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Typically, diffraction theory is treated from the perspective of the diffractive element. That is, depending on the size of the diffractive element, we describe light-matter interaction in a way or another. Typically, a vectorial electromagnetic description is needed for subwavelength objects. Whereas objects that are greater than 5 times the wavelength can usually be described using scalar electromagnetic theory [1].

Figure 1: Realm of validity of different diffraction models as a function of the ratio between the smallest grating period in the diffractive element (Λ) and the reconstruction wavelength (λ).

But...could the incoming beam change the diffraction regime? In this project we will study how light beams can tailor the vectorial nature of light-matter interactions. We will study how certain light beams can turn vectorial light-matter interactions into scalar and the other way around.

Figure 2: A SEM image of a bull’s eye structure. These kind of the structures will be probed using the VCD technique.

To quantify that, we will use a measurement introduced in [2]: vortex beam-induced circular dichroism (VCD). VCD is a technique that consists in performing a circular dichroism (CD) measurement using a vortex beam instead of Gaussian beam (or plane wave). In [2], it was observed that a non-chiral sample can yield a CD ≠ 0 when the incident beam is a vortex beam with a phase singularity of order q = ±1. Following some later works by Xavier Zambrana-Puyalto and co-workers [3, 4], we have realised that VCD can be used to quantify how vectorial/scalar light-matter interactions are.

Figure 3: Differences between intensity, phase and polarization for a paraxial and non-paraxial beam.