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Microspheres, Photonic Atoms
and the Physics of Nothing
Light can become trapped within tiny, transparent spheres. The surprising properties
that result may turn “microsphere photonics” into an important new technology
Stephen Arnold
T
he transfer of energy between neighboring molecules plays a pivotal
role in nature. In photosynthesis, for example, a plant fuels its metabolism and
growth with sunlight by taking advantage of a curious physical phenomenon
that allows energy to hop from one
chlorophyll molecule to another situated about a half a nanometer away. A
couple of hundred chlorophyll molecules pass the energy they collect from
the sun in this way to a single reaction
center, the starting point for subsequent
chemical reactions. Without this mechanism for transferring energy between
molecules, photosynthesis would largely cease, and we would likely starve.
About 15 years ago I began to wonder whether similar forms of energy
transfer could influence photochemistry within aerosol particles. In particular, I wanted to know whether there
are subtleties in the way energy is conveyed between molecules in an isolated droplet about 10 micrometers in diameter. To most physicists, the idea
must have seemed crazy. After all, the
range of the longest substantial exchange of this sort, as Nobelist Jean
Perrin had discovered and Theodore
Förster had described in quantummechanical terms decades ago, is only
Stephen Arnold is director of the Microparticle
Photophysics Laboratory and Thomas Potts
Professor of Physics at the Polytechnic University
in New York. Arnold has also held visiting appointments at Caltech, the University of Tokyo and the
École normale supérieure in Paris. He is a Fellow of
the American Physical Society and the Optical
Society of America. Address: Department of
Physics, Polytechnic University, 6 Metrotech
Center, Brooklyn, NY 11201.
Internet: [email protected]
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about 5 nanometers. The vessels I was
proposing to use would be 2,000 times
larger. So there was no obvious reason
to expect that their tiny size would
have any influence at all. Still, research
elsewhere with similar microscopic
particles hinted of interesting physical
effects, and I urged one of my graduate
students, Lorcan Folan, to investigate.
Little did I know that the results we
and others were soon to obtain would
distinguish the lowly aerosol particle
as a high-tech item.
Such microscopic particles now
stand poised to serve in a variety of
ways, from lasers of exceptional efficiency to optical filters of unprecedented purity and chemical probes of tiny
size—to name just a few obvious applications. But before delving into how
tiny spheres can provide such valuable
functions, it is worthwhile to review
how this rapidly evolving creature of
21st-century technology first arose from
a primordial soup of basic research.
Peering at Particles
Probing something as subtle as the
transfer of energy between molecules
may seem daunting, but in truth the
procedure is straightforward. You first
energize one type of molecule, known
as a donor, by illuminating it with light
from a properly tuned laser, which
kicks ground-state electrons to a higher
energy level. Then you look for a transfer of this energy to another type of
molecule, known as an acceptor, by
sensing the characteristic color of light
it gives off when its excited electrons
fall back to a lower-energy state (a familiar enough process called fluorescence). If no energy passes between
donors and acceptors, only the donor
Copyright © 2001 American Scientist
molecules will fluoresce, giving off
their own particular color. So the ratio
of acceptor to donor fluorescence provides a convenient way to gauge the
amount of energy transferred.
To perform the measurement on a
microscopic droplet, one simply mixes
in the appropriate donors and acceptors and observes the spectrum of
laser-induced fluorescence. The not-sominor complication is that it is difficult to hold a 10-micrometer sphere of
liquid in place long enough to study
it. Lorcan and I solved this problem
by constructing an apparatus in which
he could levitate and contain an electrically charged particle indefinitely using electrostatic force to balance gravity (Figure 1), just as Robert Millikan
had done decades before in his famous
oil-drop experiment. But Millikan’s
scheme alone does not provide a
“trap”—the particle can wander freely.
