L. Aigouy

Polarization effects in apertureless scanning near-field optical microscopy: an experimental study.

Strong electric-field enhancements at the apex of a tungsten tip illuminated by an external light source were recently predicted theoretically. We present an experimental study of the dependence of this effect on the polarization angle of the incident light. It is shown that the intensity of the light scattered by the tungsten tip of an apertureless scanning near-field optical microscope is 2 orders of magnitude higher when the incident light is p polarized than when it is s polarized. This experimental result is in good agreement with theoretical predictions and provides an easy way to test the quality of the tips.

Scanning thermal imaging of microelectronic circuits with a fluorescent nanoprobe

We have developed a scanning thermal imaging method that uses a fluorescent particle as a temperature sensor. The particle, which contains rare-earth ions, is glued at the end of an atomic force microscope tip and allows the determination of the temperature of its surrounding medium. The measurement is performed by comparing the relative integrated intensity of two fluorescence lines that have a well-defined temperature dependence. As an example of application, we show the temperature map on an operating complementary metal-oxide-semiconductor integrated circuit.

Scanning thermal imaging by near-field fluorescence spectroscopy

A scanning thermal microscope that uses a fluorescent particle as a temperature probe has been developed. The particle, made of a rare-earth ion-doped fluoride glass, is glued at the extremity of a sharp tungsten tip and scanned on the surface of an electronic device. The temperature of the device is determined by measuring the fluorescence spectrum of the particle at every point on the surface and by comparing the intensity variations of two emission lines. As an example, we will show some images obtained on a nickel stripe 1 microm wide, heated by an electrical current. A good agreement is observed with a simulation of the temperature field on the device.

Efficient generation of surface plasmon by single-nanoslit illumination under highly oblique incidence

Using scanning near-field optical microscopy, we investigate the ability of nanoslits in metallic films to launch surface plasmon polaritons (SPPs) under highly oblique incidence at λ=975 nm. The SPP generation efficiency is inferred by fitting the recorded near-field data with a simple analytical model. We find a remarkably large efficiency of 20% for the front side of the slit, which is in agreement with recent theoretical predictions relying on a fully vectoral electromagnetic formalism. An even larger efficiency is predicted experimentally (44%) and theoretically (33%) for the rear side. The present near-field analysis provides a direct approach to measure SPP generation efficiencies and may find applications for characterizing SPP devices.

Parallel Collective Resonances in Arrays of Gold Nanorods

In this work we discuss the excitation of parallel collective resonances in arrays of gold nanoparticles. Parallel collective resonances result from the coupling of the nanoparticles localized surface plasmons with diffraction orders traveling in the direction parallel to the polarization vector. While they provide field enhancement and delocalization as the standard collective resonances, our results suggest that parallel resonances could exhibit greater tolerance to index asymmetry in the environment surrounding the arrays. The near- and far-field properties of these resonances are analyzed, both experimentally and numerically.

Near-field optical spectroscopy using an incoherent light source

We present a method to perform near-field optical spectroscopy. It consists of an apertureless near-field optical microscope combined with an incoherent light source and a monochromator. We show that the optical response of the tip depends both on the wavelength and the tip apex shape. The experimental optical response of the tips is in good agreement with the predicted theoretical one. The near-field optical spectrum obtained on a gold island in the visible spectrum range (λ=400–600 nm) is presented and shows that the method is reliable to perform spectroscopic measurements on submicrometer-sized objects.

Mapping and quantifying electric and magnetic dipole luminescence at the nanoscale.

We report on an experimental technique to quantify the relative importance of electric and magnetic dipole luminescence from a single nanosource in structured environments. By attaching a Eu^{3+}-doped nanocrystal to a near-field scanning optical microscope tip, we map the branching ratios associated with two electric dipole and one magnetic dipole transitions in three dimensions on a gold stripe. The relative weights of the electric and magnetic radiative local density of states can be recovered quantitatively, based on a multilevel model. This paves the way towards the full electric and magnetic characterization of nanostructures for the control of single emitter luminescence.

AC thermal imaging of a microwire with a fluorescent nanocrystal: Influence of the near field on the thermal contrast

We have studied the temperature dependence of the visible fluorescence lines of 250 nm large PbF2 nanocrystals codoped with Er3+ and Yb3+ ions. By gluing such a particle at the end of a sharp atomic force microscope tip, we have developed a scanning thermal microscope able to observe the heating of electrically excited micro- and nanowires. By modulating the electrical current that flows in the structure, the resulting temperature variations modulate the particle fluorescence giving rise to the thermal contrast. We will show that the fluorescence is affected both by the near-field optical distribution and by temperature variations. We will show that it is possible to get rid of these optical effects and to keep the thermal contribution by comparing the images to reference images obtained when the device is not driven by a current. The determination of the temperature of the devices is performed by analyzing the thermal quenching of the fluorescent particle and is in good agreement with numerical simulatio...

Local optical imaging of nanoholes using a single fluorescent rare-earth-doped glass particle as a probe

We have developed a local optical imaging technique that uses a fluorescent rare-earth-doped fluoride glass particle as a probe. This particle is glued at the end of an atomic force microscope tip and scanned over the surface of a nanostructured sample illuminated by a laser beam. The intensity of the laser-induced fluorescence of the particle is then recorded as a function of the position on the sample surface. This method has enabled us to image the light scattered by 250-nm large nanoholes made in a thin chromium film. The advantages of this material over other fluorescent probes is that it has a strong fluorescence when excited at 980 nm, it operates at room temperature, and does not present any evidence of photobleaching.

Fabrication and characterization of fluorescent rare-earth-doped glass-particle-based tips for near-field optical imaging applications

Fluorescent rare-earth-doped glass particles glued to the end of an atomic force microscope tip have been used to perform scanning near-field optical measurements on nanostructured samples. The fixation procedure of the fluorescent fragment at the end of the tip is described in detail. The procedure consists of depositing a thin adhesive layer on the tip. Then a tip approach is performed on a fragment that remains stuck near the tip extremity. To displace the particle and position it at the very end of the tip, a nanomanipulation is achieved by use of a second tip mounted on piezoelectric scanners. Afterward, the particle size is reduced by focused ion beam milling. These particles exhibit a strong green luminescence where excited in the near infrared by an upconversion mechanism. Images obtained near a metallic edge show a lateral resolution in the 180-200-nm range. Images we obtained by measuring the light scattered by 250-nm holes show a resolution well below 100 nm. This phenomenon can be explained by a local excitation of the particle and by the nonlinear nature of the excitation.