Single pulse near field study on a Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure by using a femtosecond laser

Single pulse near field study on a Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure was experimentally investigated with a laser pulse width of 200 fs at a wavelength of 775 nm. For the near field experiments, we have used polystyrene colloidal particles of 700 nm diameter deposited by spin coating on top of the multilayer structure, as well on top of Co (50 nm) and Cu (50 nm) thin films. The diameter and the morphologies of the holes were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). We have estimated the fluence thresholds values for the near field and discuss their values in respect with the enhancement factor of the intensity of the electromagnetic field due to the use of the colloidal particles. We compare the depths and the widths of the holes obtained at the same peak laser fluence for the Co thin film (50 nm), Cu thin film (50 nm) and Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure. Depending on the laser fluence, the ablation depth can reach the first, the second, or the third layer. Theoretical estimations of the intensity enhancement were done using the finite-difference time-domain (FDTD) by using the RSoft software. This type of a selective distribution of the ablation depth, in the near field regime, of a planar metal/dielectric interface can open new perspective in the excitation of propagating surface plasmons.