Apertureless Near-field Optical Microscopy Research Articles

Terahertz response of monolayer and few-layer WTe2 at the nanoscale

Tungsten ditelluride (WTe2) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this report, we use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe2 in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with Delta approx -10,{{{rm{meV}}}} over the semiconductor scenario for bilayer WTe2. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons in the THz range confined to trilayer terraces in our specimens. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe2 asymptotically approaches the bulk limit.

Effect of the gold crystallinity on the enhanced luminescence signal of scanning probe tips in apertureless near-field optical microscopy.

The effect of gold tip crystallinity on their spectral amplification characteristics, monitored through the luminescence enhanced by surface plasmon resonance (SPR), is investigated experimentally. As the tip radius increases, the grains composing polycrystalline tips become larger, resulting in a blueshift of the emission while a redshift of the SPR was predicted for monocrystalline gold. This reveals that the effect of the grain size, a parameter that has not been considered so far, is dominant over that of the tip radius. This study is significant to apertureless scanning near-field optical microscopy, where the gold tip emission defines the spectral antenna range.

Apertureless near-field optical microscopy

We discuss the operating principles of the apertureless scanning near-field optical microscope (ASNOM), in which the probe acts as a rod antenna and its electromagnetic radiation plays the role of the registered signal. The phase and amplitude of the emitted wave vary depending on the ‘grounding conditions’ of the antenna tip at the sample point under study. Weak radiation from a tiny (2–15 μm long) tip is detected using optical homo- and heterodyning and the nonlinear dependence of the tip polarizability on the tip–surface distance. The lateral resolution of ASNOMs is determined by the tip curvature radius (1– 20 nm), regardless of the wavelength (500 nm–100 μm). ASNOMs are shown to be capable of providing a surface optical map with nanometer resolution and carrying out spectral- and time-resolved measurements at a selected point on the surface.

Безапертурная микроскопия ближнего оптического поля

Безапертурная микроскопия ближнего оптического поля

Grating enhanced apertureless near-field optical microscopy.

We examine the possibility of optimizing the emission and the near-field signal of apertureless silver and gold tips by using an optimized non-periodic grating. In this context, we consider the emission of a single quantum emitter in close proximity to optimized tips. Additionally, we study the far-field coupling efficiency of a tightly focused beam to the near-field of the tip. The gain in performance is compared with unstructured tips and the comparison with a pure plasmonic excitation of an unstructured tip is discussed. The optimized, structured tips show a significant enhancement of the total decay rate, as a result of standing plasmonic waves between the grating and the tip apex, leading to a resonant behavior. The resonances can be explained well with a Fabry-Pérot model. Furthermore, the total decay rate of an emitter near a structured tip can also be decreased as compared to an unstructured tip, when the grating is shifted from the optimal resonant position. The proposed scheme represents an interesting novel nano-antenna, for which the resonance as well as the directivity can be controlled by the grating.

Near-Field Mapping of Optical Modes on All-Dielectric Silicon Nanodisks

We measure, for the first time to our knowledge, the near-field amplitudes and phases of localized optical modes of high-index all-dielectric nanoparticles using apertureless near-field optical microscopy. For individual silicon nanodisks, we observe a four-lobed mode pattern and the formation of deep-subwavelength hot-spots. Our numerical calculations of the optical near-fields of the nanodisks in combination with a multipole expansion of the scattered field based on vector spherical harmonics reveal that the observed modes are dominated by electric quadrupole contributions. The observed mode is of particular interest for the design of low-loss all-dielectric metasurfaces and nanoantennas for a broad range of applications, such as directional and complex-polarization controlled emission, light extraction from multipolar atomic transitions, and coherent multiple-emitter-nanocavity interactions.

Probe-sample optical interaction: size and wavelength dependence in localized plasmon near-field imaging

The probe-sample optical interaction in apertureless near-field optical microscopy is studied at 633 nm and 808 nm excitation wavelengths using gold nanodisks as model systems. The near-field distributions of the dipolar and quadrupolar surface plasmon modes have been mapped successfully using metal coated probes with different polarization combinations of excitation and detection except when the incident and the scattered light polarizations are chosen to be parallel to the probe axis. For the parallel polarization of the incident and the scattered light, the pattern of the near-field distribution differs from the inherent plasmon mode structures of the sample, depending sensitively on the sample size and excitation energy. For a given excitation energy, the near-field amplitude shifts from one pole to the other as the sample size increases, having nearly equal amplitude at the two poles when the plasmon resonance peak spectrally overlaps with the excitation energy.

High aspect ratio nanoneedle probes with an integrated electrode at the tip apex

Many nanoscale characterization techniques require high aspect ratio nanoneedle probes with an integrated electrode that is electrically insulated everywhere except at the tip apex. We report the utilization of electron beam induced deposition, focused ion beam milling, and atomic layer deposition to fabricate such probes at the sub-100 nm length scale. This fabrication method is highly reproducible and enables precise control of the probe dimensions. Subsequent electrodeposition at the integrated electrode enables customized functionalization of the tip apex. These probes have clear applications in scanning electrochemical microscopy-atomic force microscopy, magnetic force microscopy, apertureless near-field optical microscopy, and tip-enhanced Raman spectroscopy.

