Apertureless Near-field Microscopy Research Articles

Phonon-polariton Bragg generation at the surface of silicon carbide

Phonon-polaritons are known to emerge at the surface of solids under infrared (IR) irradiation at frequencies close to the optical phonon resonance. Metal, patterned on the top of the polariton-active surface, locally blocks the excitation of surface waves due to plasmonic screening and can be used for the design of wave patterns. We excite polaritonic waves at the surface of SiC under the irradiation of a CO2 laser (λ∼10μm) and visualize them using apertureless near-field interference scanning probe microscopy. From the near-field scans in the vicinity of gold film periodical strip structures, we identify the Bragg scattering (diffraction) outside the grating with the contribution from separate strips coherently summed up, provided that the wavelength matching condition is fulfilled. The observed phenomena agree with wavefield calculations. Our observations demonstrate the potential of metal-patterned silicon carbide for the fabrication of on-chip polaritonic IR circuits.

Near-Field IR Nanoscale Imaging of the Solid Electrolyte Interphase on a HOPG Electrode

The SEI layer on graphitic carbon electrodes is well known to protect effectively the electrode from further electrolyte reduction during long-term charge-discharge cycling process. Many different techniques have been applied to characterize the chemical and structural composition of this complex surface film. The standard vibrational optical spectroscopies, which offer molecular-level information are subject to the diffraction limit, which restricts their ability to probe at the nanoscale level of the SEI building blocks. This work exploits infrared apertureless near-field microscopy that operates below the diffraction limit to characterize the SEI layer on a model HOPG electrode. Variations in surface topography and chemical contrast are discussed in the context of SEI composition and function. The promise of near-field techniques for characterization of electrochemical interfaces is briefly evaluated.

Three-dimensional winged nanocone optical antennas.

We introduce 3D optical antennas based on winged nanocones. The antennas support particle plasmon oscillations with current distributions that facilitate transformation of transverse far-field radiation to strong longitudinal local fields near the cone apices. We characterize the optical responses of the antennas by their extinction spectra and by second-harmonic generation microscopy with cylindrical vector beams. The results demonstrate a new 3D polarization-controllable optical antenna for applications in apertureless near-field microscopy, spectroscopy, and plasmonic sensing.

Apertureless near-field microscopy using a knife blade as a scanning probe at millimeter wavelengths

We report on the use of a knife blade as a scanning probe for apertureless near-field microscopy at millimeter wavelengths. Since the knife blade probe is a wider version of the metal tip probe commonly used in this technique, and therefore the interaction area between the probe tip and the sample is larger, an improvement in the intensity of the measured near-field signal is expected. The knife blade probe can also work as a part of a resonator in the illumination optics used in this microscopy format to enhance the strength of the near field that interacts with the sample, resulting in a further improvement in the signal intensity. A scanning method and an image reconstruction algorithm based on computerized tomography are adopted to obtain 2-D near-field images. Experiments performed at 60 GHz using a knife blade with a tip radius of 6 μm (∼λ/1000) show that the signal intensity is enhanced by ∼20 dB compared with an equivalent metal tip probe, and that an image resolution approaching the tip radius of the knife blade is achieved.

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.

High Resolution Imaging of Actin Filaments in Living Cells Under Physilogically Relevant Conditions Using Apertureless Near-Field Microscopy

Living cells were used to demonstrate the potential biological applications of the apertureless SNOM when operating under fluid. An oral epithelial squamous cell carcinoma cell line (H357) was imaged under physiological-like conditions using apertureless SNOM following staining with FITC-Phalloidin which preferentially stains intracellular actin filaments of the cytoskeleton. Compared with simultaneously obtained AFM topographic image, the apertureless SNOM data provides greater detail on these cellular structures and the spatial resolution of the apertureless SNOM fluorescence image appears to be about 100 nm.

