Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force

We develop a novel optical microcantilever for scanning near-field optical microscopy controlled by atomic force mode (SNOM/AFM). The optical microcantilever has the bent channel waveguide, the corner of which acts as aperture with a large tip angle. The resonance frequency of the optical microcantilever is 9 kHz, and the spring constant is estimated to be 0.59 N/m. The optical microcantilever can be operated in contact mode of SNOM/AFM and we obtain the optical resolution of about 200 nm, which is as same size as the diameter of aperture. We confirm that the throughput of optical microcantilever with an aperture of 170 nm diameter would be improved to be more than 10⁻⁵.     

Microfabricated silicon dioxide cantilever with subwavelength aperture

We have developed a microfabricated SiO₂ cantilever with subwavelength aperture for scanning near-field optical microscopy (SNOM), to overcome the disadvantages of conventional optical fibre probes such as low reproducibility and low optical throughput. The microcantilever, which has a SiO₂ cantilever and an aperture tip near the end of the cantilever, is fabricated in a reproducible batch process. The circular aperture with a diameter of 100–150 nm is formed by a focused ion-beam technique. Incident light is directly focused on the aperture from the rear side of the cantilever using a focusing objective, and high optical throughput (10⁻² to 10⁻³) is obtained. The microcantilever can be operated as a SNOM probe in contact mode or in dynamic mode.     

High-resolution constant-height imaging with apertured silicon cantilever probes

We present high-resolution aperture probes based on non-contact silicon atomic force microscopy (AFM) cantilevers for simultaneous AFM and near-infrared scanning near-field optical microscopy (SNOM). For use in near-field optical microscopy, conventional AFM cantilevers are modified by covering their tip side with an opaque aluminium layer. To fabricate an aperture, this metal layer is opened at the end of the polyhedral probe using focused ion beams (FIB). Here we show that apertures of less than 50 nm can be obtained using this technique, which actually yield a resolution of about 50 nm, corresponding to λ/20 at the wavelength used. To exclude artefacts induced by distance control, we work in constant-height mode. Our attention is particularly focused on the distance dependence of resolution and to the influence of slight cantilever bending on the optical images when scanning at such low scan heights, where first small attractive forces exerted on the cantilever become detectable.     

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 20nm 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.     

Scanning near-field optical microscopy (SNOM)

SNOM is a non-contact stylus microscopy analogous to STM. Optical near-field interaction is used to sense approach and optical properties on the nanometre scale (?1 nm normal, 20–50 nm lateral). SNOM was demonstrated in transmission and reflection, in a topographic mode, and with amplitude as well as phase objects. The excitation of plasmons in the SNOM ‘tip’, a very recent development, greatly enhances sensitivity and permits intriguing new optical experiments. Overcoming the limit of diffraction, SNOM turns a long-held dream of optical microscopists into reality.     

Near-field optical microscopy with video signal processor to apply a CCD imaging device as a variable area photo-sensor for improving operability and spectrum mode imaging

We have developed a video signal processor for improving the operability and function of scanning near-field optical microscopy (SNOM). The video signal processor applies a CCD imaging device as a variable area photo-sensor in the SNOM unit instead of conventional photo-detectors. The signal processor converts the intensity of a selected area in video frames to a numerical value with a rate of 30 Hz. Consequently, the CCD imaging device can be used as a photo-detector of variable areas and positions for detecting a small area of a optical probe position. The need for a precise optical axis alignment is relaxed due to the large sensing area of the CCD device. Using the video signal processor, near-field optical and topographic images have been obtained by SNOM/AFM system simultaneously. By adding a spectrometer between the SNOM unit and the CCD device, the spectrum signal of selected wavelength ranges has been monitored by the video signal processor to provide an optical image.     

Use of a near-field optical probe to locally launch surface plasmon polaritons on plasmonic waveguides: a study by the finite difference time domain method

We used the finite difference time domain (FDTD) method to study the use of scanning near field optical microscopy (SNOM) to locally excite the nanometric plasmonic waveguides. In our calculation, the light is funneled through a SNOM probe with a sub-wavelength optical aperture and is irradiated on one end of two types of plasmonic waveguides made of 50 nm Au sphere arrays and Au nanowires. The incident light was well localized at one end of the waveguides and consequently propagated toward the other end, due to the excitation of surface plasmon polaritons. We found that the propagation length of the nanosphere array type waveguide varies from 100 to 130 nm depending on the light wavelength, the size of the probe aperture, and the launching heights. Our result shows that reducing the aperture size and using the light of the plasmon resonance wavelength of the nanosphere array could increase the propagation length and, thus, the efficiency of electromagnetic energy transportation through nanosphere arrays.     

