In this study, a slot waveguide structure is designed based on the principle of external reflection. Further, this structure is optimized to limit the propagation of light in low-refractive-index materials and concentrate energy. The slot waveguide structure comprises two symmetrical high-refractive-index slabs, a middle low-refractive-index slot, and a peripheral cladding. The effects of the cladding size, the height and width of the middle low-refractive-index slot, and the width of the slab are simulated under two-dimensional conditions with an incident wavelength of 1550 nm based on the finite element analysis method. The aforementioned parameters are optimized to ensure that energy is more concentrated in the middle low-refractive-index slot. In addition, the distribution of the electric field in a three-dimensional space is studied when the optimized parameters are used to establish a three-dimensional model, proving that light can propagate in low-refractive-index materials when the proposed structure is used.