# Directional Plasmonic Excitation by Helical Nanotips

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Nanotip Fabrication

_{3}N

_{4}membrane) and coated with a thin gold layer. The skeleton of the tip is made of S1813 optical resist exposed with secondary electron during its milling to create the desired shape. With respect to the previously reported tip fabrication [17,18], here we introduced an additional step where the smooth tip shape is finalized with an embedded Archimede’s spiral with radius $R\left(\phi \right)$ and m arms according to the following equation:

#### 2.2. Leakage Microscopy for k-Space Microscopy

## 3. Results

_{z}, is nonzero, this phase matching can be only achieved in a specific azimuthal direction ϕ . This is where one can observe the maximum in the plasmonic resonance ring in the k-space. For purely symmetric structures and an accurate alignment of the sample with the beam axis, the normal component vanishes and the SP distribution is uniform as in Figure 2a,b. We believe that by introducing the helical groove on the tip’s surface, we excite a z-dipole that breaks the symmetry in a spin-dependent fashion and leads to the directionality observed in Figure 2c–f.

_{R}and I

_{L}are the intensities for right and left polarization, respectively, and $\chi $ is the ellipticity angle. This way a spatial CD spectrum can be obtained. In contrast with a most common temporal CD (frequency-dependent ellipticity variation), the results presented here show k-space maps of the ellipticity. Clearly, the most significant contributions to these maps appear at the central part of the k-space due to the light scattering and from the SP resonance circle at ${k}_{\rho}={k}_{SP}$.

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**SEM micrographs of the prepared tips. (

**a**) Bare tip; (

**b**) tip with embedded m = 1 spiral; (

**c**) tip with embedded m = 3 spiral; (

**d**) top view of m = 1 tip; (

**e**) set up of Leakage Radiation Microscopy. The Laser beam’s polarization is controlled by a set of a linear polarizer (LP) and a half or a quarter wave plate (HWP/QWP) and then focused by an objective O1 (details in the text). The imaging objective O2 extracts the leakage radiation through an index-matching oil, which then passes through a tube lens (TL) and a Fourier lens (FL) to obtain the k-space image.

**Figure 2.**Measured intensity distribution in the k-space from the nanotips for R and L polarizations incident light; (

**a**) and (

**b**) symmetric tip; (

**c**) and (

**d**) m = 1 tip; (

**e**) and (

**f**) m = 3 tip. Directionality Factor appears in each panel.

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**MDPI and ACS Style**

Singh, L.; Maccaferri, N.; Garoli, D.; Gorodetski, Y. Directional Plasmonic Excitation by Helical Nanotips. *Nanomaterials* **2021**, *11*, 1333.
https://doi.org/10.3390/nano11051333

**AMA Style**

Singh L, Maccaferri N, Garoli D, Gorodetski Y. Directional Plasmonic Excitation by Helical Nanotips. *Nanomaterials*. 2021; 11(5):1333.
https://doi.org/10.3390/nano11051333

**Chicago/Turabian Style**

Singh, Leeju, Nicolò Maccaferri, Denis Garoli, and Yuri Gorodetski. 2021. "Directional Plasmonic Excitation by Helical Nanotips" *Nanomaterials* 11, no. 5: 1333.
https://doi.org/10.3390/nano11051333