FORGET everything you know about lasers. Forget the pure colours of their
light, forget the explosion of white sparks as their beams carve through steel,
forget the light shows, death rays and the thousand other uses. Just lift the
lid and focus on what lies at a laser鈥檚 heart鈥攁 light amplifier.
Kick-start the machine with a blast of light and watch a small bunch of photons
tumble back and forth in the cavity between two mirrors. As they bounce back and
forth, their numbers swell, the trickle becomes a flood and the cavity fills
with light. Let part of this seething avalanche of photons escape and you have a
laser beam. But this cavity can also serve another purpose鈥攊t鈥檚 the
perfect place to discover the secrets of infinitesimal concentrations of
chemicals.
In the early 1970s, Soviet researchers studying minute amounts of ammonia by
bouncing laser beams through the gas realised that if they turned the experiment
inside out鈥攑utting the ammonia inside the laser rather than
outside鈥攖hey could create a technique more than ten million times as
sensitive as conventional methods. But it was the height of the Cold War. Soviet
journals were often inaccessible to Western researchers, and the idea was slow
to spread beyond the Iron Curtain. And when it did reach the West, it was all
but ignored. 鈥淚 think American researchers didn鈥檛 trust the technique,鈥� says
Frederic Stoeckel, from the Laboratory of Physical Spectrometry at Joseph
Fourier University in Grenoble, France. 鈥淎t that time it was more of a
肠耻谤颈辞蝉颈迟测.鈥�
Picking out the chemical equivalent of a needle in a haystack is a challenge
that scientists often face. For example, designing energy-efficient engines that
could reduce air pollution relies on understanding the complex chemistry of
combustion. It鈥檚 no easy task. How do you study the tiny populations of
transient molecules created and destroyed in the violent heat of a flame?
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Faintest signals
Understanding the chemistry of freezing gases on faraway planets presents
similar problems. The composition of light is altered as it passes through alien
atmospheres on its way to Earth, but unless astronomers know how light is
affected by different gases in various conditions, they cannot interpret what
they see. The answer is to follow the Russian lead: move the sample inside the
laser, where even the faintest signals will be amplified until they are easily
detected.
And the technique isn鈥檛 just useful to chemists and astronomers鈥攊t
could even save lives. Using a tiny semiconductor laser, researchers in the US
have built a revolutionary sensor that can analyse blood for poisons, spot
sickle-cell anaemia and detect cancer cells in a fraction of the time other
types of sensors take.
The Soviet method is based on one of the first techniques that chemists turn
to in their hunt for the identity of an unknown substance鈥攁bsorption
spectroscopy. Take light containing a broad spectrum of wavelengths, shine it
through a sample and measure which wavelengths the sample absorbs. The electrons
of every element or compound have a unique set of energy levels, and so will
absorb photons of specific energies or wavelengths. If light that has passed
through the sample is spread out into a spectrum, one or more black
lines鈥攎issing wavelengths鈥攁re visible. These provide a fingerprint
from which the sample can easily be identified.
But what if there just isn鈥檛 enough of the mystery chemical to give a clearly
identifiable fingerprint? What the Soviet scientists realised is that the
amplifiers responsible for laser beams could also be used to amplify extremely
faint signals.
Lasers consist of two components: a material in which light amplification
takes place and a pair of mirrors. The material鈥攃alled the 鈥済ain
medium鈥濃�攎ight be argon gas, a single crystal of yttrium aluminium garnet
or a slice of semiconductor such as gallium aluminium arsenide, and the two
mirrors sit on either side of this gain medium to form the laser 鈥渃avity鈥�. To
generate laser light, a powerful flash of light or blast of current is used to
excite the electrons in the gain medium. Random photons of light bouncing about
in the gain medium stimulate these excited electrons to release their excess
energy as photons of light. Since the gain medium sits between two mirrors, the
amplified light reflects backwards and forwards millions of times a second, and
each time the photons passes through the gain medium, they stimulate even more
electrons to give off their energy as light, forming a laser beam (鈥淢aking light
work鈥�, Inside Science, 17 June 1989).
Most lasers are designed to generate light at a single wavelength. These are
known as single mode lasers and produce a beam that contains one pure colour.
