快猫短视频

IT LOOKED something like a cross between a lamp and an organ pipe and it was
the brainchild of Douglas Shields, an acoustic engineer at the University of
Mississippi. His idea was simple: take a metre-long glass tube full of nitrogen
gas, pump energy into the molecules with a crackling spark, and then inject a
pulse of sound into the gas. He reckoned that as the pulse bounced up and down
the tube, the gas molecules would release their pent-up vibrations, making the
sound louder. And it worked, up to a point. 鈥淲e did see evidence for
amplification,鈥� Shields reports of the trials he ran in the 1980s. But he needed
to pump in so much energy that the gas overheated, and the device went
kaput.

Shields鈥檚 amplifier eventually fell prey to the vagaries of research funding.
鈥淭he programme sponsors wanted something useful out,鈥� says Henry Bass, one of
Shields鈥檚 co-workers and now director of the National Center for Physical
Acoustics at the University of Mississippi. 鈥淎t the time, it didn鈥檛 seem like
the device had anything to offer.鈥�

But while Shields has moved on to other things, physicists in labs from
Belarus to Brazil have been pursuing similar ideas. The end result could be a
huge range of applications, from acoustic microscopes that probe tiny circuits
and sensors that listen in on submarines or high-energy particles, to devices
that quieten the noise inherent in all electrical circuits.

The most promising of these applications rely on phonons鈥攓uantum
particles associated with all kinds of high-frequency compression waves, including ultrasound
(see Diagram).
Just as a packet of light waves can be viewed as a photon,
so it is with the waves that permeate solid materials: a
packet of these waves can be treated as a particle called a phonon. 鈥淧honons are
high-frequency sound waves,鈥� says physicist James Wolfe from the University of
Illinois at Urbana-Champaign. And even in a grain of salt there are more than a
million billion of them at any one time.

As these phonons bounce around in solids, they knock into anything that gets
in their way, bumping into electrons and scattering from impurities or the edges
of tiny crystal grains. Just as spectroscopists use light鈥攅specially laser
light鈥攖o study the structure of atoms and molecules in gases and liquids,
it should be possible to unravel the structure and properties of solids by
finding out how they scatter and absorb beams of phonons. However, there鈥檚 a
problem: 鈥淟aser light has a well defined energy,鈥� says Wolfe. And
spectroscopists can easily adjust the energy of their laser beams. Shine laser
light onto a bunch of atoms, scan the energies of your photons, and you can
probe their quantum energy states one at a time.

But devices that generate phonons鈥攕imple oscillators or heaters, for
instance鈥攁re more like light bulbs than lasers. They emit a jumbled mix of
phonons with different energies and directions that interact with a whole bunch
of quantum states rather than with one or two. What is needed is a
鈥渟aser鈥濃�攕ound amplification by stimulated emission of radiation, or a
laser for phonons.

In essence, lasers are intensely powerful and versatile amplifiers. Under the
right conditions, a laser turns a trickle of light into an avalanche of
identical photons. These photons reflect back and forth between the two mirrors
that make up a laser鈥檚 cavity. Make one of these mirrors slightly transparent
and the light streams out as a bright, pencil-thin beam
(鈥淚nside Story鈥�, 快猫短视频, 4 April 1998, p 38, and
Inside Science No 24, 快猫短视频, 17 June 1989).

Build a saser that is based on the same principles and you can create a
鈥渓aser beam鈥� of phonons with a narrow range of energies. 鈥淵ou might use phonons
like light,鈥� says Wolfe, to pick out the fine detail in a material, in much the
same way that you learn more about a tiny object by increasing the magnification
on a microscope. A tunable beam of phonons could help physicists discover
exactly how electrons vary their energy as a material heats up or cools down,
for example. Eventually, says Wolfe, this could reveal the inside story of
things like heat dissipation, electrical resistance and superconductivity.

Of all the researchers trying to build a saser that can emit a phonon beam,
Harold de Wijn and his colleagues from the Debye Research Institute at Utrecht
University in the Netherlands are probably the closest to their goal. 鈥淚f we are
being nice to ourselves, we say we have a saser,鈥� says de Wijn. 鈥淚f we are a
little bit more critical, then we say, well, there鈥檚 a lot of work to be
诲辞苍别.鈥�

De Wijn鈥檚 saser is made from a 5 millimetre long rectangular crystal of
ruby鈥攁luminium oxide lightly peppered with chromium ions. To freeze out
unwanted sound waves that might interfere with the performance of the saser, de
Wijn and his colleagues bathe their ruby block in liquid helium to cool it to
1.8 kelvin. Then they focus a laser beam into a spot near the centre of the
crystal just a third of a millimetre across
(see Diagram). At this
point, electrons on the chromium ions absorb the light energy, jump to a higher
energy level and then drop back to a lower level, giving out their excess energy
as light.

