Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

Abstract

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about the tip’s field. In particular, they unambiguously demonstrate both the nanometric spatial confinement of the tip field and the evanescent nature of the nanosource excited at the tip’s end. Most of the experimental results are illustrated by numerical calculations based on the finite element method and commented using the literature on the subject. Additionally, we discuss the origin of the optically induced topography on a nanometer scale and present some preliminary results of the apertureless near-field optical lithography based on local field enhancement. Our approach constitutes a useful tool to investigate the near-field of apertureless probes and should enable the optimization of the nanosource for any experiment requiring local optical excitation of the matter.

A Review of Heat Transfer Physics

“…Ultrahigh-resolution fluorescence microscopy utilizing two-photon excitation and sharp metal tips and spectroscopy of single molecules have been demonstrated using similar configurations [92]. Direct observations of the enhanced field using photosensitive films have confirmed the nanoscopic spatial confinement of the electric field, hence excluding the possibility of surface modification due to the tip expansion [91]. The favorable sharp tip geometry (leading to the lightning rod effect discussed earlier) and the multiple scattering between the resonantly coupled probe tip and the sample structure thus effectively lead to the focusing of the optical power by collecting over the larger, nearly flat probe surface and reradiating over the smaller and sharper tipapex structure.…”

“…Several investigators have attempted to explain the physical phenomena responsible for surface nanostructuring [89][90][91]. In general, material removal by ablation can be associated with photothermal, photomechanical, and photochemical effects.…”

“…While the nominal optical absorption depth z s ¼ 12 nm, a modified optical absorption depth z s ¼ 1 nm was fitted to the computed near-field absorbed energy density profile in the target obtained from the FDTD near-field simulations. Several investigators have attempted to explain the physical phenomena responsible for surface nanostructuring [89][90][91]. In general, material removal by ablation can be associated with photothermal, photomechanical, and photochemical effects.…”

Probing photonic and optoelectronic structures by Apertureless Scanning Near‐Field Optical Microscopy

“…• Several theoretical works (e. g. Novotny et al, 1997;Martin and Girard, 1997) and experiments (e.g., Aigouy et al, 1999;H'Dhili et al, 2001;Bachelot et al, 2003, Bouhelier et al, 2003 have showed that the end of apertureless metal tip can present an electromagnetic singularity resulting in a strongly confined nano-source. This effect possibly involves local second harmonic generation or white light emission.…”

Near-field optical imaging of noble metal nanoparticles

“…This is due to the high density of charges at the apex that interact with the illumination field to create enhanced evanescent fields confined near the probe [85]. As a result, this opens an entirely new dimension to SNOM, in that spectroscopies can be performed on nanostructures with advantages over conventional aperture-based methods.…”