated optical sensor shown in Fig.l with dimensions mentioned in Table 2. The X axis represents the width of the structure, Y axis represents the intensity of the optical field and the Z axis represents the length of the structure in micro-meters. The length of the structure is 5.5um (2 um being the length of the input waveguide, 1.5 um being the gap between the waveguides and 2um being the length of Output waveguide). The Z-axis also represents the direction of field propagation. From, Figs. 7 (a) and (b), it is clear that the optical field intensity is pronounced in the Graphene-based SOI waveguide as compared to the Standard SOI, along the length of propagation in the Z axis. In other words, as the length of propagation increases, there is a sharp reduction in the optical field intensity in a pure SOI waveguide structure, while this reduction is not so notable in the Graphene-based SOI structure. This is because the inclusion of the additional cover layer over the SOI waveguide reduces the optical power loss especially due to absorption and hence it can be effectively used for integrated optical sensors operating at the green wavelength (540nm). In Fig. 8 the sensitivity is determined by considering molar concentration The Highest sensitivity is found to be 3.12x10-6 RIU for graphene based optical sensor and 2.77x10-6 RIU for pure SOI based optical sensor at 540 nm. This sensitivity has been obtained for a gap distance of 0.6 um.Table 3 shows the comparative analysis between the pure and graphene-based SOI waveguide sensor at various visible wavelengths, 530 nm, 535 nm and 540 nm. It is evident that for the same physical dimensions, Single mode Operation with minimum loss is achieved when the wavelength is set to 540 nm. Table 5 also shows that the optical loss in SOI based sensors at 540 nm 5.499x105 dB/cm, which is quite high, in comparison graphene-based SOI waveguide sensor, the loss is restricted to only 4.63x105 dB/cm (% reduction in loss). The presence of the graphene layer over the SOI increases the effective index from 3.02 to 3.773, and sensitivity from 2.77x10-6 to 3.12x10-6 RIU, which accounts for 24.94% and 12.635% increase in effective index and sensitivity respectively. Thus graphene is a suitable material for use in optical waveguide sensors at visible wavelength.
above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-discussed embodiments, but should be defined only in accordance with

the following Claims and their equivalents.

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CLAIMS
I/we claim,
1. Coating Graphene layer on top of Silicon layer in pure SOI waveguide reduces the optical power loss.
2. High optical losses occurring in SOI waveguide at green wavelength, 540nm can be reduced by coating a thin layer of graphene on top of Silicon layer SOI waveguide.
3. A method, System and apparatus providing one or more features as described in the paragraphs of this specification.