FIBER OPTIC ATOMIC FORCE MICROSCOPE

Scanning Tunneling Microscope (STM) is the device for studying and imaging of the structures at the nanometer lengthscale (or even individual atoms on the surfaces of materials). The instrument was invented in the early 1980s by Gerd Binnig and Heinrich Rohrer, who were awarded the 1986 Nobel prize in physics for their work. The underlying principle of the microscope is the tunneling of electrons between the sharp tip of a probe and the surface of the sample under study. The flow of electrons is extremely sensitive to the distance between the tip and the sample. As the tip is swept over the surface the height of the tip is continually adjusted so as to keep the flow of electrons constant. A map of the “bumps” on the surface is then obtained by accurately recording the height fluctuations of the tip. Further details can be found at the website of IBM Zurich Research Laboratory

Despite of the great success of the scanning tunneling microscopy it was obvious that STM has fundamental disadvantage - with STM one can investigate only the conductive or conductive layers coated samples. This disadvantage was overcomed due to the invention of Atomic Force Microscope (AFM) by Binnig in 1986. He was first who have guessed that under interaction with sample surface macroscopic cantilever provided with sharp tip can be bended by atomic forces to sufficiently large amount to be measured by the common facilities. The AFM works by scanning a tip over a surface much the same way as a phonograph needle scans a record. The tip is positioned at the end of a cantilever beam as shown in animation. As the tip is repelled by or attracted to the surface, the cantilever beam deflects. The magnitude of the deflection is detected optically. Together with other new-technology force microscopes (e.g. Magnetic Force Microscope, Electrostatic Force Microscope, etc.) STM and AFM belong to the group called Scanning Probe Microscopes (SPM).

The use of fiber optic interferometer Fabry-Perot allows to make AFM extremely compact, economic and sensitive. In fiber optic interferometer Fabry-Perot the interference occurs at the partially reflecting end face surface of the fiber and cantilever surface which play the role of an external mirror. The size of the sensitive element based on this principle can be as small as diameter of the fiber, i.e. about 0.1 mm, and the sensitivity to deflections of cantilever can achieve sub-angstrom level. A plot of the cantilever deflection versus horizontal tip position on the sample surface provides the resolution of the hills and valleys that constitute the topography of the surface.

Typical working mode of AFM is so called Contact Mode, where AFM tip remains (more or less) in contact with surface (in this way some friction may be observed). Another possibility is so called Tapping Mode, where from point to point tip is pulled from the surface, moved over next point and lowered again. Animation shows one of the variants of Fiber Optic AFM (FOAFM), which works in tapping mode. Tip is "flying" under the surface. Deviations (flexures) of the cantilever repeat the surface topology. The displacements of the sensitive cantilever are detected by changes of the interferometer "working point" position (i.e. mean separation between interferometer mirrors).

A rich variety of forces can be sensed by atomic force microscopy. In the non-contact mode (of distances greater than 10A between the tip and the sample surface), Van der Waals, electrostatic, magnetic or capillary forces produce images of topography, whereas in the contact mode, ionic repulsion forces take the leading role. Because its operation does not require a current between the sample surface and the tip, the AFM can move into potential regions inaccessible to STM or image fragile samples which would be damaged irreparably by the STM tunnelling current. Insulators, organic materials, biological macromolecules, polymers, ceramics and glasses are some of the many materials which can be imaged in different environments, such as liquids, vacuum, and low temperatures. Most AFMs have vertical resolution below 0.1 nm range, whereas lateral is usually lower. Further details on AFM can be found here: [1], [2], [3], [4]

One more mode of AFM operation is the mode with the vibration of the cantilever. As the tip is moved across the surface the interaction between the tip and the surface causes a change in the resonance frequency of the cantilever. The variations of the resonance frequency is the parameter which gives to us an information about the topology of the surface. Vibrating can proceed in direct contact of the tip with the sample surface, without touching the surface under vibration and with intermittent-contact (semicontact) under vibration. Scanning can be many-passing, each next pass can give additional information concerning sample under investigation.

Animation shown the resonance mode of AFM operation, when the cantilever is excited with the constant frequency (equal to resonance frequency of the cantilever in the absence of the surface). Flying under the surface the tip is acted upon by the interaction forces from the side of the surface, which change the natural frequency of the cantilever. Because the frequency of excitation is constant the amplitude of oscillation diminishes as much as stronger the forces. Force of interaction between the tip and the surface depends upon the distance between them, so variation of the amplitude of oscillation will repeat the profile of the surface.  The oscillation of the cantilever are detected interferometrically with the aid of fiber optic Fabry-Perot interferometer. The output of the interferometer is provided to a computer for processing of the data for providing a topographical image of the surface with atomic resolution. 

In constant-amplitude mode the amplitude of oscillation is kept constant by a regulation circuit that excites a piezoactuator with a sinusoidal voltage of the oscillation frequency f and an amplitude Vexc . The actuator shakes the fixed end of the cantilever. When the cantilever oscillation is damped due to the tip-sample interaction, Vexc will increase to maintain the oscillation amplitude constant. By recording Df and Vexc simultaneously, forces and dissipation can be measured. [ Phys. Rev. B 62, 13674 (2000); Phys. Rev. B 61, 11151 (2000) ]. Using PLL-circuit we can track the resonance frequency of the cantilever. The oscillation of cantilever can be excited photothermally by intensity-modulated optical radiation. This idea is explained in the following paper: Microresonator fiber optic sensors.

More detail information on Atomic Force Microscopy can be found at the website of the company Surface Imaging Systems - a manufacturer of high quality, most versatile Scanning Probe Microscopy components.

And, finally, the topography of a surface can be investigated by means of the optical fiber itself without the use of micromachined cantilever. In such an approach semireflecting tip of the optical fiber and the surface of interest form interferometer Fabry-Perot. Variation of the distance between the fiber and the surface will lead to variation of the interferometer signal. Fiber is fixed on piezoelectric XYZ-micropositioner, so it can be manipulated in all three directions. Z-channel of micropositioner is fed by amplified interferometer signal, so the distance between the fiber and surface remains unchangeable. Other two channels of micropositioner are used to scan the fiber along the surface. There is only one limitation of such an approach: surface of the sample should be smooth enough and homogeneously reflecting.