Microspheres, Photonic Atoms and the Physics of Nothing: Light can become trapped within tiny,...

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Microspheres, Photonic Atoms and the Physics of Nothing: Light can become trapped withintiny, transparent spheres. The surprising properties that result may turn "microspherephotonics" into an important new technologyAuthor(s): Stephen ArnoldSource: American Scientist, Vol. 89, No. 5 (SEPTEMBER-OCTOBER 2001), pp. 414-421Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/27857529 .

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Microspheres, Photonic Atoms

and the Physics of Nothing

Light can become trapped within tiny, transparent spheres. The surprising ptoperties that result may turn "microsphere photonics" into an important new technology

Stephen Arnold

The

transfer of energy between neigh boring molecules plays a pivotal

role in nature. In photosynthesis, for ex

ample, a plant fuels its metabolism and

growth with sunlight by taking advan

tage of a curious physical phenomenon that allows energy to hop from one

chlorophyll molecule to another situat ed about a half a nanometer away. A

couple of hundred chlorophyll mole cules 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 mech anism for transferring energy between

molecules, photosynthesis would large ly cease, and we would likely starve.

About 15 years ago I began to won der whether similar forms of energy transfer could influence photochem istry within aerosol particles. In partic ular, I wanted to know whether there are subtleties in the way energy is con

veyed between molecules in an isolat ed droplet about 10 micrometers in di ameter. To most physicists, the idea must have seemed crazy. After all, the

range of the longest substantial ex

change of this sort, as Nobelist Jean Perrin had discovered and Theodore F?rster had described in quantum mechanical 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 appoint

ments at Caltech, the University of Tokyo and the

?cole normale sup?rieure in Pans. 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]

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 mteresting 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 effi

ciency to optical filters of unprecedent ed purity and chemical probes of tiny size?to name just a few obvious appli cations. 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 illummating it with light from a properly tuned laser, which kicks ground-state electrons to a higher energy level Then you look for a trans fer 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 fa

miliar enough process called fluores

cence). If no energy passes between donors and acceptors, only the donor

molecules will fluoresce, giving off their own particular color. So the ratio of acceptor to donor fluorescence pro vides 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 accep tors and observes the spectrum of laser-induced fluorescence. The not-so

minor complication is that it is diffi cult 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 elec

trically charged particle indefinitely us

ing electrostatic force to balance gravi ty (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 appara tus superimposed an oscillating elec tric field on the constant l?vitation field, following the example of Wolf

gang 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 F?rster

exchange. So we did not expect any en

414 American Scientist, Volume 89



Figure 1. Charged droplet of glycerol floats motionlessly between two metallic electrodes as it is irradiated from the left with laser light The au

thor 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 in

vestigate 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 ra tio of acceptor to donor fluorescence

proved to be more than 10 percent. As Lorcan increased the concentra

tion of acceptor molecules in a series of

droplets under study, we became in

creasingly puzzled. Had the well known F?rster mechanism operated, the amount of energy transferred in such dilute solutions should have been

proportional to the concentration of ac

ceptors. But these experiments re vealed 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 ac

ceptors, we observed spikes that were as obvious as fence pickets (Figure 4). Such distinctive features had never ap peared in experiments that probed these

very same molecules in a centimeter

scale test tube. Although the explanation was not immediately obvious, these two

pieces of evidence were ultimately to re

vamp our view of how energy was be

ing passed between molecules. At the outset, we were planning to

probe the subtleties of F?rster transfer, whereby the energy in an excited elec tron shifts to another molecule without

ever generating a photon. How, one

might reasonably ask, does that hap pen? Such transfer takes place because an excited molecule behaves some

thing like a transmitting radio anten na. Close to this nanoscopic source, the

oscillating electric field is especially intense (although it drops off extreme

ly rapidly, with the cube of the dis

tance). This field can, in fact, be suffi

ciently strong to induce oscillation in the electron cloud of a nearby mole cule, and this coupling conveys energy if the acceptor sits close enough that the probability of transfer overwhelms the natural probability for the donor to

