Application Of Scanning Near-field Optical Microscopy Research Articles

Terahertz nanoscopy: Advances, challenges, and the road ahead

Exploring nanoscale material properties through light-matter interactions is essential to unveil new phenomena and manipulate materials at the atomic level, paving the way for ground-breaking advancements in nanotechnology and materials science. Various elementary excitations and low-energy modes of materials reside in the terahertz (THz) range of the electromagnetic spectrum (0.1–10 THz) and occur over various spatial and temporal scales. However, due to the diffraction limit, a slew of THz studies are restricted to drawing conclusions from the spatially varying THz responses around half of the probing wavelengths, i.e., from tens to a couple of hundred micrometers. To address this fundamental challenge, scanning near-field optical microscopy (SNOM), notably scattering-type SNOM (s-SNOM), combined with THz sources has been employed and is fueling growing interest in this technique across multiple disciplines. This review (1) provides an overview of the system developments of SNOM, (2) evaluates current approaches to understand and quantify light-matter interactions, (3) explores advances in THz SNOM applications, especially studies with THz nano-scale spatial responses employing an s-SNOM, and (4) envisions future challenges and potential development avenues for the practical use of THz s-SNOM.

Invited Review Article: A review of techniques for attaching micro- and nanoparticles to a probe’s tip for surface force and near-field optical measurements

Cantilevers with single micro- or nanoparticle probes have been widely used for atomic force microscopy surface force measurements and apertureless scanning near-field optical microscopy applications. In this article, I critically review the particle attachment and modification techniques currently available, to help researchers choose the appropriate techniques for specific applications.

Dynamic force microscopy with quartz tuning forks at high oscillation amplitudes

Dynamic force microscopy (DFM) with the self-oscillator (SO)method allows reasonably high scanning rates even with highQ-factors of the resonant force sensor, typical of cantilevers in ultra-high vacuum and ofquartz tuning forks. However, due to simpler interpretation of force spectroscopymeasurements, small oscillation amplitudes (sub-nm level) are generally preferred. Inapplications like ‘apertureless’ scanning near-field optical microscopy (SNOM), oscillationamplitudes of the order of 5–10 nm are needed to increase optical sensitivity and toapply standard optical artefact suppression methods. This motivates the studyof the behaviour of tuning forks driven at such high amplitudes, as comparedto usual air-operated cantilevers. Both constant-excitation-amplitude (CE) andconstant-oscillation-amplitude (CA) modes of SO-DFM are analysed, since the CA mode ismore convenient for SNOM applications, denoting remarkable differences. In particular,possible instability effects, previously found in CE mode, are not anticipated for CAmode. It is shown how resonance and approach (‘isophase’) curves in both modescan be conveniently described in terms of the usual ‘normalized frequency shift’γ and of a‘normalized gain’ η, defined as a measurement of surface dissipation.

Methods for Molecular Nanoanalysis

This contribution reviews methods based on scanning probe microscopy for molecular analysis and identification on a length scale of 20–200 nm. Aperture scanning near-field optical microscopy (SNOM) was initially developed in our group for chemical analysis and imaging using fluorescence and Raman spectroscopies, with a resolution beyond the optical diffraction limit. In recent years, tip-enhanced Raman spectroscopy was developed into an analytical tool capable of a spatial resolution less than 50 nm and, using the 'gap' mode of operation, the promise for single molecule sensitivity and even higher spatial resolution. Aperture SNOM with pulsed laser irradiation can be used for nanoscale laser ablation. With an interface to a very sensitive mass spectrometer, this provides a platform for nanoscale mass spectrometry using atmospheric pressure sampling. The preparation of fiber tips for aperture SNOM as well as metallized a FM tips and etched metal wires for aperture less SNOM applications is reviewed. Finally, applications for the chemical analysis of thin molecular films and of biological samples are presented.

Analysis of mode coupling due to spherical defects in ideal fully metal-coated scanning near-field optical microscopy probes

We investigate the effect of defects in the metal-coating layer of a scanning near-field optical microscopy (SNOM) probe on the coupling of polarization modes using rigorous electromagnetic modeling tools. Because of practical limitations, we study an ensemble of simple defects to identify important trends and then extrapolate these results to more realistic structures. We find that a probe with many random defects will produce a small but significant coupling of energy between a linearly polarized input mode and a radial/longitudinal polarization mode, which is known to produce a strongly localized emitted optical field and is desirable for SNOM applications.

