Scanning Optical Microscope Probe Research Articles

Near-field scanning optical microscopy probes with heat sinks for higher power operations

The metal coating of a near field scanning optical microscope (NSOM) probe will melt when a few mW of laser light is introduced into the fiber. For higher power operation a process to coat the tip of the NSOM probe with epoxy without damaging the probe was developed. The tip protrudes no more than a few micron outside the epoxy surface such that the evanescent wave from the tip is still available for applications. The epoxy heat sink dissipates heat away from the NSOM probe keeping the metal coating cooler, which allows the input laser power to be increased to 21.4 mW at a wavelength of 405 nm without damaging the NSOM probe.

Selective electric and magnetic sensitivity of aperture probes.

We report the effect of geometrical factors governing the polarization profiles of near-field scanning optical microscope (NSOM) probes. The most important physical parameter controlling the selective electric or magnetic field sensitivity is found to be the width of the metal rim surrounding aperture. Probes with metal rim width w < λ/2 selectively senses the optical electric field, while those with w > λ/2 selectively senses the optical magnetic field. Intensity variation of optical Hertz standing wave formed upon reflection at oblique incidence shows a phase difference of π/2 between electric and magnetic probes: an analogue of the classical Wiener's experiment. Our work paves way towards electromagnetic engineering of nanostructures.

Transient temperature response of near field scanning optical microscope probes under pulsed illumination

Time resolved thermoreflectance imaging with a nanosecond laser is applied to study the transient thermal response of near field scanning optical microscope (NSOM) probes driven by either microsecond (μs) or nanosecond (ns) laser pulses. It is found that when the laser energies are the same, μs and ns laser pulses show similar trend of close to adiabatic heating during the laser pulse and close to conductive cooling after the laser pulse. The peak temperature is almost linear with respect to input laser energy for the energy selected. The peak temperature of the NSOM probe is increased when the aperture size of the probe is smaller and when the incident laser wavelength is shorter. However, compared with μs laser pulses, small physical damages appear around the NSOM aperture, which gradually decreases the optical-thermal energy conversion efficiency in the NSOM probe when ns laser pulses are applied.

Existence of a stable resonance zone with nearly unchanging vibration characteristics for a near-field scanning optical microscope probe dipped partially into a liquid

The vibration characteristics of a near-field scanning optical microscope (NSOM) probe, whose vibration was controlled by two-nodal-wedge method and tip was partially dipped into a liquid, were investigated theoretically and experimentally. The resonant frequency and the Q value of the NSOM probe were found to remain nearly unchanged irrespective of the dipping depth of the probe, if the probe was kept within a dipping depth range of 0.4∼1.0 mm. With the achievement of a high Q value, the existence of this stable resonance zone implies that bio-samples immersed in a liquid can be accurately and stably measured using a NSOM controlled by using two-nodal-wedge method.

Plasmon Polariton Imaging Characteristics of Near-Field Optical Fiber Probes

The influence of different near-field optical (near-field scanning optical microscopy) probes on the imaging of surface plasmon polaritons propagating on thin metal films is investigated. Metal-coated fiber probes exhibit a suppression of the measured plasmon signal close to the metal film surface and increased local scattering of the plasmon field. Purely dielectric fiber probes are shown to be largely free of these effects.

Non-intrusive temperature measurement of NSOM probes with thermoreflectance imaging

A non-intrusive optical measurement technique (i.e. the thermoreflectance method) was adopted to study the light-induced heating of near-field scanning optical microscope (NSOM) probes for regions ≳5 µm from the tip of the probes. The thermoreflectance measurements indicate that there is a hot spot at 20–30 µm away from the tip of the probe. The location of the hot spot is not a strong function of laser wavelength for a fixed type of NSOM probe. The temperature of the NSOM probes increases almost linearly with respect to the input laser power. The shorter wavelength light (405 nm) provides stronger heating of the gold-coated NSOM probes compared with the longer wavelength light (650 nm). The modulation rate of the input laser also contributes to the heating of the NSOM probes: the temperature variation of the NSOM probes decreases with increase in the modulation frequency of the input light. In addition to laser conditions, the NSOM probe with a smaller aperture suffers stronger heating compared with that with a larger aperture.

Near-Field Optical Tweezers Based on NSOM and AFM Probes

By applying the direct calculation of Maxwell stress tensor using finite difference time domain method, the feasibility of using a metal-coated fiber probe and using the combination of a near-field scanning optical microscopy (NSOM) probe and an atomic force microscopy (AFM) probe to create near-field optical tweezers is investigated. Numerical results indicate that these schemes are able to trap particles in the range of tens of nano-meters. In experiments 120 nm polystyrene particles are trapped in multi-circular shape with a minimum size of 400 nm, and the polystyrene cluster consisting of three polystyrene particles is obtained for the fabrication of stable polymer nano-structures.

