Apertureless Probes Research Articles

Numerical calculation of near-field in vicinity of three-dimensional nano-optical probes using boundary element method

The diffraction limit is one of the main obstacles in the development of microscopes to analyze the morphology and structure of materials. The main idea of near field scanning optical microscopy (NSOM) is to overcome the diffraction limit using sub-wavelength apertures. In this work, the near-field is simulated in the vicinity of three-dimensional nano-optical apertureless probes. For this purpose, the Helmholtz equation is solved using the boundary element method (BEM). The effects of different parameters on the near field generated in the vicinity of the optical probe are studied. These parameters consist of the length and radius of the probe, the size of the aperture, the angle of the tapered tip, and the geometry of the probe tip. The main advantages of the proposed method are the high accuracy, the very short calculation time, and the ability to calculate the near field inside and outside the optical probe without any approximation.

New development of nanoscale spectroscopy using scanning probe microscope

Nanoscale spectroscopy and imaging, a hybrid technique that combines a scanning probe microscope (SPM) with spectroscopy, can provide nanoscale topographical, spectral, and chemical information of a sample. In recent years, developments in nanofabrication technology have dramatically advanced the field of nanospectroscopy for applications in various fields including nanoscale materials, electronics, catalysis, and biological systems. However, challenges in nanofocusing of light for excitation and extracting weak signals of individual molecules from the background signal persist in conventional nanoscale spectroscopy including tip-enhanced Raman spectroscopy, scanning near-field microscopy (SNOM/NSOM), and photoluminescence spectroscopy. This article reviews new approaches to design plasmonic SPM probes that improve important aspects of nanospectroscopy such as nanofocusing, far-to-near-field-coupling efficiency, background suppression, and ease of fabrication. The authors survey a diverse range of novel schemes to excite propagating surface plasmon polaritons on the probe surface to attain highly enhanced nanofocused light at the apex for nanoscale spectroscopies. These schemes include grating coupler configurations on the plasmonic SPM probes, aperture and apertureless plasmonic SNOM probes, nanostructured resonators coupled with a high-quality-factor photonic cavity, interfacing of the optical fiber with plasmonic nanowires, and nanoparticle-coupled plasmonic nanowires. These innovative probes merge the field of fiber optics, plasmonics, quantum optics, and nanomaterials. The authors provide a perspective on new approaches that combine the advantages of these probes and have the potential for significant advancement in nanoscale imaging and other types of nanoscale spectroscopies including scanning quantum spin spectroscopy and scanning thermal imaging microscopy.

Optical imaging of nanosized structures by using plasmonically excited cascade near-field coupling with a carbon nanotube probe

Near-field scanning optical microscopy (NSOM) overcomes the diffraction limit, thereby realizing a spatial resolution far beyond the wavelength of light used. However, NSOM still has a problem in repeatable imaging at the high spatial resolution and high contrast with conventional aperture or apertureless probes that are needed for practical applications. Here, we describe an optical imaging technique based on plasmonically excited cascade near-field coupling that has the potential to achieve single-nanometer spatial resolution with high imaging repeatability. This technique makes use of a plasmon waveguide coupled with a high-stiffness carbon nanotube optical probe. Through the action of surface plasmon polaritons, the input far-field light is converted into an optical near field that is used as an excitation source. This excitation near field is strongly enhanced and concentrated on the probe tip such that it generates a second near field as a nanosized probe spot on the apex of the tip. Extremely high-resolution optical imaging is accomplished by scanning the sample surface with the probe spot. At a wavelength of 850 nm, a 5-nm-wide metallic striped pattern on a cross-sectional superlattice sample was clearly resolved as a permittivity distribution.

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).

Directional and enhanced spontaneous emission with a corrugated metal probe.

A three-dimensional corrugated metal tapered probe with surface corrugated gratings at the tip apex is proposed and investigated theoretically, which leads to an obvious emission beaming effect of spontaneous emission from a single emitter near the probe. In contrast with conventional apertureless metal probes, where only the enhancement of an optical near-field is concerned, the corrugated probe is able to manipulate local excitation intensity and far-field emission direction simultaneously. The angular emission from a single dipole source, being placed close to the corrugated probe, falls into a cone with a maximum directivity angle of ±11.6°, which improves the collection efficiency 25-fold. Such a probe simultaneously increases the localized field intensity to about twice as strong as the conventional bare tip. In addition, the radiation pattern is sensitive to the working wavelength and the dipole to tip-apex separation. These findings make a promising route to the development of plasmonic spontaneous emission manipulation based on corrugated tapered antenna-for instance, tip-enhanced spectroscopy, single-molecule sensing, and single-photon source.