2001 September-October 415



to microscope camera and

spectrometer

Figure 2. Custom-designed "picopipette" generates one charged 10

micrometer 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 mi

croscope to which a spectrometer is coupled. It can be studied in this

way because it is levitated in air using three electrodes. A large, con

stant electric force (blue arrow) balances the downward force of grav

ity (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 atom

ic 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.)

fluoresce. Photons are not involved in such an exchange; that is to say, the donors do not have to give off electro

magnetic radiation. The F?rster process is thus akin to what happens to people

who mysteriously hear radio programs because the electric field of a nearby ra dio transmitter is so powerful that it in duces 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

100

S10 o 2 S c

1 . o 1 ?? o

o.H 10"8 10"7 io-6 io-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 F?rster 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 mole

cule to another within a levitated droplet.

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 look

ing 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 as

tronomically 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 F?rster 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 expla nation 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 electro

magnetic 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 wave

lengths along its length. The electro

magnetic modes of small particles? commonly known as Mie resonances or whispering-gallery modes?are

analogous and are normally described

by their wave character, too, in a way that takes full account of such compli cations as polarization and diffraction. However there is a more visceral de

scription of these modes that clearly underscores the ability of a photon to revisit a region many times. H. M.

Nussenzveig of the Universidade Fed eral do Rio de Janeiro realized that if the sphere is much larger than the

wavelength of light involved, whisper ing-gallery modes can be represented as geometrical orbits. With diffraction

stripped away, photons can be thought of as bouncing around inside the parti cle in well-defined trajectories, con fined by total internal reflection?the same phenomenon that makes the sur face 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 be cause rays mtting the surface from un derneath 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 spheri cal surface at a shallow angle, it just bounces back inside. A ricocheting photon thus remains within the parti cle for much longer than it would oth erwise. Ultimately the photon lifetime is limited by diffraction, which causes

photon trajectories to be less certain, al




lowing the energy eventually to "leak" out. The effects of diffraction grow as the particle shrinks, and when the ra dius approaches the wavelength of

light passing through the interior, reso nances are lost altogether.

With these considerations in mind, we began to make sense of the obser vations and started to appreciate the

importance of the special electromag netic 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 pho tons traveling along simple polygons (Figure 6), whereas for the shorter-lived

modes the paths close after more than one revolution around the interior. Us

ing 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. Ein stein had shown long ago that a photon has momentum proportional to its ener

gy, which in turn is proportional to the

optical frequency. Trie 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 con stant 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 osciUations 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 stu dent I was advising at the time, set about to shore up the hypothesis by taking pictures of a glowing micro

droplet through discriminating color filters that captured the light of donor and acceptor emissions separately

(Figure 7). His acceptor image boosted our confidence enormously, because

light was concentrated close to the sur

face, where the photons in a photonic atom circulate. Holler's donor image was less mteresting, with almost all of the interior aglow, but it displayed a distinctive asymmetry, which arose be cause 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 dis

sipated when we realized that donor molecules excited preferentially at the back of the droplet launch photons at

every "azimuth." Imagine a busy air

port 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 mi

crosphere sends photons on great-circle trajectories, which intersect at two an

tipodal points, seen as bright points in the image of acceptor fluorescence. Be cause our image averaged many revo

lutions 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 im

age showed that this transfer only takes

place in a thin shell near the surface; most donor molecules do not partici pate. Indeed, only one out of five are

involved, mdicating that for these the transfer efficiency is about 50 percent.

Noel Goddard, a master's student

working in my lab, was later able to ob tain such an enhanced efficiency by em

bedding all donor molecules just under the surface of the droplet. But even with all donors in the right place, a key ques tion remains: Why should they emit

photons so efficiently into the special di rections needed for photonic-atom

modes? The answer requires a brief con sideration 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 station

ary. 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 nanosec

onds, prompting me to reply, "That's

stationary?" My professor had little more to offer except to remind me that

c

? o 3

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.