Near field phase mapping exploiting intrinsic oscillations of aperture NSOM probe

An innovative, simple, compact and low cost approach for phase mapping based on the intrinsic modulation of an aperture Near Field Scanning Optical Microscope probe is analyzed and experimentally demonstrated. Several nanoscale silicon waveguides are phase-mapped using this approach, and the different modes of propagation are obtained via Fourier analysis. The obtained measured results are in good agreement with the effective indexes of the modes calculated by electromagnetic simulations. Owing to its simplicity and effectiveness, the demonstrated system is a potential candidate for integration with current near field systems for the characterization of nanophotonic components and devices.

Simulation of near-field optical manipulator using the combination of a near-field scanning optical microscope probe and an atomic force microscope metallic probe

We propose a physical model to calculate the trapping force on a nanoparticle trapped by the system using the combination of a near-field scanning optical microscope (NSOM) probe and an atomic force microscope (AFM) metallic probe. Such a near-field trap is produced by evanescent illumination from the NSOM probe and light scattering at the tip of the AFM metallic probe. By using the Maxwell stress tensor through the electric field distribution obtained with the three-dimensional finite difference time domain (3-D FDTD) method, the dependence of the trapping force on the system parameters is discussed, and trapping properties including near-field distribution, trapping position, and the role of other forces versus trapping force are revealed. The results indicate that a particle down to tens of nanometers in size can be trapped toward the tip of an AFM probe with a lower laser intensity (∼1040 W/mm2) than that required by conventional optical manipulators (∼105 W/mm2).

Near-field optical data storage using C-apertures

We demonstrate the all-optical recording of deeply subwavelength data bits in Ge2Sb2Te5 using a near-field scanning optical microscope (NSOM) probe that utilizes a C-aperture fabricated using through membrane focused ion beam milling. Data bits recorded with various optical powers were read out optically by C-aperture NSOM and the physical bit size was measured by atomic force microscopy (AFM). Both optical and AFM measurements were found to be in excellent agreement with simulation. We achieved a minimum physical bit size of 53.5×50.2 nm2 at a wavelength of 980 nm (λ/20) indicating a data density of 223 Gbit/in.2.

Subwavelength focusing of light by a tapered microtube

We propose a mechanism for subwavelength focusing at optical frequencies based on the use of a tapered microtube fabricated from a glass capillary tube. Using coherent illumination at 671 nm and a near-field scanning optical microscope probe which was mounted on a three-axis piezonanopositioning stage, a sequence of two-dimensional intensity profiles were obtained. Our experimental results reveal the smallest focal spot with a near diffraction-limited full width at half-maximum of ∼435 nm (0.65λ) at a distance of ∼1.47 μm (2.2λ) from the output end face of microtube. The experimental results are in agreement with our numerical simulation.

Novel split-tip proximal probe for fabrication of nanometer-textured, in-plane oriented polymer films

Novel fabrication schemes are required to deposit nanoscale materials that contain molecules oriented in the plane of the surface. The breakage of in-plane symmetry allows devices to be fabricated in this plane, enabling molecular electronics to follow this successful paradigm of semiconductor devices. The authors discuss here the fabrication of a unique split-tip optical nanoprobe that can be used to both orient molecules on a surface with a strong, localized electric field and deposit them with nanoscale resolution. Ultraviolet light injected through the probe into the region of aligned molecules causes the deposition. The production of the split-tip probe is significantly different than that of the related near-field scanning optical microscope (NSOM) probe, since the stresses in the metal layer must be held by the metal film–silica interface rather than within the film as it encircles the silica of a NSOM probe. Mounting of the probe to ensure reliable electrical contacts is also described.

Optical and thermal energy transport from a NSOM probe to a pure silicon target under intense ns pulsed light

An integrated wave-based optical analysis and diffusion-based thermal analysis are constructed to understand the optical and thermal energy transport from a near-field scanning optical microscope (NSOM) probe to a pure silicon target. Based on a comparison between the simulated results and experimentally observed melting structure on the silicon targets in the previous study, it is concluded that a direct contact between the NSOM probes and the target occurs when high intense nanosecond (ns) laser pulses are carried with NSOM probes. Significant thermal energy transport from the NSOM probe to the Si target occurs during the direct contact and is responsible for the melted structure on silicon targets observed in previous experiments. It is also predicted from the integrated optical-thermal analysis for the NSOM probes that (a) the thermal energy transport from the NSOM probe to the target can be orders higher than the near-field optical energy transport under intense ns laser pulses, (b) light transport efficiency from the NSOM probe to the Si target is a strong function of the tapering angle, aperture size and the constructing material of the metal cladding of the probe, (c) the highest temperature of the NSOM probe is a weak function of the tapering angle and aperture diameter of the NSOM probe. However, the maximum temperature of the NSOM can be significantly changed by selecting the appropriate cladding material of the NSOM probe.

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.