To prevent such drift, Lorcan’s apparatus superimposed an oscillating electric field on the constant levitation
field, following the example of Wolfgang Paul, who had shown with his
Nobel Prize–winning work of the
1950s that a dynamic field can be used
to trap an atomic ion (Figure 2). With a
properly fashioned oscillating field, a
microscopic droplet just hangs there in
air, an easy target to hit with a laser
and to view through a microscope.
The first spectrum Lorcan obtained
from a levitated droplet sent a wave of
surprise through our laboratory. He
had planned a series of experiments at
varying concentrations, starting on the
dilute side where the average distance
between donors and acceptors was 20
times the maximum range for Förster
exchange. So we did not expect any en-
Figure 1. Charged droplet of glycerol floats motionlessly between two metallic electrodes as it is irradiated from the left with laser light. The author and his students used this experimental arrangement to study how energy passes between two different kinds of dye molecules mixed into
the droplet. Their discovery that optical energy moved from one kind of molecule to the other with surprising efficiency prompted them to investigate the intriguing physics of light traveling within microscopic spheres made of liquid, plastic and glass. The novel optical properties of such
microspheres suggests their use in many different applications, from signal processing to biological sensing. (Photograph courtesy of the author.)
ergy transfer to be apparent. Yet the ratio of acceptor to donor fluorescence
proved to be more than 10 percent.
As Lorcan increased the concentration of acceptor molecules in a series of
droplets under study, we became increasingly puzzled. Had the wellknown Förster mechanism operated,
the amount of energy transferred in
such dilute solutions should have been
proportional to the concentration of acceptors. But these experiments revealed a range of concentrations nearly
two orders of magnitude wide for
which we saw little change in the
amount of energy conveyed between
molecules (Figure 3). What is more, in
the light spectra for both donors and acceptors, we observed spikes that were
as obvious as fence pickets (Figure 4).
Such distinctive features had never appeared in experiments that probed these
very same molecules in a centimeterscale test tube. Although the explanation
was not immediately obvious, these two
pieces of evidence were ultimately to revamp our view of how energy was being passed between molecules.
At the outset, we were planning to
probe the subtleties of Förster transfer,
whereby the energy in an excited electron shifts to another molecule without
Copyright © 2001 American Scientist
ever generating a photon. How, one
might reasonably ask, does that happen? Such transfer takes place because
an excited molecule behaves something like a transmitting radio antenna. Close to this nanoscopic source, the
oscillating electric field is especially
intense (although it drops off extremely rapidly, with the cube of the distance). This field can, in fact, be sufficiently strong to induce oscillation in
the electron cloud of a nearby molecule, and this coupling conveys energy
if the acceptor sits close enough that
the probability of transfer overwhelms
the natural probability for the donor to
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415
picopipette
to microscope,
camera and
spectrometer
laser beam
fluoresce. Photons are not involved in
such an exchange; that is to say, the
donors do not have to give off electromagnetic radiation. The Förster process
is thus akin to what happens to people
who mysteriously hear radio programs
because the electric field of a nearby radio transmitter is so powerful that it induces currents to flow in their fillings.
Of course, radio transmitters are not
built to energize the mouths of people
standing nearby: They depend on their
antennas broadcasting electromagnetic
waves to far-flung receivers. So too
fluorescence ratio (percent)
100
10
1
Förster
transfer
0.1
10–8
10–7
10–6
10–5
acceptor concentration
(moles per liter)
Figure 3. Transfer of energy between donor and
acceptor dye molecules (as measured by their
fluorescence) proved to be much larger (dots)
than predicted according to Förster theory (line).
This discrepancy, and the weak dependence
on concentration over much of the range, led
the author and his students to consider other
ways that energy could flow from one molecule to another within a levitated droplet.
