Apertureless Scanning Near-field Optical Microscopy Research Articles

An Apertureless Scanning Near-Field Optical Microscope Probe with a Lateral Resolution of 10 – 15 nm Observed with a Semiconductor Structure

An Apertureless Scanning Near-Field Optical Microscope Probe with a Lateral Resolution of 10 – 15 nm Observed with a Semiconductor Structure

Near-infrared nanospectroscopy using a low-noise supercontinuum source

Unlocking the true potential of optical spectroscopy on the nanoscale requires development of stable and low-noise laser sources. Here, we have developed a low-noise supercontinuum (SC) source based on an all-normal dispersion fiber pumped by a femtosecond fiber laser and demonstrate high resolution, spectrally resolved near-field measurements in the near-infrared (NIR) region. Specifically, we explore the reduced-noise requirements for aperture-less scattering-type scanning near-field optical microscopy (s-SNOM), including inherent pulse-to-pulse fluctuation of the SC. We use our SC light source to demonstrate the first NIR, spectrally resolved s-SNOM measurement, a situation where state-of-the-art commercial SC sources are too noisy to be useful. We map the propagation of surface plasmon polariton (SPP) waves on monocrystalline gold platelets in the wavelength region of 1.34–1.75 μm in a single measurement, thereby characterizing experimentally the dispersion curve of the SPP in the NIR. Our results represent a technological breakthrough that has the potential to enable a wide range of new applications of low-noise SC sources in near-field studies.

Topography-induced variations of localized surface plasmon resonance in tip-enhanced Raman configuration

We report on topography-induced changes of the localized surface plasmon resonance (LSPR) enhanced luminescence of gold tip on SrTiO3 nanostructures with apertureless scanning near-field optical microscopy (aSNOM) in tip-enhanced Raman spectroscopy (TERS) configuration. Our experimental and simulated results indicate that the averaged refractive index of the dielectric environment of the tip apex containing both air and SrTiO3 in variable volume ratios, is dependent on the topography of the sample. This reveals that the local topography has to be taken into consideration as an additional contribution to the position of the LSPR.

ASNOM mapping of SiC epilayer doping profile and of surface phonon polariton waveguiding

Apertureless scanning near-field optical microscopy mapping of a slightly doped 4H-SiC epitaxial layer grown on a heavily doped 4H-SiC substrate was performed in a cleaved edge geometry. Surface phonon polariton waves excited by an external coherent light were observed on a sample surface that contains such an epilayer-defined strip near its edge. The light frequency was tuned close to the lattice resonance. Due to a low doping level in an epilayer, its electromagnetic response is determined mainly by the SiC lattice resonance. The rest of the sample surface corresponds to a substrate whose electromagnetic response is determined mainly by the free carriers so that phonon polariton phenomena get suppressed. Such an epilayer-defined strip (vanishing at 895cm−1 frequency) becomes more pronounced at 920cm−1 and, finally, the excited state gets completely confined within such a strip (938cm−1) due to the differences in the electromagnetic properties of doped and undoped SiC.

A scanning apertureless near-field optical microscope as an instrument for characterizing the optical properties of a surface with nanometer spatial resolution

The operating principles of an apertureless scanning near-field optical microscope (ASNOM) are described. The metalized needle of an atomic force microscope is a probe in the device, and the optical interaction with objects on the surface is localized near its tip, which is a few nanometers in size. The needle’s body is several microns long, ensuring high efficiency of its electromagnetic interaction with the light waves incident on it from the outside and emitted by it into space. The nano-antenna formed by the needle thus raises the efficiency of the optical interaction between nano-objects and the electromagnetic ether by 4–5 orders of magnitude. Results from scanning semiconductor and polymer structures are presented that demonstrate the ability of ASNOM to produce high-contrast images of objects’ optical properties (absorption, reflection, and thermal expansion) with resolutions of 10–50 nm, regardless of wavelength.

Correlative imaging of biological tissues with apertureless scanning near-field optical microscopy and confocal laser scanning microscopy.

