Theoretical principles of near-field optical microscopies and spectroscopies

Near-field transmission and reflection spectroscopy for revealing absorption and scattering characteristics of single silver nanoplates

Near-field optical microscopy visualizes spatial characteristics of elementary excitations induced in metal nanostructures. However, the microscopy is not able to reveal the absorption and scattering characteristics of the object simultaneously. In this study, we demonstrate a method for revealing the absorption and scattering characteristics of silver nanoplate by using near-field transmission and reflection spectroscopy. Near-field transmission and reflection images show characteristic spatial features attributable to the excited plasmon modes. The near-field refection image near the resonance shows a reversed contrast depending on the observed wavelength. Near-field reflection spectra show unique positive and negative resonant features. We reveal that the optical characteristics and the wavelength dependency of the optical contrast originate from the scattering and absorption properties of the plasmons, with the aid of the electromagnetic simulations.

Probing vectorial near field of light: imaging theory and design principles of nanoprobes

Near-field microscopy is widely used for characterizing electromagnetic fields at nanoscale, where nanoprobes afford the opportunity to extract subwavelength optical quantities, including the amplitude, phase, polarization, chirality, etc. However, owing to the complexity of various nanoprobes, a general and intuitive theory is highly desired to assess the vectorial responses of nanoprobes and interpret the mechanism of the probe-field interaction. Here, we develop a general imaging theory based on the reciprocity of electromagnetism and multipole expansion analysis. The proposed theory closely resembles the multipolar Hamiltonian for light-matter interaction energy, revealing the coupling mechanism of the probe-field interaction. Based on this theory, we introduce a new paradigm for the design of functional nanoprobes by analyzing the reciprocal dipole moments, and establish effective design principles for the imaging of vectorial near fields. As application examples of the proposed theory, we numerically analyze the responses of two typical probes, a split-ring probe and a nanoparticle probe, which can quantitatively reproduce and well explain the experimental results of previously reported measurements of the optical magnetism and the transverse spin angular momentum. Our work provides a powerful tool for the design and analysis of new functional probes that may enable the probing of various physical quantities of the vectorial near field.

Local Density of States for Nanoplasmonics

We obtain the local density of states (LDOS) for any nanoplasmonic system in the frequency range dominated by a localized surface plasmon. By including the Ohmic losses in a consistent way, we show that the plasmon LDOS is proportional to the local field intensity normalized by the absorbed power. We obtain explicit formulas for the energy transfer (ET) between quantum emitters and plasmons as well as between donors and acceptors situated near a plasmonic structure. In the latter case, we find that the plasmon-assisted ET rate is proportional to the LDOS product at the donor and acceptor positions, obtain, in a general form, the plasmon ET enhancement factor, and establish the transition onset between Förster-dominated and plasmon-dominated ET regimes.

Unveiling nanometer scale extinction and scattering phenomena through combined electron energy loss spectroscopy and cathodoluminescence measurements

Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nanometer-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Polarization-resolved near-field mapping of plasmonic aperture emission by a dual-SNOM system

We study the polarization characteristics of light emission and collection in the near field by the tips of a Dual-SNOM (two scanning near-field optical microscopes) setup. We find that cantilevered fiber probes can serve as emitters of polarized light, or as polarization-sensitive detectors. The polarization characteristics depend on the fiber type used for tip fabrication. In Dual-SNOM measurements, we demonstrate mapping of different field components of the plasmonic dipole pattern emitted by an aperture probe.

Mapping magnetic near-field distributions of plasmonic nanoantennas

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

Understanding Plasmonic Properties in Metallic Nanostructures by Correlating Photonic and Electronic Excitations

A large number of optical phenomena rely on the excitation of localized surface plasmon resonances (LSPR) in metallic nanostructures. Electron-energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has emerged as a technique capable of mapping plasmonic properties on length scales 100 times smaller than optical wavelengths. While this technique is promising, the connection between electron-driven plasmons, encountered in EELS, and photon-driven plasmons, encountered in plasmonic devices, is not well understood. This Perspective highlights some of the contributions that have been made in correlating optical scattering and STEM/EELS from the exact same nanostructures. The experimental observations are further elucidated by comparison with theoretical calculations obtained from the electron-driven discrete dipole approximation, which provides a method to calculate EEL spectra for nanoparticles of arbitrary shape. Applications of plasmon mapping to the electromagnetic hot-spots encountered in single-molecule surface-enhanced Raman scattering and electron beam induced damage in silver nanocubes are discussed. It is anticipated that the complementarity of both techniques will address issues in fundamental and applied plasmonics going forward.

