Optimization of the Transverse Electric Photonic Strip Waveguide Biosensor for Detecting Diabetes Mellitus from Bulk Sensitivity

Diabetes mellitus is a chronic metabolic condition that affects millions of people worldwide. The present paper investigates the bulk sensitivity of silicon and silicon nitride strip waveguides in the transverse electric (TE) mode. At 1550 nm wavelength, silicon on insulator (SOI) and silicon nitride (Si3N4) are two distinct waveguides of the same geometry structure that can react to refractive changes around the waveguide surface. This article examines the response of two silicon-based waveguide structures to the refractive index of urine samples (human renal fluids) to diagnose diabetes mellitus. An asymmetric Mach–Zehnder interferometer has waveguide sensing and a reference arm with a device that operates in the transverse electric (TE) mode. 3D FDTD simulated waveguide width 800 nm, thickness 220 nm, and analyte thickness 130 nm give the bulk sensitivity of 1.09 (RIU/RIU) and 1.04 (RIU/RIU) for silicon and silicon nitride, respectively, high compared to the regular transverse magnetic (TM) mode strip waveguides. Furthermore, the proposed design gives simple fabrication, contrasting sharply with the state-of-the-art 220 nm wafer technology.

Optical Anisotropy in van der Waals materials: Impact on Direct Excitation of Plasmons and Photons by Quantum Tunneling

Inelastic quantum mechanical tunneling of electrons across plasmonic tunnel junctions can lead to surface plasmon polariton (SPP) and photon emission. So far, the optical properties of such junctions have been controlled by changing the shape, or the type of the material, of the electrodes, primarily with the aim to improve SPP or photon emission efficiencies. Here we show that by tuning the tunneling barrier itself, the efficiency of the inelastic tunneling rates can be improved by a factor of 3. We exploit the anisotropic nature of hexagonal boron nitride (hBN) as the tunneling barrier material in Au//hBN//graphene tunnel junctions where the Au electrode also serves as a plasmonic strip waveguide. As this junction constitutes an optically transparent hBN–graphene heterostructure on a glass substrate, it forms an open plasmonic system where the SPPs are directly coupled to the dedicated strip waveguide and photons outcouple to the far field. We experimentally and analytically show that the photon emission rate per tunneling electron is significantly improved (~ ×3) in Au//hBN//graphene tunnel junction due to the enhancement in the local density of optical states (LDOS) arising from the hBN anisotropy. With the dedicated strip waveguide, SPP outcoupling efficiency is quantified and is found to be ∼ 80% stronger than the radiative outcoupling in Au//hBN//graphene due to the high LDOS of the SPP decay channel associated with the inelastic tunneling. The new insights elucidated here deepen our understanding of plasmonic tunnel junctions beyond the isotropic models with enhanced LDOS.

Study of Silicon Nitride Waveguide Platform for On-Chip Virus Detection: Prospects for COVID-19

This work presents a rigorous sensitivity analysis of silicon nitride on silicon dioxide strip waveguide for virus detection, focusing on COVID-19. In general, by functionalizing the waveguide surface with specific antibodies layer, we make the optical sensor sensitive only to a particular virus. Unlike conventional virus detection methods such as polymerase chain reaction (PCR), integrated refractive index (RI) optical sensors offer cheap and mass-scale fabrication of compact devices for fast and straightforward detection with high sensitivity and selectivity. Our analysis includes a wide range of wavelengths from visible to mid-infrared. We determined the strip waveguide's single-mode dimensions and the optimum dimensions that maximize the sensitivity to the virus layer attached to its surface at each wavelength. We also compared the strip waveguide to the widely used slot waveguide. Our study shows that silicon nitride strip waveguide working at lower wavelengths is the optimum choice for virus detection as it maximizes both the waveguide sensitivity (Swg) and the figure of merit (FOM) of the sensor. Furthermore, the optimized waveguide can work for a range of viruses. Balanced Mach-Zehnder interferometer (MZI) sensors were designed at different wavelengths showing high FOM at λ = 450nm ranging from 500 RIU-1 up to 1231 RIU-1 with LMZI=500 µm. Different MZI configurations were also studied and compared. Finally, edge coupling from the fiber to the sensor was designed, showing insertion loss (IL) at λ = 450nm of 4.1 dB for the design with FOM = 500 RIU-1. The obtained coupling efficiencies are higher than recently proposed fiber couplers.

Design and Simulation Investigation of Si3N4 Photonics Circuits for Wideband On-Chip Optical Gas Sensing around 2 µm Optical Wavelength

We theoretically explore the potential of Si3N4 on SiO2 waveguide platform toward a wideband spectroscopic detection around the optical wavelength of 2 μm. The design of Si3N4 on SiO2 waveguide architectures consisting of a Si3N4 slot waveguide for a wideband on-chip spectroscopic sensing around 2 μm, and a Si3N4 multi-mode interferometer (MMI)-based coupler for light coupling from classical strip waveguide into the identified Si3N4 slot waveguides over a wide spectral range are investigated. We found that a Si3N4 on SiO2 slot waveguide structure can be designed for using as optical interaction part over a spectral range of interest, and the MMI structure can be used to enable broadband optical coupling from a strip to the slot waveguide for wideband multi-gas on-chip spectroscopic sensing. Reasons for the operating spectral range of the system are discussed.