Apertureless scanning near-field optical microscopy (ASNOM) has attracted considerable interest over the past years as a result of its valuable contrast mechanisms and capabilities for optical resolutions in the nanoscale range. However, at this moment the intersections between ASNOM and the realm of bioimaging are scarce, mainly due to data interpretation difficulties linked to the limited body of work performed so far in this field and hence the reduced volume of supporting information. We propose an imaging approach that holds significant potential for alleviating this issue, consisting of correlative imaging of biological specimens using a multimodal system that incorporates ASNOM and confocal laser scanning microscopy (CLSM), which allows placing near-field data into a well understood context of anatomical relevance. We demonstrate this approach on zebrafish retinal tissue. The proposed method holds important implications for the in-depth understanding of biological items through the prism of ASNOM and CLSM data complementarity.

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.

Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS), a combination of Raman spectroscopy and apertureless near-field scanning optical microscopy using a metallic tip which resonates with the local mode of the surface plasmon, can provide a high-sensitive and high-spatial-resolution optical analytical approach. The basic principle of TERS, common experimental setups, various SPM technologies, and excitation/collection configurations are introduced as well as recent research progress with respect to TERS.

Near-Field IR Imaging and Spectroscopy of Interfaces and Interphases in Li-Ion Electrodes

Most Li-ion battery chemistries involve the creation or transformation of materials under non-equilibrium environments. Because these chemical energy storage systems operate beyond the thermodynamic stability limits of the electrolytes, reactions between electrodes and electrolyte result in the formation of new phases and interphases, which hinder further electrolyte decomposition and assure battery operation over the lifespan of the application. The basic physico-chemical properties and long-term stability of these surface layers known as the solid electrolyte interphase (SEI) determine the performance and durability of the entire system [1,2]. However, our understanding of how and why some materials function, and, equally importantly, why some materials with many ideal characteristics actually fail, is mainly rudimentary and empirical. The requirements for long-term stability of Li-ion batteries are extremely stringent and necessitate control of the chemistry at a wide variety of temporal and structural length scales. However, the spatial resolution of standard optical spectroscopic and imaging techniques is limited by diffraction to light beam sizes on the order of the wavelength of the source i.e., ∼10 μm for IR and∼500 nm in the visible range. The advent of near-field optical methods during the past decades has led to the development of new advanced techniques for chemical analysis. This presentation provides an overview of novel imaging and spectroscopic optical methodologies, which exploit micro and nano-manipulation techniques and single crystal model electrodes to provide sufficient sample definition suitable for advanced multifunctional far- and near-field optical scanning probes and electrochemical characterization techniques. The SEI chemistry is known to be highly heterogeneous at the nanoscale, and although many compounds have been suggested as contributing, their distribution, relative concentration and functionality remain poorly understood [2]. To address this challenge we used the apertureless near-field scanning optical microscopy (aNSOM) (3-9), which combines the high resolution of AFM with the chemical specificity of IR spectroscopy, to resolve the SEI chemistry at high (∼20 nm) spatial resolution. Examples of detailed in situ molecular characterization of graphite Sn and Si electrodes surface and bulk processes at the nano-level scale exceeding the diffraction limit will be discussed [7-9]. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, under contract no. DE-AC02-05CH11231.

Photoluminescence emission from a nanofabricated scanning probe tip made of epitaxial germanium

We discuss a new type of nanoscale photon source constituted by a micro-pyramid made of epitaxial semiconductor material (Ge) attached to a silicon cantilever optimized for Atomic Force Microscopy and shaped by means of low-current Focused Ion Beam (FIB) milling. The optical characterization of these tips demonstrates sizeable photoluminescence emission at the direct energy gap of Ge (1550nm). We have checked by independent measurements on the nano-patterned epitaxial material prior to the tip shaping process that the photoluminescence yield is not significantly affected by FIB milling, indicating that the optical properties of the material in this wavelength range are preserved. We envisage an application of this tool as a scanning probe in apertureless scanning near-field optical microscopy.

