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Nanolasers Nanolasers
Semiconductor lasers have shrunk to dimensions even smaller
than the wavelength of the light they emit. In that realm, quantum
behavior takes over, enabling more efcient and faster devices
by Paul L. Gourley
MICRODISK LASERS are each only a couple of microns, or millionths of a meter,
in diameter and just a fraction of a micron thick. The disks are made of semicon-
ductor material and are supported by pedestals. Light is generated within the disk
and skims along its circumference before escaping radially, as shown by the red
wave pattern in the computer simulation (
inset at right). The depressions in the
center of the disks and the tiny, random particles are artifacts of the chemical etch-
ing process used to fabricate the structures.
56
Scientific American
March 1998
Copyright 1998 Scientific American, Inc. Scientific American
March 1998 57
F
or decades, silicon transistors have become smaller
and smaller, allowing the fabrication of tiny but pow-
erful chips. Less well known is the parallel revolution
of semiconductor lasers. Recently researchers have shrunk
some of the dimensions of such devices to an astonishing
scale of nanometers (billionths of meters), even smaller than
the wavelength of the light they produce. At such sizes less
than one hundredth the thickness of a human hair curious
aspects of quantum physics begin to take over. By exploiting
this quantum behavior, researchers can tailor the basic char-
acteristics of the devices to achieve even greater efciencies
and faster speeds.
Nanolasers could have myriad applications, for instance,
in optical computers, where light would replace electricity for
transporting, processing and storing information. Even though
light-based computing may not occur anytime soon, other
uses, such as in ber-optic communications, have now be-
come increasingly practical. With other researchers, I am also
investigating the new lasers for novel purposes, such as the
early detection of disease.
Jumping Electrons
A
lthough nanolasers push the boundaries of modern phys-
ics, the devices work much like their earliest ancestor, a
contraption fashioned from a rod of dark ruby more than 35
years ago. Essentially, a lasing material for example, a gas
such as helium or neon, or a crystalline semiconductor is
sandwiched between two mirrors. The substance is pumped
with light or electricity. The process excites the electrons in
the material to hop from lower to higher energy levels. When
the electrons return to the lower stations, they produce light,
DA
VID SCHARF
; SAM M
C
C
ALL (
inset
)
Copyright 1998 Scientific American, Inc. which is reected between the mirrors.
The bouncing photons trigger other
excited electrons those in higher en-
ergy states to emit identical photons,
much like recrackers that pop and set
off other recrackers. This chain reac-
tion is called stimulated emission. (Hence
the name laser, which is an acronym
for light amplication by stimulated
emission of radiation.) As the number
of photons grows, they become part of
a communal wave that intensies, nal-
ly bursting through one of the mirrors
in a concentrated, focused beam.
But not all the photons take part in
this wave. In fact, many are emitted
spontaneously, apart from the chain re-
action. In a large space to a subatomic
particle, the size of a typical laser cavity
is immense photons are relatively free
to do what they want. Thus, many of
the free-spirited photons are literally on
a different wavelength, and they can
scatter in all directions, often hitting the
sides of the laser and generating un-
wanted heat instead of bouncing be-
tween the mirrors. For some types of la-
sers, only one photon in 10,000 is useful.
Because of this enormous waste, a
certain threshold of energy is necessary
to ensure that the number of excited
electrons is large enough to induce and
maintain stimulated emission. The re-
quirement is analogous to the mini-
mum amount of heat needed to bring a
pot of water to boil. If the hurdle is not
cleared, the laser will fail to attain the
self-sustaining chain reaction crucial to
its operation. This obstacle is why semi-
conductor lasers have required relative-
ly high currents to work, in contrast to
silicon transistors, which are much more
frugal. But if semiconductor lasers could
stop squandering energy, they could be-
come competitive with their electronic
counterparts for a host of applications,
including their use in computers.
Recently the concept of threshold-
less operation has become increasingly
favored by many physicists. Proposed
by Yoshihisa Yamamoto of NTT Basic
Research Laboratories and Stanford
University and Takeshi Kobayashi of
Osaka University in Japan, threshold-
less operation calls for all photons, even
those spontaneously born, to be drafted
into lasing duty. In theory, the device
would require only the tiniest amount
of energy, almost like a special kettle that
could boil water with the heat of just a
single match. Researchers disagree about
the best design of such a laser. The con-
sensus, though, is that the dimensions
must be extraordinarily small on the
order of the wavelength of light emit-
ted so that the devices could take ad-
vantage of quantum behavior.
A New Generation
T
he groundwork for thresholdless
operation was set in the late 1970s,
when Kenichi Iga and other researchers
at the Tokyo Institute of Technology
demonstrated a radically different type
of semiconductor laser [see Microla-
sers, by J. L. Jewell, J. P. Harbison and
A. Scherer; Scientic American, No-
vember 1991]. Popularly referred to as
microlasers because of their micron-size
dimensions, these devices are cousins to
the semiconductor diode lasers widely
found in compact-disc players. (Diode
refers to a one-way ow of electricity
during operation.)
Microlasers, however, differ from their
common diode relatives in several fun-
damental ways. The latter are shaped
like rectangular boxes that must be
cleaved, or diced, from a large wafer,
and they issue light longitudinally from
the cut edges. Microlasers are smaller,
cylindrical shapes formed by etching,
and they emit light from the top per-
pendicular to the round layers of semi-
conductor material that make up the
device. Therefore, microlasers produce
more perfectly circular beams. In addi-
tion, they can be built and tested many
at a time in arrays on a wafer, similar to
the way in which computer chips are
fabricated. In contrast, diode lasers must
generally be tested individually after
having been diced into separate units.
Perhaps more important, microlasers
exploit the quantum behavior of both
electrons and photons. The devices are
built with a well an extremely thin
layer of semiconductor only several
atoms thick. In such a minute space,
electrons can exist only at certain dis-
crete, or quantized, energy levels sepa-
rated by forbidden territory, called the
band gap of the semiconductor. By sand-
wiching the quantum well with other
material, researchers can trap electrons
and force them to jump across the band
gap to emit just the right kind of light.
Microlasers must also imprison pho-
tons to function. To accomplish this feat,
engineers take advantage of the same
effect that causes a transparent window
to display a faint reection. This phe-
nomenon results from glass having a
higher refractive index than air that is,
photons move more slowly through
glass. When light passes between mate-
rials with different refractive indices,
some of the photons are reected at the
border. The mirrors of microlasers con-
sist of alternating layers of semiconduc-
tors with different refractive indices
(such as gallium arsenide and aluminum
arsenide). If the layers are just one quar-
ter of a wavelength thick, the geometry
of the structure will allow the weak re-
ections to reinforce one another. For
the coupling of gallium arsenide and
aluminum arsenide, a dozen pairs of lay-
ers will bounce back 99 percent of the
light a performance superior to that of
polished metal mirrors commonly found
in bathrooms.
Already the rst crop of microlasers
has found commercial applications in
ber-optic communications. Other uses
are currently under investigation [see
box on page 60]. Meanwhile ongoing
work continues to rene the structures.
In one recent device, certain layers are
selectively oxidized, which helps to raise
the population of excited electrons and
bouncing photons in the well area, re-
sulting in an operating efciency great-
er than 50 percent. In other words, the
laser is able to convert more than half
the input energy into output laser light.
This performance far exceeds that of
semiconductor diode lasers, which are
typically not even 30 percent efcient.
Microlasers have led to a new gener-
ation of devices that exploits electronic
quantum behavior further. Scientists
have now built structures such