Adiabatic nanofocusing on ultrasmooth single-crystalline gold tapers creates a 10-nm-sized light source with few-cycle time resolution

We demonstrate adiabatic nanofocusing of few-cycle light pulses using ultrasharp and ultrasmooth single-crystalline gold tapers. We show that the grating-induced launching of spectrally broad-band surface plasmon polariton wavepackets onto the shaft of such a taper generates isolated, point-like light spots with 10 fs duration and 10 nm diameter spatial extent at its very apex. This nanofocusing is so efficient that nanolocalized electric fields inducing strong optical nonlinearities at the tip end are reached with conventional high repetition rate laser oscillators. We use here the resulting second harmonic to fully characterize the time structure of the localized electric field in frequency-resolved interferometric autocorrelation measurements. Our results strongly suggest that these nanometer-sized ultrafast light spots will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.

SUBWAVELENGTH OPTICAL DEVICES FOR NANOMETER SCALE APPLICATIONS

Recent progress in near-field optics instrumentation led to a new class of subwavelength optical experiments in which it is intended to use either the optical tunnel effect (OTE) or the lower mode based transmission (LMBT) in order to control the optical transfer between several delocalized detection or injection centers. This paper presents a panel of new theoretical and experimental results computed or observed near various dielectric or metallic patterns, linear, curved, or dashed, integrated in coplanar geometry. In particular, we demonstrate, how an efficient control of light evanescent waves can allow structures of subwavelength cross sections to be addressed.

Real-time terahertz near-field microscope

We report a terahertz near-field microscope with a high dynamic range that can capture images of a 370 x 740 μm2 area at 35 frames per second. We achieve high spatial resolution (14 μm corresponding to λ/30 for a center frequency at 0.7 THz) on a large area by combining two novel techniques: terahertz generation by tilted-pulse-front excitation and electro-optic balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a terahertz pulse.

Transmitting hertzian optical nanoantenna with free-electron feed

A pair of coupled gold nanorods excited by a beam of free electrons acts as a transmitting Hertzian antenna in the optical part of the spectrum. Significantly enhanced resonant emission is observed from the antenna when the electron beam is injected around the junction between the rods, where the local density of electromagnetic states is elevated.

Calculation of the local optical density of states in absorbing and gain media

The local optical density of states plays a key role in a wide range of phenomena. Near to structures displaying optical absorption or gain, the definition of the photonic local density of states needs to be revised. In this case two operative different definitions can be adopted to characterize photonic structures. The first (ρ(A)(r, ω)) describes the light intensity at a point r when the material system is illuminated isotropically and corresponds to what can be measured by a near-field microscope. The second (ρ(B)(r, ω)) gives a measure of vacuum fluctuations and coincides with ρ(A)(r, ω) in systems with real susceptibility. Scattering calculations in the presence of dielectric and metallic nanostructures show that these two definitions can give rather different results, the difference being proportional to the thermal emission power of the photonic structure. We present a detailed derivation of this result and numerical calculations for nanostructures displaying optical gain. In the presence of amplifying media, ρ(B)(r, ω) displays regions with negative photon densities, thus failing in describing a power signal. In contrast, ρ(A)(r, ω), positive definite, properly describes the near-field optical properties of these structures.

Probing physical properties of confined fluids within individual nanobubbles

Spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has been used to investigate a He fluidic phase in nanobubbles embedded in a metallic Pd(90)Pt(10) matrix. Using the 1s-->2p excitation of the He atoms, maps of the He density and pressure in bubbles of different diameters have been realized, to provide an indication of the bubble formation mechanism. Detailed local variations of the He K-line characteristics have been measured and interpreted as modifications of the electromagnetic properties of the He atom close to a metallic interface, which affects a correct estimation of the densities within the smallest bubbles.

Surface plasmon mediated near-field imaging and optical addressing in nanoscience

We present an overview of recent progress in "plasmonics". We focus our study on the observation and excitation of surface plasmon polaritons (SPPs) with optical near-field microscopy. We discuss in particular recent applications of photon scanning tunnelling microscope (PSTM) for imaging of SPP propagating in metal and dielectric wave guides. We show how near-field scanning optical microscopy (NSOM) can be used to optically and actively address remote nano objects such as quantum dots. Additionally we compare results obtained with near-field microcopy to those obtained with other optical far-field methods of analysis such as leakage radiation microscopy (LRM).

