Number 853, January 11, 2008
by Phillip F. Schewe and Jason S. Bardi
Unprecedented Spectroscopy Using the Best Ever Ruler for Light
Physicists at NIST-Boulder have carried out a powerful new spectroscopic study of a sample of gas using optical frequency combs. The NIST work, which might well change the way spectroscopy
is done, is remarkable in that it provides the full spectrum of the
gas over a broad spectral region and with frequency accuracy that
can reach 1 Hz (for spectral frequencies of the order of 2 x 10^14
Hz). The NIST spectroscopic feat is equivalent to simultaneously
sending 155,000 individual single frequency lasers through the
sample and measuring the resulting amplitude and phase shift on each
individual laser. Moreover, the spectrum is measured rapidly, using
a device with no moving mechanical parts.
The invention of the optical frequency comb method was a great step
forward in laser science. John Hall (NIST) and Ted Haensch (Max
Planck) the Nobel prize in 2005 for their pioneering work in this
area. (For a tutorial on frequency combs, see http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm)
In the comb process, a pulsed laser emits light not merely at a
single frequency, but at a series of frequencies. A frequency
spectrum of this composite laser output looks like a comb, with
light occurring at regularly spaced frequencies, covering the
infrared part of the light spectrum.
In many ways the frequency comb is an ideal tool for spectroscopy.
Its light covers enormous amounts of the optical spectrum and the
frequency of each individual comb line can be known to 1-Hz
precision. When you pass a frequency comb through a gas cell a
given comb line will, like any laser beam, be absorbed when it is
resonant with any of the many quantum energy levels of the gas.
The challenge with frequency combs is to figure out which of the
more than one-hundred thousand comb lines experience absorption and
which do not. To solve this problem NIST researchers take the comb
used for spectroscopy and mix it with a second carefully crafted
frequency-comb. This ensemble of light pulses results in a “beat-frequency” pulse which can be measured with conventional
electronics. From this beat-frequency pulse the absorption and
phase shift experienced by each individual comb line can be
separately observed. This work represents by far the largest number of frequency comb teeth that
have been individually observed.
The present NIST experiment interrogates the effect of the
absorption from the gas on 155,000 comb lines, spanning a wavelength
range of 125 nm. The NIST precision of 1 Hz for spectral lines is to
be compared with tens of MHz precision characterizing other
spectroscopic techniques. NIST researchers believe that this new
work might change the way people perform spectroscopy.
(Coddington{ian@nist.gov, 303-497-4889}, Swann, Newbury, Physical
Review Letters, 11 January 2008; PRL editors designate this as a Suggested
Article)
Acoustic Cloaking
Computer simulations and the use of wave
scattering theory have demonstrated that, contrary to earlier
predictions, it should be possible to produce a
3-dimensional material shell which is invisible to sound waves,
analogous to “optical cloaking,” the process in which light waves
are guided around an object and then refocused on the far side and
in the same direction (with no reflected light to betray position)
so as to make the object seem invisible. Full optical cloaking has
not been achieved yet, but researchers expect to be able to do it.
Can the same thing be done with sound waves?
In principle there is no reason why it couldn’t be done. The leader
of a group of scientists examining this issue, Steven Cummer at Duke
University, says that many of the principles that pertain to the
channeling of light waves around an object also apply to sound
waves. To be sure, there are differences. Sound waves oscillate in
the direction of their motion while the electric and magnetic
fields composing light waves oscillate
perpendicularly to the wave motion. In the optical case, cloaking
will require a material (actually a meta-material) tailored, highly
anisotropic (varying widely according to the direction through the
material) index of refraction.
In practice, the index of refraction
for electromagnetic waves depends on the permittivity, a measure of
the material's response to an applied electric field, and
permeability, its response to an applied magnetic field (for an
account of the demonstration of negative-index materials, see
http://www.aip.org/pnu/2000/split/pnu476-1.htm).
The acoustic equivalent of these two parameters are the mass density
and the bulk modulus (the springiness) of the background fluid (usually air or
water) in which the object sits.
Cummer (919-660-5256, cummer@ee.duke.edu) says that in the short run
acoustic cloaking might be more practical than optical cloaking.
A
limitation of electromagnetic cloaking, he says, is that it requires
portions of the wave to move faster than the speed of light (in full
accordance with special relativity); this can be done for very
limited frequency ranges but not for wider ranges, limiting the
applicability of optical cloaking. This limitation does not apply
to sound waves moving through matter. Furthermore, the acoustic
properties of most materials means that sound waves might not be
absorbed as readily in acoustic cloaking as light waves are absorbed
in optical cloaking (in which case the cloaking would be something
less than perfect).
Applications of acoustic cloaking come easily to mind:
hiding submarines from sonar, for example. Another potential
practical application might be in architecture, where acoustic
considerations (reducing noise) might not have to be sacrificed in
the interest of structural integrity. Among Cummer’s collaborators
are David Smith of Duke (one of the early pioneers in the field of
negative-index materials) and John Pendry of Imperial College (the
early theorist of negative-index studies). (Cummer et al., Physical
Review Letters, 18 January 2008; considered an editor’s Suggested
article in PRL)
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