Longitudinally polarized fields in near-field imaging systems

The evolution path of nanotechnologies moves through the domain of microscopy. Among all the kinds of imaging tools available today, optical imaging systems are of primary importance. When using optical imaging systems, we take profit of the information remotely delivered by light to the detector, after interaction with the sample. Nevertheless, as light moves away from the place of interaction, a part of this information is inevitably lost. With the help of suitable probes, light in close proximity of the illuminated sample can be detected. In this way, an extraordinary richness of information about the sample becomes accessible and we can manage to "see the invisible". Light confined in proximity of the sample is called near-field and near-field microscopy is the branch of microscopy dealing with the detection of such an optical field. In the domain of near-field microscopy, the rigorous electromagnetic and the optical description of light meet together. When images are formed by detecting optical fields close to illuminated samples, the vectorial and electromagnetic nature of light can not be neglected. In this thesis we focused our attention on the special case of electromagnetic fields oriented parallel to the optical axis (longitudinal fields). The subject is particularly intriguing, as longitudinally polarized fields cannot be detected using standard microscope devices. We mainly considered the interaction of longitudinal fields with microfabricated, fully metal-coated quartz nano-probes, and managed to give a description of their role in the near-field imaging process. The work will be presented as follows. In the first chapter, an introduction to Scanning Near-field Optical Microscopy (SNOM) is given. Starting from the concept of super-resolution in paraxial imaging systems, we introduce the concept of near-field microscopy. For two of the best known SNOM techniques we demonstrate that a resolution beyond the capabilities of classical microscopes can be, in principle, achieved. Moreover, we present a rigorous electromagnetic description of a simplified SNOM system, and stress the necessity of adequate tools for the understanding and the interpretation of near-field images. The second chapter is devoted to the description of the SNOM system with which the main part of the experimental work has been performed. The near-field microscope is particularly interesting since it employs an innovative kind of probes: microfabricated, metal-coated quartz probes with no aperture at their apex. The presence of a full metal coating has strong effects in the near-field imaging characteristics of the probes. Results form standard imaging tests are also presented in this chapter. We demonstrate that fully-metalized probes can actually couple evanescent fields and are capable to attain super-resolution. Microfabricated probes can be used either as light sources or as light collectors. In the third chapter we study their optical response when 3D-oriented fields are collected. We find a non trivial selective collection behavior for longitudinally and transversely polarized fields. Moreover, the lateral resolution of our system shows a clear dependence on the polarization state of the collected field. A phenomenological explanation of this effect, involving a polarization-dependent coupling mechanism of light into the probes, is proposed. In the fourth chapter we study the behavior of fully-coated transparent probes as light emitters. With the help of a three-dimensional numerical model, we characterize the guiding properties of the probes and analyzed the near-field and far-field emitted patterns. A set of experimental measurements confirms the results expected by simulations and validates our phenomenological model describing the interaction of light with the probes. In addition, the essential role of longitudinal fields in an illumination-mode near-field scanning system is underlined. We managed to directly measure the longitudinally-polarized, ultra-small hot spot produced at the metallic apex, after injection of a radially polarized mode into the probe. An alternative way of producing longitudinally polarized light source for near-field imaging is proposed in the fifth chapter. With a set of theoretical calculations, we demonstrate that Surface Plasmon Polaritons (SPP) excited on structured metallic thin films can be exploited as non-radiating, strongly localized light sources. The study presented in this chapter is purely computational: no practical implementation of the proposed device has been performed yet. A conclusive summary of this work is presented in the sixth chapter.

Investigation of Surface Electromagnetic Waves with Multi-Heterodyne Scanning Near-Field Optical Microscopy

Tristan Sfez

In this thesis, we present the development of a tunable Multi-Heterodyne Scanning Near-Field Optical Microscope (MH-SNOM). This instrument has been built and evaluated for the investigation of optical near fields in amplitude, phase and polarization. With this microscope, the response of a structure illuminated with two orthogonally polarized beams can be simultaneously measured both in amplitude and phase. Moreover, the integral state of polarization at the surface of a specimen can be retrieved under specific conditions. We demonstrate the capabilities of the system through a series of measurements involving Surface Electromagnetic Waves (SEWs). We have mainly focused our attention on a particular class of SEWs known as Bloch Surface Waves (BSWs). The propagation of BSWs on the outer surface of a silicon nitride multilayer has been studied in detail. Furthermore, we show that this propagation is affected by the presence of shallow dielectric corrugations such as a subwavelength grating or at the straight interface with a coated portion of the multilayer. In particular, we demonstrate that ultra-thin (thickness < λ/10) dielectric ridges may act as BSW waveguides. Combining the detection capabilities of the MH-SNOM with a numerical treatment of the experimental data, we are able to separate the transverse and longitudinal field components of the three modes propagating within a specific BSW waveguide. This new structure provides interesting opportunities in waveguide-based biosensing schemes in which the ridge is realized with functionalized molecular layers of nanometric thickness. Finally, we investigate a structure sustaining another type of SEW: Surface Plasmon Polaritons (SPPs). This structure is designed for the asymmetrical coupling of SPPs at normal incidence. Through a detailed analysis of the spatial spectra, we show that, in addition to SPPs, the field contains other near-field components. All these experiments demonstrate the expected MH-SNOM capabilities of measuring the amplitude, phase and polarization of optical near fields. The MH-SNOM therefore serves as a powerful tool for the investigation with subwavelength resolution of optical near fields generated in structures such as integrated optics, photonic crystals, cavities, resonators, etc.

