Uncoated Fiber Tip Research Articles

Experimental investigation of field electron emission from uncoated and coated graphite fiber tips

Abstract This study investigates the field emission characteristics of coated and uncoated graphite fiber tips under high vacuum conditions in the pressure range of 10–6 Pa. A 2 M Sodium hydroxide (NaOH) solution was used in the electrochemical etching procedure to produce an uncoated etched graphite fiber emitter with a diameter of 436.5 nm. An epoxy coating 2301 dielectric material with a thickness of 61 ± 1 nm was used to coat the fiber tip. Field-emitter emission micrographs, current stability, and current-voltage (I-V) characteristics were recorded. Murphy’s Good plots were used to examine and analyse the I-V characteristics. An orthodoxy test was performed to evaluate the Murphy Good plots. In the reported results, at low applied voltages, the uncoated graphite fiber emitter passed orthodoxy tests, and generated a steady current with 0.03 µA uncertainty. However, the coated graphite fiber emitter showed an increase in the applied voltage and threshold voltages but had no impact on the sample emission characteristics.

Detection of non-paraxial optical fields by optical fiber tip probes

The optical response of a single-mode uncoated fiber tip to a 3D polarized field, including longitudinal components, is investigated. The 3D field is produced by an opportune superposition of TE and TM plane waves. The contribution of the different field components to the detected signals was discriminated by integrating the scanning probe microscope with a multi-heterodyne detection technique. A simple coupling model for the tip is introduced. The longitudinal field component was assumed to couple to the transverse fiber modes through complex coupling coefficients. Coupling coefficients were obtained by fitting the parameters of the model to the experimental data. These results demonstrate that the longitudinal components of the field are coupled by this probe with an efficiency approximately equal to that of the transverse polarization components.

Local excitation of surface plasmon polaritons in random surface nanostructures

We investigate local excitation of surface plasmon polaritons (SPPs) at a 55-nm-thick gold layer covered with randomly located scatterers (density ∼75 μm −2) by using an uncoated fiber tip of a near-field optical microscope as a radiation source and detecting the radiation scattered in the direction of resonant (at flat regions) SPP excitation. In the wavelength range of 725–773 nm, we observe a considerable decrease in the detected signal from the random scattering structures as compared to that from the flat (corrugation-free) regions. We relate the obtained optical images to the electromagnetic local density of states corresponding to SPPs. Correspondence between these images (recorded in the illumination mode) and those obtained in the collection mode with similar structures is also discussed.

Ringlike emission profiles in scanning near-field photoluminescence images of single InGaAs quantum dots

In scanning near-field photoluminescence images of single InGaAs quantum dots taken with uncoated fiber tips, we observed sharp ringlike features varying in diameter with detection energy. This is in contrast to the typical single emission peaks observed using metal-coated fiber tips. These features can be explained by electrical charges induced by the uncoated fiber tip that lead to a Stark shift of the transition energy depending on the tip position. Simulations of the electric field at the quantum dot agree nicely with the experimental observation.

Second-harmonic generation from individual surface defects under local excitation

Enhancement of optical second-harmonic generation (SHG) at individual defects on metal surface has been studied. SHG has been excited locally at chosen defects using a near-field optical microscope with an uncoated fiber tip. SH intensity enhancement up to ten times has been observed at the apex of micron size defects on a gold surface while the average enhancement is of about 1.2 times. Observed SHG enhancement has been described by lightning rod effect. Specific features of SHG enhancement due to local excitation are briefly discussed.

Energy transfer from fluorescent thin films to metals in near-field optical microscopy: Comparison between time-resolved and intensity measurements

The fluorescence intensity, fluorescence decay time, and shear-force images of a thin film have been simultaneously investigated by reflection scanning near-field optical microscopy using an uncoated fiber tip. The sample is made of a europium chelate embedded in a 32-nm thick polymer layer that coats a periodic structure of gold and chromium. It is contended that the three images carry different and somewhat complementary information; the shear force supplying the sample profile while the intensity mainly depends on the local sample’s reflectance. Moreover, the decay time exhibits the local-energy-transfer process that takes place between the metallic substrate and the dye layer.

Transfer functions in collection scanning near-field optical microscopy

It is generally accepted that, if in collection near-field optical microscopy the probe—sample coupling can be disregarded, a fiber probe can be considered as a detector of the near-field intensity whose size can be accounted for via an intensity transfer function. We show that, in general, this perception is wrong and it is impossible to introduce such a transfer function for the detected signal. Instead, we introduce an amplitude coupling function that relates the near-field amplitude and the amplitude of a mode guided in a probe fiber toward a detector. Different experimental configurations are considered with respect to the relation between near-field optical images and the corresponding intensity distributions. Our conclusions are supported with numerical simulations and experimental results obtained by using a photon scanning tunneling microscope with an uncoated fiber tip.

