Interfacial structure designs with impedance-matching for ideal broadband antireflections

Abstract

This work focuses on the ideal broadband antireflection structure designs based on the impedance-matching and the effective medium theory. Graded refractive index profiles that satisfy the impedance-matching condition between two media result in zero reflection over the entire wavelength range. Our studies found that both the thickness of the graded refractive index layer and the refractive indices of the adjacent two media determine the dispersion properties of the graded refractive index profiles. Specifically, we case-studied the dispersion properties of the gradient refractive index profiles for silicon, GaN, and glass substrates. The effective medium theory was utilized to design interface structures that match the ideal graded refractive index profiles. The accuracy of this design approach was assessed by comparing the filling factor as a function of thickness by using effective medium theory with zeroth-order and second-order approximations. A novel interface structure with concaved-dome geometrical shape was studied as a new type of impedance-matching antireflection structure (concaved-dome impedance-matching II), which has the advantage of reduced effective feature size and thus can better match the ideal graded refractive index profiles by applying the effective medium theory more accurately. The interface reflection properties of the impedance-matching II structure were computed via a three-dimensional finite difference time domain method. The interface reflections were compared with that of a conventional flat surface, a previously proposed micro-dome structure, and a traditional impedance-matching structure (impedance-matching I), which revealed that the concaved-dome impedance-matching II structure has the best antireflection performance over a broad wavelength range and wide incidence angles.