Near-field Scanning Optical Microscopy Tip Research Articles

Simulations of tip‐enhanced optical microscopy reveal atomic resolution

We have performed finite element electromagnetic analysis of 'apertureless' scanning near-field optical microscope tips. A variety of radii was considered, from 40 nm down to 1 nm, and the enhancement of optical scattering from the region beneath the probe apex was modelled in air and water. We observed sizeable spectral shifts when the radius decreases, together with a surprising increase in the amount of scattering from small tips. The lateral resolution is considered, which is shown to be always smaller than the tip radius, and for 1-nm-radius tips can allow atomic resolution imaging with sufficient optical enhancement. Gold, silver, copper and aluminium were modelled as tip materials.

SNOM signal near plasmonic nanostructures: an analogy with fluorescence decays channels

Scanning Near-field Optical Microscope (SNOM) is based on local excitations of nanostructures deposited on a substrate (illumination mode). Ideally, the local source behaves like a dipolar emitter so that the SNOM signal is strongly similar to the fluorescence decay rates of an excited molecule that would be located at the SNOM tip position. We present here how the SNOM signal near plasmonic nanostructures can be used to analyze radiative and non-radiative contribution to the fluorescence decay rate.

Direct Fabrication of Nanoscale Light Emitting Diode on Silicon Probe Tip for Scanning Microscopy

We have fabricated a silicon microprobe integrated with a nanometer-sized light emitting diode (Nano-LED) on the tip. This paper describes the fabrication procedure and preliminary topographic testing results. The silicon probe with electrode pattern was made by wet-etching a silicon-on-insulator wafer using oxide as the mask. Subsequently, the probe tip was cut using a focused ion beam (FIB) to form a 150 nm-wide gap. Semiconductor nanoparticles (CdSe/ZnS core-shell nanoparticles) were electrostatically trapped and excited within the electrode gap made on the probe tip. The LED-tip is approximately 150 nm 150 nm. The nano-LED light intensity and current were measured as a function of the driving voltage up to 25 V. In addition to the electroluminescence peaks from the CdSe particles, possible emission from silicon dioxide doped in the FIB milling process was also observed in the measured spectra. Basic mechanical characteristics of the silicon probe were measured by mounting the probe on a tuning fork in a standard near-field scanning optical microscopy (NSOM) set up. It was observed that the drag force reduces the probe oscillation as the vibrating tip approached the near-field of the sample surface. The topographic images of a chromium test pattern on a glass substrate were successfully acquired by keeping the probe tip within roughly 5 nm from the sample surface. Although the probe tip shape and the location of the Nano-LED are yet to be further optimized before realizing near-field optical scanning experiment, the result showed its great promise as a new type of NSOM tip with the ldquoon-proberdquo light-source.

Analytical tools for the nano world

Nanotechnology and nanoscience are very important areas of modern research. They are supposed to give us judicious control over the properties of matter at the nanoscale (i.e., lengths of well below 1 μm in at least two dimensions). Nano objects include structures such as subcellular compartments, “molecular machines” and small biological objects, nanocomposites, and components of molecular electronics. However, there is currently a lack of chemical diagnostics and characterization methods for these nano objects and nanoscale structures, particularly methods for characterizing their molecular compositions. Established methods for studying matter with nanoscale lateral resolution, such as atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning electron microscopy (SEM), typically give very little or no chemical information. Some exceptions exist—for example derivatized AFM tips that can be used for chemical recognition. In general, however, only the shape of a nanostructure, its surface topography, and its local electric properties can be measured, albeit in many cases with excellent spatial resolution, down to the atomic scale. However, with the advent of nanotechnology and nanoscience, the need for chemical and in particular molecular analysis has become urgent. Several new approaches to spectroscopic observation and imaging with nanometer-scale resolution have appeared recently, and will continue to set trends in the fields of nanoscience and nanotechnology. Scanning near-field optical microscopy (SNOM), the “optical cousin” of STM and AFM, emerged in the 1980s [1]. Standard aperture SNOM works by guiding light from a laser into an optical fiber whose tip is sharpened and metallized on the outside such that only a very small aperture (diameter ≈50 nm) passes laser light. If brought close to the object under investigation (within a few nm, i.e., into the optical near-field), this nanometer light source can be used to illuminate a spot on a sample that is considerably smaller than the optical diffraction limit. In order to obtain a high-resolution optical image of the sample, the SNOM tip is scanned over the sample surface with nanometer accuracy, using piezo elements for motion control. SNOM observation was initially restricted to fluorescence [2], but was later extended to Raman spectroscopy [3] and even to laser ablation mass spectrometry [4, 5]. One of the main limitations of SNOM is its optical throughput when using very small apertures. This limits both the achievable spatial resolution and the optical brightness of SNOM nano light sources. A way to circumvent this limitation is to use apertureless near-field methods, such as tip-enhanced Raman scattering (TERS). In this case, Raman scattering is amplified by the presence of a metallic nanoparticle brought close to the sample, using an AFM cantilever for example. Conceptually, this is an inversion of the geometry used in surface-enhanced Raman scattering (SERS). The advantage of this approach is that the sample can be illuminated using conventional optics, avoiding the much lower transmission of an aperture SNOM probe. Such apertureless or “tip-enhanced” nearfield spectroscopy experiments have been realized using coated AFM tips or sharpened metal tips, and are used for Anal Bioanal Chem (2008) 390:215–221 DOI 10.1007/s00216-007-1640-1

