Physics news Update 779

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Number 779, June 2, 2006 by Phil Schewe and Ben Stein

A New Kind of Acoustic Laser

Sound amplification by stimulated emission of raciation, or SASER, is the acoustic analog of a laser. Instead of a feedback-built potent wave of electromagnetic radiation, a saser would deliver a potent ultrasound wave.
The concept has been around for years and several labs have implemented models with differing features. In a new version, undertaken by scientists from the University of Nottingham (Anthony Kent, anthony.kent@nottingham.ac.uk) in the U.K. and the Lashkarev Institute of Semiconductor Physics in Ukraine, the gain medium -- that is, the medium where the amplification takes place -- consists of stacks (or a superlattice) of thin layers of semiconductors which together form "quantum wells."
In these wells, really just carefully confined planar regions, electrons can be excited by parcels of ultrasound, which typically possess millielectronvolts of energy, equivalent to a frequency of 0.1-1 terahertz. And just as coherent light can build up in a laser by the concerted, stimulated emission of light from a lot of atoms, so in a saser coherent sound can build up by the concerted emission of phonons from a lot of quantum wells in the superlattice.
In lasers the light buildup is maintained by a reflective optical cavity. In the U.K.-Ukraine saser, the acoustic buildup is maintained by an artful spacing of the lattice layer thicknesses in such a way that the layers act as an acoustic mirror (see figure at Physics News Graphics).
Eventually the sound wave emerges from the device at a narrow angular range, as do laser pulses. The monoenergetic nature of the acoustic emission, however, has not yet been fully probed. The researchers believe their saser is the first to reach the terahertz frequency range while using also modest electrical power input. Terahertz coherent sound is itself a relatively new field of research. Essentially ultrasound with wavelengths measured in nanometers, terahertz acoustical devices might be used in modulating light waves in optoelectronic devices.
Kent et al., Physical Review Letter, 2 June 2006
Contact Anthony Kent, anthony.kent@nottingham.ac.uk
Figure at Physics News Graphics

Existence of Atoms Reaffirmed

A new experiment has reproduced a landmark 1908 study that demonstrated the physical existence of atoms, even to many of those (such as the chemist William Ostwald) who had doubted that matter consisted of microscopic particles rather than being continuous in nature.
The new experiment, conducted partly as an educational exercise for undergraduates at Harvard, reproduced (with modern equipment) the work in 1908 of Jean-Baptiste Perrin, a French physicist, who in turn was seeking to test a prediction of Albert Einstein.
Einstein's miraculous 1905 output included famous papers on special relativity (bearing on features of space-time and on the equivalence of matter and energy) and the photoelectric effect (explaining the quantum nature of light). The propositions of relativity and quantum theory proved to be extremely fruitful and are put to frequent experimental test.
A third paper from that year, one devoted to explaining Brownian motion, is perhaps less well known, but also of great importance. Brownian motion, first observed by Robert Brown in 1827, is the jostling of one set of tiny particles (in this case, pollen grains) by other, even smaller, particles (the surrounding water molecules).
Einstein interpreted the jostling as the incessant and fluctuating aggregate effect of all the presumed atoms or molecules on the grains; occasionally the net force on the grain would push it to the side. Einstein worked out a formula relating the size of the pollen grains and their median momentary excursion (part of what we would now call a "random walk") and the size of the surrounding and invisible buffeting particles (atoms and molecules).
Perrin performed his experiment using emulsions containing microscopic particles of gamboge (a type of pigment) or mastic (a clear plastic). Using a microscope he painstakingly watched, measured, and tabulated many displacements of individual gamboge particles. From this he confirmed Einstein's predictions about the statistical nature of the agitations, and from this one could calculate Avogadro's Number, the number of atoms or molecules in a single mole of that substance. And this in turn supported the atomistic view of matter.
The new Harvard version of this experiment is faithful to the 1908 work except that a CCD camera viewed the particle movements and analyzed the displacements by means of a computer program.
Newburgh, Peidle, and Rueckner, American Journal of Physics, June 2006
Contact Ronald Newburgh, rgnew@verizon.net

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