Theoretical study of electromagnetic scattering by metal nanoparticles

Progress in near-field optical spectroscopy research on metal nanoparticles demands a better understanding of the role of particle-particle and tip-sample interactions. In this perspective, we investigate theoretically, at a very moderate level of sophistication, the optical behavior of simple silver nanoparticle aggregates, in terms of a formalism involving a multipolar expansion of the fields involved, along with a simplified model for the optical behavior of nanostructures previously developed. In particular, the tip-sample interaction is taken into account roughly, treating the tip as an additional, single particle, characterized by proper dielectric behavior.

Near-field optical imaging of plasmon modes in gold nanorods

We have investigated optical properties of single gold nanorods by using an apertured-type scanning near-field optical microscope. Near-field transmission spectrum of single gold nanorod shows several longitudinal surface plasmon resonances. Transmission images observed at these resonance wavelengths show oscillating pattern along the long axis of the nanorod. The number of oscillation increases with decrement of observing wavelength. These spatial characteristics were well reproduced by calculated local density-of-states maps and were attributed to spatial characteristics of plasmon modes inside the nanorods. Dispersion relation for plasmons in gold nanorods was obtained by plotting the resonance frequencies of the plasmon modes versus the wave vectors obtained from the transmission images.

Dielectric optical elements for surface plasmons

Basic optical elements for surface plasmons are fabricated and their functionality (focusing, refraction, and total internal reflection) is demonstrated experimentally. The optical elements consist of dielectric structures of defined geometry on top of a gold film. The working principle of these structures is discussed on the basis of calculated surface plasmon dispersion relations.

Radiative and nonradiative decay rates of a molecule close to a metal particle of complex shape

We present a model to evaluate the radiative and nonradiative lifetimes of electronic excited states of a molecule close to a metal particle of complex shape and, possibly, in the presence of a solvent. The molecule is treated quantum mechanically at Hartree-Fock (HF) or density-functional theory (DFT) level. The metal/solvent is considered as a continuous body, characterized by its frequency dependent local dielectric constant. For simple metal shapes (planar infinite surface and spherical particle) a version of the polarizable continuum model based on the integral equation formalism has been used, while an alternative methodology has been implemented to treat metal particles of arbitrary shape. In both cases, equations have been numerically solved using a boundary element method. Excitation energies and nonradiative decay rates due to the energy transfer from the molecule to the metal are evaluated exploiting the linear response theory (TDHF or TDDFT where TD--time dependent). The radiative decay rate of the whole system (molecule + metal/solvent) is calculated, still using a continuum model, in terms of the response of the surrounding to the molecular transition. The model presented has been applied to the study of the radiative and nonradiative lifetimes of a lissamine molecule in solution (water) and close to gold spherical nanoparticles of different radius. In addition, the influence of the metal shape has been analyzed by performing calculations on a system composed by a coumarin-type molecule close to silver aggregates of complex shape.

New dimension in nano-imaging: breaking through the diffraction limit with scanning near-field optical microscopy

In recent years scanning near-field optical microscopy (SNOM) has developed into a powerful surface analytical technique for observing specimens with lateral resolution equal to or even better than 100 nm. A large number of applications, from material science to biology, have been reported. In this paper, two different kinds of near-field optical microscopy, aperture and scattering-type SNOM, are reviewed together with recent studies in surface analysis and biology. Here, near-field optical techniques are discussed in comparison with related methods, such as scanning probe and standard optical microscopy, with respect to their specific advantages and fields of application.

Application of evolution strategies for the solution of an inverse problem in near-field optics

We introduce an inversion procedure for the characterization of a nanostructure from near-field intensity data. The method proposed is based on heuristic arguments and makes use of evolution strategies for the solution of the inverse problem as a nonlinear constrained-optimization problem. By means of some examples we illustrate the performance of our inversion method. We also discuss its possibilities and potential applications.

Surface plasmon polaritons in metal stripes and wires

Surface plasmon polaritons (SPPs) are collective electron oscillations coupled to a light field which are propagating along the interface of a metal and a dielectric. As a surface wave, SPP modes feature properties essentially different from light-field modes in all dielectric structures. These properties could allow the realization of novel photonic devices that overcome certain limitations of conventional devices. Specifically, the realization of two-dimensional optics and light-field transport in sub-wavelength SPP waveguides seems feasible. In this review we discuss recent experimental advances regarding SPP waveguides, i.e. laterally confined metal thin films that guide SPPs. Electron-beam lithography is applied to tailor these films with widths ranging from a few micrometres (stripes) to nanoscopic values (wires). We investigate SPP properties such as propagation length, mode field profile and reflection or scattering at interfaces. Various techniques for SPP excitation and detection are discussed.

