Near-field Scanning Optical Microscope System Research Articles

Surface plasmon coupled nano-probe for near field scanning optical microscopy.

Near-field scanning optical microscopy (NSOM) is a powerful tool for study of the nanoscale information of objects by measuring their near-field electric field distributions. The near-field probe, which determines NSOM system performance, can be either a scattering-type or an aperture-type. Both types have strengths and weaknesses. Here we propose and study a surface plasmon-coupled type nano-probe, which works as a hybrid scheme and could potentially combine the advantages of the two NSOM probe types. The key element of the proposed probe is a nanoparticle-on-film structure designed on a tapered fiber tip. On the one hand, the probe can yield the signals scattered in the near field by a nanoparticle with a scattering mechanism; on the other hand, the scattered signals can be transmitted by the metal film and coupled into the fiber via surface plasmon coupled emission, thus providing a collection mode similar to an aperture-type NSOM. This will lead to signal enhancement, while greatly suppressing background noise. This surface plasmon-coupled nano-probe thus has great potential for near-field optical microscopy applications.

Evaluation of mitochondrial activity by two-photon absorption with near-field multioptical fiber probes

pH measurements enable the direct monitoring and evaluation of mitochondrial activity. We constructed a scanning near-field optical microscopy system with multioptical fiber probes using the two-photon absorption of a pH-sensitive fluorescent dye, SNARF-4F, to measure the activity difference of mitochondrial aggregates. pH can be monitored through the fluorescence intensity ratio (FIR) of SNARF-4F. We derived a calibration curve of the FIR as a function of pH. The FIR dynamic responses were measured by adding hydrochloric acid to the buffer solution. Using the developed system, we simultaneously measured the pH changes at two different locations in the SNARF-4F solution. Mitochondrial samples were prepared using optical tweezers to control the number and position of mitochondria. Mitochondrial pH changes (ΔpH) between 0.05 and 0.57 were observed after adding a nutritional supplement (malate and glutamate). In addition, in the comparative experiment on the activities of two mitochondrial populations, the obtained result suggested that the activity differs depending on the difference in the number of mitochondria.

Investigation of erbium dopant distribution in silica optical fibers with fluorescence-based measurements using a near-field scanning microscope

High spatial resolution information of the rare-earth dopant distribution in optical fibers enriches our understanding of the fiber manufacture processes and enables improvement in the design of active photonic devices including optical fiber lasers and amplifiers. Here, data from an investigation of the backscattered fluorescence signal off the end-face of an erbium (Er3+) doped silica optical fiber obtained with a near-field scanning optical microscope (NSOM) are presented. It has been recently confirmed via a theoretical model that information about the relative Er3+ ion distribution in fibers can be inferred by simply monitoring the fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. A comparison of the Er3+ ion distribution profiles extracted from the fluorescence measurements acquired through the NSOM system with those obtained from the application of a powerful analytical ion probe is also presented.

Scanning near-field optical microscopy system based on frequency-modulation atomic force microscopy using a piezoelectric cantilever

We developed a scanning near-field optical microscopy (SNOM) system based on frequency-modulation atomic force microscopy (FM-AFM), using a microfabricated force-sensing cantilever with a lead zirconate titanate (PZT) thin film as an integrated deflection sensor. In the system, an evanescent field is generated on the sample surface by total internal reflection, and the light transmitted through the optically transparent tip is collected by a photomultiplier while the gap distance between the tip and the sample surface is regulated by the FM detection method. We obtained a SNOM image of a sample with the nanometer-scale resolution of polystyrene particles dispersed in a polyvinyl alcohol thin film deposited onto a glass substrate with the developed SNOM/AFM system, under a completely dark condition.

A resonance tracking method for stable operation of a near-field scanning optical microscope in liquid environment

We report on the novel design of the near-field scanning optical microscope (NSOM) which operates in liquid environment. A resonance tracking digital scanning method is applied to compensate the resonance shift due to the evaporation of the liquid in the atmospheric pressure. By this method, stable operation of NSOM system is demonstrated by showing topographic images of the metallic grating embedded in liquid environment.

