Near-field Scanning Optical Microscopy Tip Research Articles

Mechanism of near-field optical nanopatterning on noble metal nano-films by a nanosecond laser irradiating a cantilevered scanning near-field optical microscopy probe.

To overcome the diffraction limit, a laser irradiating cantilevered scanning near-field optical microscopy (SNOM) probe has been used in near-field optical nanopatterning. In this paper, the mechanism of nanopatterning on noble metal nano-films by this technique is investigated by the finite element method. It is proposed that the main mechanism of this phenomenon is the melt and reshaping of the nano-film under the SNOM tip. The melt is caused by the surface plasmon polariton-assisted enhancement and restriction within the SNOM tip aperture. The impacts of the gap g between the tip and substrate and the polarization of the laser are further analyzed.

Fabrication of nanostructure on Au nano-film by nanosecond laser coupled with cantilevered scanning near-field optical microscopy probe

Diffraction limit has been the constraint of the nanostructure fabrication. Because the scanning near-field optical microscopy (SNOM) can work in the evanescent near-field region, its application in nano-processing has received extensive attention from researchers globally. In this paper, we combined nanosecond laser with cantilevered SNOM probe. Utilizing the high precision of the confinement and enhancement effect of probe tip and the high instantaneous energy of the laser, we realized nanostructure fabrication and in situ detection on Au nano-film. Feature sizes down to 47 nm full width at half maximum were fabricated. We investigated the laser propagation through the SNOM tip aperture and the light field intensity distribution on the surface of substrate theoretically. The calculation results demonstrate that the laser is highly restricted within the SNOM aperture and enhanced on the exit plane at the rim of aperture. After the transmission, the light field intensity distribution on the surface of the Au nano-film was enhanced due to the localized surface plasmon resonance. The thermal distribution on the surface of Au nano-film indicates that the peak of the temperature distribution appeared at the surface right underneath the center of the aperture. It is proved that the simulation results are consistent with the experimental results.

Mechanism of nanostructure processing on Au and Ag nano-film by a nanosecond laser illuminating cantilevered scanning near-field optical microscopy tip.

Nanostructure processing by a laser illuminating cantilevered scanning near-field optical microscopy (SNOM) tip is a novel technology that has received extensive attention from researchers. In this paper, theoretical investigations of the mechanism for nanostructure fabrication on Au and Ag nano-film by this technology are realized by the finite element method. The light field intensity and temperature distribution on Au and Ag surfaces at the near-field of the SNOM tip apex after illumination is simulated. The results reveal that the laser is restricted and enhanced within the SNOM tip aperture during illumination. Locally excited surface plasmon polaritons, which induce near-field enhancement on the Au and Ag nano-film at the vicinity of the aperture, are significant for nanostructure fabrication. The impacts of several parameters such as aperture width w, gap between the apex and substrate g, and the initial electric field intensity |E0| of the laser on the temperature of the Au and Ag substrate surfaces during fabrication are deeply studied. It reveals that the surface temperature depends on both the enhancement of the light field intensity and the transmitted laser. The enhancement is dominant in affecting temperature when the gap is small, while the transmittance becomes the main factor influencing the surface temperature with the increase of the gap.

Mapping optical standing-waves of an open-access Fabry-Perot cavity with a tapered fiber.

We describe a method to map the standing-wave pattern inside an open-access Fabry-Perot optical cavity with sub-wavelength resolution by perturbing it with a commercially available tapered fiber. The method is applied to a fiber Fabry-Perot microcavity. We demonstrate its use in determining the relative position of the antinodes at two different wavelengths. In addition, we use the tapered optical fiber as a point-like source, allowing precise positioning of a microscope objective with respect to the cavity mode.

Site-Controlled Single-Photon Emitters Fabricated by Near-Field Illumination.

Many of the most advanced applications of semiconductor quantum dots (QDs) in quantum information technology require a fine control of the QDs' position and confinement potential, which cannot be achieved with conventional growth techniques. Here, a novel and versatile approach for the fabrication of site-controlled QDs is presented. Hydrogen incorporation in GaAsN results in the formation of N-2H and N-2H-H complexes, which neutralize all the effects of N on GaAs, including the N-induced large reduction of the bandgap energy. Starting from a fully hydrogenated GaAs/GaAsN:H/GaAs quantum well, the NH bonds located within the light spot generated by a scanning near-field optical microscope tip are broken, thus obtaining site-controlled GaAsN QDs surrounded by a barrier of GaAsN:H (laterally) and GaAs (above and below). By adjusting the laser power density and exposure time, the optical properties of the QDs can be finely controlled and optimized, tuning the quantum confinement energy over more than 100 meV and resulting in the observation of single-photon emission from both the exciton and biexciton recombinations. This novel fabrication technique reaches a position accuracy <100 nm and it can easily be applied to the realization of more complex nanostructures.

Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio

Near-field scanning optical microscopy (NSOM) enables observation of light-matter interaction with a spatial resolution far below the diffraction limit without the need for a vacuum environment. However, modern NSOM techniques remain subject to a few fundamental restrictions. For example, concerning the aperture tip (a-tip), the throughput is extremely low, and the lateral resolution is poor; both are limited by the aperture size. Meanwhile, with regard to the scattering tip (s-tip), the signal-to-noise ratio (SNR) appears to be almost zero; consequently, one cannot directly use the measured data. In this work, we present a plasmonic tip (p-tip) developed by tailoring subwavelength annuli so as to couple internal radial illumination to surface plasmon polaritons (SPPs), resulting in an ultrastrong, superfocused spot. Our p-tip supports both a radial symmetric SPP excitation and a Fabry-Pérot resonance, and experimental results indicate an optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28%, and an outstanding SNR of up to 18.2 (nearly free of background). The demonstrated p-tip outperforms state-of-the-art NSOM tips and can be readily employed in near-field optics, nanolithography, tip-enhanced Raman spectroscopy, and other applications.

Apertureless Near-Field Scanning Probes Based on Graphene Plasmonics

We present a novel approach in designing high-throughput high-resolution apertureless near-field scanning probes with enhanced nanofocusing based on graphene plasmonics. Extremely localized plasmons on graphene are mingled with nanofocusing of surface plasmon polaritons to confine and steer the plasmon waves into the apex of a near-field scanning optical microscopy tip. The Fermi level, localized emission sites on graphene, and the angle of excitation play a critical role in exciting graphene surface plasmons on the lateral walls of the designed conical probes. The optimized probes feature full-width at half-maximum (FWHM) around 25 nm, which is at least two times smaller than conventional metallic plasmonic tips. The near-field electromagnetic properties of the designed probes are characterized in detail and compared to the conventional single-aperture and typical apertureless metallic plasmonic (silver and gold) probes. Over three orders of magnitude electric field enhancement compared to metallic probes on SiO 2 substrate has been achieved.

Determination of critical diameters for intrinsic carrier diffusion-length of GaN nanorods with cryo-scanning near-field optical microscopy

Direct measurements of carrier diffusion in GaN nanorods with a designed InGaN/GaN layer-in-a-wire structure by scanning near-field optical microscopy (SNOM) were performed at liquid-helium temperatures of 10 K. Without an applied voltage, intrinsic diffusion lengths of photo-excited carriers were measured as the diameters of the nanorods differ from 50 to 800 nm. The critical diameter of nanorods for carrier diffusion is concluded as 170 nm with a statistical approach. Photoluminescence spectra were acquired for different positions of the SNOM tip on the nanorod, corresponding to the origins of the well-defined luminescence peaks, each being related to recombination-centers. The phenomenon originated from surface oxide by direct comparison of two nanorods with similar diameters in a single map has been observed and investigated.

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 ...

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).

Mapping and quantifying electric and magnetic dipole luminescence at the nanoscale.

We report on an experimental technique to quantify the relative importance of electric and magnetic dipole luminescence from a single nanosource in structured environments. By attaching a Eu^{3+}-doped nanocrystal to a near-field scanning optical microscope tip, we map the branching ratios associated with two electric dipole and one magnetic dipole transitions in three dimensions on a gold stripe. The relative weights of the electric and magnetic radiative local density of states can be recovered quantitatively, based on a multilevel model. This paves the way towards the full electric and magnetic characterization of nanostructures for the control of single emitter luminescence.

Near‐Field Analysis of Bright and Dark Modes on Plasmonic Metasurfaces Showing Extraordinary Suppressed Transmission

Plasmonic metasurfaces are investigated that consist of a sub‐wavelength line pattern in an ultrathin (∼10 nm) silver film, designed for extraordinary suppressed transmission (EOST) in the visible spectral range. Measurements with a near‐field scanning optical microscope (NSOM) demonstrate that far‐field irradiation resonantly excites antenna‐like (bright) modes that are localized on the metal ridges. The re‐radiation of these modes into the far‐field interferes destructively with the transmitted wave, thus suppressing transmission almost completely. In contrast, a second type of mode, bound (dark) surface plasmon polaritons (SPPs) launched from an NSOM tip, propagates well across the metasurface, preferentially perpendicular to the grating lines.

Simulation of a metallic SNOM tip illuminated by a parabolic mirror

We investigate numerically a Scanning Near field Optical Microscope (SNOM) that uses a Parabolic Mirror (PM) to focus a radially polarized beam on a metallic tip. In order to overcome problems--like overestimated near fields or resonances--that arise when only considering finite tips, we have introduced a semi-infinite continuation of the tip, which incorporates the analytic solution of surface waves. For a realistic modeling the right description of the incident field is essential and we have complied with this requirement by a Bessel expansion of the focal fields, which is also applicable to an aplanatic objective. The established numerical model is used for an extensive study of model parameters like tip geometry, illumination directions and tip materials (Ag, Au, Al and Cu). Compared with a simplified inverted microscope configuration, the PM setup shows an increased field enhancement (factor of 2-2.5), which can be ascribed to the efficient coupling of the exciting field to tip surface plasmons.

