A. L. Lereu

NEAR FIELD OPTICAL MICROSCOPY: A BRIEF REVIEW

Near Field Optical Microscopy (NSOM) has evolved into a mature member of the family of scanning probe microscopy. In this article, we briefly go over the principle of NSOM, its breakthroughs and setbacks. We will describe some of the most commonly used NSOM modalities and conclude with the recent advances based on optical nanoantennas. We will then highlight the potential of this high–resolution optical microscopy for chemical and biological applications as well as for materials sciences.

Thermoplasmonic shift and dispersion in thin metal films

In 2004, the authors reported two coupling schemes based on the thermo-optic properties of thin metallic films and their associated sub- and superstrates, by utilizing surface plasmons. These studies showed a potential for all-optical modulation at low rates that may be used for sensing purposes. In this article, they continue by investigating thermal processes involved in thin metallic films with different approaches. They first experimentally imaged the shift of the surface plasmon dispersion relation in the visible spectrum, as the thin film temperature is externally varied. They then reinforce the previous observations by collecting the absorption curves at selected visible photon energies of excitation, as the film temperature in the excitation region increases. Utilizing the absorption measurements, they briefly address how one may obtain the real and imaginary parts of the index of refraction of the thin film as a function of temperature for each involved wavelength. Finally, they investigate the l...

Individual gold dimers investigated by far- and near-field imaging.

Plasmon resonances in 3D nanoparticle arrangements can produce strong localized optical fields, which are of importance for any application involving interaction of light with subwavelength volumes of matter down to the molecular level. In particular, remarkable field enhancement and confinement occur in a dimer geometry formed by two identical closely spaced particles. Although, recent advances in nanofabrication have rendered the fabrication of complex plasmon architectures more accessible, addressing their local fields in a nonperturbative fashion remains not straightforward, because metallic nanostructures are rather sensitive to their local environment. Here we study gold dimers fabricated by e‐beam lithography. Individual dimers are imaged both by far‐ and near‐field methods. First, the near‐field electromagnetic interaction in an ensemble of dimers is investigated by scattering spectroscopy, using dark field microscopy. Next, to probe their local field, we explore the luminescence of individual gold dimers utilizing a confocal microscope with single molecule detection sensitivity. We provide a statistical analysis of the dimer luminescence for different incident polarizations, with direct comparison to single particles (monomers). Finally, the near‐field transmission of the resonant dimers is mapped with a subwavelength resolution using polarized controlled near‐field scanning optical microscopy. Surprisingly, no clear evidence of the high mode density in the dimer gap is observed. This result may be attributed to the limited coupling of the field emitted by the aperture probe to the dimer mode.

Giant optical field enhancement in multi-dielectric stacks by photon scanning tunneling microscopy

Dielectric optical thin films, as opposed to metallic, have been very sparsely explored as good candidates for absorption-based optical field enhancement. In such materials, the low imaginary part of the refractive index implies that absorption processes are usually not predominant. This leads to dielectric-based optical resonances mainly via waveguiding modes. We show here that when properly designed, a multi-layered dielectric thin films stack can give rise to optical resonances linked to total absorption. We report here, on such dielectric stack designed to possess a theoretical optical field enhancement above 1000. Using photon scanning tunneling microscopy, we experimentally evaluate the resulting field enhancement of the stack as well as the associated penetration depth. We thus demonstrate the capability of multi-dielectric stacks in generating giant optical field with tunable penetration depth (down to few dozens of nm).

Nanometrology of delignified Populus using mode synthesizing atomic force microscopy

The study of the spatially resolved physical and compositional properties of materials at the nanoscale is increasingly challenging due to the level of complexity of biological specimens such as those of interest in bioenergy production. Mode synthesizing atomic force microscopy (MSAFM) has emerged as a promising metrology tool for such studies. It is shown that, by tuning the mechanical excitation of the probe-sample system, MSAFM can be used to dynamically investigate the multifaceted complexity of plant cells. The results are argued to be of importance both for the characteristics of the invoked synthesized modes and for accessing new features of the samples. As a specific system to investigate, we present images of Populus, before and after a holopulping treatment, a crucial step in the biomass delignification process.

Effect of thermal variations on the Knudsen forces in the transitional regime

When objects are maintained at different temperatures and separated by distances of the order of the mean free path of the surrounding host gas molecules, gas kinetic forces called Knudsen forces may be involved. The understanding of this effect may result in some improvements in microelectromechanical devices and measurement systems. We present the thermal dependence of these forces in the transitional regime for different gases. In this mode, the Knudsen effect can be significant and, therefore, become a problem in microscale devices. For this study, a silicon microcantilever, mounted close to a substrate, is used and changes in temperature are observed by measuring bending of the microcantilever.

Photon tunneling via surface plasmon coupling

The measurement of a photonic signal via plasmon-plasmon coupling in curved thin metal films is presented. In domains of subwavelength dimension, we calculate the resonant dispersion relations by modeling the curved thin film as a single sheeted hyperboloid of revolution. We show that several such surface modes are accessible optically at frequencies below the plasma frequency of the metal.

Plasmon assisted thermal modulation in nanoparticles

Single-particle interactions hold the promise of nanometer-scale devices in areas such as data communications and storage, nanolithography, waveguides, renewable energy and therapeutics. We propose that the collective electronic properties possessed by noble metal nanoparticles may be exploited for device actuation via the unapparent mechanism of plasmon-assisted heat generation and flux. The temperature dependence of the dielectric function and the thermal transport properties of the particles play the central role in the feasibility of the thermally-actuated system, however the behavior of these thermoplasmonic processes is unclear. We experimentally and computationally analyzed modulation via thermoplasmonic processes on a test system of gold (Au) nano-islands. Modulation and energy transport in discontinuous domains exhibited quantitatively different characteristics compared to thin films. The results have implications for all surface plasmon based nano-devices where inevitable small-scale thermal processes are present.

Plasticity, elasticity, and adhesion energy of plant cell walls: nanometrology of lignin loss using atomic force microscopy

The complex organic polymer, lignin, abundant in plants, prevents the efficient extraction of sugars from the cell walls that is required for large scale biofuel production. Because lignin removal is crucial in overcoming this challenge, the question of how the nanoscale properties of the plant cell ultrastructure correlate with delignification processes is important. Here, we report how distinct molecular domains can be identified and how physical quantities of adhesion energy, elasticity, and plasticity undergo changes, and whether such quantitative observations can be used to characterize delignification. By chemically processing biomass, and employing nanometrology, the various stages of lignin removal are shown to be distinguished through the observed morphochemical and nanomechanical variations. Such spatially resolved correlations between chemistry and nanomechanics during deconstruction not only provide a better understanding of the cell wall architecture but also is vital for devising optimum chemical treatments.

Spectroscopy and imaging of arrays of nanorods toward nanopolarimetry

The polarization dependence of the optical scattering properties of two-dimensional arrays of metal nanostructures with sub-wavelength dimensions (nanoantennas) has been investigated. Arrays of 500 nm × 100 nm gold nanorods covering a 100 × 100 µm(2) area were fabricated with varying orientations on an electrically conductive substrate. The experimental and computational analysis of the angularly organized nanorods suggest potential use toward the development of an integrated polarimeter. Using the gold nanorods on a transparent substrate as a preliminary system, we show that in the proper spectral range the scattering properties of the structures may be tuned for such an application.