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American Scientist, Volume 89
Figure 2. Custom-designed “picopipette” generates one charged 10micrometer droplet of glycerol, which passes through an access hole
into the experimental chamber. This tiny sphere is then irradiated
with a laser, and the resulting emissions are observed through a microscope to which a spectrometer is coupled. It can be studied in this
way because it is levitated in air using three electrodes. A large, constant electric force (blue arrow) balances the downward force of gravity (orange arrow). But according to a well-known theorem of physics,
a static electric field cannot be fashioned to hold a charged object in
place without drifting. An alternating saddle-shaped potential can,
however, be used to create an electric field that switches back and
forth (green arrows), trapping the droplet. During his Nobel address in
1988, Wolfgang Paul, who first constructed such traps to study atomic ions, used a mechanical analogue (above) to illustrate how such an
oscillating saddle can be made to contain a small ball. (Photograph
by Hans-Joachim Becker, courtesy of the Deutsches Museum.)
with excited molecules. If the donor has
no near-field neighbors to capture this
energy, the excited molecule emits a
photon into the far field (at least one
wavelength away). An acceptor looking back at the donor from this distance
normally finds itself so far away that
the probability of being struck by the
photon and absorbing its energy is astronomically small. Yet this was the
scale of separation between donors and
acceptors in Lorcan’s first experiment.
What was going on? Given the low
concentration of donor and acceptor
molecules within the droplet, we knew
that Förster transfer was not operating.
Photons must have been leaving the
donors and hitting the acceptors, but
they were doing so with an unexpected
efficiency. The only reasonable explanation was that each of the emitted
photons was returning many times to
the same region, so that it had many
chances to collide with an acceptor.
The spikes we saw in the spectra gave
us a good clue to the mechanism.
These spikes are resonance peaks,
which correspond to special electromagnetic modes for the entire particle.
The situation is akin to a violin string,
which supports vibrational modes
only at those frequencies that provide
for an integral number of half wavelengths along its length. The electromagnetic modes of small particles—
commonly known as Mie resonances
or whispering-gallery modes—are
Copyright © 2001 American Scientist
analogous and are normally described
by their wave character, too, in a way
that takes full account of such complications as polarization and diffraction.
However there is a more visceral description of these modes that clearly
underscores the ability of a photon to
revisit a region many times. H. M.
Nussenzveig of the Universidade Federal do Rio de Janeiro realized that if
the sphere is much larger than the
wavelength of light involved, whispering-gallery modes can be represented
as geometrical orbits. With diffraction
stripped away, photons can be thought
of as bouncing around inside the particle in well-defined trajectories, confined by total internal reflection—the
same phenomenon that makes the surface of a swimming pool look like a
mirror when you peer up from under
water and look to the side (Figure 5).
The swimming-pool effect arises because rays hitting the surface from underneath at a shallow angle reflect
completely back into the water. To a
large extent this is also true inside a
transparent particle, so long as it is
much larger than the wavelength.
When a ray of light strikes the spherical surface at a shallow angle, it just
bounces back inside. A ricocheting
photon thus remains within the particle for much longer than it would otherwise. Ultimately the photon lifetime
is limited by diffraction, which causes
photon trajectories to be less certain, al-
(Figure 7). His acceptor image boosted
our confidence enormously, because
light was concentrated close to the surface, where the photons in a photonic
atom circulate. Holler’s donor image
was less interesting, with almost all of
the interior aglow, but it displayed a
distinctive asymmetry, which arose because the spherical droplet focused
much of the incident laser light onto its
back side. Surprisingly, the acceptor
image showed no difference between
back and front. Rather, it revealed
two curious bright spots, diametrically
opposed.
The cloud of mystery eventually dissipated when we realized that donor
molecules excited preferentially at the
back of the droplet launch photons at
every “azimuth.” Imagine a busy airport sending jets in every direction,
with each of the pilots instructed to fly
a great-circle route. These aircraft will
eventually converge at the antipodal
point on the other side of the globe.
Unless they collide in midair, they will
then return to their starting point, and
so forth. Similarly, the concentration of
the donor emission at the back of a microsphere sends photons on great-circle
trajectories, which intersect at two antipodal points, seen as bright points in
the image of acceptor fluorescence. Because our image averaged many revolutions of the photons from one side
of the sphere to the other, we saw a
completely symmetrical picture—just
as if we had taken a time exposure of a
swinging pendulum over an interval
much longer than one oscillation.
