Muhammad Ali ButtWarsaw University of Technology, Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662 Warszawa, Poland* Correspondence: ali.butt@pw.edu.plPlasmonic sensors based on metal-insulator-metal (MIM) waveguides are renowned for their miniaturization and high sensitivity in various sensing applications. A broad spectrum of researchers is numerically investigating the characteristics of MIM waveguide-based plasmonic sensors with diverse cavity shapes. However, practical demonstrations of these sensors have not yet been realized, primarily due to the overlooked aspect of the light coupling mechanism into these waveguides. In this context, two distinct methods for coupling light into and out of plasmonic chips based on MIM waveguides are presented.Keywords: Metal-insulator-metal waveguide; plasmonic sensor; mode coupling techniques.Plasmonic sensors based on metal-insulator-metal (MIM) waveguides (WGs) hold significant promise and impact in various fields such as biosensing, food control, environmental monitoring, and telecommunications[1], [2], [3], [4], [5], [6], [7]. A MIM WG is a type of WG used for guiding and controlling electromagnetic waves, particularly in the optical and infrared spectrum. The structure typically consists of two parallel metallic layers separated by a thin insulating layer. The metal layers serve as the WG’s walls, confining the electromagnetic energy within the insulating layer. The insulating layer, often a dielectric material like air, silicon dioxide (SiO2) or a polymer, acts as a spacer, maintaining the separation between the metal layers and preventing direct electrical contact between them. The unique properties of plasmonic materials allow for the confinement of surface plasmon polaritons (SPPs) at the metal-dielectric interfaces within the WG structure. This confinement results in highly localized electromagnetic fields, making MIM WGs ideal platforms for sensing applications with enhanced sensitivity.Plasmonic sensors offer the ability to detect slight changes in refractive index or surface interactions, enabling label-free and real-time detection of biomolecules, gases, or chemicals[8], [9]. Furthermore, MIM WG sensors can operate at optical frequencies, providing advantages such as miniaturization, compatibility with existing photonic technologies, and the potential for integration into compact and portable devices for point-of-care diagnostics and environmental monitoring. Research in this area is driving advancements towards highly sensitive, selective, and versatile plasmonic sensor platforms with broad applications in diverse industries[10], [11]. Figure 1 illustrates the trend in published papers on MIM WG-based devices indexed in the Scopus database from 2010 to 2024. The search was conducted using key terms including ”MIM waveguide,” ”Metal-insulator-metal waveguide,” and ”MIM sensor.” The data represents the cumulative number of papers over this period, highlighting the increasing interest and research activity in this field.Metals used in the conception of plasmonic sensors play a fundamental role in determining the performance and characteristics of these sensors. Commonly employed metals include gold (Au)[12], silver (Ag)[13], and aluminum (Al), each offering unique advantages and considerations. However, Al has not been widely considered by researchers in the realization of MIM WG sensors. Au is favored for its excellent stability, biocompatibility, and relatively low optical losses in the visible to near-infrared (NIR) spectral range, making it ideal for biomedical sensing applications. Ag exhibits strong plasmonic properties, particularly in the visible spectrum, providing high sensitivity and enhancement factors. However, Ag can be prone to oxidation and degradation over time, requiring careful handling and protection in sensor design. Al is a cost-effective alternative with plasmonic resonance in the ultraviolet (UV) to visible spectrum, suitable for applications requiring sensitivity in these wavelengths. However, Al can also oxidize easily, impacting its long-term stability. The selection of metal depends on the definite sensing needs, operational wavelength range, desired sensitivity, and environmental factors, highlighting the importance of material selection in optimizing plasmonic sensor performance[10]. Ongoing research focuses on exploring new metal combinations and nanostructuring techniques to further enhance the capabilities and versatility of plasmonic sensor platforms.Currently, research on plasmonic sensors utilizing MIM WGs predominantly operates at both analytical and numerical levels. This encompasses comprehensive theoretical investigations alongside sophisticated computational simulations. The finite element method (FEM) and finite difference time domain (FDTD) methods are powerful numerical techniques used extensively in the simulation of photonic devices. FEM discretizes the domain into smaller, simpler elements, allowing the complex behavior of light to be modeled with great accuracy. In photonic simulations, FEM can handle irregular geometries effectively and is versatile for analyzing different types of devices, such as WGs, resonators, and photonic crystals. On the other hand, FDTD is a time-domain numerical technique that divides space into a grid of finite cells and computes electromagnetic field equations over time. Both methods play vital roles in the design and optimization of photonic devices, advancing complementary benefits in terms of flexibility, accuracy, and computational efficiency depending on the specific requirements of the simulation[4].