Composition variation in Al-based dilute nitride alloys using apertureless scanning near-field optical microscopy.

We use apertureless scanning near-field optical microscopy to study the phase separation in chemical beam epitaxy grown Al0.1Ga0.9NxAs1-x alloys. Pits attributed to nitrogen-clustering observed on the Al0.1Ga0.9NxAs1-x surface grown at 420 °C become larger at higher growth temperatures, and 3D islands appear on the surface at 565 °C. Atomic force microscopy phase measurements reveal a composition difference between the islands and the pits, whereas the sample grown at 420 °C appears to be homogeneous. Confocal Raman spectra show that all the N atoms are bonded to Al instead of Ga. Using apertureless scanning near-field optical microscopy, the luminescence of a gold tip is mapped over the surface of the sample grown at 565 °C. We extract the shift of the tip's surface plasmon resonance and determine the variation in the refractive index between the islands and the pits to be close to 0.2. Numerical simulations of the tip luminescence while in contact with the sample predict a similar variation of ∼0.3 in the refractive indices between AlGaAs islands and AlN pits, a substantially smaller value than the difference in the bulk refractive indices of the two media (∼1.8), which we attribute to a convolution of material distribution in an uneven topography. The excellent agreement between simulation and experiments supports the hypothesis of nitrogen-clustering in the pits.

Functional characterization of the novel DES mutation p.L136P associated with dilated cardiomyopathy reveals a dominant filament assembly defect

BackgroundDilated cardiomyopathy (DCM) could be caused by mutations in more than 40 different genes. However, the pathogenic impact of specific mutations is in most cases unknown complicating the genetic counseling of affected families. Therefore, functional studies could contribute to distinguish pathogenic mutations and benign variants. Here, we present a novel heterozygous DES missense variant (c.407C>T; p.L136P) identified by next generation sequencing in a DCM patient. DES encodes the cardiac intermediate filament protein desmin, which has important functions in mechanical stabilization and linkage of the cell structures in cardiomyocytes. Methods and resultsCell transfection experiments and assembly assays of recombinant desmin in combination with atomic force microscopy were used to investigate the impact of this novel DES variant on filament formation. Desmin-p.L136P forms cytoplasmic aggregates indicating a severe intrinsic filament assembly defect of this mutant. Co-transfection experiments of wild-type and mutant desmin conjugated to different fluorescence proteins revealed a dominant affect of this mutant on filament assembly. These experiments were complemented by apertureless scanning near-field optical microscopy. ConclusionIn vitro analysis demonstrated that desmin-p.L136P is unable to form regular filaments and accumulate instead within the cytoplasm. Therefore, we classified DES-p.L136P as a likely pathogenic mutation. In conclusion, the functional characterization of DES-p.L136P might have relevance for the genetic counseling of affected families with similar DES mutations and could contribute to distinguish pathogenic mutations from benign rare variants.

Design and fabrication of a tip-on-aperture probe for resolution enhancement of optical patterning

A variety of near-field scanning optical microscope (NSOM)-based lithography techniques have been developed for different applications through the use of a sub-wavelength aperture probe. The light transmission efficiency of an NSOM probe decreases markedly as the aperture diameter decreases. Apertureless NSOM yields a much higher resolution by concentrating the light field near the tip apex. However, far-field illumination by a focused laser beam deteriorates the resolution in optical patterning process. In this report, a tip-on-aperture (TOA) probe was fabricated to achieve high pattern resolution beyond that obtained from conventional NSOM probes. The shape of the probes was designed using the results of numerical analysis based on a finite-difference time-domain (FDTD) algorithm. The results showed that a probe with a triangular shape tip confined the electromagnetic energy with a 6.7 times larger intensity than the value obtained from a probe with a cylindrical shape tip. The TOA probes were fabricated by generating a triangular tip on a metal-coated NSOM probe using a focused ion beam (FIB) process. The resolution enhancement was experimentally demonstrated by the formation of the developed pattern on photo-resist.