Development of new apertureless near-field scanning optical microscope tip using finite-differential time-domain calculation

We calculated the electric field enhancement factor of the apertureless near-field scanning optical microscope (ANSOM) tip using the finite-differential time-domain method. The results revealed that the enhancement factor of the silver tip remarkably decreased as the rounded tip radius increased. On the other hand, aggregated silver nanoparticles resulted in a strong field enhancement. Therefore, we developed a silver nanoparticles immobilized tip for the ANSOM probe and attempted to measure the surface enhanced Raman scattering spectrum of carbon onion molecules using it. We successfully detected the Raman spectra of individual carbon onions with a high sensitivity.

Near-field optical patterning on chloromethylated polyimide

Near-field scanning optical microscopy (NSOM) coupled with laser is used in nano-scale processing to make nano-scale dots or nano-scale structure. Nano-sclae processing using NSOM coupled with laser can be applied to photo- chemical etching process on crstalline silicon, to additive processes on some polymers, to subtraction processes on SAMs and other polymers. And it can be used to change material's optical properties in nano-scale geometry. As above, nano-scale processing using NSOM coupled with laser has an advantage that it can be applied to various processes. In this work, by using NSOM coupled with 266㎚ UV laser, nano-scale patterns were fabricated on chloromethylated polyimide(CMPI) films coated on silicon wafer. CMPI undergoes a fast photolysis under UV light. So, in the case of pattern fabrication on CMPI it is possible to fabricate patterns without development process. Possibilities for SMPI to be applied to nano-scale patterns fabrication were demonstrated. Compared to usual lithographic processes, the process proposed in this work is simple because development, one of steps to fabricate nano-scale patterns, is not needed. And the finite-difference-time-domain(FDTD) method was employed to simulate the energy intensity distribution in the near-field. The simulation was executed for NSOM tip and UV laser. The influence of aperture size and tip-sample distance on the resolution of the lithographic process is discussed from the simulation results. Comparison of some simulation results with corresponding experimental results could confirm the validity of the simulation model proposed.

Laser nanotraps and nanotweezers for cold atoms: 3D gradient dipole force trap in the vicinity of scanning near-field optical microscope tip

Using a two-dipole model of an optical near-field of scanning near-field optical microscope tip, i.e., taking into account contributions of magnetic and electric dipoles, we propose and analyze a new type of 3D optical nanotrap found for certain relations between electric and magnetic dipoles. Electric field attains a minimum value in vacuum in the vicinity of the tip and hence such a trap is quite suitable for manipulations with cold atoms.

Hybrid near-field scanning optical microscopy tips for live cell measurements

We report a near-field scanning optical microscopy (NSOM) probe that enables high-resolution imaging of living cells under physiological buffered conditions. The hybrid design combines a conventional fiber optic near-field probe with a standard atomic force microscopy cantilever. Imaging of fluorescent latex spheres suspended in an acetate matrix demonstrates the subdiffraction limited fluorescence and topography capabilities of the tips. The reduced spring constant of the hybrid tip is also shown to be amenable to measurements on living cells. NSOM fluorescence and topography measurements on living human arterial smooth muscle cells under buffered conditions are demonstrated.

Formation of fine near-field scanning optical microscopy tips. Part I. By static and dynamic chemical etching

The probe tip is the key component in scanning probe microscopes and their applications in the nanoscale imaging and nanofabrication. In this work, we have investigated the formation of near-field scanning optical microscopy probe tips from optical fiber by chemical etching. Static and dynamic etchings and their combinations are studied. The etching process is optimized, and tips with short tapers, small apertures (about 50 nm) and large aperture cone angles (40°) are successfully obtained. Multiple-tapered tips are also fabricated by using different dynamic regimes. It is found that the taper profiles are determined by the nonlinear dynamic evolution of the meniscus of the etchant near the fiber.

