Observation of Intense Far Infrared Picosecond Pulses of Coherent Transition Radiation

Transition radiation is produced by the passage of a charged particle through the interface between media with different dielectric constants. It is caused by a collective response of the matter surrounding the particle trajectory to readjust to the electromagnetic field of the charged particle. Part (a) of the figure is a schematic corresponding to the geometry used in our experiment. A negatively charged particle is incident at 45° with respect to a metallic mirror. In the low-frequency region where the metal acts like a perfect conductor, its electric shielding effects can be represented in the nonrelativistic limit by an image charge placed at an equal distance behind the metal surface. As the charge approaches the boundary, so does this image charge. The emitted radiation pattern with a frequency spectrum extending from the microwave to the x-ray region [see (b)] is emitted when the charged particle enters the metal and the charge and its image cancel.

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