Three-dimensional winged nanocone optical antennas
Mikko J. HuttunenDomenico AndrianoJouni MäkitaloKlas LindforsGodofredo BautistaMarkus LippitzMartti Kauranen
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Abstract:
Optical antennas convert propagating far-field radiation into localized near-fields. Perhaps the simplest optical antenna is a sharp metal tip, which is commonly utilized in near-field microscopy and spectroscopy. Usually, the coupling of far-field light to the near-field requires either a field polarized along the tip axis (longitudinal field) or a surface grating on the tip. Here, we demonstrate that efficient transformation of incident far-field radiation to localized energy at the cone apex can be achieved using nanocones with wings oriented perpendicular to the tip axis.Keywords:
Near-field optics
The ability to achieve subwavelength optical resolution for microscopy through the use of a mechanically scanned near-field optical probe has recently been reported.¹ This technique provides a means of exceeding the far-field diffraction limit associated with the use of an objective lens in conventional optical microscopy, and is known as near-field scanning optical microscopy (NSOM). The near-field probe may be realized by forming a subwavelength-sized aperture in an opaque screen. Such a probe may be implemented in practice by tapering an optical fiber down to the desired diameter and coating its exterior (except for the very tip) with aluminum. The spatial resolution achieved is dependent on the aperture diameter and the tip-to sample separation, and the resolution is approximately equal to the aperture diameter for separations less than one aperture radius.
Aperture (computer memory)
Near-field optics
Numerical aperture
Opacity
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Near-field scanning optical microscopy (NSOM) is a scanning probe technique that uses a tapered optical fiber to probe optical characteristics of a surface in registry with topography. Light can either be injected into the sample or collected from the sample via the subwavelength aperture formed at the tip of the probe. While operating in injection mode, variations in the optical power delivered to the probe, and consequently variations in the optical flux through the aperture, place limits on the imaging of spatial variations in optical properties. We present a novel method utilizing bend loss in an optical fiber to correct for variations in the optical flux of the aperture of a NSOM probe.
Optical power
Aperture (computer memory)
Near-field optics
Scanning Probe Microscopy
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Aperture (computer memory)
Near-field optics
Tapering
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The most important part of a scanning near-field optical microscope (SNOM) is an optical probe that is scanned along the object at a very small distance. The probe tip is usually the tapered end of an optical monomode fiber. In various types of SNOM tips with and without metal coating are: utilized. In the aperture SNOM (a-SNOM) the tip is overcoated by a metal except for a small aperture at the apex. In the photon scanning tunneling microscope (PSTM) the bare quartz tip is used to detect the evanescent field near the sample.
Near-field optics
Aperture (computer memory)
Scanning Probe Microscopy
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광 도파로를 따라 전파하는 빛의 특성을 측정하기 위해 근접장 주사 광학현미경(Near-field scanning optical microscope, NSOM)으로 광 도파로의 표면에 형성된 에바네슨트 파 evanescent wave)의 분포를 측정하였다. 사용된 NSOM은 photon scanning tunneling microscope방식으로 본 연구의 목적에 적합하도록 직접 제작한 것이다. 광원 파장 1550㎚에서 단일 모드 다중 모드 채널형 광 도파로에 대해 도파로 표면에 형성된 에바네슨트 파의 분포를 측정하였으며, 3차원 빔전파방법(Beam Propagation Method)으로 계산된 수치 해석 결과와 두 모드 간의 간섭 형상을 직접적으로 확인할 수 있었다. The propagation characteristic of an optical waveguide was investigated by measuring with a near-field scanning optical microscope (NSOM) the evanescent field formed at the neighbor of its core-cladding interface. For this purpose, the NSOM system was developed specially as a form of Photon scanning tunneling microscope. The evanescent field distributions of several channel waveguides were measured at the wavelength of 1550 ㎚, and the usefulness of the system was verified by comparing experimental results with simulation results. In particular, the interference phenomena of the guided modes during their propagation along a multimode channel waveguide could be observed directly from the measured evanescent field distribution.
Cladding (metalworking)
Near-field optics
Waveguide
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In this work we use an approach that allows a determination of the spatial distribution of the electromagnetic field in the near-field region of the near field scanning optical microscope (NSOM) probe. We studied a MBE grown sample, which contains a 9 nm AlAs layer embedded in a GaAs matrix. The optical contrast arises from the difference in the index of refraction of the two materials. Topography related contrast is avoided by studying a freshly cleaved surface. With the NSOM working in emission mode the reflected light was collected in the far field.
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We investigate the influence of single and multiple defects in the metal coating layer of microfabricated apertureless scanning near-field optical microscopy (SNOM) probes on the polarization-dependent emitted optical near field using rigorous electromagnetic modeling tools.
Near-field optics
Scanning Probe Microscopy
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Scanning near field optical microscopes provide access to a variety of interesting material properties with a resolution in the nanometric size of scale. However, the quality of the optical fiber tip is of decisive importance. Because the production process of pulled and coated glass fiber tips is still highly empirical and full of defects, a technique would be useful to determine the tips' quality before they are shipped to the user or mounted in the microscope. This contribution shows an easy and fast full field method for the characterization of common 633 nm glass fiber SNOM tips. Size and shape as well as disturbances at the aperture can be recognized by means of evaluating the far field distribution of the emitted intensity and phase which are recorded by a CCD target. A numerical model is introduced which solves the reverse task that allows to draw conclusions from the measured intensity and phase distributions to the shape of the tip itself. Experimental investigation in a simple and robust setup and comparisons with combined near/far-field calculations show the working principle of this measurement technique for the analysis of SNOM tips.
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Intensity
Characterization
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The tetrahedral tip is used as a light emitting probe for scanning near-field optical microscopy (SNOM). It has no aperture as an element for the confinement of light and the techniques of scanning tunneling microscopy and SNOM can be combined with the same probing tip. Silver grains are distinguished from gold grains by their specific near-field optical contrast in SNOM transmission mode images of mixed films of silver and gold at a lateral resolution in the nanometer range and an edge resolution of 1 nm for selected grains. The contrast is explained in terms of a quasielectrostatic model of a local light-emitting source interacting with the object.
Near-field optics
Aperture (computer memory)
Scanning Probe Microscopy
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Scanning near-field optical microscope (SNOM) can provide optical imaging with ultrahigh resolution owing to its breakthrough the limit of optical diffraction. Metal coated optical fiber probe in nano-scale is one of the most important parts in aperture type of SNOM. Tip diameter and structure determine the final spatial resolution and experimental utility of SNOM. In order to understand the behavior of light propagation in the probes, we have investigated two kinds of 3D probe models (metal coated and uncoated) by solving Maxwell equations with the Finite- Difference Time-Domain method. The 3D computation reveals that the field distribution of light in the probes are some patterns due to the polarization of light and the structure of the probe. This result can guide to find optimized tip design.
Near-field optics
Aperture (computer memory)
Nanophotonics
Scanning Probe Microscopy
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