Nanoslits have various applications, including localized surface plasmon resonance (LSPR)-based nanodevices, optical biosensors, superfocusing, high-efficiency refractive index sensors and chip-based protein detection. In this study, the effect of substrates on the optical properties of gold nanoslits placed in free space is discussed; for this purpose, glass BK7 and Al${}_2$O${}_3$ are used as substrates and the wavelength of incident light is supposed to be 650 nm. The optical properties, power flow and electric field enhancement for gold nanoslits are investigated by using the finite element method (FEM) in COMSOL Multiphysics software. The effect of polarization of an incident electromagnetic wave as it propagates from a gold nanoslit is also analyzed. As special case, the effect of glass and alumina substrate on magnetic field, power flow and electric field enhancement is discussed. The goal of this research is to investigate the phenomenon of power flow and electric field enhancement. The study of power flow in gold nanoslits provides valuable insights into the behavior of light at the nanoscale and offers opportunities for developing novel applications in the field of nanophotonics and plasmonics. The consequences of this study show the significance of gold nanoslits as optical nanosensors.

An aluminum-based metal–insulator–metal (MIM) Bragg grating waveguide filter operating in the near- to mid-infrared spectral range is numerically investigated. The structure employs air as the dielectric core and periodic geometrical modulation to achieve wavelength-selective Bragg reflection and plasmonic confinement. Finite-element simulations reveal pronounced field localization at resonant wavelengths between 1000 nm and 4600 nm. To ensure a meaningful comparison, the field maps are reported relative to the incident fields of a uniform (non-grated) aluminum–air MIM waveguide under the same mode-port excitation, evaluated at the input plane z = 0. The peak enhancements reach approximately 4.5 for the electric field at the aluminum–air interfaces and 3.8 for the magnetic field within the slot region. Parametric analysis demonstrates that increasing the longitudinal period length (here from L _p = 825 to 1728 nm) systematically shifts the Bragg condition toward longer wavelengths, while increasing the total number of periods (from Λ = 18 to Λ = 36) deepens the transmission minima. Within the stopband region, electric-field penetration is strongly suppressed, resulting in reflectance approaching unity. These results demonstrate a compact numerical proof-of-concept aluminum–air MIM Bragg grating filter for near- to mid-infrared wavelength-selective operation and field-engineered nanophotonic design. Practical implementation would require careful control of material quality and geometric tolerances.