Text 791, 82 rader
Skriven 2006-06-02 15:34:39 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 779
===============
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 779 June 2, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu
        
SOUND AMPLIFICATION BY STIMULATED EMISSION OF RADIATION, 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 UK and the
Lashkarev Institute of Semiconductor Physics in the 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
milli-electron-volts of energy (meV), equivalent to a frequency of
0.1-1 THz.  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 UK-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 www.aip.org/png
).  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 THz
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 nm,
THz acoustical devices might be used in modulating light waves in
optoelectronic devices.  (Kent et al., Physical Review Letter, 2
June 2006)

EXISTENCE OF ATOMS REAFFIRMED.  A new experiment has reproduced a
landmark 1908 study demonstrating 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)

---
 * Origin: Big Bang (1:106/2000.7)