Aperture Probe Research Articles

Light distribution analysis of optical fibre probe-based near-field optical tweezers using FDTD

Optical fibre probe-based near-field optical tweezers overcomes the diffraction limit of conventional optical tweezers, utilizing strong mechanical forces and torque associated with highly enhanced electric fields to trap and manipulate nano-scale particles. Near-field evanescent wave generated at optical fibre probe decays rapidly with the distance that results a significant reduced trapping volume, thus it is necessary to analyze the near-field distribution of optical fibre probe. The finite difference time domain (FDTD) method is applied to characterize the near-field distribution of optical fibre probe. In terms of the distribution patterns, depolarization and polarization, the near-field distributions in longitudinal sections and cross-sections of tapered metal-coated optical fibre probe are calculated. The calculation results reveal that the incident polarized wave becomes depolarized after exiting from the nano-scale aperture of probe. The near-field distribution of the probe is unsymmetrical, and the near-field distribution in the cross-section vertical to the incident polarized wave is different from that in the cross-section parallel to the incident polarized wave. Moreover, the polarization of incident wave has a great impact on the light intensity distribution.

Near-field optical probes provide subdiffraction-limited excitation areas for fluorescence correlation spectroscopy on membranes

Abstract Near-field optical probes have been used to produce a subdiffraction-limited observation area for fluorescence correlation spectroscopy (FCS) experiments on supported membranes. The design of a bent, etched fiber probe that is compatible with biological imaging in an aqueous environment is described. This probe design is used for proof of principle experiments to measure lipid diffusion in a fluid-supported bilayer. A reduction in excitation area of approximately one order of magnitude (relative to a confocal FCS experiment) is obtained with a probe aperture diameter of 140 nm. We also demonstrate a simple approach for modeling the autocorrelation decay due to diffusion within the excitation profile at the near-field scanning optical microscopy (NSOM) probe aperture. The use of probes with smaller apertures is expected to provide an additional order of magnitude reduction in the observation area, thus enabling the study of cellular membranes with higher concentrations of fluorophores than is currently possible with diffraction-limited techniques.

Near‐field modulation of AlGaInP laser diode emissions by an aperture probe of near‐field optical microscope

AbstractIn situ emission profiles of AlGaInP multiple quantum well laser diodes (LD's) observed by a near‐field scanning optical microscope (NSOM) aperture probe shows a single broad elliptic profile similar to a far‐field observation, whereas the local emission spectrum was different depending where the aperture probe was positioned within the emission profile. This observation suggests the existence of a near‐field coupling between the aperture and the LD cavity, which modulates the lasing wavelength among the longitudinal multimodes determined by the LD cavity length. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fluorescence Correlation Spectroscopy with Sub-Diffraction-Limited Resolution Using Near-field Optical Probes

!Received 10 May 2008; accepted 20 September 2008; published online 24 October 2008We report fluorescence correlation spectroscopy !FCS measurements using near-field scanningoptical microscopy !NSOM probes to produce a sub-diffraction-limited observation area. An orderof magnitude reduction in the area compared to confocal FCS has been achieved. We alsodemonstrate a simple means to model the autocorrelation decay due to diffusion within theexcitation profile at the NSOM probe aperture. The use of probes with smaller apertures is expectedto provide an additional order of magnitude reduction in the observation area, thus enabling thestudy of cellular membranes with higher concentrations of fluorophores than is currently possiblewith diffraction-limited techniques. ©

An Aperture Probe of Near-Field Optical Microscope: Near-Field Tool to Modulate the Laser Diode Emissions

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LD's) observed by a near-field scanning optical microscope aperture probe shows a single broad elliptic profile similar to a far-field observation, whereas the local emission spectrum was different depending where the aperture probe was placed within the emission profile. This observation suggests the existence of a near-field coupling between the aperture and the LD cavity, which modulates the lasing wavelength among the longitudinal multimodes determined by the LD cavity length. [DOI: 10.1380/ejssnt.2009.757]

