Aperture Probe Research Articles

Nondestructive Online <emphasis>In Vitro</emphasis> Monitoring of Pre-Osteoblast Cell Proliferation Within Microporous Polymer Scaffolds

We present a system for the online, in vitro, nondestructive monitoring of tissue growth within microporous polymer scaffolds. The system is based on measuring the admittance of the sample over a frequency range of 10-200 MHz using an open-ended coaxial probe and impedance analyzer. The sample admittance is related to the sample complex permittivity (CP) by a quasi-static model of the probe's aperture admittance. A modified effective medium approximation is then used to relate the CP to the cell volume fraction. The change of cell volume fraction is used as a measure of tissue growth inside the scaffold. The system detected relative cell concentration differences between microporous polymer scaffolds seeded with 0.4, 0.45, 0.5, and 0.6 x 10(6) pre-osteoblast cells. In addition, the pre-osteoblast proliferation within 56 scaffolds over 14 days was recorded by the system and a concurrent DNA assay. Both techniques produced cell proliferation curves that corresponded to those found in literature. Thus, our data confirmed that the new system can assess relative cell concentration differences in microporous scaffolds enabling online nondestructive tissue growth monitoring.

Near-Field Transmission Imaging by 60 GHz Band Waveguide-Type Microscopic Aperture Probe

Near-field imaging has been intensively investigated to observe the shape and the physical properties of objects, aiming at wide applications in the areas of science and engineering. In this research, by using 60 GHz band waveguide-type microscopic aperture probe, the characteristics of the near-field imaging in transmission mode have been studied by simulation and experiment. The probe is made of a WR-15 rectangular waveguide with end-shielded metal plate and a 0.5 mm-diameter aperture. In the simulation, at first, the electric field distribution at the aperture, at the rear (waveguide) and the front positions (free space) are presented. Second, the transmitted electric fields are presented for three cases: (a) scanning of a dielectric slit, (b) by varying the distance between the aperture and a dielectric sample, and (c) scanning of a dielectric groove. In the experiment, the lateral resolution with a two-slit and the depth resolution with grooves having various depths in rectangular format are described and the results show both resolutions to be much shorter than the wavelength. Finally, the scanned images of the letter N punched through a dielectric material and a leaf are demonstrated.

Fabrication of Cantilevered Tip-on-Aperture Probe for Enhancing Resolution of Scanning Near-Field Optical Microscopy System

The scanning near-field optical microscopy (SNOM) system achieves a resolution beyond the diffraction limit of the conventional optical microscopy system by subwavelength aperture probe scanning. The problem is that the light throughput decreases very markedly with decreasing aperture diameter. Apertureless scanning near-field optical microscopes obtain a much better resolution by concentrating light field near the tip apex. However, 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 presented in previous works. In this study, the fabrication of a cantilevered tip for SNOM and scanning force microscopy (SFM) has been described. The nano-probes are batch-fabricated on a silicon wafer. The Si3N4 has excellent optical transparent characteristics, higher Young's modulus and yield strength so that it should provide a better probe for SNOM and SFM. For this purpose, a Si3N4 thin film was deposited using low-pressure chemical vapor deposition (LPCVD). To form the aperture and TOA in the probe, we applied focused ion beam (FIB) machining at the end of the sharpened tip. For verification of the efficiency of the micromachined TOA probes, numerical analysis using the finite-difference time domain (FDTD) analysis and experimental measurement using an inverted microscope based the SNOM system were performed.

Near-field spectroscopy of surface plasmons in flat gold nanoparticles

We use near-field interference spectroscopy with a broadband femtosecond, white-light probe to study local surface plasmon resonances in flat gold nanoparticles (FGNPs). Depending on nanoparticle dimensions, local near-field extinction spectra exhibit none, one, or two resonances in the range of visible wavelengths (1.6-2.6 eV). The measured spectra can be accurately described in terms of interference between the field emitted by the probe aperture and the field reradiated by driven FGNP surface plasmon oscillations. The measured resonances are in good agreement with those predicted by calculations using discrete dipole approximation. We observe that the amplitudes of these resonances are dependent upon the spatial position of the near-field probe, which indicates the possibility of spatially selective excitation of specific plasmon modes.

High transmission nanoscale bowtie-shaped aperture probe for near-field optical imaging

A near-field scanning optical microscope probe integrated with nanoscale bowtie aperture for enhanced optical transmission is demonstrated. The bowtie-shape aperture allows a propagating mode in the bowtie gap region, which enables simultaneous nanoscale optical resolution and enhanced optical transmission. The optical characteristics of the bowtie aperture are demonstrated by measuring the optical near fields produced by the aperture. It is shown that bowtie aperture probes have one order of magnitude increase in transmission over probes with a regular shape aperture of the same resolution. The imaging results using bowtie aperture are in agreement with those obtained from numerical calculations.

