Revisiting Newton's rings with a plasmonic optical flat for high-accuracy surface inspection

Abstract

Two parallel optical surfaces often exhibit colorful fringes along the lines of equal thickness because of the interference of light. This simple phenomenon allows one to observe subwavelength corrugations on a reflective surface by simply placing on it a flat reference dielectric surface, a so-called optical flat, and inspecting the resultant interference pattern. In this work, we extend this principle from dielectric surfaces to two-dimensional plasmonic nanostructures. Optical couplings between an Au nanodisk array and an Au thin film were measured quantitatively using two different techniques, namely, the classical Newton's rings method and a closed-loop nano-positioning system. Extremely high spectral sensitivity to the inter-surface distance was observed in the near-field coupling regime, where a 1-nm change in distance could alter the resonance wavelength by almost 10 nm, >40 times greater than the variation in the case without near-field coupling. With the help of a numerical fitting technique, the resonance wavelength could be determined with a precision of 0.03 nm, corresponding to a distance precision as high as 0.003 nm. Utilizing this effect, we demonstrated that a plasmonic nanodisk array can be utilized as a plasmonic optical flat, with which nanometer-deep grooves can be directly visualized using a low-cost microscope.

Keywords: near-field optical coupling; optical flat; plasmonics.

Figure 1

Plasmonic Newton’s rings experiment. (a) Schematic drawing of the experimental setup. (b) SEM image of a typical plasmonic optical flat consisting of a 2D plasmonic nanodisk array on a quartz substrate. (c) Plasmonic Newton’s rings captured by a color camera. (d) Reflection spectra measured along the dashed line in c. The inset shows the detailed spectral behaviors of the two plasmonic metasurfaces in the near-field coupling regime. In a, PT and TF denote the piezo tube and tuning fork, respectively.

Figure 2

Resonance behavior of the coupled plasmonic metasurfaces. (a) Resonance wavelength, λ_res, as a function of gap size, g, as measured using the Newton’s rings method and the lifting method, in which the gap size was tuned using a piezo tube. (b) Distance sensitivity as a function of g retrieved from the experimental data presented in a. The distance sensitivity, s, increases markedly when entering the near-field coupling regime.

Figure 3

Optical profiling using a plasmonic optical flat. (a) Schematic drawing of the experiment. (b) Plasmonic optical flat fabricated using a nanoimprint-based method. The inset shows an SEM image of the 2D plasmonic nanodisk array. (c) AFM image of the 15-nm deep NJU-shaped grooves and their cross section along the dashed line. (d) Optical image of the nanogrooved film pressed onto the plasmonic optical flat. AFM, atomic force microscope.

Figure 4

Simulated resonance behaviors of the coupled plasmonic metasurfaces. (a–c) show the field distributions in a unit cell of the coupled system with g=∞, 10 and 50 nm, respectively; (d–f) the respective charge distributions. In the simulation, a periodic boundary was used, with a 250-nm pitch size; the unit Au nanodisk was 100-nm wide and 40-nm thick; and the Au film was 40-nm thick.