I was happy to see that you
used the correct nomenclature
for the trapping regimes in
your article on optical tweezers
(July 2009). I was also pleased
to read that Arthur Ashkin was
recently named an honorary
member of the Optical Society
of America for his pioneering
work in optical trapping and
the development of optical
tweezers (January 2010).
My purpose in this letter is
to note how the optical trapping community is separating
trapping regimes—in some
cases inappropriately. They are
doing so mainly to simplify
theoretical calculations by
making initial approximations
based on the trapped particle
or scatterer dimension with
respect to wavelength. However, in the last few years, one
of these regimes is continuously being misrepresented—
the Mie regime.
Some authors attribute
the Mie regime to arbitrary
particle size with respect
to wavelength, while others consider the term Mie
regime to be synonymous
with geometrical optics or the
ray optics regime, where the
scatterer is much bigger than
the wavelength.
In 2008, we commemorated 100 years of Gustav Mie’s
1908 publication, entitled:
“Contributions on the Optics
of Turbid Media, Particularly
Colloidal Metal Solutions”
(Ann. d. Physik 4[ 25], 377).
The emphasis on colloidal gold
was at the time a current topic.
Nonetheless, his approach,
based on Maxwell’s electromagnetic theory, provided a
rigorous solution for the scattering of a plane monochromatic wave by a homogeneous
sphere of arbitrary diameter
and composition.
In his paper, he limited
the spheres to 180 nm, due
to the computational limitations at the time. (
Calculations were done by hand.) Yet
many authors still describe the
Rayleigh and Mie regimes as
distinct and non-intersecting.
The work on Mie scattering
refers to any sized sphere. As
a matter of fact, the Rayleigh
regime is just a subset of the
Mie theory (the first expansion
term). In many papers today,
we can read about the Mie
regime as being used exclusively for a scatterer larger
than the wavelength. When
this came about or why this
belief still propagates I do not
know—but I hope that other
careful and more experienced scientists might help to
explain. For now, ray optics
and Rayleigh regimes are just
a subset of the Mie regime,
where the Mie theory applies.
Antonio A.R. Neves
Lecce, Italy
aneves@gmail.com
The history of gas lasers
I was amused by your articles
on defunct lasers and the history of continuous wave gas
lasers (January 2010), and I
look forward to more.
I think you should be aware
of a forgotten laser that I
helped to discover: the optically pumped molecular
iodine laser (R.L. Byer et al.
Appl. Phys. Lett. 20, 463-
466 [June 1972]). There must
be more than 1,000,000
laser lines possible in that
system! Unfortunately, it self-terminates
and must
be optically
pumped
with a laser.
However,
if someone
is counting
lines, that is
a bunch.
The
laser works
because
greenish light
can pump
some molecules in a populated rotational-vibrational
level of the ground state into
a specific sublevel of the B
electronic state, creating a
population inversion with
respect to higher-lying levels
of the ground electronic state.
Since there are more than 100
vibrational levels of each electronic state, each with more
than 100 rotational levels (and
two transitions into and out
of each one), a great number
of combinations is possible.
Selecting the desired transition takes a tunable pump
laser, though. Tunable green
isn’t easy, often requiring laser
pumping, OPOs, doubling
and worse. So the iodine laser
is totally impractical and long
defunct. But it is a gas!
Marc Levenson
Saratoga, Calif., U.S.A.
microlith@aol.com
I WAS THRILLED to find my
picture on the cover of my
favorite magazine (January
2010). Who ever heard of the
cesium laser or that it had
allowed Gordon Gould to
obtain the patent rights to
all optically pumped lasers,
including ruby, rare earths,
and perhaps Ti:sapphire? I
am grateful to
you for mak-
ing these little
known points.
Here are a
couple of other
tidbits about
laser history:
The story
goes that the
Bell Labs team,
anticipating
the difficulty
of aligning the
meter-long,
superflat HeNe
cavity into resonance, used
an apparatus to systematically
search for the laser oscillation
flash. When this failed, they
succeeded instead by hitting
the invar cavity spacer with a
dull hammer thud.
At a time when the military was clamoring for ruby
logs, Gould had the foresight
to see that lasers would make
coherent detection possible.
Using the superheterodyne,
he did with light what long
ago had been done with
radio waves. I tried unsuccessfully to obtain Gould’s
classified proposal from
ARPA’s records; it was a
magnificent piece of writing.
He did report success at The
3rd International Quantum Electronics Congress
in Paris in 1963 (Quantum
Electronics, Vol. 1, Columbia
University Press, N.Y., 1964,