EPFL2010

Developing luminescent Brownian probes for near-field investigations of the intracellular environment

Flavio Maurizio Mor

Life sciences have a constantly growing need for novel methodological approaches suitable to investigate the intracellular environment with increased temporal and spatial resolutions. Recently, optical near-field probes, such as laser-irradiated pointed metal, uncoated/metal-coated tapered optical fibers, as well as nano-emitters, such as single molecules or nanoparticles, have attracted increasing attention as key components of high-resolution microscopes. In parallel, super-resolution fluorescence microscopy, operating in far-field regime and overcoming the light diffraction limit, has markedly improved imaging resolution. Although both near- and far-field optical approaches drastically improve image resolution, they still represent numerous drawbacks, such as, technical difficulties concerning probe preparation (near-field) or potential damaging through very high light intensities (far-field). The aforementioned shortcomings of high-resolution optical microscopy can be overcome to a great extent by photonic force microscopy (PFM). PFM employs a strongly focused near-infrared (NIR) laser light to hold a dielectric or metallic particle as a local probe. Such an optical trap enables one to ‘cage’ a mesoscopic particle and track its three-dimensional (3D) thermal fluctuations in the surrounding environment, e.g. a viscous liquid. Therefore, besides offering near-field 3D imaging, PFM reports also on other important quantities concerning local mechanical properties, including force and viscosity as well as dynamical properties of the surrounding medium. This additional information is derived from the careful analysis of the jittery motion (so-called Brownian motion) of the trapped single-particle probe that collides with thermally-activated surrounding molecules. In this thesis, we first study in detail the Brownian motion of a single spherical particle that is confined within the harmonic potential of the optical trap. To this end, we optimize the NIR light path and electronic noise floor of our custom-built PFM set-up for detecting and quantifying resonances in the Brownian motion. These resonances arise from the coupling between the hydrodynamic memory and strong strength of the optical trap. Due to the high sensitivity of the short-time dynamics, the size of the particle can be measured by simultaneous fitting of the velocity autocorrelation function and power spectral density of its thermal fluctuations. In order to choose the best-suitable spherical probe for a given experiment in PFM, computational modeling based on a Matlab toolbox is performed. The generalized Lorenz-Mie theory is computed using the T -matrix method for various experimental conditions, including changes in the size and refractive index of the sphere, as well as different laser polarization states. The axial equilibrium position is examined to predict its location compared with the position of the laser focus. Optical forces acting on the sphere are investigated in 3D to highlight, in particular, the limits of the perfectly harmonic potential assumption. These limits are quantified and discussed. Subsequently, we identify different types of inorganic nano/micro-sized particles that can be trapped by the NIR laser of the PFM and used simultaneously as mechanical and near-field light sources. To this end, we take advantage of the visible light emission from single probes confined in the optical trap and excited by the NIR laser light. The emphasis is on particles revealing nonlinear optical properties. In particular, nonlinear optical field enhancement resulting from excitation of surface plasmon resonance (SPR) on trapped gold particles of 200 nm is demonstrated by measuring the emission of that second-harmonic generation (SHG). Furthermore, we show, under optical trapping conditions, SHG emission from single potassium niobate (KNbO3) nano/micro-sized particles. We also report on the multicolor upconversion luminescence (UCL), at the single particle level, for randomly shaped crystals of sodium yttrium fluoride codoped with ytterbium and erbium, NaYF4:Yb,Er (UCC), when they are held in the optical trap. In parallel, 3D Brownian fluctuations of a single trapped particle are quantified. Moreover, luminescence resonance energy transfer (LRET) between a KNbO3 particle or an UCC and molecules of an organic dye (rose bengal) adsorbed on their surface is highlighted and analyzed. Finally, randomly shaped UCCs and hexagonal nanoplates are characterized in detail at the single particle level. The hexagonal β phase in the polycrystalline and monocrystalline crystals is identified, in both randomly and hexagonally shaped particles by electron microscopy. UCL emission from the trapped crystal is quantified in terms of power and found to be dependent on the laser power density, size and shape of the particle as well as its orientation within the trap. Most importantly, the different colors in the UCL spectrum do not vary with the same trend for different orientations of the randomly or hexagonally shaped crystal upon trapping. This suggests the existence of crystalline anisotropy-dependent UCL. Thereby, we should be able to design the best-suited particle probe for LRET experiments with subwavelength resolution.

EPFL2013

Investigation of Surface Electromagnetic Waves with Multi-Heterodyne Scanning Near-Field Optical Microscopy

Tristan Sfez

EPFL2010

Developing luminescent Brownian probes for near-field investigations of the intracellular environment

Flavio Maurizio Mor

EPFL2013