Souvik Ghosh

An Innovative Straight Resonator Incorporating a Vertical Slot as an Efficient Bio-Chemical Sensor

A compact and integrated label-free refractometric bio-chemical sensor based on silicon-on-insulator (SOI) is proposed and comprehensively studied at the telecommunication wavelength of λ = 1550 nm. This device incorporated a three-dimensional (3D) Fabry-Perot cavity in the nano-scale regime with maximum footprint area around 470 × 473 nm2. A resonance shift (Δλres) of 5.2 nm is reported for an ultrathin (5 nm) bio-layer sensing. Besides, an improved maximum sensitivity (S = 820 nm/RIU) is also achieved for bulk refractive index change in surroundings. As a chemical sensor, very low detection limit (DL = 6.1 × 10-6 RIU) also can be possible to achieve by this device. All the numerical investigations and optimizations were carried out in frequency domain by a numerically efficient and rigorous full vectorial H-field based 2-D and 3-D finite element methods (FEM). A 3D-FEM code is developed and used to find out the wavelength dependencies of the resonator. Possibility of easy CMOS fabrication and integration opportunities make this structure as a prospective and efficient lab-on-chip device.

A Compact Mach–Zehnder Interferometer Using Composite Plasmonic Waveguide for Ethanol Vapor Sensing

The finite element method (FEM) has attracted a considerable interest in the past few decades for the analysis of a wide range of dielectric waveguides. This method can handle isotropic and anisotropic material properties and arbitrary-shaped complex dielectric discontinuities more efficiently and accurately than any other methods. A modified H-field based full-vectorial finite element method is used for a rigorous analysis of a composite plasmonic waveguide as an efficient ethanol vapor sensor where a porous ZnO (P-ZnO) layer is used as low index material in between high index silicon and silver metal layer. Enhanced field confined into low index slot is utilized for ethanol vapor sensing which has many potential applications in chemical industries. It is reported here that a high waveguide sensitivity over 0.7 per RIU could be realized with our proposed design depending on the porosity of the ZnO layer. For accurate detection of refractometric changes, a compact Mach–Zehnder interferometer is designed where maximum phase sensitivities of 0.30, 0.34, 0.38, and 0.40 are shown to be achieved for

Evolution of Plasmonic Modes in a Metal Nano-Wire Studied by a Modified Finite Element Method

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Finite Element Method for Sensing Applications

50% volume fraction of ethanol into porous ZnO layer with porosity, P = 30%, 40%, 50%, and 60%, respectively. The complete investigation has been carried out at the well-known telecommunication wavelength 1550 nm and with our in-house, accurate full-vectorial FEM code.

Compact Photonic SOI Sensors

A finite width thin metal film plasmonic nanowire with its unique feature of subwavelength light guiding is finding many applications in compact integrated nanophotonic circuits and sensors. Full-vectorial finite element method (FV-FEM) is becoming an important simulation tool for the analyses of such exotic waveguides. Instead of a penalty approach reported earlier, a more direct divergence formulation considering each discretized elements optical properties to eliminate nonphysical modal eigenvectors has been exploited and is reported here. Long and short-range fundamental and higher order plasmonic modes and supermodes of a pure metal nanowire and their evolutions with waveguide geometry, surrounding identical, and nonidentical dielectric cladding materials and operating wavelength are thoroughly studied. Interesting long-range modal properties such as, supermode formation, complex phase matching, and mode evolution in identical and non-identical clad metal nanowires have been observed and explained in detail including supermode profiles. This study is expected to help in understanding the evolution of plasmonic guided modes in compact active and passive integrated photonic devices containing metal narrow strips.

Tailoring light-sound interactions in a single mode fiber for the high-power transmission or sensing applications

In this chapter, the fundamentals of the nodal finite element method (FEM) are presented, including the first-order element and second-order element. The nodal FEM is introduced for the scalar concept of the propagation constant of 2D waveguide cross section. Then, it is extended to include the time domain analysis under perfectly matched layer absorbing boundary conditions. A simple sensor based on optical grating is thereafter simulated using the time domain FEM. Also, the full vectorial analysis is discussed through the application of the penalty function method on the nodal FEM and the vector finite element method (VFEM). For the penalty function method, a global weighting factor is used to incorporate the effect of the divergence-free equation. In the VFEM, nodes are used to represent the orthogonal component of the field while the edges are used to represent the tangential component for accurate application of the boundary conditions. Finally, surface plasmon resonance photonic crystal fiber biosensor is introduced as an example of the full vectorial analysis using the VFEM.

