U. C. Fischer

The tetrahedral tip as a probe for scanning near-field optical microscopy at 30 nm resolution

The tetrahedral tip is introduced as a new type of a probe for scanning near‐field optical microscopy (SNOM). Probe fabrication, its integration into a scheme of an inverted photon scanning tunnelling microscope and imaging at 30 nm resolution are shown. A purely optical signal is used for feedback control of the distance of the scanning tip to the sample, thus avoiding a convolution of the SNOM image with other simultaneous imaging modes such as force microscopy. The advantages of this probe seem to be a very high efficiency and its potential for SNOM at high lateral resolution below 30 nm.

Dynamic force distance control suited to various probes for scanning near-field optical microscopy

A dynamic force distance control for scanning near-field optical microscopy (SNOM) based on a quartz tuning fork as piezoelectric force sensor is introduced. In contrast to a similar design for shear-force feedback, the tuning fork is aligned in such a way that forces perpendicular to the surface are detected. Various near-field probes can be attached to the end of the tuning fork and serve as force sensing tip. The high force sensitivity is demonstrated for two different near-field probes by imaging the topography of organic samples. The tetrahedral tip, an apertureless high-resolution near-field probe used up to now mainly with tunnel current distance control, is for the first time successfully combined with a force distance control for SNOM. A similar distance control was used in conjunction with an only few millimeters short aluminum-coated tapered fiber tip as near-field probe. The suitability of this design for near-field optical fluorescence imaging is demonstrated. Furthermore, the dynamic force d...

A low-temperature scanning confocal and near-field optical microscope

A scanning near-field optical microscope equipped with an Al-coated glass fiber tip for operation in a He cryostat is described. The instrument is designed for fluorescence detection of nanoscopic particles (single dye molecules and nanocrystallites), and allows optical imaging and spectroscopy of surfaces or thin objects with subwavelength spatial resolution at low temperatures. A shear-force control loop keeps the probing fiber tip aperture at a constant distance, i.e., a few nanometers away from the sample. The shear-force method results in a vertical stability of better than 2 nm. For light collection and confocal imaging, a transmission objective with a high numerical aperture is operated inside the cryostat. For rapid sample inspection, and in cases where no high lateral resolution or topographic imaging is necessary, the instrument can also be used without a tip as a low-temperature conventional or confocal microscope, due to the large scan range of an improved scanner. Details and characteristics ...

Photonic nanopatterns of gold nanostructures indicate the excitation of surface plasmon modes of a wavelength of 50-100 nm by scanning near-field optical microscopy.

Scanning near‐field optical microscopy images of metal nanostructures taken with the tetrahedral tip (T‐tip) show a distribution of dark and bright spots at distances in the order of 25–50 nm. The images are interpreted as photonic nanopatterns defined as calculated scanning near‐field optical microscopy images using a dipole serving as a light‐emitting scanning near‐field optical microscopy probe. Changing from a positive to a negative value of the dielectric function of a sample leads to the partition of one spot into several spots in the photonic nanopatterns, indicating the excitation of surface plasmons of a wavelength in the order of 50–100 nm in metal nanostructures.

1 MHz quartz length extension resonator as a probe for scanning near-field acoustic microscopy

Abstract The feasibility of a rod-shaped 1 MHz length extension quartz resonator as a probe for scanning near-field acoustic microscopy was investigated. A corner of the quartz rod which is exposed at an angle of 45 ° to the surface of the sample is used as a probing tip. The damping of the vibration increases with the approach of the tip to the sample and is used as a signal to obtain a topographic image of the sample at sub-micron resolution.

Acoustic and dynamic force microscopy with ultrasonic probes

Ultrasonic quartz resonators with quality factors ≳5000 allow one to probe surface morphologies directly, i.e., without any additional sensors determining the vertical position of the probing tip. With tip radii of several tenths of a micrometer, hydrodynamic friction forces dominate the tip/surface interaction, while, by using sharp atomic force microscopy tips glued onto rod‐shaped 1 MHz length extension quartz resonators, the conventional dynamical quasi noncontact atomic force microscopy mode is obtained. This allows one to resolve atomically sized structures on mica in air. The ultrasonic sensors with spring moduli of 3000 to 400 000 N/m exhibit an extremely high sensitivity allowing one to probe even monomolecular organic layers without destruction.

The Tetrahedral Tip as a Probe for Scanning Near-Field Optical and for Scanning Tunneling Microscopy

It was recently shown, that the tetrahedral tip can be used as a probe for Scanning Near- Field Optical Microscopy (SNOM) at a lateral resolution of 30 nm. The optical signal as obtained in an Inverse Photon Scanning Tunneling Microscope configuration was used to control the distance between tip and object during the scan. The optical signal is however not only determined by the distance between tip and object. To explore the resolution limits of our SNOM we need an independent signal for controlling the distance during a scan more accurately. We tested the possibility to use the tetrahedral tip for simultaneous SNOM and STM operation by using the tetrahedral tip as a probe for STM alone. In first experiments atomic resolution was obtained on graphite, and a resolution in the nm range, superior to the one obtained by us in AFM images could be demonstrated on test samples, which are comparable to the ones used for the SNOM experiments.

Near-field optical tools to read, write, and copy information at the 100- to 10-nm scale

Contact imaging by energy transfer was the first application of the optical near-field for imaging beyond the diffraction limit. It is a method by which surface nanostructures can be copied onto a monomolecular layer of a dye. Near field microscopy using tapered metal coated fibers with an aperture at their tip as a submicron source of light can be used as a tool to write structures at a resolution of 80 nm. These near-field optical methods are well suited to create-- by local photochemical reactions--patterns of locally differing chemical composition and reactivity. Such structures serve as matrices for a site selective binding of colloidal particles. Unlike other methods, light microscopy and near-field microscopy have a sensitivity to detect photochemical processes at the single molecular level. Near- field microscopy is not limited to a resolution of 50 nm. As recently shown, the resolution can be extended to the 1 - 10 nm range using the tetrahedral tip as a probe. We expect, that a convergence of these recent developments should result in a very powerful near-field optical toolbox to read, write and copy information at the 10 nm scale.

The shape of transmission spectra of thin films deposited on a substrate.

On the basis of the previous considerations [E. G. Bortchagovsky and U. C. Fischer, J. Chem. Phys. 117, 5384 (2002)] we analyze here the shape of the spectral lines created in transmission spectra by thin films deposited on a substrate. We demonstrate that such lines have as a rule the shape of an asymmetrical Fano line with the minimum shifted from the resonance position. Thus we present the additional argument that the popular visual estimation of the parameters of resonances from the shape of those lines should be made with care.

6 NM Lateral Resolution in Scanning Near Field Optical Microscopy with the Tetrahedral Tip

We realized a combination of a scanning near field optical (SNOM) and a scanning tunneling microscope (STM) using the tetrahedral tip as a probe. The SNOM and the STM signal are acquired simultaneously during the scan. In the STM mode atomic resolution on pyrolytic graphite is routinely obtained. Simultaneous SNOM/STM investigations of thin silver films evaporated on glass show a lateral resolution of 6 nm in the near field optical signal. Absorption contrast in the optical image is obtained in images of evaporated silver films as well as of patches of purple membrane deposited on indium tin oxide (ITO) as a substrate.