The overall efficiency of energy
transfer we measured for a sphere was
about 10 percent. But the acceptor image showed that this transfer only takes
place in a thin shell near the surface;
most donor molecules do not participate. Indeed, only one out of five are
involved, indicating that for these the
transfer efficiency is about 50 percent.
Noel Goddard, a master ’s student
working in my lab, was later able to obtain such an enhanced efficiency by embedding all donor molecules just under
the surface of the droplet. But even with
all donors in the right place, a key question remains: Why should they emit
photons so efficiently into the special directions needed for photonic-atom
modes? The answer requires a brief consideration of why an excited atom or
molecule emits light in the first place.
Much Ado About Nothing
When I took quantum mechanics as a
graduate student, I recall being told by
my professor that the excited states of
hydrogen are mathematically stationary. I realized that the atom should
emit a photon and so asked, “Does this
mean that if I put an excited hydrogen
atom in my pocket, go home and come
back tomorrow, it will still be excited?”
The instructor confirmed that it would
not, showing some discomfort about
where the discussion was headed. Still,
I pushed on: “How long, then?” He
said it was on the order of nanoseconds, prompting me to reply, “That’s
stationary?” My professor had little
more to offer except to remind me that
fluorescence
lowing the energy eventually to “leak”
out. The effects of diffraction grow as
the particle shrinks, and when the radius approaches the wavelength of
light passing through the interior, resonances are lost altogether.
With these considerations in mind,
we began to make sense of the observations and started to appreciate the
importance of the special electromagnetic modes of a tiny droplet. We called
them photonic-atom modes, because the
trajectories of the photons resemble the
orbits of electrons in atoms. It turns out
that the longest-lived modes have photons traveling along simple polygons
(Figure 6), whereas for the shorter-lived
modes the paths close after more than
one revolution around the interior. Using our data and the photonic-atom
picture, we estimated that before being
absorbed or leaking out, the photons
circulating within an acceptor-free
droplet of 10-micrometer radius cover
a total distance of about 0.6 meter—
30,000 times the width of the sphere!
Just as with an electron in a Bohr
atom, the photon in a microsphere, or
“photonic atom,” has a quantized amount
of orbital angular momentum. Photons
have momentum? Indeed they do. Einstein had shown long ago that a photon
has momentum proportional to its energy, which in turn is proportional to the
optical frequency. The orbital angular
momentum of a photon in a photonic
atom is just the momentum Einstein had
shown times the inner radius of the
polygon (which for grazing photons is
nearly the same as the particle radius).
So having modes separated by a constant increment of angular momentum
is equivalent to their being regularly
spaced in frequency—the spectral fence
pickets Lorcan had uncovered.
One can think of a given mode as a
wave that circumnavigates the interior
of the tiny sphere and returns in step
with the oscillations at its starting point.
The mode with the next higher value
of angular momentum has a frequency
increased by just the right amount to
squeeze an additional wavelength into
this circuit. Such properties of photonic
atoms seemed neatly to explain what
we were seeing in our experiments.
Steve Holler, an undergraduate student I was advising at the time, set
about to shore up the hypothesis by
taking pictures of a glowing microdroplet through discriminating color
filters that captured the light of donor
and acceptor emissions separately
540
550
560
wavelength (nanometers)
570
Figure 4. Spectrum of light emitted by acceptor molecules within a droplet reveals a series of
peaks, corresponding to resonant electromagnetic modes. The peaks shown represent modes
of three families (pink, blue and yellow) with different polarizations or radial order. For each
family, the peaks are regularly spaced, which reflects the quantization of angular momentum
for the photons circulating within.