A study of background signals in terahertz apertureless near-field microscopy and their use for scattering-probe imaging

Apertureless near-field microscopy is an imaging technique in which a small metal tip is held close to a surface, converting evanescent waves to propagating waves and permitting extreme subwavelength spatial resolution. This technique has recently been adapted for use in the terahertz region of the spectrum. Here, the interpretation of the measured signals and the suppression of background scattering can be complicated by the extremely broad bandwidth of the terahertz source and by the coherent (i.e., phase-sensitive) detection of the scattered radiation. We have analyzed the use of tip-sample distance modulation for the removal of background signals. We find that significant background signals, originating from scattering off the probe tip, can be observed even after modulation. These background signals result from path-length difference modulation, and thus are relevant when phase-sensitive detection is used. We use a dipole antenna model to explain the spatial variation in the scattered signal. Since this signal originates from the tip only, it can be used to characterize free-space terahertz wave fronts with subwavelength resolution.

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.

Strongly subluminal regime of optical-to-terahertz conversion in GaP

We introduce and investigate a special regime of terahertz generation in electro-optic crystals with ultrashort laser pulses, in which neither phase-matching nor Cherenkov radiation mechanism are efficient. This regime occurs under the conditions that the optical group refractive index is larger than the low-frequency phase refractive index of the crystal, and the spectral bandwidth of the laser pulse is smaller than the frequency of the phase-matched terahertz wave. In this regime, that can be realized for a Ti:sapphire laser in GaP, terahertz generation is provided by transient processes at the crystal boundaries. We study the peculiarities of the terahertz emission from a slab of GaP excited with a Ti:sapphire laser. In particular, we demonstrate a possibility to implement a pointlike source of terahertz radiation that can increase the resolution of terahertz apertureless near-field microscopy.

Surface plasmons in terahertz metamaterials

We characterize terahertz metamaterials by applying apertureless near-field microscopy with a bandwidth that covers the entire spectral response of the structures. The observations agree with the interpretation of the fundamental mode of the metamaterial. But the high frequency resonance shows properties that deviate from the common interpretation. We show that the high frequency response is governed by surface Plasmon excitations, which have a comparable oscillator strength as the fundamental mode.

Apertureless Near-Field Distance-Dependent Lifetime Imaging and Spectroscopy of Semiconductor Nanocrystals

Apertureless near-field scanning optical microscopy, along with time-resolving capabilities, is used to produce optical imaging and spectroscopy measurements of single-semiconductor nanocrystals, in correlation with the AFM topography scan. The strongly distance-dependent energy transfer between the excited particle and silicon or metallic-coated AFM tips provides a contrast mechanism for subdiffraction-limited optical imaging. Fluorescence lifetime optical images show excellent contrast, sharpness, sensitivity, and resolution equivalent to that of the AFM topography images (sub 20 nm) and significantly improved over fluorescence intensity images. The sharper resolution of lifetime images is consistent with model predictions of energy transfer between an emitting dipole and a dielectric surface. Lifetime images also enable resolving multiple emitters located in the excitation spot. The comprehensive time and distance dependent data is used to study the imaging mechanism and the properties of silicon tips and platinum-coated tips as energy acceptors and quenchers. The findings provide a basis for use of lifetime imaging, in conjunction with apertureless near field microscopy, for simultaneous high-resolution topography and optical imaging.

Chemical Imaging of the Surface of Self-Assembled Polystyrene-b-Poly(methyl methacrylate) Diblock Copolymer Films Using Apertureless Near-Field IR Microscopy

The nanoscale chemical composition variations of the surfaces of thin films of polystyrene- b-poly(methyl methacrylate) (PS- b-PMMA) diblock copolymers are investigated using apertureless near-field IR microscopy. The scattering of the incident infrared beam from a modulated atomic force microscopy (AFM) tip is probed using homodyne detection and demodulation at the tip oscillation frequency. An increase in the IR attenuation is observed in the PMMA-rich domains with a wavenumber dependence that is consistent with the bulk absorption spectrum. The results indicate that even though a small topography-induced artifact can be observed in the near-field images, the chemical signature of the sample is detected clearly.

