Light diffraction by a slit and grooves with a point source model based on wave dynamics

A point source model based on wave dynamics is proposed to study the fundamental light diffraction physics by a subwavelength slit and grooves in a metallic film. In this model, two opposite traveling waves are considered in each indentation; the resultant outgoing wave can propagate along the film surface to couple each other or radiate into free space as a point source. With small-system simulations, the tangential electric field at each opening determines its source temporal phase; then the energy conservation of each point source radiation and of the total radiant wave determine the source amplitudes. Besides these, this model reveals more physics regarding the wave interactions. In the strong-wave-coupling case studied, the surface waves created by the grooves flow into the slit and delay the Fabry-Pérot-like resonance. When adding the grooves concentrates the light field into a directional beam, the total transmitted energy through the slit significantly decreases. However, the energy in the original nearby grooves increases so that the groove radiation increasingly shares the transmitted energy. As the total transmitted energy decreases, the slit radiation energy decreases further due to the energy conservation. In the weak-wave-coupling cases, the groove radiation still interferes with that from the slit; as a result, the diffracted light is split into two beams. It is interesting to find that, due to the groove radiation, the slit radiation energy is enhanced to become larger than that transmitted through it. Detailed physical interpretations will be given.