To create phonons rather than photons, de Wijn switches on a powerful
magnetic field that nudges the electrons in the chromium ions into slightly
different energy levels. With the field switched on, the electrons absorb light
but lose their energy in small steps rather than a single leap. These steps are
too small to give a photon, but just enough to create vibrations in the crystal
lattice鈥攎aking phonons rather than photons.

These phonons travel the length of the crystal and reflect off the end walls,
making five or six passes in all. Each time they whizz through the region where
the laser light is focused, they stimulate excited electrons on the chromium
ions to lose their energy and give out more phonons鈥攖he process known as
stimulated emission. 鈥淭he basic ingredients of a saser are there,鈥� says de
Wijn.

So far so good: the phonons inside his ruby crystal behave just like photons
in a laser cavity. The snag is that they remain imprisoned within the cavity as
the sudden density change at the edge of the crystal acts like a highly polished
mirror. To make the device useful, de Wijn must find a way for the phonon beam
to escape into other materials. 鈥淵ou could just glue another crystal to it,鈥� he
says. 鈥淏ut we haven鈥檛 tried that yet.鈥�

Eventually, de Wijn might build sasers inside the material he wants to study,
or the sasers may simply be stuck onto the side. He is also looking at ways to
alter the shape of the ruby cavity to improve the amplification. Maybe, he
suggests, simply making it shorter will do the trick. 鈥淭his is all far away from
applications at this point,鈥� he says. 鈥淎ll we want to do is show that it can be
诲辞苍别.鈥�

At the University of Paris-South, Jean-Yves Prieur and his colleagues have
put together a different sort of saser. Rather than relying on laser power,
Prieur鈥檚 saser has a pair of tiny piezoelectric transducers that convert a
fluctuating voltage into high-frequency vibrations. These transducers are
mounted on opposite ends of a small block of glass just 2 centimetres long. One
creates a 鈥減ump鈥� pulse that travels along the block, passing its energy to the
atoms as it goes. Its partner creates a pulse of high-frequency phonons that
stimulates the energised atoms to release this energy, amplifying the pulse in
the process.

With its flat end faces, the block is meant to form a resonant cavity like de
Wijn鈥檚 crystal that will reflect the sound pulses back into the glass where they
can stimulate still more phonons. Unfortunately, it hasn鈥檛 panned out that way.
鈥淢ultiple passes don鈥檛 seem to work,鈥� says Prieur. When the pump pulse reaches
the end of the cavity, it reflects back along the block and interferes
destructively with the phonon pulse, eliminating some of the phonons it has just
created. Despite this, Prieur鈥檚 saser design can amplify a sound pulse by a
factor of thirty or so.

Prieur鈥檚 saser may eventually provide a source of phonons that will probe the
interior of solid materials. Combined with a phonon detector such as a
bolometer, these phonon sources could act as 鈥渁coustic microscopes鈥� that can
pick out tiny defects inside the material. You should be able to use phonons to
stare inside integrated circuits or composite materials, says Wolfe. Small
defects or breaks in a material interact strongly with phonons, so they stand
out like beacons. This could be especially valuable for measuring the thickness
and quality of the thin metal connections that make up the circuits within a
microprocessor.

Sasers could also be the basis of sensitive particle detectors, Prieur
suggests. As high-energy particles slam into a piece of silicon, they create
faint ripples in the silicon鈥檚 atomic lattice. Amplifying the ripples with a
saser could turn such a device into an ultra-sensitive detector, analogous to a
photomultiplier. Physicists could use it to search for the weird and wonderful
particles believed to flood space and contribute to the dark matter of the Universe
(鈥淪pace oddity鈥�, 快猫短视频, 16 January 1999, p 24).

Sergio Makler at Fluminense Federal University at Niteroi in Brazil is also
building a saser. Five years ago, Makler, together with Russian theorist Mikhail
Vasilevski at Nizhni Novgorod State University, outlined a device thousands of
times smaller than even de Wijn or Prieur鈥檚 tiny cavities. It is based on a
quantum well, an artificial atom made from layers of semiconductors such as
gallium arsenide that can trap an electron in quantised energy levels. Inject an
electron into the well with a small voltage and it jumps between these energy
levels, blasting out a stream of phonons at ultra-high frequencies.

If Makler can make this device work he will have sidestepped the complexities
of other saser designs. It could be incorporated into larger semiconductor
devices at the manufacturing stage. Best of all, it will create high-energy
phonons corresponding to frequencies in the terahertz region and beyond. Makler
predicts that such phonons will reveal semiconductor structures just tens of
nanometres across鈥攊deal for studying the details of microchips. At even
higher frequencies, acoustic microscopes may eventually probe solids down to the
atomic level.

