Coupled-wave analysis of lamellar metal transmission gratings for the visible and the infrared

Abstract

We theoretically and experimentally investigate the response, in the visible and in the near infrared, of micrometer- and submicrometer-period lamellar metal transmission gratings in vacuum and on silica and GaAs substrates. We use a coupled-wave analysis to characterize the grating response as a function of wavelength, period, grating profile, and dielectric constant of the metal and the substrate. Losses to the metal, which have been neglected in prior studies, are shown to be as large as 80% of the incident optical power. Absorption in the metal and the substrate, associated with complex refractive indices, leads to a broadening and a reduction in amplitude of Rayleigh wavelength resonance features in the transmission efficiency and reduces the extinction between orthogonal polarizations in the wire-grid polarizer limit. The results of transmission and photocurrent studies performed on metal–semiconductor–metal photodiodes fabricated on GaAs or GaAs–AlGaAs heterostructure substrates demonstrate the rigorous nature of the coupled-wave analysis, indicate experimental limitations for the application of an infinite grating approximation to model finite-period structures, and provide evidence for the presence of surface electromagnetic waves in the forward-diffracted optical intensity distribution. Qualitative agreement is also obtained between coupled-wave analysis results and transmission data reported in the literature for gold gratings on silica.