Plasmonic Tip Based on Excitation of Radially Polarized Conical Surface Plasmon Polariton for Detecting Longitudinal and Transversal Fields

We study experimentally the excitation of the radially polarized conical surface plasmon polariton (SPP) in a fully metal-coated conically tapered M-profile fiber which works as a “plasmonic tip” for the scanning near-field optical microscope (SNOM). This structure extends the Kretschmann configuration to the conical geometry. In this plasmonic tip, the radially polarized waveguide mode, propagating inside the fiber, resonantly excites the radially polarized SPP on the metal surface, which consequently gets confined at the apex where the field oscillates longitudinally along the tip axis. We also demonstrate the reverse process, where a longitudinal field excites the radially polarized SPP mode which then resonantly excites the radially polarized waveguide mode. This plasmonic tip combines the advantageous properties of near-field optical probes. Though, it has the shape of an apertureless SNOM tip, it can simplify the detection/excitation procedure and suppresses the background signal by its fiber-based ...

Nanoscale IR Near-Field Imaging of the SEI Layer on an HOPG Electrode

The solid electrolyte interphase (SEI) layer [1] is essential to Li-ion battery functionality as it ideally forms an ionically conductive passivation layer, which protects the electrolyte from further reduction at the electrode surface. Although the SEI has been studied extensively in a number of electrode/electrolyte systems, its chemical composition, structure, and operation mechanism still remain a subject of considerable debate. Among the principal obstacles to detailed SEI characterization is the fact that the signal spatial resolution of traditional optical techniques such as Fourier transform infrared (FTIR) and Raman spectroscopy/microscopy is limited by the diffraction limit c.a. 500 nm. The standard optical response signal from the SEI layer convolves spectral features from many different compounds, which makes characterization of the inherently nanoscale chemical building blocks of the SEI layer nearly impossible. Infrared apertureless near-field scanning optical microscopy (IR aNSOM) [2] can easily overcome the diffraction limit. In aNSOM, light is focused onto a metal-coated atomic force microscope (AFM) tip, which causes a strongly enhanced field at the tip apex. Since the tip is held in close proximity to the sample surface, the microscope is capable of imaging at a resolution only limited by the tip’s radius of curvature (~20 nm). The scattering from the tip is modulated by the dielectric function of the sample surface, allowing for a direct surface-selective measurement of local absorption at the nanoscale. This capability is valuable in characterization of Li-ion electrode materials [3]. In this work we apply IR aNSOM to image the surface film on highly oriented pyrolitic graphite (HOPG). Linear sweep voltammetry (LSV) was conducted on ZYH-grade HOPG electrode in 1 M LiPF6 in EC:DEC 1:2 (w:w) electrolyte from open-circuit potential (OCP ~ 2.9 V vs. Li/Li+) to several potentials that correspond to the SEI layer formation. The HOPG electrodes were removed from the cells and imaged ex situ under a dry nitrogen atmosphere with the IR aNSOM at a variety of wavelengths within the IR laser tuning range 927-1094 cm-1. IR aNSOM images of the HOPG electrode are shown in Figure 1. The image intensity Asinϕis proportional to the IR absorption at the tip location. The variation in IR contrast with wavenumber demonstrates the nanoscale chemical inhomogeneity of the layer. Variation in wavenumber produces near-complete contrast reversal, highlighting the locations of different compounds with non-overlapping absorption bands within the laser tuning range. These results show the capability of near-field microscopy to explore the composition and structure of interfaces in electrochemical systems. Correlating near-field absorption data with spectra of reference compounds will enable full identification of the SEI’s chemical constituents at the nanoscale. Acknowledgement This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 under the Batteries for Advanced Transportation Technologies (BATT) Program. References E. Peled, J. Electrochem. Soc., 126, 2047 (1979).L. Novotny and B. Hecht, Principles of Nano-Optics, Cambridge University Press, Cambridge (2006).I. Lucas et al. Nano Lett., DOI: 10.1021/nl5010898. Figure 1: Ex situ IR aNSOM images of the HOPG electrode at (a) 1.36 V, 940 cm-1, (b) 1.36 V, 1070 cm-1, (c) 0.9 V, 1030 cm-1 (d) 0.9 V, 1088 cm-1 Figure 1

IR Near-Field Study of the Solid Electrolyte Interphase on a Tin Electrode.