Efficient apertureless scanning probes using patterned plasmonic surfaces

We present a novel concept to design apertureless plasmonic probes for near-field scanning optical microscopy (NSOM) with enhanced optical power throughput and near-field enhancement. Specifically, we combine unidirectional surface plasmon polariton (SPP) generation along the tip lateral walls with nanofocusing of SPPs through adiabatic propagation towards an apertureless tip. Three key design parameters are considered: the nanoslit width, the pitch period of nanogrooves for unidirectional plasmonic excitation and the pyramidal geometry of the NSOM probe for SPP focusing. Optimal design parameters are obtained with 2D analysis and two realistic probe geometries with patterned plasmonic surfaces are proposed using the optimized designs. The electromagnetic properties of the designed probes are characterized in the near-field and compared to those of a conventional single-aperture probe with same pyramidal shape. The optimized probes feature FWHM around 150nm, comparable with conventional NSOM designs, but over 3 orders of magnitude larger field enhancement, without degrading its spatial resolution. Our ideas effectively combine the resolution of apertureless probes with throughput levels much larger than those available even in aperture-based devices.

Cloaking apertureless near-field scanning optical microscopy tips

We numerically demonstrate that properly designed plasmonic covers can be used to enhance the performance of near-field scanning optical microscopy (NSOM) systems based on the employment of apertureless metallic tip probes. The covering material, exhibiting a near-zero value of the real permittivity at the working frequency, is designed in such a way to dramatically reduce the undesired scattering due to the strongly plasmonic behavior of the tip. Though the light scattering by the tip end is necessary for the correct operation of NSOMs, the additional scattering due to the whole probe affects the signal-to-noise ratio and thus the resolution of the acquired image. By covering the whole probe but not the very tip, we show that unwanted scattering can be effectively reduced. A realistic setup, working at mid-IR frequencies and employing silicon carbide covers, has been designed and simulated to confirm the effectiveness of the proposed approach.

Effect of Polarization Direction on the Electric Field Distribution at the Near-Field of a Tip-on-Aperture Near-Field Scanning Optical Microscope Probe

A tip-on-aperture (TOA) probe is a novel near-field scanning optical microscope (NSOM) probe, which combines the features of aperture and apertureless probes. It has a small metallic tip at the end of an aperture NSOM's probe. The tip is illuminated through an aperture at the near field of the probe and electromagnetic interaction between the structured sharp tip at the end of the probe and the surface is used for measurements and materials processing. In this study, the electric field distributions at the near-field of a TOA probe were calculated numerically to analyze the effects of polarization direction on the characteristics of measurement and processing. A TOA probe is asymmetric for the axis, since it has a probe at metal coated layer on the aperture. The geometrical relationship between this asymmetric shape and polarization direction is the reason for the change of the electromagnetic energy distributions. Numerical analysis shows that a TOA probe can make the best use of the tip for high resolutions when the tip is located on the parallel axis with the polarization direction.

Optimized apertureless optical near-field probes with 15 nm optical resolution

Back-illuminated full body glass tips coated with a thin metal layer can be used aslocal probes for apertureless scanning near-field optical microscopy (SNOM). Inorder to achieve high spatial resolution, high electric field intensities and lowbackground illumination, the thickness of the metal coating, angular illuminationdirection, and polarization have to be optimized. Optimal conditions have beencalculated and experimentally verified for 10–15 nm thick aluminium and 15–25 nmthick silver layers. Upon imaging single dye molecules, characteristic single anddouble-peak patterns with peak widths down to 15 nm could be measured, exhibiting anoptical resolution which exceeds the classical diffraction limit of Abbé significantly.

Microarrays of near-field optical probes with adjustable dimensions

We present the fabrication and the characterization of high-density microarrays comprising thousands of near-field optical probes. Two types of microarrays have been prepared by adapting the SNOM methodology: arrays of uncoated fiber nanotips (i.e. apertureless probes) and arrays of apertures with adjustable subwavelength dimensions. Such arrays were fabricated by retaining the coherent structure of monomode optical fiber bundles and therefore keeping their imaging properties. The size of the apertures in a microarray was tuned at the nanometer scale by modifying the fabrication parameters. Far-field characterization of these near-field probe arrays shows completely different behavior depending both on their architecture and on their characteristic size. The angular distribution of the far-field intensity transmitted through the aperture arrays is used to determine the optical size of such diffracting apertures. Aperture radii ranging from 95 to 250 nm were found in good agreement with SEM data. Furthermore, each nanoaperture of the array is optically independent in the far-field regime. Eventually, this study demonstrates potential applications of these imaging arrays as parallel near-field optical probes in both configurations (apertureless and with apertures).

Near‐field scanning Raman microscopy using apertureless probes

AbstractWe describe an apertureless near‐field scanning Raman microscope in reflection geometry developed by integrating a near‐field scanning optical microscope and a Raman spectrometer. This set‐up offers some unique features that make it applicable to more samples. The fabrication of the metal tips is also explained in detail. Near‐field Raman mappings have been realized on real silicon devices for the first time. The results illustrate the capability of our near‐field Raman microscope in 2D Raman imaging. The apertureless configuration is a breakthrough to the limitation set by the low optical throughput of metal‐coated optical fiber tips, reducing drastically the integration time for Raman spectra. The reflection scattering geometry makes the system applicable to any samples without preparation. Copyright © 2003 John Wiley & Sons, Ltd.

Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about the tip’s field. In particular, they unambiguously demonstrate both the nanometric spatial confinement of the tip field and the evanescent nature of the nanosource excited at the tip’s end. Most of the experimental results are illustrated by numerical calculations based on the finite element method and commented using the literature on the subject. Additionally, we discuss the origin of the optically induced topography on a nanometer scale and present some preliminary results of the apertureless near-field optical lithography based on local field enhancement. Our approach constitutes a useful tool to investigate the near-field of apertureless probes and should enable the optimization of the nanosource for any experiment requiring local optical excitation of the matter.

Enhanced light confinement in a near-field optical probe with a triangular aperture.

We present a probe concept for scanning near-field optical microscopy combining the excellent background suppression of aperture probes with the superior light confinement of apertureless probes. A triangular aperture at the tip of a tetrahedral waveguide (full taper angle approximately 90 degrees ) shows a strong field enhancement at only one rim when illuminated with light of suitable polarization. Compared to a circular aperture of equivalent size, the resolution capability is doubled without loss of brightness. For a approximately 60 nm sized triangular aperture, we measured an optical resolution <40 nm and a transmission of approximately 10(-4).

Ion and electron beam assisted growth of nanometric SimOn structures for near-field microscopy

We report the fabrication of nanometric, conformal, smooth dielectric coatings on the ends of apertureless near-field optical probes by both ion beam and electron beam assisted deposition techniques (IBAD and EBAD). The ion beam provides a higher SimOn growth rate than the electron beam, though the undesirable implantation of Ga ions may outweigh the benefits of rapid growth. Wavelength dispersive x-ray spectroscopy reveals that the electron beam deposited dielectric has a stoichiometry of SinO2n. We present two near-field optics applications of EBAD and IBAD grown dielectric layers. EBAD deposited coatings can be used to reduce fluorescence quenching in apertureless near-field scanning optical microscopy, while IBAD fabricated structures are suited for micro/nano-optics.

Apertureless near-field scanning Raman microscopy using reflection scattering geometry

The combination of near-field scanning optical microscopy and Raman spectroscopy provides chemical/structural specific information with nanometer spatial resolution, which are critically important for a wide range of applications, including the study of Si devices, nanodevices, quantum dots, single molecules of biological samples. In this paper, we describe our near-field Raman study using apertureless probes. Our system has two important features, critical to practical applications. (1) The near-field Raman enhancement was achieved by Ag coating of the metal probes, without any preparation of the sample, and (2) while all other apertureless near-field Raman systems were constructed in transmission mode, our system works in the reflection mode, making near-field Raman study a reality for any samples. We have obtained the first 1D Raman mapping of a real Si device with 1 s exposure time. This is a very significant development in near-field scanning Raman microscopy as it is the first demonstration that this technique can be used for imaging purpose because of the short integration time. In addition, the metal tips used in our set-up can be utilized to make simultaneous AFM and electrical mappings such as resistance and capacitance that are critical parameters for device applications.

Design of near-field optical probes with optimal field enhancement by finite difference time domain electromagnetic simulation

We report the three-dimensional electromagnetic simulation of gold nanoparticles with specific geometries as a means to the rational design of apertureless near-field scanning optical microscopy (NSOM) probes. Analytical solutions for field enhancement by spheroidal particles are used to provide physical insight for probe design. These solutions indicate that probes need to be not only sharp, but also finite in length in order to generate the highest field enhancement. Finite difference time domain (FDTD) simulations of gold particles illuminated by near infrared radiation are performed. Field enhancements for right trigonal pyramids are found to be size and wavelength dependent. Furthermore, the enhancements for these pyramidal particles are higher than for similar length conical particles, which in turn perform better than quasi-infinite conical probes. The particles we design with FDTD can be made using current nanofabrication techniques, and therefore hold great promise as apertureless NSOM probes. These right trigonal pyramids are particularly well suited for use in tip enhanced nonlinear optical microscopy or near-field Raman microscopy.

Near-field optics: Direct observation of the field enhancement below an apertureless probe using a photosensitive polymer

We report the direct observation of the optical near-field enhancement at the nanometric extremity of a metallic probe for apertureless scanning near-field optical microscopy. Our approach consists in making the “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end via a photosensitive polymer. This distribution is coded by polymer surface topography which is characterized in situ by atomic force microscopy using the same probe. Results clearly reveal nanometric dots corresponding to local field enhancement below the tip end. The field enhancement is shown to be crucially dependent on the polarization state of the incident laser beam as well as the tip material and geometry. The experimental results are found to agree with the results of preliminary calculations. This experiment both constitutes a useful tool for investigating field enhancement below apertureless probes and has potential applications in nanophotolithography.

A novel fabrication method for fluorescence-based apertureless scanning near-field optical microscope probes.

We report a novel method for the fabrication of probes with localized sub-wavelength fluorescing media at their extremities. We present our first results and discuss future plans to extend this technique to the systematic fabrication of fluorescent probes for apertureless scanning near-field optical microscopy.