Near- to Far-Field Coupling of Evanescent Waves by Glass Microspheres

Rayenne Boudoukha,Nacer-Eddine Demagh,Paul Montgomery,Assia Demagh,Stephane Perrin,Sylvain Lecler

doi:10.3390/photonics8030073

Abstract

Through rigorous electromagnetic simulations, the natural coupling of high-spatial-frequency evanescent waves from the near field to the far field by dielectric microspheres is studied in air. The generation of whispering gallery modes inside the microspheres is shown independently of any resonance. In addition, the conversion mechanism of these evanescent waves into propagating waves is analysed. This latter point leads to key information that allows a better physical understanding of the super-resolution phenomenon in microsphere-assisted microscopy where sub-diffraction-limit revolving power is achieved.

Highlights

In optical imaging, the spatial distribution of an object leads to a spatial modulation of the reflected light
The magnification factor and the resolving power seem to be higher in microsphere-assisted microscopy than in nanolens-based microscopy
To investigate the evanescent wave collection, a rigorous electromagnetic simulation of microsphere assisted microscopy was implemented in the visible range using a finite element method (COMSOL Multiphysics®)

Summary

Introduction

The spatial distribution of an object leads to a spatial modulation of the reflected (or transmitted) light. In 2009 and in 2011, the ability of nanoscale hemispheres [5] and of microspheres [6] to perform super resolution imaging was experimentally revealed. Fort this purpose, a dielectric spherical lens is deposited on the object surface to be tested. A magnified image carrying the fine details is generated [7] in order to be collected by a microscope objective. Both make it possible to observe sub-diffraction-limit features while being label-free, scanning-free and easy-to-implement. The microspheres are able to improve the resolving power of an optical microscope by a factor of up to ×4 in air [6] and ×6 in immersion [8] while spherical nanolenses offer a magnification factor of 1.6 in air [5]

Methods

Results

Conclusion