Formation of fine near-field scanning optical microscopy tips. Part II. By laser-heated pulling and bending

We have developed a simplified heating and pulling method for formation of near-field scanning optical microscopy probing tips from optical fibers. Laser power and continuous pulling force are two key processing parameters investigated. We found a narrow working range of laser power of 1.85–1.95 W and the pulling force of 0.5–0.9 lb, with the optimum conditions of 1.90 W and 0.74 lb. Tips with short taper (∼300 μm), small apex (∼50 nm), and large aperture cone angles (∼45°) have been achieved. The as-prepared tips are subsequently bended by pulsed laser heating and metal coating. Digitized bending from 10° to 90° can be achieved by linearly adjustment of the laser dose. The fabricated tips have shown good light guiding.

Sub-Micron Scale Optical Read/Write/Erase on Azo-Polymethacrylate Thin Films by Scanning Near-Field Optical Microscopy

Optical writing and subsequent optical reading of sub-micron size features has been obtained at room temperature on a spin-coated film of polymethecrylate added with azobenzene mesogenic side groups (PMA4), by means of scanning near-field optical microscopy (SNOM). Illumination with blue or UV light through the SNOM aperture induces conformational transitions in the side units, obtaining their orientation in a localized region. Optical writing and topographic reading with subwavelength resolution have been demonstrated. The pure optical readback is obtained by laser light at 690 nm through the aperture, in the polarization-modulation mode that provides sensitivity to dichroism and birefringence. Equally spaced lines have been written and subsequently imaged, presenting about one micron width in the birefringence image and about 600 nm width in the scattering optical image. Local erasure of optical information can be obtained by using green light at 543 nm with modulated polarization through the SNOM tip, in order to destroy the molecular alignment.

Unique Functional Micro/nano-structures Created by Femtosecond Laser Irradiation

AbstractFemtosecond (fs) laser application in three-dimensional (3D) optical recording is introduced. The laser irradiation on transparent glass and polymer matrix doped with fluorescent material is carried out, which changes the physical or chemical properties of the recording media and records information bits. With the change of the focusing positions inside the transparent substrates, 3D optical recording can be available for ultrahigh capacity data storage. Feasibility on fs laser drilling of poly-caprolactone (PCL) thin films for tissue engineering is investigated. It is found that precisely defined micro-hole arrays can be formed on the sample surfaces. Hydrophilic property of the processed samples is much improved, which provides good conditions for tissue cells to anchor on the man-made skin. Fs laser applications to form nanostructures on substrate surfaces are studied. Fs laser combination with near-field scanning optical microscopy (NSOM) to induce surface property modification in the sub 50-nm under NSOM tip and nanoparticles is also discussed.

Fabrication and characterization of probes for combined scanning electrochemical/optical microscopy experiments.

A technique that combines scanning electrochemical microscopy (SECM) and optical microscopy (OM) was implemented with a new probe tip. The tip for scanning electrochemicaVoptical microscopy (SECM/OM) was constructed by insulating a typical gold-coated near-field scanning optical microscopy tip using electrophoretic anodic paint. Once fabricated, the tip was characterized by steady-state cyclic voltammetry, as well as optical and electrochemical approach experiments. This tip generated a stable steady-state current and well-defined SECM approach curves for both conductive and insulating substrates. Durable tips whose geometry was a ring with < 1 microm as outer ring radius could be consistently fabricated. Simultaneous electrochemical and optical images of an interdigitated array electrode were obtained with a resolution on the micrometer scale, demonstrating good performance of the tip as both an optical and an electrochemical probe for imaging microstructures. The SECM feedback current measurements were successfully employed to determine tip-substrate distances for imaging.

Near-field cathodoluminescence of nanoscopic diamond properties

Near-field detection cathodoluminescence (NF-CL) analyses on MWCVD diamond has been carried out in a scanning near-field optical microscope (SNOM)/scanning electron microscope hybrid system to ensure that even the smallest structures with modified material properties can be evaluated. The CL signal is detected directly with the SNOM tip above the recombination center while the sample is irradiated simultaneously as well as homogeneously with an electron beam. The achievable resolution of this NF-CL analyses has been improved to approximately 50 nm. In addition, it was possible to determine electronic properties such as the local diffusion lengths by keeping the position of the SNOM probe constant and scanning the electron beam. Using this technique, distributions of defects within small diamond grains became visible without any sample preparation. Dislocations, hydrogen plasma induced defects, and nitrogen impurities were separated on a H post-treated MW-CVD diamond single crystal. Clusters of the 575-nm luminescence system were detected. A shift from blue to green of the Band-A emission depicted a dislocation.

On the coupling efficiency of metal-coated near-field probes.