Copyright © 2001 American Scientist
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Steve Thomton/Corbis
Figure 5. Light rays refract as they pass from air into water or vice versa. Beyond a critical angle, the light one sees while viewing the surface of
a pool from below comes from total internal reflection (pink line, left). The outside of the pool is visible only within a circle overhead; elsewhere
one sees reflections from within the pool (right).
we had already done calculations on
stimulated emission.
In stimulated emission, a passing
photon induces an excited electron to
return to the ground state. The energy
of the triggering photon must match the
difference between these two levels, and
the emission produces a second, identical photon. Although this process
makes lasers possible, it is not nearly as
interesting to me as spontaneous emission, which provides almost all of the
light we see. The sun, incandescent
bulbs, fluorescent lamps, even fireflies
shine without requiring external photons to trigger their emissions. Einstein
used thermodynamic arguments to
show the need for both stimulated and
spontaneous emission, but he was not
able to offer an explanation for the latter. Nor could the mechanism be obtained from the quantum mechanics of
atoms. The answer appeared only after 1948, when physicists began to appreciate that the electromagnetic field
of empty space is quantized.
As high school students, we were
taught to think that if we removed all
the atoms, molecules and photons from
a vessel, it would then contain nothing.
But this notion is incorrect. Even in a
cold enclosure held near absolute zero,
an excited atom is bathed in electromagnetic fluctuations, which are particularly
intense when the atom’s emission wavelength matches a resonant mode. They
are the result of an electromagnetic
Heisenberg uncertainty principle: The
product of the electric and magnetic
fields of a mode has a fixed minimum—
both fields cannot simultaneously be
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zero. Even in a completely dark, empty
enclosure, each electromagnetic mode
will have a residual “zero-point energy,”
equal to a constant times its frequency.
The time-varying fields associated
with zero-point energy trigger spontaneous emission, which in this sense is
quite similar to stimulated emission, although the source of stimulation is
rather subtle. Such quantum fluctuations also apply pressure, as the Dutch
physicist H. B. G. Casimir realized
decades ago. Two neutral metallic plates,
for example, attract each other with a
force that increases as their separation
diminishes. A simple explanation is
that the limited space between the
plates supports fewer electromagnetic
modes than the surrounding regions,
giving rise to a net inward push.
The universe is thought to have an
infinite number of electromagnetic
modes and a zero-point energy density
that increases continuously with frequency. Within an enclosure, the spectrum of the energy density becomes
bunched around discrete frequencies,
each one associated with a particular
mode. As the container is reduced in
size, a mode of a given frequency occupies a smaller volume and consequently has a higher zero-point energy density. If the physical enclosure measures
just a few wavelengths on a side, the
zero-point energy densities within a
mode can exceed the energy densities
of free space by orders of magnitude.
As a result, an excited molecule, which
would otherwise radiate over a broad
band of frequencies, is easily induced
to emit photons into such a mode—so
Copyright © 2001 American Scientist
long as the emission band contains the
frequency required for that mode.
Although the tiny droplets my students and I were studying are not
evacuated cavities per se, they confine
photons within modes in a small region and thus act in essentially the
same way. So it is not surprising that
the physics of nothingness applies to
them as well. Indeed, detailed calculations confirm that enhanced quantum
fluctuations within the photonic-atom
modes account for the efficiency of the
energy transfer we observed. Consistent with this, we find experimentally
that the energy-transfer efficiency is increased even further as the particle size
is reduced to a diameter of 5 microme-
Figure 6. Light rays skimming at a shallow
angle below the surface of a spherical droplet
are subject to total internal reflection. In the
example shown here, a photon completes a
revolution of the sphere with eight bounces
and follows an octagonal trajectory. A typical
photon might orbit in this way many thousands of times within a microscopic droplet
before it is absorbed or “leaks” out.
a
Figure 7. Light emitted from donor molecules
within a droplet shows a distinct asymmetry
(a), because the laser beam that excites them is
focused at the left (b). Light emitted from the
acceptor molecules reveals a ringlike pattern
(c), which demonstrates that they are excited
only near the surface of the droplet. Most of
the light emitted by donor molecules escapes
before intercepting an acceptor. Only photons
in a resonant mode, which circulate just beneath the surface (d), remain within the
droplet long enough to have an appreciable
chance of colliding with acceptor molecules.