光纤探针化学镀镍

As a component of near-field scanning optical microscope (NSOM), optical fiber probe is an important factor influncing the equipment resolution. Electroless nickel plating is introduced to metallize the optical fiber probe. The optical fibers are etched by 40% HF with Turner etching method. Through pretreatment, the optical fiber probe is coated with Ni-P film by electroless plating in a constant temperature water tank. Atomic absorption spectrometry (AAS), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometry (EDXS) are carried out to characterize the deposition on fiber probe. We have reproducibly fabricated two kinds of fiber probes with a Ni-P film: aperture probe and apertureless probe. In addition, reductive particle transportation on the surface of fiber probe is proposed to explain the cause of these probes.

Electric field-induced direct delivery of proteins by a nanofountain probe.

We report nanofabrication of protein dot and line patterns using a nanofountain atomic force microscopy probe (NFP). Biomolecules are continuously fed in solution through an integrated microfluidic system, and deposited directly onto a substrate. Deposition is controlled by application of an electric potential of appropriate sign and magnitude between the probe reservoir and substrate. Submicron dot and line molecular patterns were generated with resolution that depended on the magnitude of the applied voltage, dwell time, and writing speed. By using an energetic argument and a Kelvin condensation model, the quasi-equilibrium liquid-air interface at the probe tip was determined. The analysis revealed the origin of the need for electric fields in achieving protein transport to the substrate and confirmed experimental observations suggesting that pattern resolution is controlled by tip sharpness and not overall probe aperture. As such, the NFP combines the high-resolution of dip-pen nanolithography with the efficient continuous liquid feeding of micropipettes while allowing scalability to 1- and 2D probe arrays for high throughput.

Fluorescence correlation spectroscopy with sub-diffraction-limited resolution using near-field optical probes

We report fluorescence correlation spectroscopy (FCS) measurements using near-field scanning optical microscopy (NSOM) probes to produce a sub-diffraction-limited observation area. An order of magnitude reduction in the area compared to confocal FCS has been achieved. We also demonstrate a simple means to model the autocorrelation decay due to diffusion within the excitation profile at the NSOM probe aperture. The use of probes with smaller apertures is expected to provide an additional order of magnitude reduction in the observation area, thus enabling the study of cellular membranes with higher concentrations of fluorophores than is currently possible with diffraction-limited techniques.

Imaging contrast under aperture tip–nanoantenna array interaction

Periodic arrays of paired and single gold nanorods were imaged in the near field using reflection and transmission modes of a near-field scanning optical microscope at various wavelengths and polarizations of light in the visible range. The paired nanorods act like nanoantenna, and an array of them was initially designed as a negative-index material for the near infrared. Reverse contrast in reflection and transmission images is observed under illumination from the small aperture of a metal-coated fiber probe. By changing the relative orientation of the rods to the polarization, the reverse contrast switches to the normal contrast of near-field imaging. Coupling between the aperture and the nanorod array makes the contrast higher. Transmission through the aperture is enhanced if the aperture probe is positioned between the nanorods. The average near-field transmission exhibits an opposite sign of anisotropy relative to the far-field case. Aperture probes with larger diameters always show normal imaging contrast. The results demonstrate that the broad angular spectra of small-aperture sources play a crucial role in near-field interactions with nanorod arrays. The results also show that angular redistributions of these spectra after transmission or reflection from the nanorod array are likely due to excitation of localized and propagating plasmons.

Lean worm-like specimen indicates mechanical trouble in stereotactic vacuum-assisted breast biopsy—A rare incident and successful resolution

