Near-field imaging by a micro-particle: a model for conversion of evanescent photons into propagating photons

Djenan Ganic,Xiaosong Gan,Min Gu

doi:10.1364/opex.12.005325

Abstract

In this letter we present a physical model, both theoretically and experimentally, which describes the mechanism for the conversion of evanescent photons into propagating photons detectable by an imaging system. The conversion mechanism consists of two physical processes, near-field Mie scattering enhanced by morphology dependant resonance and vectorial diffraction. For dielectric probe particles, these two processes lead to the formation of an interference-like pattern in the far-field of a collecting objective. The detailed knowledge of the far-field structure of converted evanescent photons is extremely important for designing novel detection systems. This model should find broad applications in near-field imaging, optical nanometry and near-field metrology.

Highlights

Optical near-field imaging has become a rich research field in recent years due to its ability to achieve optical resolution well below the classical diffraction limit of approximately half the wavelength of illuminating light
While the conversion mechanism by a metallic needle has been studied [18], to our knowledge, the underlying physical principle of the mechanism for the evanescent photon conversion by a microscopic particle probe, such as the one utilized with the trapped particle scanning near-field optical microscopy (SNOM) [16,17], has not been dealt with. Such a physical understanding is needed in nanometry for single molecule detection, in which case a single molecule is attached to a laser trapped microscopic particle immersed into evanescent field, and is monitored by measuring the scattered field [19]
We have presented, both theoretically and experimentally, a physical model for the mechanism for conversion of evanescent photons into propagating photons by a small particle. This model consists of two physical processes, near-field Mie scattering enhanced by morphology dependent resonance (MDR) and vectorial diffraction

Summary

Introduction

Optical near-field imaging has become a rich research field in recent years due to its ability to achieve optical resolution well below the classical diffraction limit of approximately half the wavelength of illuminating light. While the conversion mechanism by a metallic needle has been studied [18], to our knowledge, the underlying physical principle of the mechanism for the evanescent photon conversion by a microscopic particle probe, such as the one utilized with the trapped particle SNOM [16,17], has not been dealt with. Such a physical understanding is needed in nanometry for single molecule detection, in which case a single molecule is attached to a laser trapped microscopic particle immersed into evanescent field, and is monitored by measuring the scattered field [19].

Physical model

Theoretical and experimental results

Signal level

Objective

Conclusion