Time-Resolved Ultraviolet Near-Field Scanning Optical Microscope for Characterizing Photoluminescence Lifetime of Light-Emitting Devices

We developed a instrument consisting of an ultraviolet (UV) near-field scanning optical microscope (NSOM) combined with time-correlated single photon counting, which allows efficient observation of temporal dynamics of near-field photoluminescence (PL) down to the sub-wavelength scale. The developed time-resolved UV NSOM system showed a spatial resolution of 110 nm and a temporal resolution of 130 ps in the optical signal. The proposed microscope system was successfully demonstrated by characterizing the near-field PL lifetime of InGaN/GaN multiple quantum wells.

Transmission Near-Field Scanning Optical Microscopy Investigation on Cellular Uptake Behavior of Iron Oxide Nanoparticles

Cellular uptake behavior of iron oxide nanoparticles is investigated using a transmission near-field scanning optical microscopy (NSOM) without the need of fluorescent labeling. By using the transmission NSOM system, we could simultaneously explore the near-field optical analysis of the cell interior and record the topographic information of the cell surface. The cell endocytosis of iron oxide nanoparticles by normal breast MCF10A cells is first studied by this transmission NSOM system, and this dual functional nanoscale-resolution microscopy shows the capability of mapping the spatial localization of nanoparticles in/outside cell surface without the need of fluorescence labeling. Nanoscale optical signature patterns for iron oxide nanoparticle-loaded vesicles inside the cells were observed and analyzed.

Employment of metamaterial cloaks to enhance the resolution of near-field scanning optical microscopy systems based on aperture tips

In this paper, we numerically demonstrate that plasmonic covers designed through the scattering cancellation approach can be successfully used to enhance the performance of near-field scanning optical microscopy (NSOM) systems based on the employment of aperture tip probes. The material used to partially cover the tip exhibits a near-zero value of the real part of the permittivity function at the working frequency and is designed in such a way to dramatically reduce the undesired electromagnetic coupling between the sample to be imaged and the metallic coating of the aperture tip. We show that minimizing such an unwanted interaction may lead to enhance the maximum achievable resolution of the NSOM system. The approach proposed is numerically tested at optical frequencies through a proper set of full-wave simulations, taking into account material losses and frequency dispersion. The proposed results represent a proof-of-concept and can be scaled at lower frequencies (infra-red and THz), where fabrication issues are more relaxed and possible implementation can be made practical using alternating plasmonic/non-plasmonic multi-layers or even some natural materials.

Near-Field Spectroscopy with Infrared Synchrotron Radiation Source

We report the results of near-field spectroscopy measurements performed using a scanning near-field optical microscopy system combined with an infrared synchrotron radiation source. The infrared synchrotron radiation is a highly brilliant white light source, and is tightly focused onto the probe tip. Strong background scattering is suppressed by modulating the distance between the probe and the sample. Higher harmonic components are extracted from the scattered light using a lock-in amplifier and examined for the presence of near-field signals. Near-field spectra in the mid-infrared region are measured by loading the higher harmonic components into a Fourier transform infrared spectroscopic apparatus. A striped metal pattern with a width of 1 μm printed on a glass substrate is found to be resolved at a wavelength of 9.8 μm. [DOI: 10.1380/ejssnt.2011.63]

Analysis of light scattering from human breast tissue using a custom dual‐optical scanning near‐field optical microscope

In this paper we introduce a custom scanning near-field optical microscope (SNOM) that simultaneously collects reflection and transmission near-field images along with topography. This dual-optical SNOM uses a bent probe, which allows for axial reflection imaging, accurate surface scanning, and easy identification of topographic artifacts. Using this novel dual-optical SNOM, we image desiccated and non-desiccated human breast epithelial tissue. By comparing the simultaneous SNOM images, we isolate the effects of tissue morphology and variations in refractive indices on the forward- and back-scattering of light from the tissue. We find that the reduction in back-scattering from tissue, relative to the glass slide, is caused by dense packing of the scattering sites in the cytoplasm (morphology) in the desiccated tissue and a thin-film of water adhering to the glass slide (refractive index) in the non-desiccated tissue sample. Our work demonstrates the potential of our customized dual-optical SNOM system for label-free tissue diagnostics.