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, identi cal photon. Although this process

makes lasers possible, it is not nearly as

mteresting to me as spontaneous emis

sion, which provides almost all of the

light we see. The sun, incandescent bulbs, fluorescent lamps, even fireflies shine without requiring external pho tons 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 lat ter. Nor could the mechanism be ob tained from the quantum mechanics of atoms. The answer appeared only af ter 1948, when physicists began to ap preciate 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 electromag netic fluctuations, which are particularly intense when the atom's emission wave

length 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

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 sponta neous emission, which in this sense is

quite similar to stimulated emission, al

though the source of stimulation is rather subtle. Such quantum fluctua tions 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 fre

quency. Within an enclosure, the spec trum 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 occu

pies a smaller volume and consequent ly has a higher zero-point energy den

sity. 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

long as the emission band contains the

frequency required for that mode.

Although the tiny droplets my stu dents and I were studying are not evacuated cavities per se, they confine

photons within modes in a small re

gion 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 calcula tions confirm that enhanced quantum fluctuations within the photonic-atom modes account for the efficiency of the

energy transfer we observed. Consis tent with this, we find experimentally that the energy-transfer efficiency is in creased 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 thou

sands of times within a microscopic droplet before it is absorbed or "leaks" out.




ters. This is a result of the so-called cav

ity quantum electrodynamic effect. An important application of this ef

fect involves miniature lasers. Everyone is familiar with these semiconductor

devices, which are found in laser point ers, laser printers and compact-disc players. Indeed examples are now so

commonplace that many people forget what the word laser means: light ampli fication by stimulated emission of radi ation. A fair fraction of the power fed into semiconductor lasers is wasted be cause there is a threshold current need ed to start the lasing process. But power losses associated with this threshold can be much diminished by taking ad

vantage of the cavity quantum electro

dynamic effect. How? By making small lasers even smaller.

Laser action always begins with

spontaneous emission. But in tradition al 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 avail able to start the laser going. Here is where shrinking dimensions and the

cavity quantum electrodynamic effect

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 over

lap just one mode. The tiny size of the

cavity then provides enhanced quan tum fluctuations and rapid emission into this mode. Thus very little power is needed to start the laser working.

Recently investigators at several labo ratories 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 confine ment is lost as the size of the sphere ap proaches 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, be cause they hold the energy of a photon over a long time in comparison with the period of one oscillation in the cor

responding 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.

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 be

neath 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), cor

responding to the two diametrically opposed

bright spots in the acceptor image (c). (Images a and c courtesy of the author.)

A correlate of this property is that such a wineglass responds to external stim ulation over a narrow range of fre

quency?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 rela

tionship says that for a given mode, the

product of the frequency width and the lifetime of a photon has a minimum

value, Vi . We determined from our en

ergy-transfer experiments that a pho ton typically bounces around the inte rior of a 10-micrometer microsphere for about 3 nanoseconds before it is ab sorbed by the material or flies out.

Consequently, the corresponding mini mum frequency width is around 50

megahertz. Although this may seem like a lot of bandwidth to a radio engi neer, for optical communication it is re

ally 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.




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 ac

tion for this particular electromagnetic mode.

(Image courtesy of Kerry Vahala, Caltech.)

How would one do something like this? The ideal way would be to use

multiple microspheres of slightly dif ferent sizes, so that each one would serve to select a particular band of in formation. The number of separate channels available depends on distin

guishing one resonance on a given sphere from the adjacent modes (those having the next higher or lower al lowed value of angular momentum). For a sphere that is about 10 microme ters in radius, these modes are 3,000 gi gahertz apart. So in principle one could

distinguish (3 1012)/(5 IO7) or 60,000 different channels. In practice, distortion of the sphere would proba bly 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 re search 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 with in a wavelength of the core of an opti cal fiber, a photon traveling through the fiber has a fair probability of excit

ing 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 tun nels resonantly across this gap.

