Dominique Barchiesi

A 3-D multilayer model of scattering by nanostructures. Application to the optimisation of thin coated nano-sources

A multilayer model is applied to near field optical nano-sources. This model is based on the resolution of the Maxwell equations fitting specific boundary conditions. Because of the thin metal coating at the end of a sort of nano-source, a perturbative approach is suitable. The first part will describe the 3D multilayer theory and algorithm, the second part will introduce geometrical considerations on the metal coating of the optical fiber in order to discuss on nano-source coating. The benefit of this model is the interpretation of the phenomenon with the help of spatial Fourier spectrum of the source.

A theoretical model for the Inverse Scanning Tunneling Optical Microscope (ISTOM)

Abstract Recently experiments have been performed with a new kind of Scanning Near-field Microscope. The apparatus is derived from a Scanning Tunneling Optical Microscope by simply inverting the direction of light propagation: it is thus an Inverted Scanning Tunneling Optical Microscope (ISTOM) where the tip is used in emission mode and where detection can be mediated via homogeneous or evanescent waves. We propose a theoretical calculation of the detected intensity measured in ISTOM experiments. In this model, the sample is a relief on the hemisphere surface and the source is an aperture tip described within the Bethe-Bouwkamp approximation. The use of plane wave expansions of the various fields and of a perturbation method for matching the boundary conditions lead to concise analytical formulas. The discussion of the influence of the various parameters on the detected intensity is thus quite easy. We illustrate the formulas by some examples showing the variations of the detected signal versus tip-surface distance, tip radius and detection angle.

Theoretical Problems in Scanning Near-Field Optical Microscopy

The rigorous calculation of Scanning Near-field Optical Microscope (SNOM) images is a difficult task which needs the resolution of a complicated problem of diffraction in unusual conditions: two diffracting objects are present, they contain details smaller than the wavelength and are very close. Various approaches have been proposed to attack this problem and the purpose of this paper is to present, classify and compare these theoretical works.

Energy transfer from fluorescent thin films to metals in near-field optical microscopy: Comparison between time-resolved and intensity measurements

The fluorescence intensity, fluorescence decay time, and shear-force images of a thin film have been simultaneously investigated by reflection scanning near-field optical microscopy using an uncoated fiber tip. The sample is made of a europium chelate embedded in a 32-nm thick polymer layer that coats a periodic structure of gold and chromium. It is contended that the three images carry different and somewhat complementary information; the shear force supplying the sample profile while the intensity mainly depends on the local sample’s reflectance. Moreover, the decay time exhibits the local-energy-transfer process that takes place between the metallic substrate and the dye layer.

Near-field theoretical study of a magneto-optical grating

Abstract We propose a theoretical description of a near-field observation of a magneto-optical grating. We consider a scanning tunnelling optical microscopy (STOM) experiment where the sample is a thin isotropic magnetic material. We choose a transverse geometry: the magnetization is perpendicular to the plane of incidence and parallel to the grating grooves. For calculating the detected signal we use a perturbative diffraction theory of a multilayer anisotropic structure. The formalism has been tested by modelling a far-field diffraction experiment. Then we calculate STOM near-field signal above the grating and compare optical images and magneto-optical images.

Scanning Tunneling Optical Microscopy (STOM): Theoretical Study of Polarization Effects with two Models of Tip

Two kinds of probe are actually used in scanning near-field optical microscopy: dielectric tip and small aperture probe. The purpose of this paper is to compare theoretically the two different probes by calculating and comparing the images of a same sample. We choose to describe a Scanning Tunneling Optical Microscope (S.T.O.M.) where the sample is illuminated by total internal reflection. The dielectric tip is modeled as a small scattering dipolar center. The intensity detected by the other probe is calculated by using the diffraction theory of Bethe and Bouwkamp. It is shown that the two probes do not detect the same things: the dielectric tip picks up the square modulus of the electric near-field. The other probe is sensible both to the electric and magnetic fields. The models are used to calculate images of a two-dimensional object (a letter) of dimension smaller than the wavelength. The images are quite different and are very sensitive to polarization of the incident field.

Surface imaging in near-field optical microscopy by using the fluorescence decay rate: a theoretical study

In this paper, we study the fluorescence decay rate of a molecule above a corrugated interface, and particularly the variations of the decay rate as a function of the lateral position of the molecule. As a first step, one has to determine the field diffracted by a corrugated interface when the incident field is the field emitted by a dipole. For this purpose, we have used a perturbative Rayleigh method, and we show that the decay rate variations can be connected to the surface profile via a transfer function. Some numerical calculations of this transfer function and of decay rate variation images are presented for dielectric and metallic samples. The visibility of the theoretical images is up to 20% and, moreover, resolution of the images is good enough to use the fluorescence lifetime of molecules as signal in a life‐time scanning near‐field optical microscope. The technical problems are discussed briefly.

The inverse scanning tunneling near-field microscope (ISTOM) or tunnel scanning near-field optical microscope (TSNOM) 3D simulations and application to nano-sources

Abstract We propose an application of the ISTOM to characterize nano-sources used in Scanning Near-Field Microscopies. The model takes into account the coupling between the nano-source and the hemispherical lens of the ISTOM set-up. By changing the angle of detection, experimental data are related to the Fourier spectrum of the source. We show “images” calculated with two different distances between tip end and lens surface.

Near Field Instrumentation

Unlike electron scanning tunneling microscopy, near field microscopy has known an erratic evolution from its presumed birth on 1928 up to now. As an example, the main progresses in microscopy before the advent of Fourier optics are due to biologists or at least to biological needs (the Kohler illumination is an example). It is still usual to call the transmission microscope, biological microscope and the reflection one, metallurgical microscope. Concerning the concepts themselves, electromagnetism is well known from the works of Maxwell at the end of the 19th century and light propagation was described long time ago thanks to the works of Huygens (late 17th century) and Fresnel 30 years later. These works lead to the wave theory of imaging proposed by Abbe around 1870. This wave theory applied to imaging systems pointed out the existence of a resolution limit due to diffraction of light by the very small object details. From this time (and probably before) several attempts have been made to get around this limit. The introduction of Fourier optics and of the subsequent notion of modulation transfer function after the last world war dynamized the research dealing with superresolution of optical systems. A few techniques have been proposed allowing one to transmit more information beyond the allowed transfer region. Unfortunately such ways did not circumvent the Abbe limit stricto sensit, since the resolution was always greater than λ/2nsinθ.

Near-field calculations of the electromagnetic field in a metallic sample illuminated by a metallized nano-source

A non perturbative Rayleigh method is used to calculate the electromagnetic field in the vicinity of an interface between metal and air. The metallic surface is enlightened by a thin coated nano-source localized a few nanometers above the interface. We study the influence of the size of the tip apparatus and of the distance between the tip end and the sample on the confinements of the spot in the metallic sample. The knowledge of the electromagnetic field in metal could enable to calculate temperature and dilatation of the surface of the metal.