There has been a dearth of suitable techniques for studying the chemical composition of solid electrolyte interphase (SEI) on Li-ion negative electrodes at a resolution of its basic building blocks' length scale. Infrared apertureless near-field scanning optical microscopy (IR aNSOM) is an emerging tool in the chemical characterization of interfacial layers on the nanometer scale. This work demonstrates an IR aNSOM imaging of the SEI layer on a model Sn electrode. IR aNSOM images reveal significant chemical contrast variations tied to specific topographic features and possible corresponding distribution of lithium carbonate and lithium ethylene dicarbonate on the Sn electrode surface.

ОСЕСИММЕТРИЧНОЕ ВОЗБУДЖЕНИЕ БИКОНУСА, СФОРМИРОВАННОГО ПОЛУБЕСКОНЕЧНЫМ И СРЕЗАННЫМ ПОЛУБЕСКОНЕЧНЫМ КОНУСАМИ

The problem of axially-symmetric electromagnetic wave diffraction by a perfectly conducting bi-conical surface, which one arm is a semi-infinite cone and the other is a semi-infinite truncated cone, is solved rigorously using the mode-matching method and analytical regularization technique. The bi-cone is excited by a ring magnetic source. This structure is analyzed under the condition of the cutting cone approaching the plane. The influence of the geometrical parameters and the exciting modes on the wideband properties of the bi-cone and on the electromagnetic energy penetration through its aperture is analyzed. Key wordes: infinite bicone, truncated apex, analytical regularization, round hole, conical conductor Manuscript submitted 23.12.2014 Radio phys. radio astron. 2015, 20(1): 76-85 REFERENCES 1. ASH, E. A. and NICHOLLS, G., 1972. Super-resolution Aperture Scanning Microscope. Nature. vol. 237, no. 5357, pp. 510–512. DOI: https://doi.org/10.1038/237510a0 2. LEE, T., LEE, E., OH, S. and HAHN, J. W., 2013. Imaging heterogeneous nanostructures with a plasmonic resonant ridge aperture. Nanotechnology. – vol. 24, no. 14, id. 145502. DOI: https://doi.org/10.1088/0957-4484/24/14/145502 3. KUZNETSOVA, T. I., LEBEDEV, V. S. and TSVELI, A. M., 2004. Optical fields inside a conical waveguide with a subwavelength-sized exit hole. J. Opt. A: Pure Appl. Opt. vol. 6, no. 4, pp. 338–348. DOI: https://doi.org/10.1088/1464-4258/6/4/008 4. LINDQUIST, N. C., JOHNSON, T. W., NAGPAL, P., NORRIS D. J. and OH, S., 2013. Plasmonic nanofocusing with a metallic pyramid and an integrated C-shaped aperture. Scientific Reports. vol. 3, id. 1857. DOI: https://doi.org/10.1038/srep01857 5. LEE, Y., ALU, A. and ZHANG, J. X. J., 2011. Efficient apertureless scanning probes using patterned plasmonic surfaces. Opt. Express. vol. 19, no. 27, pp. 25990–25999. DOI: https://doi.org/10.1364/OE.19.025990 6. DREZET, A., WOEHL, J. C. and HUANT, S., 2002. Diffraction by a small aperture in conical geometry: Application to metal coated tips used in near-field scanning optical microscopy. Phys. Rev. E. vol. 65, is. 4, id. 046611. DOI: https://doi.org/10.1103/PhysRevE.65.046611 7. BILOTTI, F., TRICARICO, S., PIERINI, F. and VEGNI, L., 2011. Cloaking apertureless near-field scanning optical microscopy tips. Opt. Lett. vol. 36, no. 2, pp. 211–213. DOI: https://doi.org/10.1364/OL.36.000211 8. FURUKAWA, H. and KAWATA, S., 1998. Local field enhancement with an apertureless near-field-microscope probe. Opt. Commun. vol. 148, is. 4–6, pp. 221–224. DOI: https://doi.org/10.1016/S0030-4018(97)00687-1 9. MUSTONEN, A., BEAUD, P., KIRK, E., FEURER, T. and TSUJINO, S., 2012. Efficient light coupling for optically excited high-density metallic nanotip arrays. Scientific Reports. vol. 2, id. 915. DOI: https://doi.org/10.1038/srep00915 10. SHESTOPALOV, V. P., KIRILENKO, A. A. and MASALOV, S. A. 1984. Matrix Convolution-Type Equations in Diffraction Theory. Kyiv, Ukraine: Naukova Dumka Publ. (in Russian). 11. KURYLIAK, D. B. and SHARABURA, O. M., 2013. Electromagnetic Excitation of Bicone Formed by Semi-Infinite and Finite Trancated Conical Surfaces. Radio Phys. Radio Astron. vol. 18, no. 2, pp. 138–146 (in Ukrainian). 12. KURYLIAK , D. B. and SHARABURA , O. M., 2013. Axially-Symmetric Radiation Field of Conical Monopole. Radio Phys. Radio Astron. vol. 18, no. 4, pp. 323–330 (in Ukrainian). 13. KURYLIAK, D. B. and SHARABURA, O. M., 2014. Axially-Symmetric Electromagnetic Excitation of Metallic Disc-Conical Scatterer. Radiofizika i Electronica. vol. 5 (19), no. 2, pp. 3–9 (in Ukrainian). 14. KURYLIAK , D. B. and NAZARCHUK, Z. T., 2006. Analytical-numerical methods in the theory of wave diffraction on conical and wedge-shaped surfaces. Kyiv, Ukraine: Naukova Dumka Publ. (in Ukrainian). 15. NEFEDOV, E. I., 1982. Open coaxial resonance structures. Moscow, Russia: Nauka Publ. (in Rissian). 16. DROBAKHIN, O. O., PRIVALOV, E. H. and SALTYKOV, D. Yu., 2002. Calculation of resonant frequencies of coaxial beconical resonator with dielectric cylinder placed in split centre conductor. Visnyk Dnipropetrovs'kogo universitetu. Seriya Fizika, radioelektronika. is. 9. pp. 91–94 (in Russian).