High-speed scanning by dual feedback control in SNOM/AFM

We have developed a high-speed scanning near-field optical microscope (SNOM)/atomic force microscope (AFM) system including dual feedback controllers. The system includes an additional piezoelectric actuator with fast response in the z direction and a correction circuit to eliminate unnecessary components from the feedback signal. From the measurement of a patterned chromium layer of 2 x 2 microm2 checks on a quartz glass plate, we confirmed that our system had more effective feedback control and faster scanning than current SNOM/AFM systems that use only a piezo-tube. The scanning speed of the present system was estimated to be about five times faster than that of current SNOM/AFM systems.     

Nano-scale imaging of chromosomes and DNA by scanning near-field optical/atomic force microscopy

Nano-scale structures of the YOYO-1-stained barley chromosomes and lambda-phage DNA were investigated by scanning near-field optical/atomic force microscopy (SNOM/AFM). This technique enabled precise analysis of fluorescence structural images in relation to the morphology of the biomaterials. The results suggested that the fluorescence intensity does not always correspond to topographic height of the chromosomes, but roughly reflects the local amount and/or density of DNA. Various sizes of the bright fluorescence spots were clearly observed in fluorescence banding-treated chromosomes. Furthermore, fluorescence-stained lambda-phage DNA analysis by SNOM/AFM demonstrated the possibility of nanometer-scale imaging for a novel technique termed nano-fluorescence in situ hybridization (nano-FISH). Thus, SNOM/AFM is a powerful tool for analyzing the structure and the function of biomaterials with higher resolution than conventional optical microscopes.     

High-speed scanning by dual feedback control in SNOM/AFM

We have developed a high-speed scanning near-field optical microscope (SNOM)/atomic force microscope (AFM) system including dual feedback controllers. The system includes an additional piezoelectric actuator with fast response in the z direction and a correction circuit to eliminate unnecessary components from the feedback signal. From the measurement of a patterned chromium layer of 2 × 2 μm² checks on a quartz glass plate, we confirmed that our system had more effective feedback control and faster scanning than current SNOM/AFM systems that use only a piezo-tube. The scanning speed of the present system was estimated to be about five times faster than that of current SNOM/AFM systems.     

SNOM/STM using a tetrahedral tip and a sensitive current-to-voltage converter

Scanning near-field optical microscopes (SNOM) using the tetrahedral-tip (T-tip) with scanning tunnelling microscopy (STM) distance control have been realized in transmission and reflection mode. Both set-ups used ordinary STM current-to-voltage converters allowing measurement of metallic samples. In the transmission mode, a resolution of 10 nm to 1 nm with regard to material contrast can be achieved on binary metal samples. Because of the great near-field optical potential of the T-tip with respect to the optical resolution, it is a challenging task to find out whether these results can be transferred to non-metallic sample systems as well. This paper reports on a newly designed SNOM/STM transmission mode set-up using the tetrahedral-tip. It implements a sensitive current-to-voltage converter to widen the field of measurable sample systems. Beyond this, mechanical and optical measuring conditions are substantially improved compared to previous set-ups. The new set-up provides a basis for the routine investigation of metal nanostructures and adsorbed organic monolayers at resolutions in the 10 nm range.     

Near-field optical excitation as a dipole-dipole energy transfer process

The process of fluorescence excitation in the scanning near-field optical microscope (SNOM) is considered as a dipole-dipole resonance energy transfer process between a molecule under study and a SNOM aperture, which can be treated as a magnetic-type point dipole. It is shown that such an approach satisfactorily describes the conditions of the usual SNOM fluorescence experiments. Fluorescence excitation dependence on the polarization of the incident light and medium refraction index have been obtained. The equation to calculate the resonance dipole-dipole energy transfer radius (which is a natural unit of a SNOM's longitudinal resolution) is derived. Those cases where such a radius is of the order of the SNOM aperture, and thus single dipole, can strongly influence the radiation conditions are discussed briefly.     