Design of dispersion-engineered As2Se3 channel waveguide for mid-infrared region supercontinuum generation

Besides well matured optical fiber-based sensors, emerging compact down-scaled nanowires, slot waveguides and resonators are now under researcher’s consideration due to their high sensitivities and on-chip fabrication possibilities. Along with pure dielectric based waveguides and resonators, clever engineering of sub-wavelength field confinement and modal propagation loss in plasmonic nanowire and hybrid plasmonic slot waveguides also showing promising results in the field of photonic sensing. Numerically efficient, versatile finite element method based approaches are used for rigorous analyses, design, and optimizations of these complex optical guided-wave structures. All these sensor devices can exploit the well-developed state-of-the-art fabrication technologies.

Design of ultra-compact composite plasmonic Mach-Zehnder interferometer for chemical vapor sensing

A full-vectorial numerically efficient Finite Element Method (FEM) based computer code is developed to study complex light-sound interactions in a single mode fiber (SMF). The SBS gain or SBS threshold in a fiber is highly related to the overlap between the optical and acoustic modes. For a typical SMF the acoustic-optic overlap strongly depends on the optical and acoustic mode profiles and it is observed that the acoustic mode is more confined in the core than the optical mode and reported overlap is around 94 % between these fundamental optical and acoustic modes. However, it is shown here that selective co-doping of Aluminum and Germanium in core reduces the acoustic index while keeping the optical index of the same value and thus results in increased acoustic- optic overlap of 99.7%. On the other hand, a design of acoustic anti-guide fiber for high-power transmission systems is also proposed, where the overlap between acoustic and optical modes is reduced. Here, we show that by keeping the optical properties same as a standard SMF and introducing a Boron doped 2nd layer in the cladding, a very low value of 2.7% overlap is achieved. Boron doping in cladding 2nd layer results in a high acoustic index and acoustic modes shifts in the cladding from the core, allowing much high power delivery through this SMF.

Cross-slot waveguide and compact straight slotted resonator based bio-chemical sensors

In recent years, low cost and scalable integrated optics compatible planar waveguides have emerged for an ultrabroadband supercontinuum generation between ultraviolet and mid-infrared region applications. A 20-mm-long integrated photonics compatible highly nonlinear As2Se3 channel waveguide, which exhibited wider as well as lower magnitude and nearly flat anomalous dispersion region, designed and modeled by employing GeAsSe glass for its upper and lower claddings. Using pump source at 6 μm with a pulse duration of 170-fs, an ultrabroadband long wavelength region supercontinuum broadening covering the wavelength from 3.5 μm to 15 μm could be predicted with the largest input peak power of 10 kW. Increasing the power further to 20 kW does not enhance the supercontinuum expansion noticeably beyond 15 μm. This numerical demonstration could be the longest supercontinuum generation by an on-chip integrated photonics compatible planar waveguide which can be used for a variety of mid-infrared region applications.

Efficient strategy to increase higher order inter-modal stability of a step index multimode fiber

Following the Industrial advancements in the last few decades, highly flammable chemicals, such as ethanol (CH3CH2OH) and methanol (CH3OH) are widely being used in daily life. Ethanol have some degrees of carcinogenic effects in human whereas acute and chronic exposer of methanol results blurred vision and nausea. Therefore, accurate and efficient sensing of these two vapors in industrial environment are of high priorities. We have designed a novel, ultra-compact chemical vapor sensor based on composite plasmonic horizontal slot waveguide (CPHSW) where a low-index porous-ZnO (P-ZnO) layer is sandwiched in between top silver metal and lower silicon layers. Different P-ZnO templates, such as nano-spheres, nano-sheets and nanoplates could be used for high-selectivity of ethanol and methanol at different temperatures. The Lorentz-Lorenz model is used to determine the variation of P-ZnO refractive index (RI) with porosity and equivalent RI of P-ZnO layer for capillary condensation of different percentage of absorbed vapor. An in-house, new divergence modified finite element method is used to calculate effective index and attenuation sensitivity. Plasmonic modal analyses of dominant quasi-TM mode shows a high 42% power confinement in the slot. Next, an ultra-compact MZI incorporating a few micrometres long CPHSW is designed and analysed as a transducer device for accurate detection of effective index change. The device performance has been studied for different percentage of ethanol into P-ZnO with different porosity and a maximum phase sensitivity of >0.35 a.u. is achieved for both the chemical vapors at a mid-IR operating wavelength of 1550 nm.