Polarization-resolved imaging with a reflection near-field optical microscope

Using a rigorous microscopic point-dipole description of probe–sample interactions, we study imaging with a reflection scanning near-field optical microscope. Optical content, topographical artifacts, sensitivity window—i.e., the scale on which near-field optical images represent mainly optical contrast—and symmetry properties are considered for optical images obtained in constant-distance mode for different polarization configurations. We demonstrate that images obtained in cross-polarized detection mode are free of background and topographical artifacts and that the cross-circular polarization configuration is preferable to the cross-linear one, since it ensures more isotropic (in the surface plane) near-field imaging of surface features. The numerical results are supported with experimental near-field images obtained by using a reflection microscope with an uncoated fiber tip.

Picosecond Multiphoton Scanning Near-Field Optical Microscopy

We have implemented simultaneous picosecond pulsed two- and three-photon excitation of near-UV and visible absorbing fluorophores in a scanning near-field optical microscope (SNOM). The 1064-nm emission from a pulsed Nd:YVO 4 laser was used to excite the visible mitochondrial specific dye MitoTracker Orange CM-H 2TMRos or a Cy3-labeled antibody by two-photon excitation, and the UV absorbing DNA dyes DAPI and the bisbenzimidazole BBI-342 by three-photon excitation, in a shared aperture SNOM using uncoated fiber tips. Both organelles in human breast adenocarcinoma cells (MCF 7) and specific protein bands on polytene chromosomes of Drosophila melanogaster doubly labeled with a UV and visible dye were readily imaged without photodamage to the specimens. The fluorescence intensities showed the expected nonlinear dependence on the excitation power over the range of 5–40 mW. An analysis of the dependence of fluorescence intensity on the tip-sample displacement normal to the sample surface revealed a higher-order function for the two-photon excitation compared to the one-photon mode. In addition, the sample photobleaching patterns corresponding to one- and two-photon modes revealed a greater lateral confinement of the excitation in the two-photon case. Thus, as in optical microscopy, two-photon excitation in SNOM is confined to a smaller volume.

Experiments on the depolarization near-field scanning optical microscope

We demonstrate the operation of an apparatus which we call the depolarization near-field scanning optical microscope. It delivers subwavelength resolution with uncoated optical fiber tips without the need for additional modulation techniques. We show that—in the near field—the edges perpendicular to the incident optical polarization are imaged. This dependence on the orientation of the linear polarization as well as the influence of small ellipticities of the polarization state on the imaging process are measured on a well-defined test sample. The transition from near- to far-field imaging as a function of the tip height is demonstrated. The results are in good agreement with recent theoretical predictions.

Near-field scanning photoluminescence measurements using an uncoated fiber tip: A potential high resolution diagnostic technique for semiconductor devices

Near-field scanning optical microscopy, in which an uncoated fiber tip is used for delivering excitation light and picking up photoluminescence, is applied for diagnosing defects in semiconductor devices. Using this high resolution, potentially subdiffraction limited, and fast acquisition time scanning μ-photoluminescence (PL) technique, we are able to locate nonluminescing defects in a multiple quantum well grown by molecular beam epitaxy. Near-field characteristics of the measured spectral PL intensity are also discussed.

Continuous Wave Two-Photon Scanning Near-Field Optical Microscopy

We have implemented continuous-wave two-photon excitation of near-UV absorbing fluorophores in a scanning near-field optical microscope (SNOM). The 647-nm emission of an Ar-Kr mixed gas laser was used to excite the UV-absorbing DNA dyes DAPI, the bisbenzimidazole Hoechst 33342, and ethidium bromide in a shared aperture SNOM with uncoated fiber tips. Polytene chromosomes of Drosophila melanogaster and the nuclei of 3T3 Balb/c cells labeled with these dyes were readily imaged. The fluorescence intensity showed the expected nonlinear (second order) dependence on the excitation power in the range of 8–180 mW. We measured the fluorescence intensity as a function of the tip-sample displacement in the direction normal to the sample surface in the single- and two-photon excitation modes (SPE, TPE). The fluorescence intensity decayed faster in TPE than in SPE.

Near-field nonlinear optical spectroscopy of Langmuir–Blodgett films

Using an uncoated fiber tip as a light source and a multilayer Langmuir–Blodgett film of 2-docosylamino-5-nitropyridine as a sample, we obtain near-field images at the fundamental-harmonic (FH) and second-harmonic (SH) wavelengths for different polarizations and wavelengths of the pump light. The spatial resolution of ∼100 nm is achieved in near-field SH images that are not correlated with either the topographical or the FH images. It is directly demonstrated that SH generation is most concentrated inside submicrometer-sized (300–500 nm) crystallike domains, which are oriented differently but predominantly in the dipping direction and whose SH efficiency is maximum for pump wavelengths in the range of 770–800 nm.