Many ringlike orbits are possible, but they all
come together at two antipodal points (e), corresponding to the two diametrically opposed
bright spots in the acceptor image (c). (Images
a and c courtesy of the author.)
b
intensity
c
ters. This is a result of the so-called cavity quantum electrodynamic effect.
An important application of this effect involves miniature lasers. Everyone
is familiar with these semiconductor
devices, which are found in laser pointers, laser printers and compact-disc
players. Indeed examples are now so
commonplace that many people forget
what the word laser means: light amplification by stimulated emission of radiation. A fair fraction of the power fed
into semiconductor lasers is wasted because there is a threshold current needed to start the lasing process. But power
losses associated with this threshold
can be much diminished by taking advantage of the cavity quantum electrodynamic effect. How? By making small
lasers even smaller.
Laser action always begins with
spontaneous emission. But in traditional devices, only a tiny fraction of the
spontaneously generated photons—
fewer than one in a thousand—go into a
lasing mode. Thus considerable power
is needed to ensure that an adequate
number of these photons will be available to start the laser going. Here is
where shrinking dimensions and the
cavity quantum electrodynamic effect
d
e
help out. Reducing the size of the laser
cavity limits the number of available
modes. In principle, the cavity can be
made small enough that the emission
spectrum of the laser material will overlap just one mode. The tiny size of the
cavity then provides enhanced quantum fluctuations and rapid emission
into this mode. Thus very little power is
needed to start the laser working.
Recently investigators at several laboratories have used the confinement of
photons in small spherical particles to
create such low-threshold lasers (Figure
8). This elegant approach does, however,
have its limits, because mode confinement is lost as the size of the sphere approaches the wavelength of the light. So
it may not be possible to eliminate the
threshold current completely. Still, these
microspheres make quite efficient lasers.
Microspheres also provide optical
filters of exceptional spectral purity, because they hold the energy of a photon
over a long time in comparison with
the period of one oscillation in the corresponding light wave. A tiny sphere is
thus something like a fine wineglass,
which tapped with a spoon will sound
an extended note, although each acoustic
oscillation lasts just a millisecond or so.
Copyright © 2001 American Scientist
A correlate of this property is that such
a wineglass responds to external stimulation over a narrow range of frequency—so narrow that it takes an Ella
Fitzgerald to sing just the right pitch
and shatter the glass. Microspheres,
too, respond to excitation over a slim
range of frequencies.
For quantum optical systems, this
basic physical notion is contained
within another Heisenberg uncertainty
principle. In a refined form, this relationship says that for a given mode, the
product of the frequency width and the
lifetime of a photon has a minimum
value, 1⁄2 π. We determined from our energy-transfer experiments that a photon typically bounces around the interior of a 10-micrometer microsphere
for about 3 nanoseconds before it is absorbed by the material or flies out.
Consequently, the corresponding minimum frequency width is around 50
megahertz. Although this may seem
like a lot of bandwidth to a radio engineer, for optical communication it is really quite small. To understand just
how narrow 50 megahertz is in this
context, consider using such spheres to
sort transmissions carried by light over
an optical fiber.
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419
How would one do something like
this? The ideal way would be to use
multiple microspheres of slightly different sizes, so that each one would
serve to select a particular band of information. The number of separate
channels available depends on distinguishing one resonance on a given
sphere from the adjacent modes (those
having the next higher or lower allowed value of angular momentum).
For a sphere that is about 10 micrometers in radius, these modes are 3,000 gigahertz apart. So in principle one could
distinguish (3 × 10 12)/(5 × 10 7) or
60,000 different channels. In practice,
distortion of the sphere would probably reduce the number of channels to
something closer to 10,000, which
would still constitute a technical tour
de force (although I suspect that if this
scheme were ever employed to select
from among television transmissions,
there might still be nothing to watch).
