Inducing superconductivity at a nanoscale: photodoping with a near-field scanning optical microscope

The local modification of an insulating GdBa₂Cu₃O_6.5 thin film, made superconducting by illumination with a near-field scanning optical microscope (NSOM), is reported. A 100-nm aperture NSOM probe acts as a sub-wavelength light source of wavelength λ_exc = 480–650 nm, locally generating photocarriers in an otherwise insulating GdBa₂–Cu₃O_6.5 thin film. Of the photogenerated electron–hole pairs, electrons are trapped in the crystallographic lattice, defining an electrostatic confining potential to enable the holes to move. Reflectance measurements at λ = 1.55 μm at room temperature show that photocarriers can be induced and constrained to move on a ≈200 nm scale for all investigated λ_exc. Photogenerated wires present a superconducting critical temperature T _c = 12 K with a critical current density J _c = 10⁴ A cm⁻². Exploiting the flexibility provided by photodoping through a NSOM probe, a junction was written by photodoping a wire with a narrow (≈ 50 nm) under-illuminated gap. The strong magnetic field modulation of the critical current provides a clear signature of the existence of a Josephson effect in the junction.     

Imaging 'intact' myofibrils with a near-field scanning optical microscope

Fluorescently labelled myofibrils were imaged in physiological salt solution by near-field scanning optical microscopy and shear-force microscopy. These myofibrils were imaged in vitro , naturally adhering to glass while retaining their ability to contract. The Z-line protein structure of the myofibrils was antibody labelled and easily identified in the near-field fluorescence images. The distinctive protein banding structure of the myofibril was also seen clearly in the shear-force images without any labelling requirement. With the microscope in the transmission mode, resolution of the fluorescence images was degraded significantly by excessive specimen thickness (>1 μm), whereas the shear-force images were less affected by specimen thickness and more affected by poor adherence to the substrate. Although the exact mechanism generating contrast in the shear-force images is still unknown, shear-force imaging appears to be a promising new imaging modality.     

Fluorescence imaging with a laser trapping scanning near-field optical microscope

We investigated fluorescence imaging using a near-field scanning optical microscope which uses a laser-stabilized gold nanoparticle as a near-field probe. This microscope is suitable for observations of biological specimens in aqueous solutions because the probe particle is held by a noncontact force exerted by a laser beam. Theoretical calculations based on Mie scattering theory are presented to evaluate the near-field enhancement by a gold particle of 40 nm diameter. We also present fluorescence images of a single fluorescent bead and discuss the near-field contribution to the fluorescence image in this type of microscope.     

Fluorescence imaging with a laser trapping scanning near-field optical microscope

We investigated fluorescence imaging using a near-field scanning optical microscope which uses a laser-stabilized gold nanoparticle as a near-field probe. This microscope is suitable for observations of biological specimens in aqueous solutions because the probe particle is held by a noncontact force exerted by a laser beam. Theoretical calculations based on Mie scattering theory are presented to evaluate the near-field enhancement by a gold particle of 40 nm diameter. We also present fluorescence images of a single fluorescent bead and discuss the near-field contribution to the fluorescence image in this type of microscope.     

A storage Dewar near-field scanning optical microscope

A near-field scanning optical microscope for operation within a storage Dewar is described. It was designed for studies of opaque samples and operates in the collection mode. Illumination can be either through the tip or from the side via a separate fiber. Scans can be begun within 2 h after start of cooldown. Its rigid design allows high resolution and long scans with no additional vibration isolation. To illustrate its performance, measurements of photoluminescence in GaAs/AlGaAs heterostructures are presented. The signal and noise levels for the two illumination modes are examined.     

Mid-infrared scanning near-field optical microscope resolves 30 nm

We explore the performance of a scanning near-field infrared microscope, which works by scattering tightly focused CO2 laser radiation (lambda = 10 microm) from the apex of a metallized atomic force microscope tip. The infrared images of test samples prove a spatial resolution of 30 nm and are free of topographical and inertial artefacts, thus they should be of great interest for practical applications. We also observe that the infrared contrast vanishes when the input beam polarization is orthogonal to the tip axis, in accordance with theoretical expectations for a mechanism of longitudinal field interaction.     

