Diffraction characteristics and formation mechanism of nanogratings in tip-based down-milling

Structural coloration of metal surfaces based on the diffraction mechanism of blazed gratings has considerable value for use in anti-counterfeiting and information encryption applications. When the grating period is comparable to the wavelength of the incident light, the diffraction process will generate only two propagating orders, comprising the 0th order and the −1st order, resulting in lower dispersion. Improvement of the diffraction efficiency of the −1st order spectrum, which has a light splitting ability, is essential to improve the colouring quality of the diffraction structure. Such improvement of efficiency is closely related to the geometric features of the structure, but the relationship is insufficiently clear at present. The novelty of the approach in this paper lies in its use of a combination of optical and mechanical analyses of nano grating structures fabricated by tip-based nanoscale down-milling with the aim of establishing a connection between the diffraction characteristics of the structure and its processing formation. First, profiles of the nanograting structures are generated numerically by taking both the trajectory shape and the cutting edge radius into account. Using the finite-difference time-domain method, a diffraction model is then established to simulate the far-field spectral distributions of the generated profiles. It is concluded from these analyses that the grating height is an important factor that affects the −1st order diffraction efficiency. When the grating height is in the range of 155–185 nm with a grating period of 400 nm, the diffraction efficiency can reach up to 75%–90%, depending on the conditions of incidence. Then, with the aim of controlling the grating height, the nanograting structure fabrication processes are investigated under different revolving trajectory conditions by finite element analysis. The results show that the grating structures are formed mainly by extrusion caused by the moving tip, which results in the grating heights being affected by the undeformed chip thickness in each cycle and by the plastic deformation of the material. Under the optimal down-milling trajectory condition, nanograting structures with the expected height and good geometric consistency are obtained.