Holographic reconstruction via sub-wavelength aperture tips

Introduction In this work, we introduce a microscopy method based on full field off-axis holography 1–3 which enables performing three-dimensional (3D) reconstruction of scattered light from sub-wavelength aperture... [ view full abstract ]

Introduction

In this work, we introduce a microscopy method based on full field off-axis holography ^1–3 which enables performing three-dimensional (3D) reconstruction of scattered light from sub-wavelength aperture tips. While far-field methods, such as back focal plane imaging ^4–6, can be used to infer the directionality of angular radiation patterns, the advantage of our technique is that a single hologram recorded instantaneously contains information on the amplitude and phase of the scattered light, allowing to reverse numerically the propagation of the electromagnetic field towards the source.

Methods

Our setup is composed of a digital holographic microscope combined with a modified commercial Witec near-field scanning optical microscope (NSOM). Light from the laser is split into sample and reference beams. The sample beam is focused on the nanosized aperture of a metal coated aperture probe that gets in contact with the sample surface. Light emerging through the substrate is directed either to a PMT detector for regular NSOM measurements, or to a CCD camera where it interferes with the reference beam creating a hologram. Using back-propagation, the complex field at any point in space can be reconstructed by computing the scattered field in Fourier space.

Results

We present a comparative study of the reconstructed light from a tip located at various distances from two samples; a glass substrate, and a 40 nm gold film on a glass substrate. Fig.1 shows the field scattered through the tip aperture when placed in contact with a gold film. Directional leaky plasmons are clearly observed. FDTD simulations are performed to measure the angular radiation pattern of a magnetic dipole on a substrate, and show very good agreement with the experimentally obtained results.

Discussion

We have successfully demonstrated 3D reconstruction of scattered light from a NSOM tip through transparent and plasmonic media.
Our experimental and simulation results confirm the lateral magnetic dipole approximation previously studied in the literature for similar aperture probes ^7–9.
Further work will include applying this technique to investigate the complex field resulting from the propagation through a strongly scattering medium.

Acknowledgments

This work was supported by LABEXWIFI under references ANR-10-LABX-24 and ANR-10-IDEX-0001-02PSL.