Optical-fibre scanning near-field optical microscope for cryogenic operation

We have developed a new type of scanning near-field optical microscope (SNOM) utilizing optical fibres. The probe tip is controlled by shear force feedback with a fibre interferometer and signal light is collected directly by a multimode fibre. These features make the SNOM head more compact and less sensitive to vibration. Further advantages of this new type of SNOM are that it obviates the need for optical windows in the cryostat and offers easy optical alignment.     

Optical-fibre scanning near-field optical microscope for cryogenic operation

We have developed a new type of scanning near-field optical microscope (SNOM) utilizing optical fibres. The probe tip is controlled by shear force feedback with a fibre interferometer and signal light is collected directly by a multimode fibre. These features make the SNOM head more compact and less sensitive to vibration. Further advantages of this new type of SNOM are that it obviates the need for optical windows in the cryostat and offers easy optical alignment.     

The scanning near-field optical microscope as a tool for proteomics

The identification of the entire genetic code of human DNA is more or less completed. With this knowledge, research in identifying the real information lying in the genes, will begin. This information is contained in the proteins, which are the main biological actors in the cell. For this reason proteins will be targeted in biological investigations in the future. The structure, affinity and reactivity of each identified protein has to be determined, which is a primary goal in the field of proteomics. This will require new and better strategies to identify protein-protein interaction. Our approach, based on the detection and visualization of single proteins by scanning near-field optical microscopy (SNOM), has allowed us to visualize various fixed and fluorochrome-labelled proteins at the nanometer scale. Subsequently SNOM may then be developed to efficiently detect the specific behavior of a certain protein in response to other biomolecules.     

Vibration sensitivity of the scanning near-field optical microscope with a tapered optical fiber probe

In this paper the Rayleigh-Ritz method was used to study the scanning near-field optical microscope (SNOM) with a tapered optical fiber probe's flexural and axial sensitivity to vibration. Not only the contact stiffness but also the geometric parameters of the probe can influence the flexural and axial sensitivity to vibration. According to the analysis, the lateral and axial contact stiffness had a significant effect on the sensitivity of vibration of the SNOM's probe, each mode had a different level of sensitivity and in the first mode the tapered optical fiber probe was the most acceptive to higher levels of flexural and axial vibration. Generally, when the contact stiffness was lower, the tapered probe was more sensitive to higher levels of both axial and flexural vibration than the uniform probe. However, the situation was reversed when the contact stiffness was larger. Furthermore, the effect that the probe's length and its tapered angle had on the SNOM's probe axial and flexural vibration were significant and these two conditions should be incorporated into the design of new SNOM probes.     

The height regulation of a near-field scanning optical microscope probe tip

A nonoptical detection of the optical fibre tip has been developed. By detecting the output signal from a tiny piezoelectric detector attached to the vibrating fibre tip, the distance between the fibre tip and the sample has been successfully controlled. The frequency responses of the system composed of tip, the dither and the detector have been studied. The difference between the shear-force detection and the tapping-mode detection is discussed. It is found that the shear force exerted on the tip reduces the vibration amplitude with an unvaried resonance frequency. However, in the tapping mode, the resonance frequency varies with the tip–sample distance as the force is exerted on the fibre tip only within a half period. This requires better adjustments for the tapping-mode detection.     

Implementation of a short-tip tapping-mode tuning fork near-field scanning optical microscope

We present the implementation of a short‐tip tapping‐mode tuning fork near‐field scanning optical microscope. Tapping frequency dependences of the piezoelectric signal amplitudes for a bare tuning fork fixed on the ceramic plate, a short‐tip tapping‐mode tuning fork scheme and an ordinary tapping‐mode tuning fork configuration with an 80‐cm optical fibre attached are demonstrated and compared. Our experimental results show that this new short‐tip tapping‐mode tuning fork scheme provides a stable and high Q factor at the tapping frequency of the tuning fork and will be very helpful when long optical fibre probes have to be used in an experiment. Both collection and excitation modes of short‐tip tapping‐mode tuning fork near‐field scanning optical microscope are applied to study the near‐field optical properties of a single‐mode telecommunication optical fibre and a green InGaN/GaN multiquantum well light‐emitting diode.     