In 1995, two members of my research group—Ali Serpenguzel and
Giora Griffel—and I reported a means
for sorting optical signals in just this
way. The technique involves another
quantum-mechanical principle called
tunneling. When a microsphere—made,
say, of glass or plastic—is placed within a wavelength of the core of an optical fiber, a photon traveling through
the fiber has a fair probability of exciting a photonic-atom mode in the
sphere, so long as the frequency of this
photon corresponds to that of the
mode. Physicists say the photon tunnels resonantly across this gap.
Photons circumnavigating the sphere
can also tunnel into the fiber, if they do
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American Scientist, Volume 89
cladding
atom mode. Such dips in transmission
arise in part because leakage and absorption in the sphere reduce the probability of the photon returning to the
fiber. But this simple picture of photons
jumping between sphere and fiber neglects the wave character of light and
thus misses an important physical effect: the destructive interference that
can take place when light reenters the
fiber after having made a relatively
long visit to the sphere. Griffel and I
first described this phenomenon in
1996. More recently, another group of
investigators led by Kerry Vahala at the
California Institute of Technology obtained nearly 100 percent efficiency for
the coupling of light from an optical
fiber to a microsphere. With their experimental arrangement, there is hardly any transmission at all through the
fiber at resonance.
Many Points of Light
Our experiments with tiny plastic
spheres and optical fibers helped
launch a field that is best described by
the phrase “microsphere photonics.”
The centers of work in this area are
now at Caltech and MIT, and many
microsphere
water
core
laser
intensity
Figure 8. Microsphere laser glows brightly in a
ring, where most of the photons circulate. This
geometry arises because an optical fiber in the
plane of the ring touches the sphere at one
point along the equator. Photons sent through
the fiber pass into the sphere, exciting laser action for this particular electromagnetic mode.
(Image courtesy of Kerry Vahala, Caltech.)
not just leak out first. Although such
leakage might seem a bad thing, in fact it
is very convenient, because it provides
information about what is going on inside the microsphere. Indeed, we first
investigated the phenomenon of tunneling by observing light leaking out of a
polystyrene microsphere (Figure 9). We
had placed the tiny plastic sphere on an
optical fiber that was polished so as to
eliminate all but the slimmest sliver of
cladding. We then shined a tunable laser
into the fiber and varied the frequency
of light while viewing the microsphere
at right angles. At most frequencies the
scene was completely dark, except for
some faint glints of light scattering from
the polished surface of the fiber, but
when the frequency of the laser matched
a photonic-atom mode, the sphere lit
up, thanks to leaking photons.
Within a month of our publishing an
article about this work, physicists at
the École normale supérieure in Paris
saw a different sort of evidence for the
tunneling of photons between an optical fiber and a microsphere. They measured a diminution in the light transmitted through the fiber when the
frequency was set equal to a photonic-
598
600
602
604
606
608
wavelength (nanometers)
610
612
614
Figure 9. Microsphere placed within one wavelength of the core of an optical fiber readily absorbs photons when the frequency of light passing through the fiber matches a resonant mode
of the sphere (top). At resonance, photons enter the sphere and eventually leak out in all directions, causing distinct peaks in the intensity of light seen coming from the sphere as a
function of the wavelength of laser excitation (bottom).
Copyright © 2001 American Scientist
drop
add
input
output
Figure 10. Microsphere coupled to two optical fibers (left) constitutes an add-drop filter. An optical signal sent toward the sphere along one fiber
(green arrow, upper right) is added to the many signals traveling along the other fiber (blue arrows). This device can also be used to extract (or
“drop”) a signal (red arrow, upper left) that was originally traveling through a fiber along with many others. (Image courtesy of Kerry Vahala.)
applications have emerged. One of the
most intriguing came from Vahala’s
laboratory at Caltech, where workers
induced photons of a specific frequency to tunnel resonantly into a sphere
and then into a second fiber. This photonic device constitutes what is called
an add-drop filter, because it can be used
either to add or to remove the signal
carried on a given optical channel (Figure 10). With it, one can route information between optical fibers at select frequencies, without having to employ
electronic circuitry at all.