Analysis of nanopatterning through near field effects with femtosecond and nanosecond lasers on semiconducting and metallic targets

A systematic study was conducted to understand the size and shape of nanopatterns generated on selected semiconducting (Si and Ge) and metallic (Cr, Cu, and Ag) targets under different laser pulse durations, laser energies, and number of laser pulses. Based on the experimental results, femtosecond laser pulses, compared with nanosecond laser pulses, provide lower damage thresholds to the targets but higher damage thresholds to the near field scanning optical microscope (NSOM) probes at the wavelength (∼400–410 nm) studied. Furthermore, the resulting nanopatterns showed a significant dependence on the optical properties (i.e., absorption coefficient and surface reflectivity) of the target material. By comparing the obtained experimental results, we conclude that the optical energy transport from the NSOM probe to the target dominates the pattern generation when femtosecond laser is applied to the NSOM system. When nanosecond laser is applied, both the thermal and optical energy transported from the NSOM probe to the targets attribute to the obtained morphology of nanopatterns on different targets under the experimental conditions studied.

Experimental study of plasmonic structures with variant periods for sub-wavelength focusing: analyses of characterization errors

Characterization issues of plasmonic structures are highlighted and investigated in detail in this paper. Combining with the plasmonic structures functioning for sub-wavelength focusing, optical characterization was carried out using a near-field scanning optical microscope (NSOM) system. Characterization errors that originated from both the nanofabrication using a focused ion beam (FIB) direct milling technique and misalignment of the NSOM system were analyzed in comparison to the theoretical computational results. Our experimental results demonstrated that the focusing function of the structures is in agreement with that of the designed structure. However, the measured beam spot size is larger than the designed value due to the direct measurement error originating from the NSOM and the indirect error from the FIB fabrication process.

Confined detection volume of fluorescence correlation spectroscopy by bare fiber probes

A fiber-tip-based near-field fluorescence correlation spectroscopy (FCS) has been developed for confining the detection volume to sub-diffraction-limited dimensions. This near-field FCS is based on near-field illumination by coupling a scanning near-field optical microscope (SNOM) to a conventional confocal FCS. Single-molecule FCS analysis at 100 nM Rhodamine 6G has been achieved by using bare chemically etched, tapered fiber tips. The detection volume under control of the SNOM system has been reduced over one order of magnitude compared to that of the conventional confocal FCS. Related factors influencing the near-field FCS performance are investigated and discussed in detail. In this proof-of-principle study, the preliminary experimental results suggest that the fiber-tip-based near-field FCS might be a good alternative to realize localized analysis at the single-molecule level.

Direct mapping of the UV surface plasmons

Researchers employed various well-developed concepts from conventional optics in designing novel plasmonic devices, which allow us to construct a framework to describe the propagation, diffraction, and interference of surface plasmon polaritons (SPPs) on a chip. Here we present what we believe to be the first direct mapping of the UV SPPs on an Al2O3/Al surface using a UV-compatible near-field scanning optical microscope system. UV SPP modes launched by one-dimensional slits or two-dimensional groove arrays and corresponding interference phenomenon were both observed, which may enrich the studies on subwavelength optics on a chip.

Design, Fabrication, and Characterization of Near-Field Apertures for 1 Tbit/in$^{2}$ Areal Density

Today, conventional magnetic recording schemes are coming to an end because of the superparamagnetic limit. Heat-assisted magnetic recording (HAMR) may ultimately extend data densities beyond 1 TB/in2. HAMR systems utilize the phenomenon during which the magnetic properties of the recording media could be locally modified via heating (optionally, by an optical source in the near field) to temperature in the vicinity of the Curie value of the media material. As a result, heat induced by the optical source can temporarily reduce the magnetic coercivity of high anisotropy material to a level attainable by the magnetic writing head, thus making it feasible to record on relatively small ultra-high anisotropy (and thermally stable) grains, consequently enhancing the areal density dramatically. The key challenge is to develop a near-field transducer capable of delivering over 50 nW into a spot diameter of 30 nm. Traditional fiber schemes are barely capable of 0.1 nW. To resolve the issue, a laser diode could be placed with the emitting edge only a few nanometers away from the recording media. The light can propagate through a nanoaperture on the surface of an aluminum-coated emitting edge. This paper will present an experimental study of recording characteristics of various near-field transducers fabricated via focused ion beam (FIB). To count the number of photons emitted in the near field, a scanning near-field optical microscopy system has been implemented. The experiments indicate that the FIB-fabricated transducers could deliver power of over a few microwatt into a 30-nm spot (Fig. 7).