Numerical analysis of a SNOM tip based on a partially cladded optical fiber

A Scanning Nearfield Optical Microscope (SNOM) tip with partial metallic cladding is presented. For its design, a very demanding 2D eigenvalue analysis of an optical waveguide with material and radiation losses is carried out by the Multiple Multipole Program (MMP) and by the Finite Element Method (FEM). These simulations require some special tricks that are outlined. The computed 2D MMP and FEM results are compared and discussed. This 2D analysis is followed by a full 3D FEM analysis of the SNOM tip. The obtained 3D results confirm the corresponding 2D predictions. Important conclusions regarding the guiding capabilities of the chosen structure and the efficiency of the applied numerical methods are presented.

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.

Cloaked Near-Field Scanning Optical Microscope Tip for Noninvasive Near-Field Imaging

Near-field imaging is a well-established technique in biomedical measurements, since closer to the detail of interest it is possible to resolve subwavelength details otherwise unresolved by regular lenses. A near-field scanning optical microscope (NSOM) tip may indeed overcome the resolution limits of far-field optics, but its proximity inherently perturbs the measurement. Here, we apply the recent concept of a "cloaked sensor" to an NSOM device in collection mode, showing theoretically how a proper plasmonic cover applied to an NSOM tip may drastically improve its overall measurement capabilities.

Near-field and far-field analysis of an azimuthally polarized slow Bloch mode microlaser

We report on the near- and far-field investigation of the slow Bloch modes associated with the Γ point of the Brillouin zone, for a honeycomb lattice photonic crystal, using near-field scanning optical microscopy (NSOM) and infra-red CCD camera. The array of doughnut-shaped monopolar mode (mode M) inside each unit cell, predicted previously by numerical simulation, is experimentally observed in the near-field by means of a metal-coated NSOM tip. In far-field, we detect the azimuthal polarization of the doughnut laser beam due to destructive and constructive interference of the mode radiating from the surface (mode TEM(01*)). A divergence of 2° for the laser beam and a mode size of (12.8 ± 1) µm for the slow Bloch mode at the surface of the crystal are also estimated.

Near-field microscopy with a single-photon point-like emitter: Resolution versus the aperture tip?

We discuss theoretically the concept of spatial resolution in near-field scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17 (2009) 19969] which reported on the achievement of active tips made of a single ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to decrease steadily in size, we assimilate a nanodiamond-based tip to a point-like single photon source and compare its ultimate resolution with that offered by standard metal-coated aperture NSOM tips. We demonstrate both classically and quantum mechanically that NSOM based on a point-like tip has a resolving power that is only limited by the scan height over the imaged system whereas the aperture-tip resolution depends critically on both the scan height and aperture diameter. This is a consequence of the complex distribution of the electromagnetic field around the aperture that tends to artificially duplicate the imaged objects. We show that the point-like tip does not suffer from this “squint” and that it rapidly approaches its ultimate resolution in the near-field as soon as its scan height falls below the distance between the two nano-objects to be resolved.

Fluorescent oxide nanoparticles adapted to active tips for near-field optics

We present a new kind of fluorescent oxide nanoparticle (NP) with properties well suited toactive-tip based near-field optics. These particles with an average diameter in the5–10 nm range are produced by low energy cluster beam deposition (LECBD) from aYAG:Ce3+ target. They are studied by transmission electron microscopy (TEM), x-ray photoelectronspectroscopy (XPS), cathodoluminescence, near-field scanning optical microscopy (NSOM)and fluorescence in the photon-counting mode. Particles of extreme photo-stability as smallas 10 nm in size are observed. These emitters are validated as building blocks ofactive NSOM tips by coating a standard optical tip with a 10 nm thick layer ofYAG:Ce3+ particles directly in the LECBD reactor and by subsequently performing NSOM imaging oftest surfaces.

The origin of fine structure in near-field scanning optical lithography of an electroactive polymer

Near-field scanning optical lithography (NSOL) has been used to produce arbitrary structures of the electroactive polymer polyphenylenevinylene at sizes comparable to optical wavelengths, which are of interest for integrated optical devices. The structures are characterized using AFM and SEM and exhibit interesting fine structure. The characteristic size and shape of the lithographic features and their associated fine structure have been examined in the context of the electric field distribution at the near-field scanning optical microscope tip. In particular, the Bethe–Bouwkamp model for electric field distribution at an aperture has been used in combination with a recently developed model for precursor solubility dependence on UV energy dose to predict the characteristics of lithographic features produced by NSOL. The fine structure in the lithographic features is also investigated and explained. Suggestions for the further improvement of the technique are made.