Photons circumnavigating the sphere can also tunnel into the fiber, if they do

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 in side the microsphere. Indeed, we first

investigated the phenomenon of tunnel

ing 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 ?cole normale sup?rieure in Paris saw a different sort of evidence for the

tunneling of photons between an opti cal fiber and a microsphere. They mea sured a diminution in the light trans

mitted through the fiber when the

frequency was set equal to a photonic

atom mode. Such dips in transmission arise in part because leakage and ab

sorption in the sphere reduce the prob ability of the photon returning to the fiber. But this simple picture of photons jumping between sphere and fiber ne

glects the wave character of light and thus misses an important physical ef fect: 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 ob tained nearly 100 percent efficiency for the coupling of light from an optical fiber to a microsphere. With their ex

perimental arrangement, there is hard

ly 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

598 600 602 604 606 608 610 612 614 wavelength (nanometers)

Figure 9. Microsphere placed within one wavelength of the core of an optical fiber readily ab

sorbs 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 di

rections, causing distinct peaks in the intensity of light seen coming from the sphere as a

function of the wavelength of laser excitation (bottom).




^^^^^^^^?^^^ ^^^^^^^^^^^^s^^ ^^^^^^^^^^^^ 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 ?olue 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 frequen cy to tunnel resonantly into a sphere and then into a second fiber. This pho tonic 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 (Fig ure 10). With it, one can route informa tion between optical fibers at select fre

quencies, 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 opti cal properties?for example, DNA mol ecules attached to their surfaces?a

possibility I am now mvestigating with

my colleague Iwao Teraoka and with Frank Vollmer of Rockefeller Universi

ty. The idea here is to affix to a micro

sphere many strands of DNA carrying one particular base sequence. Genetic material having the complementary se

quence can readily bind to the surface of such a sphere. When it does, the added coating (like the expansion that

accompanies heating) alters the effec tive 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 con

necting 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 sensi

tivity. They do, at least in principle. The constancy of angular momen

tum for a particular mode dictates that the fractional decrease in frequency

must be the same as the fractional in crease 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 rnillion. 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 multi

ple probes specific enough to detect the

activity of particular genes could be in

terrogated over a single optical fiber

using spheres of different sizes.

TTiinking 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 molecu lar realm. Just recently, scientists work

ing with Makoto Kuwata-Gonokami at the University of Tokyo built the pho tonic equivalent of a hydrogen mole cule. In H2, two protons share two elec trons in a covalent bond, which gives rise to a splitting of the atomic elec tronic states. Gonokami and his re search collaborators touched two near

ly identical fluorescent microspheres together and recorded a similar split ting in photonic-atom modes. Appro priately, 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

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 mole cules without photons. In that quest I

failed, which I realize now was the best

thing I could have hoped for.

Bibliography Arnold, S., and L. M. Folan. 1989. Energy trans

fer and the photon lifetime within an

aerosol particle. Optics Letters 14:387-389.

Cai, Ming, Guido Hunziker and Kerry Vahala. 1999. Fiber-optic add-drop device based on a silica microsphere-whispering gallery

mode system. IEEE Photonics Technology Let ters 11:686-687.

Arnold, S., S. Holler and S. D. Druger. 1996.

Imaging enhanced energy transfer in a levi tated aerosol particle. Journal of Chemical

Physics 104:7741-7748.

Folan, L. M., S. Arnold and S. D. Druger. 1985. Enhanced energy transfer within a micro

particle. Chemical Physics Utters 118:322-327.

Haroche, Serge, and Jean-Michel Raimond. 1993. Cavity quantum electrodynamics. Sci

entific American 268(4):54-62.

Mukaiyama, T., . Takeda, . Miyazaki, Y. Jim

ba and M. Kuwata-Gonokami. 1999. Tight binding photonic-molecule modes of reso nant bispheres. Physical Review Letters 82:4623-4625.

Nussenzveig, H. M. 1992. Diffraction Effects in Semiclassical Scattering. New York: Cam

bridge University Press.

Serpenguzel, A., S. Arnold and G. Griffel. 1995. Excitation of morphological resonances

from individual microparticles and clusters in contact with an optical fiber. Optics Utters 20:654-656.

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.htrnl




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