Near-field optical microscopy of nonlinear susceptibilities

Local probing of second-order susceptibilities with a near-field optical microscope is demonstrated for the first time. Using an uncoated fiber tip as a light source, near-field images of a surface of y-cut LiNbO 3 crystal and of a multilayer Langmuir–Blodgett film of 2-docosylamino-5-nitropyridine are obtained at the fundamental pump and second harmonic wavelengths while simultaneously recording surface topography. It is shown that optical second-harmonic images for different polarizations of the pump light reflect the difference in magnitude of the corresponding components of the second-order susceptibility tensor. Various degrees of correlation in contrast of optical (fundamental and second harmonic) and topographical images are observed and discussed.

Photoluminescence of buried InGaAs/GaAs quantum dots spectrally imaged by scanning near-field optical microscopy

We present scanning near-field optical microscopy (SNOM) investigations of buried quantum dots. InGaAs/GaAs quantum dots are grown in the Stranski-Krastanov growth mode and covered with a 50 nm thick GaAs-cap by metal-organic vapour phase epitaxy (MOVPE). For near field investigations with high spatial resolution, uncoated glass fiber tips are used in internal reflection mode to illuminate the sample with 2.54 eV argon ion laser at 300 K. Overall photoluminescence (PL) intensity images as well as images taken at one wavelength are compared. The overall PL intensity image shows individual spots with a resolution of about 300 nm. The corresponding shear force image (topography) shows small height corrugation of 3 nm in the surface topography at the centers of the emitting spots. These high corrugations are possibly correlated to a surface topography above the quantum dots as revealed by transmission electron microscopy (TEM). Different contributions of the near field and far field in the PL intensity are separated and the emission spectra of individual emission centers are shown to have a spectral half with larger than 15 meV.

Method for increasing shear-force detection sensitivity with uncoated fiber tips

A technique that is easy to implement and sensitive for measuring lateral oscillation amplitudes of optical fibers on the nanometer scale for shear-force microscopy is described. The measurement system analyzed here is based on using the optical fiber tip as a cylindrical lens to focus and deflect a detection beam. It is shown that for our experimental arrangement, this technique is at least 2.5 times as sensitive as merely shadowing the edge of such a beam, as in most commonly used configurations. As a result, oscillation amplitudes of the order of 2-3 nm may easily be measured. An advantage of this system is that absolute vibration amplitudes are easily measured. A simple geometric model is used to describe the operation of the system.

Near-field optics with uncoated fiber tips:light confinement and spatial resolution

Using a macroscopic self-consistent model for scanning near-field optical microscopy, we show that the field distribution of light emitted by an uncoated fiber tip near a sample surface consists of two spatially separated domains, of which only the central domain (near the tip end) contains evanescent-field components. The relative magnitude of the near-field contribution is found to be strongly dependent on the tip shape. Spatial resolution in near-field microscopy and lithography with uncoated fiber tips is discussed on the basis of the numerical results. Experimental results obtained on surface modification of polymer films, phase conjugation of optical near fields, and surface-polariton localization are presented. Using optical images with true optical contrast (i.e., not correlated to surface topography), we find the spatial resolution to be ∼100 nm for the light wavelength of 633 nm.

Reflection scanning near-field optical microscopy with uncoated fiber tips: How good is the resolution really?

We have investigated the optical resolution of a scanning near-field optical microscope in reflection collection mode using an uncoated fiber tip. We demonstrate that the apparent resolution in the optical signal (better than 70 nm) is a topography-induced effect. We believe that the purely optical resolution is only of the order of λ/2 and diffraction limited.

Self-consistent model for photon scanning tunneling microscopy: implications for image formation and light scattering near a phase-conjugating mirror

A macroscopic self-consistent model for photon scanning tunneling microscopy with an uncoated fiber tip is developed by use of integral plane-wave representations of the total electric field in two-dimensional geometry. The model framework allows one to treat the incident field with an arbitrary angular spectrum and the presence of a thin-layer medium with a subwavelength structure on the sample surface. Imaging with the photon scanning tunneling microscope and light scattering near a phase-conjugating mirror are considered with our model by use of numerical simulations. We show that the near-field optical images provided by the microscope can be quite different from the light intensity distributions that exist near the sample surface in the absence of the fiber tip. We demonstrate that the self-consistent field at the site of a scatterer placed in front of a phase-conjugating mirror can be significantly enhanced as a result of multiple phase conjugation of the scattered light.

Study of shear force technique for near-field microscopy with an uncoated fiber tip

The shear force technique, which is based on the detection of intensity variations in the diffracted light from a laser beam incident on the resonantly vibrating fiber tip, is considered. Two simple mechanical models are used for estimating the magnitude of the forces involved in the tip-surface interaction. A theoretical model describing the optical detection part of the shear force technique is developed for uncoated fiber tips. It is shown that when using an optimum configuration of the detection system, a tip vibration amplitude of 1 nm can result in up to 50 nW of the detected optical signal for the laser beam power of 5 mW. The calculated results are compared with experimental data.