Near-field Optical Microscope Probe Research Articles

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.

Waveguide analysis of heat-drawn and chemically etched probe tips for scanning near-field optical microscopy

We analyze two basic aspects of a scanning near-field optical microscope (SNOM) probe's operation: (i) spot-size evolution of the electric field along the probe with and without a metal layer, and (ii) a modal analysis of the SNOM probe, particularly in close proximity to the aperture. A slab waveguide model is utilized to minimize the analytical complexity, yet provides useful quantitative results--including losses associated with the metal coating--which can then be used as design rules.

Photoinduced Nanodots in Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+d</sub>

We report a fabrication of high-density nanodots by photodoping in overdoped Bi2Sr2CaCu2O8+d thin film (Tc = 80 K). A scanning near-field optical microscope probe is used to locally excite carrier, and photodoped region is associated with lower Tc phase (Tc = 75 K) via overdoping. Nanoscale characterizations with optical reflectivity reveal that nanodots (30-nm diameter) are regularly distributed in 50-nm step. The resultant films with photoinduced nanodots enhance Jc, a situation being similar to strong pinning effects observed in films modified by either ion irradiation or sputtered nanoparticles. These results suggest that photoinduced nanodots with lower Tc act as effective pinning centers.

Photoinduced nanodots and pinning effects in Bi2Sr2CaCu2O8+d

We report a fabrication of high-density nanodots by photodoping in overdoped Bi2Sr2CaCu2O8+d thin film (Tc=80K). A scanning near-field optical microscope probe is used to locally excite carrier and photodoped region is associated with lower Tc phase (Tc=75K) via overdoping. Nanoscale characterizations with optical reflectivity reveal that nanodots (30-nm-diameter) are regularly distributed in 50nm step. The resultant films with photoinduced nanodots enhance Jc (at least 5 times larger than the initial one), a situation being similar to strong pinning effects observed in films modified by either ion irradiation or sputtered nanoparticles. These results suggest that photoinduced nanodots with lower Tc act as effective pinning centers.

Optimized apertureless optical near-field probes with 15 nm optical resolution

Back-illuminated full body glass tips coated with a thin metal layer can be used aslocal probes for apertureless scanning near-field optical microscopy (SNOM). Inorder to achieve high spatial resolution, high electric field intensities and lowbackground illumination, the thickness of the metal coating, angular illuminationdirection, and polarization have to be optimized. Optimal conditions have beencalculated and experimentally verified for 10–15 nm thick aluminium and 15–25 nmthick silver layers. Upon imaging single dye molecules, characteristic single anddouble-peak patterns with peak widths down to 15 nm could be measured, exhibiting anoptical resolution which exceeds the classical diffraction limit of Abbé significantly.

Analysis of mode coupling due to spherical defects in ideal fully metal-coated scanning near-field optical microscopy probes

We investigate the effect of defects in the metal-coating layer of a scanning near-field optical microscopy (SNOM) probe on the coupling of polarization modes using rigorous electromagnetic modeling tools. Because of practical limitations, we study an ensemble of simple defects to identify important trends and then extrapolate these results to more realistic structures. We find that a probe with many random defects will produce a small but significant coupling of energy between a linearly polarized input mode and a radial/longitudinal polarization mode, which is known to produce a strongly localized emitted optical field and is desirable for SNOM applications.

Double-resonance probe for near-field scanning optical microscopy

A surface-contact transducer is developed for scanning probe microscopes, whose operating principle is based on the coincidence between the resonance frequency of a 32kHz quartz tuning fork and that of the probe attached to it. This allows the transducer to have a high quality factor and, if the vibration amplitude of the probe tip exceeds that of the tuning fork prongs, materially improves its force sensitivity. The resonance transducer proposed by us has an experimentally verified force sensitivity of 8pN (rms) in the 300Hz frequency band, which is of the same order of magnitude as the sensitivity of atomic force microscope (AFM) cantilever sensors. The manufacture of such transducers equipped with optical-fiber probes for near-field scanning optical microscopy and with tungsten probes for AFM is described as an example.

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.

Wide angle near-field optical probes by reverse tube etching

We present a simple modification of the tube etching process for the fabrication of fiber probes for near-field optical microscopy. It increases the taper angle of the probe by a factor of two. The novelty is that the fiber is immersed in hydrofluoric acid and chemically etched in an upside-down geometry. The tip formation occurs inside the micrometer tube cavity formed by the polymeric jacket. By applying this approach, called reverse tube etching, to multimode fibers with 200/250 μm core/cladding diameter, we have fabricated tapered regions featuring high surface smoothness and average cone angles of ∼30°. A simple model based on the crucial role of the gravity in removing the etching products, explains the tip formation process.

Subwavelength imaging of field confinement in a waveguide-integrated photonic crystal cavity

A photonic crystal microcavity is designed to obtain an original field distribution inside the cavity and the structure is etched inside a silicon-on-insulator waveguide. Spectral location of the photonic band gap and cavity resonance are identified by using transmittance measurements and by analyzing the light collected by a scanning near-field optical microscope probe exactly positioned on the center of the cavity. The results obtained with the two techniques are in very good agreement. Then the near-field distribution above the device is mapped and light confinement inside the cavity is evidenced. Moreover, this confined light presents some remarkable patterns which clearly correspond to the cavity mode field distribution computed by using a plane-wave expansion method and taking into account the probe resolution.

