DEPENDENCE OF SURFACE PLASMON POLYARITON EXCITATION EFFICIENCY ON ALUMINUM GRATINGS RELIEF DEPTH

Abstract

An experimental study of the excitation of surface plasmonpolaritons (SPP) on aluminum diffraction gratings with a fixed period of 519 ± 0,5 nm and a variable modulation depth h/a(where h is the grating depth, and a – its period) was carried out. Gratings with a sine-like profile were formed on vacuum chalcogenide photoresists films by interference lithography and covered with an opaque aluminum film. A Dimension 3000 Scanning Probe Microscope was used to determine the grating groove profile. The characteristics of the SPP were determinedfor28 gratings with h/a ranged from 0,018 to 0,20, by measuring the dependences of specular reflection of p-polarized radiation of He-Ne laser on the angle of incidence, which was defined as the angle between the normal to the substrate plane and the laser beam. It was found that there is an optimal grating relief depth for a given excitation wavelength, which provides the maximum transfer of the incident electromagnetic wave energy to the surface plasmon-polariton mode.The dependence of the SPP excitation efficiency on the grating modulation depth has a maximum at a relatively small value of h/a ≈ 0.086. At such modulation depth the absorption of electromagnetic radiation of the incident laser beam is more than two orders of magnitude higher than the absorption of aluminum film with flat surface at the same angle of incidence. The position of the angle of resonant excitation of SPP practically does not change from h/a= 0,018 up to h/a ≈ 0,06. With further increase of h/a it begins to shift to the region of smaller incidence angles, with the rate of the shift accelerating gradually. With an increase of h/a, a decrease in the depth of the plasmon resonance and a significant increase in its half-width are also observed, and the dependence of the half-width of the SPP band on the modulation depth is close to quadratic. Using this grating-coupled SPP technique, the estimated thickness of air-formed oxide layer on the aluminum gratings surface (about 3.9 nm) is close to the value obtained in the literature with a set of complicated techniques.