Beams of high-frequency sound from a saser could also create acoustic
holograms, Makler suggests. Analogous to light holograms created with two laser
beams, these could provide a way to store vast amounts of information in a small
space. 鈥淭he data density would be high because of the short wavelength [of the
sound waves],鈥� predicts Makler. 鈥淏ut they will take time to develop.鈥� Makler
even envisages that sasers will create a completely new field called
鈥減honoelectronics鈥�. Phonoelectronic devices will talk to each other with beams
of phonons rather than with light or electric current.

Franco Nori, a physicist from the University of Michigan, has another
practical application in mind for phonons: suppressing the quantum noise that
drowns out very faint signals in ordinary conductors. The trick exploits the
fact that the uncertainty principle allows a trade-off between the uncertainty
in a signal鈥檚 amplitude and that of its phase. Physicists have already done
something similar with 鈥渟queezed鈥� light to reduce the energy in a vacuum below
its normal background 鈥渮ero鈥� level (鈥淟ight gets a quantum squeeze鈥�, New
快猫短视频, 19 October 1991, p 41). Reducing the noise levels in electronic
devices such as silicon detectors or amplifiers, for instance, might allow them
to pick out even the weakest signal. 鈥淐ontrol a phonon beam and you may be able
to suppress quantum noise,鈥� says Nori. 鈥淢aybe in five years鈥� time. But we
haven鈥檛 fleshed out the theory yet.鈥�

As Shields showed more than a decade ago with his energised tube of nitrogen,
sasers don鈥檛 necessarily have to involve phonons. The principle works with
lower-frequency vibrations too, where the particle nature of vibrational wave
packets virtually disappears.

This is the line that Sergei Zavtrak, a physicist at the Belarussian State
University in Minsk, is following. His idea is for a device based on a
cylindrical vessel filled with water containing billions of tiny gas
bubbles鈥攑erhaps produced by electrolysis. Zavtrak calculates that if you
rhythmically squeeze these bubbles by subjecting them to a varying electric
field or by squashing the sides of the container, they will resonate in
response, just as a bell rings when you strike it. If you now inject a sound
pulse, it will gather energy from the vibrating bubbles as it bounces back and
forth through the cylindrical cavity.

Not only that: Zavtrak calculates that the bubbles will organise into a
series of planes at right angles to the beam direction鈥攁n 鈥渙rdering鈥�
effect that is seen by biologists when they pass ultrasound through suspensions
of cells. The final result, Zavtrak believes, should be a powerful, highly
directional, narrow beam of low-frequency sound waves emerging from the end of
the container (see Diagram).

鈥淚t鈥檚 an interesting scientific concept,鈥� says Lawrence Crum, a physicist at
the University of Washington in Seattle. 鈥淚 expect something like what he
proposes to work, but efficiency would be a real problem.鈥� Zavtrak has yet to
build his bubble-based saser, but at British Aerospace鈥檚 Sowerby Research Centre
near Preston in Lancashire, Ron McEwan and his colleagues are intrigued by the
idea of a powerful, directional source of sound. They suggest it could be used
for tasks such as detonating explosives from afar, or as a weapon, to immobilise
terrorists by stunning them with a blast of sound. But having attempted a few
simple trials with a device based on Zavtrak鈥檚 saser 鈥渏ust in the hope that we
might stumble onto something that looked encouraging鈥�, they are not particularly
optimistic about its prospects. How do you stop the bubbles from collecting
together or rising to the top of the cylinder, for instance? 鈥淭he theory appears
to be there,鈥� says McEwan, 鈥淚 just have misgivings about its practicality.鈥�

Zavtrak鈥檚 unique amplification mechanism could also be used to amplify one
set of frequencies among a soup of other sounds, explains Bass. 鈥淭his might be
especially useful in a very noisy environment,鈥� he says. One example is the
detection of submarines, where you want to pick out the sound of an approaching
vessel among all the other noises of the ocean. But as Bass points out, such
applications are still speculative, and no one yet knows whether Zavtrak鈥檚
device offers any advantages over conventional sonar equipment.

Sasers of all sizes are little more than a laboratory curiosity at the
moment, but that doesn鈥檛 dim the enthusiasm of their supporters. 鈥淭hey鈥檙e new,
and new territory had better be explored,鈥� says Nori. 鈥淓ven the inventors of the
laser did not come up with good reasons why they should study it.鈥� And you
couldn鈥檛 ask for a better role model than that.