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## Viewing Tiny Electric and Magnetic Fields in Real Time

Technique for viewing a submicroscopic object's electric and magnetic fields in real time. Three electron waves descend upon the sample. One of the waves passes directly through the object, and its wavefront shifts after it interacts with the object's electric and magnetic fields. The other two waves serve as "reference waves"; the biprisms (shaded circles) deflect these waves so that they combine with the "object wave" to form an interference pattern on the image plane, where it is recorded onto a film or camera. The interference pattern provides direct information on the object's electric and magnetic fields. Shown is an image of an electrically charged latex particle (0.5 microns in diameter) along with its interference pattern. From the image, the researchers deduced that the latex particle's electric field was created by approximately 400 electrons in the particle. (Illustration by Malcolm Tarlton, AIP; photographs courtesy of T. Hirayama et al.)

To develop their real-time technique for imaging an object's electric and magnetic fields, researchers in Japan exploited the phenomenon of three-wave interference. In their particular setup, the researchers pass an electron wave through the object of interest and combine it with two "reference waves" to form an interference pattern. If the researchers were to combine the three electron waves in the absence of an object, they would get an interference pattern consisting simply of light and dark bands. When an object is present, however, its electric and/or magnetic fields reveal itself by darkening or brightening these bands.

How can one determine the electric or magnetic fields from these interference patterns? The first thing to note is that there are two ways to depict these fields. One way is to show their lines of force; for example, a single electron produces straight lines in all directions. A bar magnet produces lines of force stretching from its north to south poles. The second way is to view its "equipotential" lines, which are perpendicular to the lines of force. For a single electron, or any pointlike charged object, the equipotential lines should form concentric circles around the particle.

In this new three-wave technique, the interference patterns yields the particle's equipotential lines. In the case of the latex particle pictured above, we see a distorted circle around the particle. The circle (and the associated bands) are somewhat distorted because the two reference waves interact with the electric fields from the particle. From their image, the researchers determined that approximately 400 electrons in and on the latex particle were responsible for creating the electric field!

With their technique, the researchers can view tiny electric and magnetic fields with details as small as tens of nanometers, at a rate of 30 images per second with their current CCD camera (and potentially more with a faster-speed camera). This is useful for studying how electric and magnetic fields emanating from a particle change with conditions such as time and temperature.

Previous electron holography techniques could image these tiny fields, but not in real time. Unlike the current technique, which combines an object wave with two reference waves, conventional electron holography techniques combine an object wave with a single reference wave. The resulting interference pattern cannot provide direct information on an electric or magnetic field unless it is subjected to a second step in which the image is reconstructed with an optical reference wave. With three-wave interference, however, direct information on the fields can be recorded.

In addition to the latex particle, the researchers observed how heating up an alumina particle (a ceramic material) causes its electric field to change because some electrons obtained enough energy from the heat to move freely through the material. Also, the researchers imaged the magnetic field lines from barrium ferrite, which is widely used for magnetic recording media.

This research was reported by Tsukasa Hirayama, Guanming Lai, Takayoshi Tanji, Nobuo Tanaka, and Akira Tonamura in the 15 July 1997 issue of the Journal of Applied Physics (vol. 82, p. 522).

• Link to related Physics News Update item on this work