A rare mechanical problem in stereotactic vacuum-assisted breast biopsy, which was recognized by a lean, worm-like appearance of the collected tissue sample, and its successful resolution are reported. A stereotactic vacuum-assisted biopsy (Mammotome ®) was performed for calcification in a 47-year-old female. The first specimen was collected, but the collected specimen was thinner than usual, and had a lean, worm-like appearance. In the second biopsy, no specimen was obtained at all. Under the speculation that breast tissue stuck in the tip of the probe aperture, and was became stretched when a vacuum was applied for the sample collection, we immediately exchanged the probe with a new one without releasing the patient's breast compression, and obtained the usual thick specimens containing ductal carcinoma in situ. The breast tissue was stuck in the tip of the aperture of the probe which we had a mechanical problem. Pathological examination of the thin, worm-like specimen showed that the collagen fibers in the breast were stretched and formed a tight bundle, which was consistent with our speculation that the tissue was stretched in the needle by the vacuum. Our experience indicates that the immediate decision of exchanging the probe without releasing the breast compression may solve the problem when the recovered specimen has a lean, worm-like appearance.

Resolution enhancing using cantilevered tip-on-aperture silicon probe in scanning near-field optical microscopy

Scanning near-field optical microscopy (SNOM) achieves a resolution beyond the diffraction limit of conventional optical microscopy systems by utilizing subwavelength aperture probe scanning. A problem associated with SNOM is that the light throughput decreases markedly as the aperture diameter decreases. Apertureless scanning near-field optical microscopes obtain a much better resolution by concentrating the light field near the tip apex. However, a far-field illumination by a focused laser beam generates a large background scattering signal. Both disadvantages are overcome using the tip-on-aperture (TOA) approach, as presented in previous works. In this study, a finite difference time domain analysis of the degree of electromagnetic field enhancement is performed to verify the efficiency of TOA probes. For plasmon enhancement, silver is deposited on commercially available cantilevered SNOM tips with 20 nm thicknesses. To form the aperture and TOA in the probes, electron beam-induced deposition and focused ion beam machining were applied at the end of the sharpened tip. The results show that cantilevered TOA probes were highly efficient for improvements of the resolution of optical and topological measurement of nanostructures.

Ultrasonically induced Effects in Electroless Nickel Plating to Fabricate a Near-Field Optical Fiber Probe

We have studied the submicron size dependence of electroless nickel plating under ultrasonic irradiation with a frequency of 1 MHz to fabricate a near-field apertured probe. Using a new ultrasonic bath with a small water depth of 80 mm, we found that a nickel film with a hollow structure is coated on the flat clad end of a fiber probe. The diameter of the hollow is 12 µm much smaller than the ultrasonic wavelength in water.

Focussed ion beam machined cantilever aperture probes for near‐field optical imaging

Near-field optical probe is the key element of a near-field scanning optical microscopy (NSOM) system. The key innovation in the first two NSOM experiments (Pohl et al., 1984; Lewis et al., 1984) is the fabrications of a sub-wavelength optical aperture at the apex of a sharply pointed transparent probe tip with a thin metal coating. This paper discusses the routine use of focussed ion beam (FIB) to micro-machine NSOM aperture probes from the commercial silicon nitride cantilevered atomic force microscopy probes. Two FIB micro-machining approaches are used to form a nanoaperture of controllable size and shape at the apex of the tip. The FIB side slicing produces a silicon nitride aperture on the flat-end tips with controllable sizes varying from 120 nm to 30 nm. The FIB head-on drilling creates holes on the aluminium-coated tips with sizes down to 50 nm. Nanoapertures in C and bow tie shapes can also be patterned using the FIB head-on milling method to possibly enhance the optical transmission. A transmission-collection NSOM system is constructed from a commercial atomic force microscopy to characterize the optical resolution of FIB-micro-machined aperture tips. The optical resolution of 78 nm is demonstrated by an aperture probe fabricated by FIB head-on drilling. Simultaneous topography imaging can also be realized using the same probe. By mapping the optical near-field from a bow-tie aperture, optical resolution as small as 59 nm is achieved by an aperture probe fabricated by the FIB side slicing method. Overall, high resolution and reliable optical imaging of routinely FIB-micro-machined aperture probes are demonstrated.