Numerical analysis of the spectral response of an NSOM measurement

Near-field Scanning Optical Microscopy (NSOM) is a powerful tool for investigating optical field with resolution greater than the diffraction limit. In this work, we study the spectral response that would be obtained from an aperture NSOM system using numerical calculations. The sample used in this study is a bowtie nanoaperture that has been shown to produce concentrated and enhanced field. The near- and far-field distributions from a bowtie aperture are also calculated and compared with what would be obtainable from a NSOM system. The results demonstrate that it will be very difficult to resolve the true spectral content of the near-field using aperture NSOM. On the other hand, the far-field response may be used as a guide to the near-field spectrum.

Scanning optical probe microscopy with submicrometer resolution using an organic photodetector

A high-resolution scanning optical microscopy technique is demonstrated, in which an organic photodetector on a silicon-based scanning probe cantilever scans a sample, simultaneously recording optical and topographic data with submicrometer resolution, while showing no measurable degradation during the scan. Potential applications of the probe include characterization of optoelectronic materials and devices, as well as simultaneous topographic and fluorescence microscopy of biological samples. Extension to these applications is aided by the fact that the probe is compatible with conventional atomic force microscopy systems and does not suffer some of the practical difficulties of existing near-field scanning optical microscopy systems.

Writing and Reading Methodology for Biochips with Sub-100-nm Chemical Patterns Based on Near-Field Scanning Optical Microscopy

This paper demonstrates a writing and reading methodology, which allows both to create and to detect sub-100-nm carboxyl-terminated patterns on light-transmissive quartz substrates by the same instrumental system. Such a technique, capable of creating carboxyl-terminated nanopatterns, offers several benefits for the miniaturization of biochips, since the carboxyl-terminated nanopatterns allow the easy immobilization of biomolecules by amide bond formation. As a consequence, increasingly miniaturized biochips require suitable analytical methods for the detection of nanopatterns. In our approach, carboxyl-terminated nanopatterns of down to 80 nm width were created using a photolabile silane coupling agent and a UV laser coupled to a near-field scanning optical microscope (NSOM). The same NSOM system was then used in a next step to detect the fabricated carboxyl-terminated nanopatterns after modification with a fluorescent label. Furthermore, as a first step towards biochip applications, the successful immobilization of several biomolecules, such as streptavidin, IgG and DNA on carboxyl-terminated nanopatterns was demonstrated. We have shown that our approach has the potential to lead to a new bioanalytical method, which enables one to write and to read biochips on a sub-100-nm scale by the same system.

Observation of localized optical near‐field generated by submicron two‐hole structure for novel SNOM probe

AbstractA novel concept for the fabrication of a submicrometer‐structured probe tip for application in scanning near‐field optical microscopy (SNOM) based on a metalized optical fiber and a focused ion beam (FIB) milling is presented and investigated experimentally. This concept has the potential to enable a novel SNOM probe having a reproducible characteristic in the generation of intense and highly localized optical near‐field at the probe tip. As the simplest case designed by the boundary element method (BEM), a fine structure, which sandwiches a small projecting portion between a pair of 100‐nm2 holes, was fabricated by FIB in a 200‐nm silver thin film on a flattened end face of the single‐mode optical fiber. This unique structure was backlit by light from an Ar+ ion laser with a wavelength of 514.5 nm, which passes along the fiber core, and combined optical and topographical measurements for detecting the localized optical near‐field around the holes. A sample scan was performed using a modified SNOM system employing a dielectric spherical probe of 500 nm in diameter, which was illuminated by the evanescent field arising from the total internal reflection of He‐Ne laser light with a wavelength of 632.8 nm. As a result, a small dimple corresponding to the two‐hole structure was observed in the topographic image and a bright spot having high locational correlation with the dimple was simultaneously recorded in the optical image. Further study is required to determine the dependence of the optical near‐field intensity and its localization on the submicrometer two‐hole structure. Copyright © 2008 John Wiley & Sons, Ltd.

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.