Optimization ofs-Polarization Sensitivity in Apertureless Near-Field Optical Microscopy

It is a general belief in apertureless near-field microscopy that the so-calledp-polarization configuration, where the incident light is polarized parallel to the axis of the probe, is advantageous to its counterpart, thes-polarization configuration, where the incident light is polarized perpendicular to the probe axis. While this is true for most samples under common near-field experimental conditions, there are samples which respond better to thes-polarization configuration due to their orientations. Indeed, there have been several reports that have discussed such samples. This leads us to an important requirement that the near-field experimental setup should be equipped with proper sensitivity for measurements withs-polarization configuration. This requires not only creation of effective s-polarized illumination at the near-field probe, but also proper enhancement of s-polarized light by the probe. In this paper, we have examined thes-polarization enhancement sensitivity of near-field probes by measuring and evaluating the near-field Rayleigh scattering images constructed by a variety of probes. We found that thes-polarization enhancement sensitivity strongly depends on the sharpness of the apex of near-field probes. We have discussed the efficient value of probe sharpness by considering a balance between the enhancement and the spatial resolution, both of which are essential requirements of apertureless near-field microscopy.

Apertureless near-field optical microscopy: Differences between heterodyne interferometric and non-interferometric images

We analyze simulations of the image generation process in apertureless Scanning Near-Field Optical Microscopy to study the differences between heterodyne interferometric and non-interferometric detection schemes implemented in experiments. Beyond the well-known possibilities of interferometry to enhance signals and extract optical phase information, we find that heterodyne interferometric detection may in fact lead to more robust and easier to interpret experimental results. Further, our results illustrate how simulations that do not include the use of interferometric detection in a given experiment can lead to significant disagreements with the measurements.

White-light total internal reflection microscopy for characterizing the optical properties of Ag-coated optical fiber tips

The optical properties of the metallized tips used in tip-enhanced Raman spectroscopy (TERS) are vital to resonance enhancement. A white-light scattering-spectrum measuring setup based on total internal reflection microscopy is developed to characterize the optical responses of the metallized tips. The scattering spectra of the Ag-coated optical fiber tips are measured as a demonstration of the methodology and the performance of the setup. A prominent resonance band of 500-550 nm is observed experimentally, which can be interpreted as the result of the excitation of the localized surface plasmon resonance at the Ag-coated optical fiber tip. The optical responses of the Ag-coated optical fiber tips are simulated by a three-dimensional finite-difference time-domain method and the results are compared with the experiments. This method has great application potential in the selection and design of high-performance metallized tips for TERS and other apertureless near-field optical microscopy.

Nanometer-scale size dependent imaging of cetyl trimethyl ammonium bromide (CTAB) capped and uncapped gold nanoparticles by apertureless near-field optical microscopy

Apertureless near-field scanning optical microscopy (ANSOM) is used to image optical near-field light scattering from uncapped gold nanoparticles and gold nanoparticles capped with the cationic surfactant cetyl trimethyl ammonium bromide (CTAB). The measurements investigate the gold-particle size-dependent signals and the modification of those signals by the spacer layer of commonly used CTAB in the visible at λ = 633 nm. Imaging of capped nanoparticles by apertureless near-field microscopy opens the possibility to predict quantitative layer thicknesses of capping agents on the surface of nanoparticles, as well as the effect of capping layers on the optical scattering properties of nanoparticles.

The metal-insulator transition in VO2 studied using terahertz apertureless near-field microscopy

We have studied the metal-insulator transition in a vanadium dioxide (VO2) thin film using terahertz apertureless near-field optical microscopy. We observe a variation of the terahertz amplitude due to the phase transition induced by an applied voltage across the sample. The change of the terahertz signal is related to the abrupt change of the conductivity of the VO2 film at the metal-insulator transition. The subwavelength spatial resolution of this near-field microscopy makes it possible to detect signatures of micron-scale metallic domains in inhomogeneous VO2 thin films.

PFabrication of gold tips by chemical etching in aqua regia

We present a method to produce sharp gold tips for applications in apertureless near-field optical microscopy and spectroscopy. Thin gold wires are tapered by chemical etching in aqua regia, covered by an isooctane protective layer. Tips with apical radii of curvature of <50 nm are obtained with a 40% yield. The tip performances have been checked by shear-force imaging of amyloid fibrils samples and compared to optical fiber probes. The analysis of the tip morphology, carried out by scanning electron microscopy, shows the existence of two different etching processes occurring in bulk and at the liquid-liquid interface. A simple analytical model is presented to describe the dynamics of the tip formation at the liquid-liquid meniscus interface that fits remarkably well the experimental results in terms of tip shape and length.