Practical Realization of Apertureless Scanning Near-field Optical Microscopy Using Hybrid Mode Atomic Force Microscopy

ABSTRACTThe local detection of optical response at the sub-wavelength scale on a materials’ surface is an invaluable characterization capability of apertureless scanning near-field optical microscopy (ASNOM). The technique is traditionally realized in amplitude modulation (AM) AFM mode. We have expanded this method by employing an alternative scheme for the detection of the near-field and far-field responses with the use of Hybrid (HD) AFM mode. In HD mode the sample is brought to the intermittent contact with the tip in a periodic oscillation at a frequency (1-2 kHz) much smaller than the probe resonance. In every oscillation cycle the probe deflection to a set-point value is used for surface profiling. For optical measurements the metal coated AFM tip was top-illuminated by visible laser. Simultaneously with surface profiling the light scattered from tip-sample junction was collected by a sensitive photomultiplier (PMT). The homodyne optical signal detection scheme was applied to discriminate near- and far-field optical components. Our method was verified by the studies of various materials (semiconductors, metals, polymers, etc.). The presented results show that the contrast of ASNOM images can be used for compositional mapping of heterogeneous systems.

Imaging of Localized Surface Plasmon by Apertureless Scanning Near-Field Optical Microscopy

Imaging of Localized Surface Plasmon by Apertureless Scanning Near-Field Optical Microscopy