High-Resolution Optical Imaging of Magnetic-Domain Structures

High-resolution optical techniques for imaging magnetic domains in ferromagnetic materials such as confocal microscopy and scanning near-field optical microscopy (SNOM) are reviewed. The imaging capabilities of different techniques and image formation are discussed in the case of in-plane as well as out-of-plane magnetic anisotropy in different illumination configurations. It is shown that the magnetooptical resolution of near-field measurements depends on the film thickness and is limited by the diffraction on magnetic domains throughout the film. For thin magnetic films, subwavelength resolution can be attained. In addition to well-known near-field magnetooptical effects (out-of plane magnetization sensitivity of linear near-field microscopy and in-plane magnetization sensitivity of nonlinear near-field measurements), linear SNOM imaging of in-plane magnetization in thin magnetic films as well as nonlinear imaging of out-of-plane domains has been demonstrated. Thus, linear and second-harmonic near-field imaging of both in-plane and out-of-plane oriented magnetic domains can be achieved in transparent and opaque specimens. This enables applications of SNOM for studies of all kinds of magnetic materials. High-resolution optical imaging is required for characterization of the micro-magnetic and magnetooptical properties of novel magnetic structures in order to adopt a bottom-up approach in the search for new materials with improved characteristics.

Micromachined photoplastic probe for scanning near-field optical microscopy

We present a hybrid probe for scanning near-field optical microscopy (SNOM), which consists of a micromachined photoplastic tip with a metallic aperture at the apex that is attached to an optical fiber, thus combining the advantages of optical fiber probes and micromachined tips. The tip and aperture are batch fabricated and assembled to a preetched optical fiber with micrometer centering precision. Rectangular apertures of 50 nm×130 nm have been produced without the need of any postprocessing. Topographical and optical imaging with a probe having an aperture of 300 nm demonstrate the great potential of the photoplastic probe for SNOM applications.

Micromachined Photoplastic Probes for Scanning Probe Microscopy

Scanning probe microscopy (SPM) is a well-established technique for surface analysis, but batch-fabricated, low-cost probes still remain a challenging issue. The design and fabrication of entirely photoplastic probes for scanning force microscopy (SFM) have been developed and used for imaging DNA molecules. Using a polymer for the cantilever facilitates the realization of mechanical properties that are difficult to achieve with classical silicon technology. Single lever and cassettes of multiple single-lever probes are presented. The probes are made of an epoxy-based photoresist, which can be structured by standard photolithography and molding techniques. The fabrication process is a simple batch process in which the integrated tips and the levers are defined in one photolithography step. Imaging soft, condensed matter with photoplastic levers, which uses laser beam deflection sensing, exhibits a resolution that compares well with that of commercially available silicon cantilevers. A similar fabrication technique was also developed to fabricate photoplastic tips for scanning near-field optical microscopy (SNOM) that are to be attached to optical fibers. This technique allows optical apertures to be integrated at the end of the well-defined tip directly by probe fabrication, without the need for any post-processing for the aperture formation. Simple fabrication, as well as topographical and optical imaging with such probes, demonstrate the potential of photoplastic-based probes for AFM and SNOM applications.

Near-field imaging of the photocurrent on a patterned Au/GaAs interface with various wavelengths and bias

This contribution presents an application of scanning near-field optical microscopy to the characterization of semi-conductors. It is based on the photocurrent mapping of a patterned Au/GaAs structure (Schottky barrier) under local illumination by the nanosource. The results obtained with different wavelengths, metallized or dielectric probes and different bias voltages exhibit photocurrent variations independent of the topography and induced by interface defects. Finally, from this study of a patterned planar structure, we propose a method to determine the mean free path of the charge carriers in the volume.

Cantilever probes for SNOM applications with single and double aperture tips

In this paper we introduce a cantilever-based probe concept combining scanning force microscopy (SFM) and scanning near-field optical microscopy (SNOM). Microstructure technology processes are used to fabricate probes with hollow-metal aperture tips of less than 60 nm aperture size integrated in the very end of a cantilever. Probe characterization revealed reproducible optical and mechanical properties for both SFM and SNOM applications. The dependence of the optical transmission of aperture tips on their aperture size was investigated as a function of the polarization state. To determine the lateral resolution in the optical transmission-mode polarization-dependent measurements on micro-fabricated test samples, e.g. grid-like structures, were performed. Preliminary results of the investigation of Garnet samples in a near-field Faraday microscope set-up are presented.