An Optimum Design Methodology for Planar-Type Coaxial Probes Applicable to Broad Temperature Permittivity Measurements

A planar-type coaxial probe applicable to wide temperature and frequency range has been developed. The probe employs a dielectric with a low coefficient of thermal expansion to minimize the effect of thermal deformation for broad temperature measurements. Additionally, a detailed design methodology has been developed to optimize the probe apertures in an effort to minimize the measurement uncertainty while maximizing the operating bandwidth. For this purpose, thorough sensitivity analysis has been employed to correlate the probe structures and dimensions to the individual sensitivity parameters. The analysis has been validated by parametric experiments. By using the dielectric with a low coefficient of thermal expansion and optimizing the probe dimensions, accurate permittivity measurements have been demonstrated from 30 C to 75 C with 40-GHz bandwidth. With the application of the error-correction method, the measurement temperature range has been further extended all the way up to the boiling temperature of water C). Furthermore, the complex permittivities of methanol have been measured from 30 C to 50 C and the dispersion parameters and full interpolation formulas have been extracted.

On the optimum form of an aperture for a confinement of the optically excited electric near field

A triangular nanoaperture in an aluminium film was used previously as a probe in a scanning near-field optical microscope to image single fluorescent molecules with an optical resolution down to 30 nm. The high-resolution capability of the triangular aperture probe is because of a highly confined spot of the electric near field which emerges at an edge of the aperture, when the incident light is polarized perpendicular to this edge. Previous numerical calculations of the near-field distribution of a triangular aperture in a planar metal film using the field susceptibility technique yielded a nearly quantitative agreement with the experimental results. Using the same numerical technique we now explored the possibility for a further confinement of the electric near field and an increase in its intensity by modifications of the form of a triangular aperture. By introducing a kink on an edge pointing into the aperture, an arrow-shaped aperture is formed with one convex and three concave metal corners. It turns out that this form leads to a substantial further confinement of the near-field intensity at the convex corner. By extending the wings of this arrow-shaped aperture a further 5-fold increase of the intensity can be obtained without a deterioration of the confined spot.

Individual gold dimers investigated by far‐ and near‐field imaging

Plasmon resonances in 3D nanoparticle arrangements can produce strong localized optical fields, which are of importance for any application involving interaction of light with subwavelength volumes of matter down to the molecular level. In particular, remarkable field enhancement and confinement occur in a dimer geometry formed by two identical closely spaced particles. Although, recent advances in nanofabrication have rendered the fabrication of complex plasmon architectures more accessible, addressing their local fields in a nonperturbative fashion remains not straightforward, because metallic nanostructures are rather sensitive to their local environment. Here we study gold dimers fabricated by e-beam lithography. Individual dimers are imaged both by far- and near-field methods. First, the near-field electromagnetic interaction in an ensemble of dimers is investigated by scattering spectroscopy, using dark field microscopy. Next, to probe their local field, we explore the luminescence of individual gold dimers utilizing a confocal microscope with single molecule detection sensitivity. We provide a statistical analysis of the dimer luminescence for different incident polarizations, with direct comparison to single particles (monomers). Finally, the near-field transmission of the resonant dimers is mapped with a subwavelength resolution using polarized controlled near-field scanning optical microscopy. Surprisingly, no clear evidence of the high mode density in the dimer gap is observed. This result may be attributed to the limited coupling of the field emitted by the aperture probe to the dimer mode.

Measurement of photo‐patterned surface potential of Alq3 thin films by Kelvin‐force microscope together with near‐field optical stimulation

AbstractUniform and extremely high surface potentials of tris(8‐hydroxyquinolinato) aluminum(III) (Alq3) thin films, formed spontaneously by vacuum evaporation of Alq3 in the dark, are patterned by contact‐mask photo‐exposure. The resulting surface‐potential patterns are observed by scanning‐probe Kelvin‐force microscope (KFM). Surface‐potential patterns on Alq3 thin films can be preserved in the dark for more than a year at − 20 °C. The dependence of photo‐induced reduction of surface potentials on photo‐exposure intensity is reproduced well by fitting calculation based on the assumption that the drift of photo‐excited electrons/holes decreases the surface potential. Two‐dimensional (2D) transfer functions of KFM probes are discussed from KFM potential profiles obtained for a step pattern of surface potential. Near‐field optical stimulation through an aperture of an optical fiber probe is demonstrated as scanning near‐field optical microscope (SNOM) combined with KFM (SNOM‐KFM). A photoemission pattern from an aperture of SNOM probes can be visualized as a surface‐potential pattern on Alq3 thin films by means of SNOM‐KFM measurements. Copyright © 2008 John Wiley & Sons, Ltd.