But a laser cavity is like a guitar string in that it can support many
frequencies simultaneously. Pluck a guitar鈥檚 E string and you hear not only the
single vibration corresponding to E, but the higher harmonics that are
vibrating, too. Lasers that are designed to support and amplify more than one
frequency are called 鈥渕ultimode鈥�, and their light contains a broader spectrum of
wavelengths than the light from a single mode laser.
If a few molecules of a gas are placed inside the cavity of a multimode
laser鈥攊n or alongside the gain medium, for example鈥攖he spectrum
changes. Molecules in the gas absorb light at just a few discrete wavelengths.
Inside the laser, the gas dampens down these wavelengths, so that the spectrum
of the laser light becomes streaked with faint dark lines. If the laser cavity
is one metre long, the light will bounce through the gas 300 times in a
millionth of a second. As the light is absorbed by the gas and then amplified on
each pass, the lines in the spectrum grow darker too. This is the secret of the
highly sensitive technique, called intracavity laser absorption spectroscopy
(ICLAS).
With conventional absorption spectroscopy, light will only bounce through the
sample a few times, giving an absorption length鈥攖he amount of sample
through which the laser passes鈥攐f just a few metres at best. But with
ICLAS, it is possible to get an equivalent absorption length of 300 kilometres,
according to Valery Baev, of the Institute for Laser Physics at the University
of Hamburg. 鈥淭he highest experimentally demonstrated absorption length in our
laboratory equals 70 000 kilometres鈥�, says Baev. By probing so much sample, weak
absorptions that would normally be invisible against the background are seen
easily. But if the light is trapped inside the laser, how is its spectrum
measured?
In theory, this isn鈥檛 difficult. One of the laser鈥檚 mirrors is designed to
let a little of the beam leak out and the beam鈥檚 spectrum can then be recorded.
But in practice, the technique relies on switching the laser beam off after a
few tens of microseconds. This gives the sample a chance to 鈥渋mprint鈥� its
spectrum onto the laser light, and for the whole pattern to be amplified. A
sensitive CCD camera is used to record the brief burst of laser light.
Stoeckel was among the first to use the intracavity method for measuring weak
absorption signals. In the early 1980s, he visited the Lebedev Physics Institute
in Moscow, where much of the early research was carried out, and returned to
Grenoble to put his new knowledge to use. He is now working with Sergey Cheskis,
of Tel Aviv University in Israel, to unlock the secrets of turbulent flames. The
quest to build engines that generate less pollution and use less fuel
requires an understanding of the chemistry of flames. So Cheskis uses ICLAS to
measure the concentration of molecules such as HCO radicals that form as methane
burns in air.
鈥淪uch information is very important for creating reliable chemical mechanisms
of combustion,鈥� says Cheskis. He has built a laser cavity with a combustion
chamber inside it, an arrangement that gives him the equivalent of a
kilometre-long flame, allowing him to pick out even the weakest absorption
signals. Because it is so sensitive, ICLAS can reveal details in the spectrum
that identify how fast molecular fragments in the flame rotate, providing a kind
of a flame thermometer. 鈥淭he rates of chemical reactions depend strongly on
temperature. So the temperature profile is very important for modelling
combustion and flames,鈥� says Cheskis.
ICLAS is also helping researchers to understand the inhospitable environments
of distant planets by compressing hundreds of kilometres of alien atmosphere
into the small space inside a laser system. Much of what we know about the
chemistry of other planets in the Solar System is gleaned from sunlight
reflected back to Earth-bound telescopes. In a NASA-funded study, James O鈥橞rien,
from the University of Missouri in St Louis, aims to create laboratory spectra
that will help in the interpretation of these reflected spectra. To do this, he
measures spectra at the low pressures and temperatures that are thought to occur
on these planets. In the reflected sunlight, the spectra of gases such as
methane are visible only because the sunlight passes through hundreds of
kilometres of atmosphere on its way to Earth. And the only way that O鈥橞rien can
reproduce this effect on Earth is with ICLAS.
鈥淭his technique gives a tremendous enhancement to the signal,鈥� he says. By
comparing his laboratory-generated signals with the real ones, O鈥橞rien hopes to
confirm what sorts of gases are out there, and in what sort of quantities. In
particular, he is looking for methane on Neptune and Uranus. Methane is
converted into heavier hydrocarbons by the action of ultraviolet light from the
Sun. Its presence hints at the existence of fascinating chemistry that might
resemble that of Earth鈥檚 young atmosphere.