The height regulation of a near-field scanning optical microscope probe tip

A nonoptical detection of the optical fibre tip has been developed. By detecting the output signal from a tiny piezoelectric detector attached to the vibrating fibre tip, the distance between the fibre tip and the sample has been successfully controlled. The frequency responses of the system composed of tip, the dither and the detector have been studied. The difference between the shear-force detection and the tapping-mode detection is discussed. It is found that the shear force exerted on the tip reduces the vibration amplitude with an unvaried resonance frequency. However, in the tapping mode, the resonance frequency varies with the tip-sample distance as the force is exerted on the fibre tip only within a half period. This requires better adjustments for the tapping-mode detection.     

Probe design optimization for a high-resolution scattering-type scanning near-field optical microscope

The scattering-type scanning near-field optical microscope (SNOM) has a probe with a sharp tip for use in high resolution imaging. As sharp a tip as possible is generally considered ideal for the observations, but actually, a sharp tip does not always provide a high resolution SNOM image. We numerically examined the scattering property of the SNOM probe by the three dimensional finite difference time domain method. In this paper, we show the criterion for the ideal scattering probe which satisfies the simple relation between radius and taper angle of the tip.     

Magnetic domain imaging with a scanning near-field optical microscope using a modified Sagnac interferometer

We report on the combination of a scanning near-field optical microscope and a modified Sagnac interferometer for magnetic-domain imaging in the reflection mode. The Sagnac interferometer is used for detection of the magneto-optical Kerr effect. Since the interferometer is inherently insensitive to polarization changes caused by topography effects, magnetic-domain imaging is not limited to samples with flat surfaces. In this way, it is possible to image magnetic bits written on the tracks of a magneto-optical disc that has a rather pronounced surface profile.     

Magnetic domain imaging with a scanning near-field optical microscope using a modified Sagnac interferometer

We report on the combination of a scanning near-field optical microscope and a modified Sagnac interferometer for magnetic-domain imaging in the reflection mode. The Sagnac interferometer is used for detection of the magneto-optical Kerr effect. Since the interferometer is inherently insensitive to polarization changes caused by topography effects, magnetic-domain imaging is not limited to samples with flat surfaces. In this way, it is possible to image magnetic bits written on the tracks of a magneto-optical disc that has a rather pronounced surface profile.     

A scanning near-field optical microscope having scanning electron tunnelling microscope capability using a single metallic probe tip

A new microscope system that has the combined capabilities of a scanning near-field optical microscope (SNOM) and a scanning tunnelling microscope (STM) is described. This is achieved with the use of a single metallic probe tip. The distance between the probe tip and the sample surface is regulated by keeping the tunnelling current constant. In this mode of operation, information about the optical properties of the sample, such as its refractive index distribution and absorption characteristics, can be disassociated from the information describing its surface structure. Details of the surface structure can be studied at resolutions smaller than the illumination wavelength. The performance of the microscope is evaluated by analysing a grating sample that was made by coating a glass substrate with gold. The results are then compared with the corresponding SNOM and STM images of the grating.     

Broadband optical near-field microscope for nanoscale absorption spectroscopy of organic materials

The cells and tissues of many marine invertebrates and their associated flora contain fluorescent pigments and proteins, many of which have been utilized commercially and provide marker molecules in other systems for fluorescence imaging technology. However, in the study of marine invertebrates and their symbioses these naturally occurring molecules have been seen to limit or confound fluorescence microscopy analyses. Here we demonstrate the endogenous fluorescence associated with two marine invertebrates (coral and foraminifera) and describe how these qualities can be utilized in fluorescence microanalyses. Understanding and imaging the diversity of fluorescent molecules provide insight into how fluorescence microscopy techniques can now be applied to these complex systems.     

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.     

Evaluation of nano-optical probe from scanning near-field optical microscope images

We studied a nanometre-sized optical probe in a scanning near-field optical microscope. The probe profile is determined by using a knife-edge method and a modulated transfer function evaluation method which uses nanometre-sized line-and-space tungsten patterns (with spaces 1 microm to 50 nm apart) on SiO2 substrates. The aluminium-covered, pipette-pulled fibre probe used here has two optical probes: one with a large diameter (350 nm) and the other with a small diameter (10 nm). The small-diameter probe has an optical intensity approximately 63 times larger than that of the large-diameter probe, but the power is about 1/25 of that of the large probe.