Measurement and calculation of the near field of a terahertz apertureless scanning optical microscope

We present measurements and calculations of the terahertz (THz) electric field measured in the near field of a metal tip used in THz apertureless near-field optical microscopy (THz-ANSOM). An analytical model in which we treat the metal tip as a linear wire antenna allows us to predict almost all of the features observed in the measurements, such as the relatively slow decay of the near-field amplitude when the tip-crystal separation increases. When the tip-crystal separation is modulated, in conjunction with lock-in detection at the modulation frequency, a smaller THz spot size is observed underneath the tip. A comparison with analytical expressions shows that in this case the electric field originates predominantly from the tip apex, with negligible contributions from the tip shaft. In the unmodulated case, the observed signal is the spatial integral of the electro-optic (EO) effect over the interaction length between the THz near field and the probe laser pulse. In the modulated case, to a good approximation, we find that the signal is proportional to the value of the THz near field at the surface of the EO crystal only.

Artifacts identification in apertureless near-field optical microscopy

The aim of this paper is to provide criteria for optical artifacts recognition in reflection-mode apertureless scanning near-field optical microscopy, implementing demodulation techniques at higher harmonics. We show that optical images acquired at different harmonics, although totally uncorrelated from the topography, can be entirely due to far-field artifacts. Such observations are interpreted by developing the dipole-dipole model for the detection scheme at higher harmonics. The model, confirmed by the experiment, predicts a lack of correlation between the topography and optical images even for structures a few tens of nanometers high, due to the rectification effect introduced by the lock-in amplifier used for signal demodulation. Analytical formulas deduced for the far-field background permit to simulate and identify all the different fictitious patterns to be expected from metallic nanowires or nanoparticles of a given shape. In particular, the background dependence on the tip-oscillation amplitude is put forward as the cause of the error-signal artifacts, suggesting, at the same time, specific fine-tuning configurations for background-free imaging. Finally a careful analysis of the phase signal is carried out. In particular, our model correctly interprets the steplike dependence observed experimentally of the background phase signal versus the tip-sample distance, and suggests to look for smooth variations of the phase signal for unambiguous near-field imaging assessment.

Tip-enhanced fluorescence microscopy of high-density samples

High-density samples of fluorescent quantum dots (QDs) were imaged using an apertureless near-field optical microscopy technique. QD fluorescence was modulated by oscillating a silicon atomic force microscope tip above an illuminated sample and a lock-in amplifier was used to suppress background from the excitation laser. Spatial resolution near 10nm and a peak signal-to-noise ratio (SNR) of ∼60 were achieved. Individual QDs within high-density ensembles were still easily resolved (SNR&amp;gt;5) at a density of 14QDs∕μm2. These results have favorable implications for the eventual nanoscale imaging of viable biological systems, such as cellular membranes.

Far-field background suppression in tip-modulated apertureless near-field optical microscopy

In apertureless near-field optical microscopy the vertical dithering of the tip, associated with demodulation at higher harmonics (n&amp;gt;1), allows us to suppress the far-field background, providing artifact free elastic scattering images. This paper analyzes, both theoretically and experimentally, the physical origin of the background signal at the different harmonics and the mechanisms underlying its rejection for the general case of propagative-field illumination. We show that Fourier components of the background must be expected at every harmonic, evidencing why demodulation at higher harmonics is not an inherently background-free technique, and assessing the experimental conditions in which it becomes like that. In particular, we put forward the fundamental roles of both the harmonic order and the tip oscillation amplitude in the background suppression mechanisms. Furthermore, we outline how the lock-in detection of the signals amplitude can enhance the nonlinear dependence of the background on the tip-sample distance. Such effect provides a more subtle source of topography artifacts since the optical maps become qualitatively uncorrelated from the topographic counterpart, requiring an upgrade of the criteria to assess the absence of artifacts from the optical maps.

Controllable method for the preparation of metalized probes for efficient scanning near-field optical Raman microscopy

A modification of the mirror reaction method has been used to metalize atomic force microscope (AFM) probes for apertureless near-field optical microscopy. The method produces a single silver particle of controllable size at the apex of the AFM tip with no detrimental effects on the AFM probe. A particle of the order 80nm in diameter appears to provide the best tip-enhanced Raman signal using 488nm excitation. The near-field Raman spatial resolution of one such probe was shown to be as high as 24nm using single-walled carbon nanotubes as a test sample.

Interferometric measurement of the tip oscillation amplitude in apertureless near-field optical microscopy

We have implemented an optical homodyne interferometer to measure the tip oscillation amplitude in apertureless near-field optical microscopy. The setup is fully embedded in the microscope’s design, avoiding the presence of external arms. Our method is based on the synchronous detection of the interference between the fields reflected by the tip and a glass sample surface, while scanning the tip–sample distance over a few wavelengths. With the help of a simple model, we show how the different interference terms arising at frequencies multiple of the tip oscillation can be exploited to easily achieve sub-Ångstrom resolution.