But perhaps the most exciting use that the intracavity technique has found is
in the 鈥渂iocavity laser鈥�. Paul Gourley, at Sandia National Laboratories in
Albuquerque, New Mexico, has dispensed with large benchtop lasers. Instead, he
has designed and built a sensor inside a microscopic vertical-cavity
surface-emitting laser (VCSEL)鈥攁 semiconductor laser about the size of a
grain of salt (鈥淗ow chips build better lasers鈥�, 快猫短视频, 11
January 1992, p 25).
Gourley has built and patented his sensor with biological and medical uses in
mind鈥攊t is designed to analyse living cells. The tiny laser contains a
glass layer into which minute channels are etched, each one about a tenth of the
thickness of a human hair across. When cells flow through these microgrooves,
they pass between the gain medium and the upper mirror
(see Diagram). Here, each
cell becomes part of the laser generation process, acting like a tiny lens and
refracting the light inside the cavity. Changing the path of the light in the
cavity changes the cavity length and the spectrum of the laser light. 鈥淐ells are
just the right size, transparency and refractivity to modify the laser
frequencies鈥�, says Gourley. Changes in the spectrum of the light emitted by the
laser can be spotted with a detector mounted above the device.
What makes the biocavity laser so potentially useful is that abnormal cells
modify laser light differently from healthy cells. The technique can distinguish
the telltale signs of blood cells affected by sickle-cell anaemia from healthy
rounded cells, for example. Gourley, who developed the biocavity laser with the
help of his brother Mark, a doctor at the Washington Hospital Center in
Washington DC, has also seen differences between the spectra of healthy and
cancerous cells, he says. 鈥淚f no cell is cancerous, we get a standard light
signal. A cancerous cell gives a bright flash at different wavelengths,鈥� he
says.
Fast screen tests
Gourley thinks the technique could have wide-ranging medical applications,
from assessing pap smears used to screen for cervical cancer to counting rare
cells and portable blood analysis. The sensor could dispense with the
time-consuming staining and mounting required by microscope-based tests, even
following changes in living cells. Its potential for analysing samples in
minutes, rather than hours or days, is attracting considerable interest from
pharmaceuticals companies.
Miniaturisation gives the biocavity laser other important advantages over
more conventional diagnosis techniques. Since the VCSEL devices are made using
conventional semiconductor technology, they are cheap to produce. A small
handheld device might cost only $5000, although a laboratory set-up would
probably be more like $70 000. It should also be easy to combine these
detectors with other instruments. 鈥淭he spectra could be piped through fibre
optics directly to high-speed computers,鈥� says Gourley.
Another advantage lies in its high sensitivity. 鈥淚t samples each cell
thousands of times,鈥� he says. Gourley predicts that his sensor should be able to
process cells at amazing speeds. Millions of blood cells, for example, could be
analysed in just a few minutes using a sensing device about the size of a
thumbnail, containing hundreds of biocavity lasers. 鈥淚t could even be used for
sequencing DNA,鈥� says Gourley. 鈥淭he applications are really exciting.鈥�
Stoeckel, too, has every reason to feel satisfied. Considering that the
intracavity technique got off to such a slow start, it may, at last, have come
in from the cold.
PETER Toschek and his colleague Valery Baev at the University of Hamburg have
used ICLAS to develop a portable but highly sensitive pollution detector. The
main challenge they faced was to dispense with the large laser used in most
laboratories. To miniaturise the cavity, Toschek has replaced the laser with a
20-centimetre-long laser, made from fluorozirconate optical fibre doped with
praseodymium and ytterbium. These elements act as the laser鈥檚 gain medium. The
power for this device comes from a tiny semiconductor laser, which generates
infrared light at 850 nanometres. This enters the fibre at one end through a
specially designed reflective coating that transmits infrared light but reflects
light of shorter wavelengths. At the other end of the fibre is the sample
chamber, beyond which is another mirror
(see Diagram). When the infrared light
excites the dopants, they emit visible light. This light is reflected by the
mirrors at the end of the cavity, and the fibre behaves as a laser. With gas
present in the sample chamber, a light-sensitive detector placed behind one of
the mirrors picks up the amplified absorption spectrum.