Plastic microspheres can also serve
as sensors, because their photonic-atom
modes change in frequency when the
temperature varies or when they come
in contact with material of similar optical properties—for example, DNA molecules attached to their surfaces—a
possibility I am now investigating with
my colleague Iwao Teraoka and with
Frank Vollmer of Rockefeller University. The idea here is to affix to a microsphere many strands of DNA carrying
one particular base sequence. Genetic
material having the complementary sequence can readily bind to the surface
of such a sphere. When it does, the
added coating (like the expansion that
accompanies heating) alters the effective radius of the particle, which forces
the resonant frequency of a given mode
to shift. Other kinds of oscillators are
sensitive to changes in dimension as
well. For instance, the frequency of a
pendulum changes when the rod connecting the bob to the pivot expands
thermally. But pendula do not make
very good thermometers, so it might
come as something of a surprise that
microspheres would have high sensitivity. They do, at least in principle.
The constancy of angular momentum for a particular mode dictates that
the fractional decrease in frequency
must be the same as the fractional increase in dimension, and vice versa.
The minimum size change that can be
detected is the smallest measurable
fractional change in frequency times
the radius of the sphere. One can easily
observe a full line-width shift, some 50
megahertz, which at a typical optical
frequency corresponds to a fractional
change in size of one part in 10 million.
So one can potentially discern a change
in radius for a 10-micrometer sphere of
10 –12 meters—one-hundredth of an
atomic diameter. This exquisitely high
sensitivity opens the door for a range
of applications, from thermometry to
biochemical sensing, for which multiple probes specific enough to detect the
activity of particular genes could be interrogated over a single optical fiber
using spheres of different sizes.
Thinking of tiny spheres as photonic
atoms indeed appears to be a fruitful
endeavor, but one can go a step further
and push the analogy into the molecular realm. Just recently, scientists working with Makoto Kuwata-Gonokami at
the University of Tokyo built the photonic equivalent of a hydrogen molecule. In H2, two protons share two electrons in a covalent bond, which gives
rise to a splitting of the atomic electronic states. Gonokami and his research collaborators touched two nearly identical fluorescent microspheres
together and recorded a similar splitting in photonic-atom modes. Appropriately, the researchers dubbed their
creation a photonic molecule.
Physicists continue to search for
clever ways to benefit from the optical
properties of tiny spheres. One can, for
example, imagine that grouping more
than two spheres together will offer yet
more interesting or useful optical
modes. Photonic “polymers” might
even be in the future. Whatever further
Copyright © 2001 American Scientist
advances grow out of this research, I
take great pleasure in remembering the
experiments of 1980s that led me into
the field of microsphere photonics in
the first place—attempts to probe how
energy is transferred between molecules without photons. In that quest I
failed, which I realize now was the best
thing I could have hoped for.
Bibliography
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Cai, Ming, Guido Hunziker and Kerry Vahala.
1999. Fiber-optic add–drop device based on
a silica microsphere-whispering gallery
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Arnold, S., S. Holler and S. D. Druger. 1996.
Imaging enhanced energy transfer in a levitated aerosol particle. Journal of Chemical
Physics 104:7741–7748.
Folan, L. M., S. Arnold and S. D. Druger. 1985.
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Mukaiyama, T., K. Takeda, H. Miyazaki, Y. Jimba and M. Kuwata-Gonokami. 1999. Tight
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Serpenguzel, A., S. Arnold and G. Griffel. 1995.
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Links to Internet resources for
“Microspheres, Photonic Atoms and the
Physics of Nothing” are available on the
American Scientist Web site:
http://www.americanscientist.org/
articles/01articles/arnold.html
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