An inverse method for determining the interaction force between the probe and sample using scanning near-field optical microscopy

This study proposes a means for calculating the interaction force during the scanning process using a scanning near-field optical microscope (SNOM) probe. The determination of the interaction force in the scanning system is regarded as an inverse vibration problem. The conjugate gradient method is applied to treat the inverse problem using available displacement measurements. The results show that the conjugate gradient method is less sensitive to measurement errors and prior information on the functional form of quality was not required. Furthermore, the initial guesses for the interaction force can be arbitrarily chosen for the iteration process.

New design for enhanced transmission and polarization control through near-field optical microscopy probes

Near field optical images depend strongly on the polarization of the incident and/or detected light. In this work, we propose a new metallized probe that exhibits a transmission enhancement in comparison with the classical one (metal coated probe with aperture). In addition, it acts as a nano-polarizer either in the detection or in the transmission modes.

Focus on Analytical Equipment

Focus on Analytical Equipment

Design and fabrication of a microlens on the sidewall of an optical fiber with a metallized 45° end face

We propose a method for designing a self-aligned microlens. We have improved its fabrication by employing metallization on a 45 degrees angled surface of the optical fiber. We designed the focal length of the microlens to be 14.0 microm, considering the dimensions of a scanning near-field optical microscopy (SNOM) probe, and we calculated possible dimensions of diameter and height by the ray-tracing method. The modeling of lens formation was also carried out with two assumptions: no volume change and no movement of peripheral parts of the photoresist (PR) on the substrate during reflow. To fabricate a microlens of diameter 16.0 microm and height 5.0 microm we exposed a coated PR to UV light guided into the optical fiber, followed by optimized reflow of 150 degrees C for 2 min. For this microlens the focal length and the beam waist were 14.0 and 1.4 microm, respectively. This lens can be used for compact optical data storage.

Vector near-field calculation of scanning near-field optical microscopy probes using Borgnis potentials as auxiliary functions

A new boundary integral equation method for solving the near field in three-dimensional vector form in scanning near-field optical microscopy (SNOM) using Borgnis potentials as auxiliary functions is presented. A boundary integral equation of the electromagnetic fields, expressed by Borgnis potentials, is derived based on Green's theorem. The harmonic expansion in rotationally symmetric SNOM probe--sample systems is studied, and the three-dimensional electromagnetic problem is partly simplified into a two-dimensional one. The boundary conditions of Borgnis potentials both on dielectric boundaries and on perfectly conducting boundaries are derived. Relevant algorithms were studied, and a computer program was written. As an example, a SNOM probe-sample system composed of a round metal-covered probe and a sample with a flat surface has been numerically studied, and the computational results are given. This new method can be used efficiently for other electromagnetic field problems with round subwavelength structures.

Controllable method for the preparation of metalized probes for efficient scanning near-field optical Raman microscopy

A modification of the mirror reaction method has been used to metalize atomic force microscope (AFM) probes for apertureless near-field optical microscopy. The method produces a single silver particle of controllable size at the apex of the AFM tip with no detrimental effects on the AFM probe. A particle of the order 80nm in diameter appears to provide the best tip-enhanced Raman signal using 488nm excitation. The near-field Raman spatial resolution of one such probe was shown to be as high as 24nm using single-walled carbon nanotubes as a test sample.

Development of a polarization-preserving optical-fiber probe for near-field scanning optical microscopy and the influences of bending and squeezing on the polarization properties

We have characterized the optical properties of a polarization-preserving near-field scanning optical microscopy (NSOM) optical fiber probe, and evaluated the influence of probe fabrication processes, such as melting-pulling and bending, on those properties. We found that the extinction ratio always decreases on the linearly polarized light parallel to the slow axis, and that the decrease is independent of the direction in which the probe is bent. The most probable cause of the low extinction ratio is the isotropic compression stress occurring when the fiber is squeezed; such stress cancels out the tensile stress applied to the core by stress applicators. Using the polarization-preserving NSOM probe, we observed the optical properties of the propagation light within a polymeric optical waveguide in guide-collection mode, and investigated those properties with various polarization conditions of incident light and probe collection.

High-transmission solid-immersion apertured optical probes for near-field scanning optical microscopy

We demonstrate significantly increased intensity transmission for two hybrid apertured near-field optical probe designs. The probes, based on traditional atomic force microscopy tips, incorporate light-confining mechanisms that yield intensity throughput several orders of magnitude greater than conventional fiber-based probes. A microlayer probe features a high-index gallium phosphide evaporated layer, while a microsphere probe incorporates a silica microsphere lens. The increase in transmitted intensity is attributed to surface plasmon field enhancement effects as well as the decreased wavelength cutoff and focusing effects of the high-index layer and integrated solid-immersion lens, respectively.

Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips.

The fabrication of silicon cantilever-based scanning near-field optical microscope probes with fully aluminium-coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic- and contact-mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone-angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near-field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near-field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.

Fabrication of near‐field optical apertures in aluminium by a highly selective corrosion process in the evanescent field

A simple, one-step process to fabricate high-quality apertures for scanning near-field optical microscope probes based on aluminium-coated silicon nitride cantilevers is presented. A thin evanescent optical field at a glass-water interface was used to heat the aluminium at the tip apex due to light absorption. The heat induced a breakdown of the passivating oxide layer and local corrosion of the metal, which selectively exposed the front-most part of the probe tip from the aluminium. Apertures with a protruding silicon nitride tip up to 72 nm in height were fabricated. The height of the protrusion was controlled by the extent of the evanescent field, whereas the diameter depended on the geometry of the probe substrate. The corrosion process proved to be self-terminating, yielding highly reproducible tip heights. Near-field optical resolution in a transmission mode of 85 nm was demonstrated.