Knife-edge method at nanoscale: Why it fails and how to correct it?

The knife-edge method is an established technique for profiling light beams. As the beams become tighter focused, sizes of their focal spots can be even smaller than the wavelength. Though the cases, when the knife-edge method... [ view full abstract ]

The knife-edge method is an established technique for profiling light beams. As the beams become tighter focused, sizes of their focal spots can be even smaller than the wavelength. Though the cases, when the knife-edge method can reconstruct projections of tightly focused beams, were reported  they rather represent an exception than the rule. The knife-edge method at the nanoscale is proven to be exceptionally sensitive to the material choice, geometry of the knife-edges, polarization of the beams and even the choice of the substrates of the knife-pads.

As a rule, two types of distortions are observed for linearly polarized Gaussian beam, see Fig 1 (a): 1) displacement d_s,p of the projection from the knife-edge and b) changes in the beamwidths w_s,p of the projections. Both distortions are polarisation sensitive. The main cause for the failure of the knife-edge method at the nanoscale is due to the plasmonic interaction of the light with the knife and its edge. A rather large number of interaction scenarios leads to the failure of the knife-edge method at the nanoscale.

Here, we systematically study different knife-edge pad geometries, materials, substrates and collection geometries. Relative shifts d_s,p are introduced for higher order HG and LG modes, see Fig. 1 (b,c) and we can report on the consistency with the cases of a linearly polarized Gaussian beam. We use our previous developments and introduce a set of corrections into the standard knife-edge evaluation method. It seems that an adapted knife-edge reconstruction technique can be successfully developed. Very first attempts to "save" the knife-edge method at the nanoscale are reported here. The algorithm can even account for various aberrations (like koma, astigmatizm etc.) introduced by the focussing system.