A study of the electric field intensity distribution of a modified probe for NSOM lithography

In this work, the shape of a near-field scanning optical microscopy (NSOM) probe was modified using the results of numerical analysis based on a finite-difference time-domain (FDTD) algorithm. This probe has a narrow slit or small tip on its aperture. Especially in the near-field area, the electric field intensity distribution is heavily dependent on the polarization of the incident light. In the plane of polarization, the electric field intensity of the NSOM aperture probe is strongly enhanced at the edge of the aperture. Simulation results show that this enhancement can be totally or partially removed according to the size of the slit, if the slit carved on the probe is aligned with the polarization direction of the incident light. When a long slit is carved on the probe, the electric field intensity of the NSOM aperture probe in the plane of polarization shows a Gaussian distribution because the points of enhanced electric field intensity are totally removed. However, when a short slit is carved on the probe, the point of enhanced electric field intensity is removed at one edge of the aperture, and consequently the electric field intensity is enhanced only at the other edge. Moreover, when a tip is put on the probe, the electric field intensity is enhanced only near the tip. This means that a higher intensity electric field can be obtained on the surface to be processed in an area smaller than the aperture size. Therefore, the modified probe is expected to improve the fabrication of extremely small patterns. Furthermore, this type of probe can be easily realized compared to any other new-type probes, because it can be manufactured directly from existing probes.

Scanning near-field optical microscopy and near-field optical probes: properties, fabrication, and control of parameters

A brief review of modern applications of scanning near-field optical (SNO) devices in microscopy, spectroscopy, and lithography is presented in the introduction. The problem of the development of SNO probes, as the most important elements of SNO devices determining their resolution and efficiency, is discussed. Based on the works of the authors, two different methods for fabricating SNO probes by using the adiabatic tapering of an optical fibre are considered: the laser-heated mechanical drawing and chemical etching. A nondestructive optical method for controlling the nanometre aperture of SNO probes is proposed, substantiated, and tested experimentally. The method is based on the reconstruction of a near-field source with the help of a theoretical algorithm of the inverse problem from the experimental far-filed intensity distribution. Some prospects for a further refinement of the construction and technology of SNO probes are discussed.

Ultrahigh interference spatial compression of light inside the subwavelength aperture of a near-field optical probe

Spatial effects of interference and interaction of light modes in the subwavelength part of the near-field optical microscopy probe have been theoretically studied. It was found that the mode interference can lead to higher spatial compression of light (wavelength is equal to 500 nm in free space) within the transverse size of 25 nm inside the probe output aperture of 100 nm in diameter. The results predict a principal possibility of higher spatial resolution in the near-field optical microscopy technique.

In situ experimental study of a near-field lens at visible frequencies

Frequency dependent near-field scanning optical microscopy (NSOM) measurements of plasmon-mediated near-field focusing using a 50nm thick Au film are presented. In these studies the tip aperture of a NSOM probe acts as a localized light source, while the near-field image formed by the metal lens is detected in situ using nanoscale scatterers placed in the image plane. By scanning the relative position of object and probe, the near-field image generated by the lens is resolved. NSOM scans performed at different illumination frequencies reveal an optimum near-field image quality at frequencies close to the surface plasmon resonance frequency.

Aperture Antennas on Probe-Fed Hemispherical Metallic Cavities

Aperture antennas cut onto probe-fed hemispherical cavities are investigated theoretically and experimentally. The result is valid for any aperture position and width. The operating frequencies of the antennas are primarily controlled by the resonant modes of the hemispherical cavity, with the input impedances easily matched by changing the probe length and/or aperture size. When the probe length and aperture size are designed properly, a very wide bandwidth of ~45% can be obtained. This bandwidth is nearly twice that of a bare monopole. The calculations are verified by measurements

Control of Droplet Size in Liquid Nanodispensing

In this Letter, the phenomena and parameters governing the nanoscale dispensing of liquid through an apertured atomic force microscope probe milled by focused ion beam lithography are characterized in detail. We show that the size of deposited droplets can be reproducibly defined by controlling the aperture size on the probe and the surface energies of both tip outer wall and substrate surface. On the basis of these findings, tips with aperture diameter as small as 35 nm enabled the deposition of regular arrays of nanodroplets with diameter down to 70 nm on an alkylamine-modified surface. The fine control of droplet volumes down to a few tens of zeptoliters (10(-21)L) provides a unique tool for creating devices and probing the fundamentals of wetting at the nanometer scale.

Surface nanostructuring by femtosecond laser irradiation through near-field scanning optical microscopy

A near-field scanning optical microscopy (NSOM) and a double-frequency femtosecond laser (400 nm, 100 fs) were applied to push the optical resolution further down to sub-50 nm on thin UV photoresist. A 20-nm feature size can be obtained. It is at a resolution of λ/20 ( λ: laser wavelength) and a/2 ( a: NSOM probe aperture diameter), respectively. It is proved that laser power and exposure time can affect feature size of lithography patterns. In this paper, the effect of probe-to-sample distance on dot-pattern features is studied, and different dot-pattern shapes are generated: dumbbell-dot, ellipsoid-dot and circle-dot. The simulated light field spatial distributions across the nano-aperture based on Bethe–Bouwkamp model is found to agree with experimental results very well.

