In a project funded by the National Science Foundation, the behavior of electrons in liquid helium is being studied. When an electron is injected into helium, it forces open a spherical cavity with a diameter of 40 Angstroms from which helium atoms are excluded. The size of this cavity is such as to minimize the sum of the zero-point energy of the electron, the surface energy and the energy associated with any externally-applied pressure. If the electron is excited to a higher quantum state, the size and shape of the cavity changes. Recently, these excited state bubbles have been detected and their properties studied. When the electron is optically excited to the higher energy state, the surface of the bubble begins to move very rapidly. As a result of this violent motion, it is possible that the bubble breaks into two parts.
By application of a negative pressure to an electron bubble it is possible to make the bubble explode. The bubble can then grow large enough to be recorded with a video camera. Currently experiments are underway in which an electron bubble is repeatedly exploded using a sound wave. Through the use of this novel technique, our group has been able to make a movie showing the motion of a single electron.
New techniques have been developed in our laboratory at Brown that make it possible to perform ultrasonic experiments at very high frequency. Femtosecond light pulses are used to generate and to detect short sound pulses and also to study the flow of heat in small structures. The Fourier spectrum of the sound pulses extends up to 1 Terahertz. Brown University has licensed the technology that has been developed to an established metrology company, and they have produced a commercial version of the equipment. This has become a standard technique for the measurement of the thickness of opaque films in semiconductor chips and is now in use by all of the leading chip fabricators world-wide.