X-ray excited optical luminescence detection by scanning near-field optical microscope: A new tool for nanoscience

Investigations of complex nanostructured materials used in modern technologies require special experimental techniques able to provide information on the structure and electronic properties of materials with a spatial resolution down to the nanometer scale. We tried to address these needs through the combination of x-ray absorption spectroscopy (XAS) using synchrotron radiation microbeams with scanning near-field optical microscopy (SNOM) detection of the x-ray excited optical luminescence (XEOL) signal. This new instrumentation offers the possibility to carry out a selective structural analysis of the sample surface with the subwavelength spatial resolution determined by the SNOM probe aperture. In addition, the apex of the optical fiber plays the role of a topographic probe, and chemical and topographic mappings can be simultaneously recorded. Our working XAS-SNOM prototype is based on a quartz tuning-fork head mounted on a high stability nanopositioning system; a coated optical fiber tip, operating as a probe in shear-force mode; a detection system coupled with the microscope head control system; and a dedicated software/hardware setup for synchronization of the XEOL signal detection with the synchrotron beamline acquisition system. We illustrate the possibility to obtain an element-specific contrast and to perform nano-XAS experiments by detecting the Zn K and W L(3) absorption edges in luminescent ZnO and mixed ZnWO(4)-ZnO nanostructured thin films.

Fluorescence correlation spectroscopy at 100 nM concentrations using near‐field scanning optical microscopic (NSOM) geometries and highly diffracting force sensing fiber probes

AbstractA tapered cantilevered optical fiber probe is introduced for fluorescence correlation spectroscopy (FCS). The probe has an aperture that is coated with metal and its dimension can be very well defined. Its design is based on cantilevered near‐field optical fiber probes that allow for on‐line atomic force feedback. This permits unprecedented stability in FCS measurements. Even such probes that have large diameters are below the cutoff wavelength of light propagation in optical fibers. Such large diameter probes produce bright spots of light that are highly diffracting. Therefore, only the fluence of light very close to the surface of the glass probe aperture is sufficient to exhibit a high probability of fluorescence excitation. From the results presented in this paper the z extent of such probes seems comparable to what can be obtained from prism‐based evanescent wave methods, but with much more flexibility and including force sensing capability. In agreement with theoretical results on prism evanescent field‐based excitation, it is shown that even with large xy dimensions for the fiber probes in this paper, fluorescence correlation spectra from 100 nM fluorophore concentrations can be recorded. This is 10 times larger than the concentrations generally required for conventional confocal FCS. The results suggest that such an approach has considerable potential for applications of FCS in membrane, near‐membrane, solution, and even within‐cell environments. The latter is achieved through atomic force feedback controlled penetration of tapered cantilevered metal‐coated glass fibers into cells, and this is also reported in this paper.

Near-field modulation of laser diode emissions by an aperture probe of near-field optical microscope

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LDs) observed by a near-field scanning optical microscope (NSOM) probe with an intentionally enlarged aperture shows a single broad elliptic profile, whereas the detected wavelength hops among those of the longitudinal multimodes observed in the far-field. This hopping suggests the near-field coupling between the aperture and the LD cavity, which is proved by the observation of strong modulation of laser intensity by the optical coupling only in the near-field regime. Probes with an intact aperture are found not to couple with the lasing TE 00 mode, but, therefore, couple only with a near-field longitudinal mode. Around the lasing region, emissions detected by an intact aperture are ascribed to amplified spontaneous emissions at wavelengths far from those of the lasing multimode emissions. These near-field modes, with spatial profile different from both the far-field TE and TM modes, are detectable only by the NSOM.