Local photocurrent mapping and cell performance behaviour on a nanometre scale for monolithically interconnected Cu(In,Ga)Se2 solar cells

The local efficiency of lamellar shaped Cu(In,Ga)Se₂ solar cells has been investigated using scanning near‐field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i‐ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i‐ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm).     

Near-field optical excitation as a dipole–dipole energy transfer process

The process of fluorescence excitation in the scanning near-field optical microscope (SNOM) is considered as a dipole–dipole resonance energy transfer process between a molecule under study and a SNOM aperture, which can be treated as a magnetic-type point dipole. It is shown that such an approach satisfactorily describes the conditions of the usual SNOM fluorescence experiments. Fluorescence excitation dependence on the polarization of the incident light and medium refraction index have been obtained. The equation to calculate the resonance dipole–dipole energy transfer radius (which is a natural unit of a SNOM's longitudinal resolution) is derived. Those cases where such a radius is of the order of the SNOM aperture, and thus single dipole, can strongly influence the radiation conditions are discussed briefly.     

A silanized mica substrate suitable for high-resolution fiber FISH analysis by scanning near-field optical/atomic force microscopy

We applied a novel silanized mica substrate with an extremely flat surface constructed according to Sasou et al. (Langmuir 19, 9845-9849 (2003)) to high-resolution detection of a specific gene on a DNA fiber by scanning near-field optical/atomic force microscopy (SNOM/AFM). The interaction between the substrate and fluorescence-dye conjugated peptide nucleic acid (PNA) probes, which causes fluorescence noise signal, was minimal. By using the substrate, we successfully obtained a fluorescence in situ hybridization signal from the ea47 gene on a λphage DNA labeled with an Alexa 532-conjugated 15-base PNA probe. As the results, no fluorescence noises were observed, indicating that the surface adsorbed almost none of the PNA probe. The combination of the substrate and SNOM/AFM is an effective tool for visualizing DNA sequences at nanometer-scale resolution.     

Dynamic etching method for fabricating a variety of tip shapes in the optical fibre probe of a scanning near-field optical microscope

A novel etching method for an optical fibre probe of a scanning near-field optical microscope (SNOM) was developed to fabricate a variety of tip shapes through dynamic movement during etching. By moving the fibre in two-phase fluids of HF solution and organic solvent, the taper length and angle can be varied according to the movement of the position of the meniscus on the optical fibre. This method produces both long (sharp angle) and short (wide angle) tapered tips compared to tips made with stationary etching processes. A bent-type probe for a SNOM/AFM was fabricated by applying this technique and its throughput efficiency was examined. A wide-angle probe with a 50° angle at the tip showed a throughput efficiency of 3.3 × 10⁻⁴ at a resolution of 100 nm.     

Dynamic etching method for fabricating a variety of tip shapes in the optical fibre probe of a scanning near-field optical microscope

A novel etching method for an optical fibre probe of a scanning near-field optical microscope (SNOM) was developed to fabricate a variety of tip shapes through dynamic movement during etching. By moving the fibre in two-phase fluids of HF solution and organic solvent, the taper length and angle can be varied according to the movement of the position of the meniscus on the optical fibre. This method produces both long (sharp angle) and short (wide angle) tapered tips compared to tips made with stationary etching processes. A bent-type probe for a SNOM/AFM was fabricated by applying this technique and its throughput efficiency was examined. A wide-angle probe with a 50 degrees angle at the tip showed a throughput efficiency of 3.3 x 10(-4) at a resolution of 100 nm.     

Near-field fluorescence imaging of single molecules with a resolution in the range of 10 nm

We demonstrate fluorescence imaging of single molecules, by near-field scanning optical microscopy (NSOM), using the illumination-collection mode of operation, with an aperture probe. Fluorescence images of single dye molecules were obtained with a spatial resolution of 15 nm, which is smaller than the diameter of the aperture (20 nm) of the probe employed. Such super-resolution may be attributable to non-radiative energy transfer from the molecules to the coated metal of the probe since the resolution obtained in the case of conventional NSOM is limited to 30–50 nm due to penetration of light into the metal.     

Analysis of fiber probes of scanning near-field optical microscope by field emission microscopy.

It is shown that field emission microscopy and related methods can be used to analyze the metal coated fiber tips, which nowadays are the most frequently used sensor for the scanning near-field optical microscopy (SNOM). Metal free and thus non field-emitting aperture for the light transmission on the tip apex can be directly seen and its parameters can be measured, which is very important for the interpretation of SNOM data.