Combined wave based optical analysis and particle based thermal analysis of nanoscale ultrafast target heating of silicon utilizing a near-field scanning optical probe and a femtosecond laser

A non-equilibrium wave based optical-thermal analysis was developed to study target heating and mass removal for irradiation with a femtosecond laser pulse utilizing a near-field scanning optical microscope probe. It was found that under the conditions when nano-craters were observed experimentally with a pure silicon target (Wen et al 2007 Appl. Phys. Lett. 91 251113) that the calculated electron temperature and electron number density are high enough to induce non-thermal mass removal. Also, the shapes of the nano-crater patterns are consistent with the calculated Joule heating distribution. The results indicate that near-field laser radiation-material interaction is a potential mechanism for forming nano-craters on semiconducting materials.

Development of apertureless near‐field scanning optical microscope tips for tip‐enhanced Raman spectroscopy

In this study, we suggested two types of novel metallized tip for the apertureless near-field scanning optical microscope probe. The first is a silver nanorod immobilized tip and the other is a double metallized probe. We calculated the electric field enhancement factor and the electric field distribution of a single sphere, aggregated spheres, an ellipse and a nanorod by the finite-differential time-domain method to improve the silver nanosphere immobilized tip developed in our previous studies. The enhanced field of the nanorod is localized at the external surfaces. The simulation results of the nanorod revealed that the position of the maximum peak is shifted to a longer wavelength and that its electric field enhancement factor increases as the aspect ratio increases. Thus, we developed the silver nanorod immobilized tip, and the tip-enhanced Raman spectrum of rhodamine 6G molecule on the substrate could be measured by the tip though it could not be detected by the previous nanosphere immobilized tip. Further, the finite-differential time-domain calculation predicted that the double metallized tips considerably enhance the electric field and that its enhancement factor in the longer wavelength region (500-600 nm) does not decrease when the tip is rounded. The results show that the proposed metallized tips were useful for the apertureless near-field scanning optical microscope system.

Near-field modulation of laser diode emissions by an aperture probe of near-field optical microscope

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LDs) observed by a near-field scanning optical microscope (NSOM) probe with an intentionally enlarged aperture shows a single broad elliptic profile, whereas the detected wavelength hops among those of the longitudinal multimodes observed in the far-field. This hopping suggests the near-field coupling between the aperture and the LD cavity, which is proved by the observation of strong modulation of laser intensity by the optical coupling only in the near-field regime. Probes with an intact aperture are found not to couple with the lasing TE 00 mode, but, therefore, couple only with a near-field longitudinal mode. Around the lasing region, emissions detected by an intact aperture are ascribed to amplified spontaneous emissions at wavelengths far from those of the lasing multimode emissions. These near-field modes, with spatial profile different from both the far-field TE and TM modes, are detectable only by the NSOM.

Near-field scanning optical microscope probe analysis

In this article results of a comparison of two NSOM probe characterization methods are presented. Scanning electron microscopy analysis combined with electromagnetic field modeling using the finite difference in time domain method are compared with measured far-field radiation diagrams of NSOM probes. It is shown that measurement of far-field radiation diagrams can be an efficient tool for daily checking of the NSOM probes quality. Moreover, it is shown that the inner probe geometry has large influence on the directional radiation of an NSOM probe and the far-field radiation diagram can be used as a simple method to distinguish between different probe geometries.

Effect of Polarization Direction on the Electric Field Distribution at the Near-Field of a Tip-on-Aperture Near-Field Scanning Optical Microscope Probe

A tip-on-aperture (TOA) probe is a novel near-field scanning optical microscope (NSOM) probe, which combines the features of aperture and apertureless probes. It has a small metallic tip at the end of an aperture NSOM's probe. The tip is illuminated through an aperture at the near field of the probe and electromagnetic interaction between the structured sharp tip at the end of the probe and the surface is used for measurements and materials processing. In this study, the electric field distributions at the near-field of a TOA probe were calculated numerically to analyze the effects of polarization direction on the characteristics of measurement and processing. A TOA probe is asymmetric for the axis, since it has a probe at metal coated layer on the aperture. The geometrical relationship between this asymmetric shape and polarization direction is the reason for the change of the electromagnetic energy distributions. Numerical analysis shows that a TOA probe can make the best use of the tip for high resolutions when the tip is located on the parallel axis with the polarization direction.

Offset-apertured near-field scanning optical microscope probes

Near-field scanning optical microscope (NSOM) probe designs consisting of a subwavelength aperture offset of either a metallic or metal-coated dielectric cantilevered tip are investigated using finite-difference time-domain calculations. The offset aperture and metal-coated dielectric tip couple surface plasmons that illuminate the tip apex, which results in a single-lobed probing optical spot having a full-width half maximum (FWHM) similar to the apex diameter. Since the surface plasmons converge at the apex, an offset-apertured probe promises significantly higher throughput light intensities than an apertured NSOM having a comparable spot FWHM.