Transient absorption measurement of organic crystals with femtosecond-laser scanning microscopes

Two types of femtosecond-laser scanning transient absorption microscopes have been developed as tools for observing ultrafast exciton dynamics in organic solids, which have recently received much attention for their roles as organic photofunctional devices, such as solar cells and light-emitting diodes. One microscope is a multiphoton excitation transient absorption microscope, and the other is an apertured probe transient absorption microscope. Their performance was evaluated by measuring transient absorption spectra, kinetics and images of perylene crystals.

High-Contrast Imaging of Nano-Channels Using Reflection Near-Field Scanning Optical Microscope Enhanced by Optical Interference

We demonstrated a contrast enhancement in a near-field scanning optical microscope (NSOM) by optical interference with an aperture probe in reflection (illumination-collection) mode operation. We observed a NiO film deposited on a sapphire substrate and clearly visualized 2-nm-deep nano-channel structures on the surface of the film. The reflection NSOM enhanced by optical interference is quite a promising instrument for high-resolution optical detection and estimation of low-contrast nanostructures.

10 nm Spatial Resolution Fluorescence Imaging of Single Molecules by Near-Field Scanning Optical Microscopy Using a Tiny Aperture Probe

We demonstrate high-resolution fluorescence imaging of single molecules using near-field scanning optical microscopy (NSOM) with a tiny aperture probe for two different wavelengths in visible range in the illumination mode of operation. The spatial resolutions obtained at both excitation wavelengths were almost the same and the highest resolution realized was about 10 nm. To discuss the achievable resolution in aperture NSOM, we also employed a computer simulation by the finite-difference time-domain method for various aperture sizes and wavelengths. The resolution of 10 nm is predicted to be contributed by the single peak of localized near-field light around the rim of the aperture.

Erratum: “Unexpected polarization behavior at the aperture of hollow-pyramid near-field probes” [Appl. Phys. Lett. 87, 223112 (2005)

Erratum: “Unexpected polarization behavior at the aperture of hollow-pyramid near-field probes” [Appl. Phys. Lett. 87, 223112 (2005)

Method to compensate for fluctuations in optical power delivered to a near-field scanning optical microscope

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.

The optimal form of the scanning near-field optical microscopy probe with subwavelength aperture

A theoretical approach to determine the optimal form of the near-field optical microscope probe is proposed. An analytical expression of the optimal probe form with subwavelength aperture has been obtained. The advantages of the probe with the optimal form are illustrated using numerical calculations. The conducted calculations show 10 times greater light throughput and the reception possibility of the more compactly localized light at the output probe aperture which could indicate better spatial resolution of the optical images in near-field optical technique using optimal probe.

Ablation of thin metal films by short-pulsed lasers coupled through near-field scanning optical microscopy probes

Short-pulsed lasers have been proven to be useful tools for precise modification of electronic materials. In conventional lens focusing schemes, the minimum feature size is determined by the diffraction limit. Finer resolution is accomplished by combining pulsed laser radiation with near-field scanning optical microscopy (NSOM) probes. In this study, short laser pulses are coupled to a fiber-based NSOM in order to ablate thin metal films. A detailed parametric study on the effects of probe aperture size, laser pulse energy, temporal width, and environment gas is performed. The significance of lateral thermal diffusion is highlighted and the dependence of the ablation process on the imparted near-field distribution is revealed.

Liquid dispensing of ultrasmall volumes using miniaturized probes

Small-scale, high-precision liquid delivery technologies are a welcome innovation in a number of areas. For example, the increasing miniaturization of microarrays for use in genomics and proteomics makes impractical conventional dispensing systems based on pipettes, pin and ring arrangements, and inkjet printing. Microfabricated probes developed for scanning probe microscopy (SPM), on the other hand, represent unique, versatile tools for working at the nanometer scale. Several techniques exist for liquid dispensing based on SPMlike nanoprobes. The first of these, dip-pen nanolithography (DPN), is based on immersing an SPM probe into molecular solution (the ‘ink’), and then writing molecular patterns with feature sizes as small as several tens of nanometers. An alternative is bioplume technology, which allows larger amounts of ink to be deposited by charging the probe with ink along the probe shaft. Surface structures created in this way, however, have larger dimensions. Here we describe a technology called nanoscale dispensing (NADIS) that is based on apertured nanoprobes. 4 Hollow probes are loaded with ink,either manually or usinga fluidicchannelsystem. When the probe tip makes contact with the substrate, a small amount of liquid is transferredthrough theaperture to the surface: see Figure 1 (left).Tocontrol the deposition process, the surface energies of both probe and substrate must be sufficiently controlled. Figure 1 (right and inset) shows a scanning electron micrograph of a NADIS probe with an integrated channel system. This allows liquid delivery from distant macroscopic reservoirs, thus opening the way to high-volume deposition applications without having to repeatedly reload the probe. Similar developments have been reported for DPN-like systems. Figure 1. (Left) In nanoscale dispensing (NADIS), liquid is deposited by an apertured probe on contact with a substrate surface. (Right) Scanning electron microscope image of a NADIS probe with a nanochannel incorporated into the cantilever. (Inset) Cross-section of a cantilever beam shows the hollow channel structure.