Near-field optical mapping of exciton wave functions in a GaAs quantum dot

Near-field photoluminescence imaging spectroscopy of naturally occurring GaAs quantum dots (QDs) is presented. We successfully mapped out center-of -mass wave functions of an exciton confined in a GaAs QD in real space due to the enhancement of spatial resolution up to 30 nm. As a consequence, we discovered that the spatial profile of the exciton emission, which reflects the shape of a monolayer-high island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. This novel technique can be extensively applied to wave function engineering in the design and the fabrication of quantum devices.

Photonic nanopatterns of gold nanostructures indicate the excitation of surface plasmon modes of a wavelength of 50-100 nm by scanning near-field optical microscopy

Scanning near-field optical microscopy images of metal nanostructures taken with the tetrahedral tip (T-tip) show a distribution of dark and bright spots at distances in the order of 25-50 nm. The images are interpreted as photonic nanopatterns defined as calculated scanning near-field optical microscopy images using a dipole serving as a light-emitting scanning near-field optical microscopy probe. Changing from a positive to a negative value of the dielectric function of a sample leads to the partition of one spot into several spots in the photonic nanopatterns, indicating the excitation of surface plasmons of a wavelength in the order of 50-100 nm in metal nanostructures.

Imaging the local density of states of optical corrals

This paper reports the experimental observation, at optical frequencies, of the electromagnetic local density of states established by nanostructures corresponding to the recently introduced concept of optical corral [G. Colas des Francs et al., Phys. Rev. Lett. 86, 4950 (2001)]. The images obtained by a scanning near-field optical microscope under specific operational conditions are found in agreement with the theoretical maps of the optical local density of states. A clear functionality of detection by the scanning near-field optical microscope is thereby identified since the theoretical maps are computed without including any specific tip model.

Polarization state of the optical near field

The polarization state of the optical electromagnetic field lying several nanometers above complex dielectric-air interfaces reveals the intricate light-matter interaction that occurs in the near-field zone. From the experimental point of view, access to this information is not direct and can only be extracted from an analysis of the polarization state of the detected light. These polarization states can be calculated by different numerical methods, well suited to near-field optics. In this paper, we apply two different techniques (localized Green's function method and differential theory of gratings) to separate each polarization component associated with both electric and magnetic optical near fields produced by nanometer sized objects. A simple dipolar model is used to get an insight into the physical origin of the near-field polarization state. In a second stage, accurate numerical simulations of field maps complete data produced by analytical models. We conclude this study by demonstrating the role played by the near-field polarization in the formation of the local density of states.

Direct interpretation of near-field optical images

The interpretation of the detection process in near-field optical microscopy is reviewed on the basis of a discussion about the possibility of establishing direct comparisons between experimental images and the solutions of Maxwell equations or the electromagnetic local density of states. On the basis of simple physical arguments, it is expected that the solutions of Maxwell equations should agree with images obtained by collecting mode near-field microscopes, while the electromagnetic local density of states should be considered to provide a practical interpretation of illumination mode near-field microscopes. We review collecting mode near-field microscope images where the conditions to obtain good agreement with the solutions of Maxwell equations have indeed been identified. In this context of collecting mode near-field microscopes, a fundamentally different functionality between dielectric and gold-coated tips has been clearly identified experimentally by checking against the solutions of Maxwell equations. It turns out that dielectric tips detect a signal proportional to the optical electric field intensity, whereas gold-coated tips detect a signal proportional to the optical magnetic field intensity. The possible implications of this surprising phenomenon are discussed.

Near-field optical addressing of single molecules in coplanar geometry: a theoretical study

Photonic transfer through elongated optical structures of submicrometre section microfabricated at the surface of dielectric or semiconductor samples can be enhanced by an appropriate structuring of the local refraction index. We show from computerized simulations that both the light localization and the spectroscopic properties of such structures can be used to selectively excite, in coplanar geometry, individuals molecules located in the near-field.

Optical addressing at the subwavelength scale

The Green dyadic formalism is applied to the study of the optical properties of dielectric subwavelength structures integrated in coplanar geometry. We first consider homogeneous wires with high refractive index featuring subwavelength cross sections. We show that such wires may have guiding properties and that they may be coupled with a local illumination produced by a focused Gaussian beam totally reflected at the substrate interface. When excited by the focused beam, these subwavelength optical waveguides (SOW's) provide a confined source of light that could be used to excite a single nanoscopic object. Well designed heteregeneous wires resulting from the alignment of dielectric particles separated from each other by a subwavelength distance are also found to propagate a Gaussian beam excitation over several micrometers. This propagation occurs with reasonable damping for incident beams in the visible frequency range. The computed transmission spectra of these heterowires may exhibit narrow gaps. Finally, we discuss the relation between the optical properties of the SOW and the calculated electromagnetic local density of states.