A theory for calculating the optical transmission of nanometric circular apertures in a thick and perfectly conducting screen coated upon an optical fibre has been developed. The theory is intended for the study of near-field probes and differs from other well known theories of radiation transmission through subwavelength apertures in the fact that it includes an optical fibre, making possible to distinguish which part of energy passing through the aperture is effectively coupled in guided modes. In a scanning near-field optical microscope tip, only the guided modes will reach the photodetector, and will contribute to the final read-out. A numerical calculation for different fibre parameters, to show the dependence of the guided transmission coefficient for the guided modes of the fibre, is presented. The agreement of the theory with earlier calculations where the optical fibre is not included is emphasized.

Demonstration of near-field scanning photoreflectance spectroscopy

A near-field scanning optical microscope (NSOM) was developed to perform photoreflectance (PR) spectroscopy experiments at high spatial resolution (∼1 μm). Representative PR spectra are shown, along with an image illustrating the capability of observing contrast in images due to the strength of a PR feature. It was found that sufficiently high intensity light from the NSOM tip can produce photovoltages large enough to limit the spatial resolution of the electric field determination by PR. The photovoltage effect is measured as a function of light intensity, and the results are discussed in terms of a simple photovoltage expression.

Theoretical and experimental study of the forces between different SNOM probes and chemically treated AFM cantilevers

A shear-force mechanism between a chemically etched scanning near-field optical microscope tip and different chemically treated atomic force microscope cantilevers has been experimentally and theoretically investigated as a function of the tip-to-sample distance for different amplitudes of the tip oscillation. The experimental results show, in agreement with the theoretical predictions, that as the tip approaches the cantilever, the electrostatic force is the most influential in the shear-force mechanism, independently of the nature of the tip or the sample. As the tip-to-sample distance decreases, other forces come into play, and the type of interaction depends on the chemical nature of tip and sample surfaces. Thus, for hydrophobic cantilevers, the decrease in the vibration amplitude is mostly due to the solid friction forces resulting from electrostatic interactions. However, if the sample surface is hydrophilic, there is a decrease in the electrostatic force, a water meniscus is formed, and the decrease in the tip amplitude is mostly due to dynamic friction related to capillarity.

Probing Biological Samples with Near-Field Optics

Abstract Near-field scanning optical microscopy (NSOM) is an emerging optical technique capable of probing samples at the nanometric level. With the NSOM technique, high spatial resolution is achieved by scanning a small light source (or collector) close to a sample surface. The light source is usually formed with special fiber optic probes that funnel light down to an aperture that is smaller than the optical wavelength. By positioning the aperture close to a sample, the emerging radiation is forced to interact with the sample before diffracting out. Therefore, the spatial resolution in NSOM is only limited by the size of the aperture and its proximity to the sample, and not the wavelength of the light as in conventional optical microscopy. Recently, we have been using the single molecule detection limits combined with the unique nature of the electric fields present near the NSOM tip aperture to probe molecular level structure in model lipid membranes.

Near-field photoconductivity of stretch-oriented poly(para-phenylene vinylene)

Near-field scanning optical microscopy (NSOM) with photoconductivity contrast is used to map the transport properties of stretch-oriented poly(p-phenylene vinylene) thin films on a 150 nm length scale. Near-field photoconductivity is highly sensitive to film morphology and contrast is observed even in films with uniform photoluminescence. Regions of relatively higher photocurrent signals are correlated to regions with higher molecular order which have contrast approaching 15%. In addition, the photocurrent signal decreases sharply as the near-field tip approaches the surface of the film while at the same time the photoluminescence signal increases. The sharp decrease is attributed to the perturbation of the applied electric field within the polymer film by the metal on the NSOM tip.

Imaging Cells in Liquid with a Near-Field Scanning Optical Microscope - Problems and Solutions

Abstract Near Field Scanning Optical Microscopy (NSOM) is the newest member of the Scanning Probe Microscopy (SPM) family, which includes Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). Like its predecessors, it uses a sharp tip and a very precise positioning system to scan the surface of samples. The important difference is that the NSOM tip is made of an optical fiber, with a sub-wavelength size aperture at its end, which is brought to near-field proximity to the sample. In the near-field regime, the resolution is only limited by the size of the light source (the aperture), and not by the wavelength of light, thus allowing NSOM to obtain images with subwavelength resolution. Applying NSOM to biological samples is very appealing, since biological research has developed a wide variety of fluorescent dyes which can be directed with high specificity to objects of interest. Moreover, biological research relies to a large extent on conventional fluorescence microscopy, which is diffraction limited. Application of NSOM to biological problems requires the ability to image rough and soft samples in liquid, a task that presents many challenges. We have developed a NSOM which is capable of imaging cells in liquid with a resolution of ∼150 nm. This has enabled us to image the organization of fluorescently labeled proteins on the membrane of cells in liquid, and demonstrate for the first time that the proteins are clustered.