Polarization-selective mapping of near-field intensity and phase around gold nanoparticles using apertureless near-field microscopy

Enhanced near-field distributions around a single gold nanosphere are imaged using scattering-type apertureless near field scanning optical microscopy (ANSOM) at a wavelength of 632.8 nm. For the first time, polarization-selected ANSOM images are obtained that show both the transverse (perpendicular to the tip axis) and the longitudinal (parallel to the tip axis) vector components of the near-field in a phase sensitive manner. A model calculation using a Green's dyadic propagator method successfully reproduces the features of the observed intensity and phase images, providing an interpretation of the ANSOM images. The results open up the possibility that the field vector directions as well as the field magnitude around plasmonic nanostructures and nanodevices can be directly mapped using the ANSOM technique.

Nanoscale Fluorescence Microscopy Using Carbon Nanotubes

We demonstrate the first reported use of single-walled carbon nanotubes as nano-optical probes in apertureless near-field fluorescence microscopy. We show that, in contrast to silicon probes, carbon nanotubes always cause strong fluorescence quenching when used to image dye-doped polystyrene spheres and Cd-Se quantum dots. For quantum dots, the carbon nanotubes induce very strong near-field contrast with a spatial resolution of 20 nm. Images of dye-doped spheres exhibit crescent-shaped artifacts caused by distortions in the surface water layer found in ambient conditions.

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.

Nanometer-Scale Dielectric Imaging of Semiconductor Nanoparticles: Size-Dependent Dipolar Coupling and Contrast Reversal

Scattering-type apertureless near-field microscopy (ANSOM) provides high-resolution dielectric maps of indium gallium nitride (InGaN) semiconductor nanoparticles at visible (633 nm) wavelengths. A specific size-dependent contrast reversal is observed in the ANSOM images of InGaN nanoparticles grown on a layer of gallium nitride (GaN). Model calculations demonstrate that the observed contrast reversal is the result of the competition between the tip-particle versus tip-substrate dipolar coupling.

Coherent scattering phenomena in apertureless scanning near-field fluorescence microscopy

We study near-field fluorescence images of samples composed of aggregates of 100 nm dye-doped latex spheres. These images have been performed by a reflection Apertureless Scanning Near-field Optical Microscope (A-SNOM). We show that the near-field distribution in fluorescence A-SNOM images arises from coherent scattering phenomena between all spheres. This is a consequence of the coherent nature of the fluorescence emission of each single sphere. The results shown here are significant for all fluorescent samples characterised by a nonnegligible topography.

Arrays of Well-Defined Size-Tunable Metallic Nano-Cones for Plasmonic Applications

ABSTRACTCones composed of noble metals with dimensions of the order of 100 nm, especially gold cones, are extremely well suited for optical near-field investigations using visible light. Their plasmon resonance frequency lies within the range of visible light frequencies, and upon illumination, strong electric field enhancement can be observed in the direct vicinity of the cone tip. For this purpose, a method to fabricate well-controlled nanocones with sharp tips is required. We present a top-down process in which arrays of cones are dry-etched from a metal stack on silicon. During the ion milling step, patterned hydrogen silsesquioxane resist is used as an etch mask. The resulting cones have tunable base diameters around 150 nm and tip radii down to less than 10 nm. Their optical characteristics are investigated by means of apertureless optical near-field microscopy, in which field enhancement at the cone tip is observed.

Fluorescence Near-Field Microscopy of DNA at Sub-10 nm Resolution

We demonstrate apertureless near-field microscopy of single molecules at sub-10 nm resolution. With a novel phase filter, near-field images of single organic fluorophores were obtained with approximately sixfold improvement in the signal-to-noise ratio. The improvement allowed pairs of molecules separated by approximately 15 nm to be reliably and repeatedly resolved, thus demonstrating the first true Rayleigh resolution test for near-field images of single molecules. The potential of this technique for biological applications was demonstrated with an experiment that measured the helical rise of A-form DNA.

Field localization in very small aperture lasers studied by apertureless near-field microscopy

Localized surface plasmon polaritons (SSPs) have been observed on very small aperture lasers using apertureless near-field microscopy. Fields around multiple apertures are shown to result from interferences of SPP point sources at each aperture and optical fields. The near-field optical pattern around a single aperture indicates the interference of SPPs with their scattered counterparts. Near-field measurements also confirmed a preferred orientation of the rectangular aperture waveguide for the signal localization in very small aperture lasers.