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Introduction
In everyday experience those who wear corrective
eyeglasses will be aware that reflections at glass/air
interfaces can be annoying. For many years this level
of annoyance has been diminished by the
development of lens coatings which act as 1/4 wave
rectifiers. Deliberate modifications of glass/air or
lasing medium/air interfaces could have very
significant impacts on internal reflections at such
interfaces
and subsequent dispersion of light. Dispersion losses
are undesirable in lasers and optic fibres because
they diminish transmission and/or lasing. Such
diminution could be measured. In optic fibres the results
would be light losses. In lasers the results would be
significant or total losses in coherent light output. It
is
suggested that measurement of such losses could
afford very rapid detection of biological agents which
increase dispersion by binding to an agent-specific
coating on the medium/air interface.
Lasers
The basic plan for a laser has been unchanged
since its first demonstration. A lasing medium, gas, liquid,
or solid is excited by some form of incident energy.
A laser, by definition, emits light. Light is trapped inside
the lasing medium by reflection at mirror surfaces
(or by total internal reflection). If one mirror surface is
partially transparent, light escapes as a coherent
series of wavefronts - laser light.
Appropriate coatings on the emitting face could
serve to block or modulate coherent light output. Most
commercial microlasers are based upon transparent
semiconductor materials which react to electrical
excitation by emitting photons of a defined
wavelength corresponding to the energy potential of the
hole
structure. Typical commercial semiconductor lasers
are red, although green lasers are also available. Blue
lasers should be commercially available soon and have
been demonstrated in the laboratory.
By coating the emitting surface with a suitable
receptor structure it should be possible to make
microlasers
which are sensitive to specific agents which bind to
the receptor.
Already, the technology for specifically binding
receptor molecules and a variety of other agents to glass,
metal and plastic surfaces has been well
demonstrated.
By tailoring suitable carriers for receptor
molecules and their binding agents, it should prove quite
straightforward to engineer an interaction layer
which addresses the issue of light output/dispersion/
rectification. This should be an incredibly sensitive
detector. Before switching off there may well be an
abrupt drop in coherent light emission, possibly
associated with very minor changes in frequency. Such
changes might easily be detected by looking for the
"beat" frequency. However, quite simply the extinction
itself should be the most obvious and sensitive
probe.
It is of course conceivable that a 2 dimensional
array of such probes, each unit or group of units treated
for
a specific agent could constitute an easily usable
monitor for multiple challenges of both epidemiological or
other interest. Groups of units could provide
internal control and protection against false
positives/negatives.
By incorporating suitable photodetector technologies
these switchable lasers could become core components
of very sensitive detection tools.
Such tools would be extremely robust. With an
appropriate power supply and telecommunications unit they
could simply be dropped - or conceivably fired - into
an area of interest. Deployment in suitable arrays would
be straightforward and small size and simple
engineering would make these cheap throw-away devices.
The major drawback of these detectors is that
where the particle density in air is low, there is
statistically
little chance of a suitable number of particles
interacting with the small surface area constituted by the
substrate surface. This drawback can be offset by
assuring that a large volume of sampled air passes over
the surface.
The very major advantage of the successful
development of the system described is the capability of
exploiting
real time detection and identification of biological
agents of disease including living organisms, viruses and
toxins.
Fibre optics
The same coating technology approach could be
applied to a fibre optic detection system. I believe that this
also
has certain attractions. Glass fibres, kilometres in
length are used for fibre optic transmission. Such fibres
are
highly flexible and incredibly transparent. They are
usually confined within a cable for data transmission
purposes.
Dispersion losses arise because of microscopic
inhomogeneities in the fibre surface or because of short
range
variations in refractive index of the glass. In
reality the fibres are very fine, much less than hair thickness,
and
very flexible. They can be crushed, without breaking,
almost like fishing line. Everyone is familiar with glass
wool, which resembles cotton wool balls. A fibre
optic glass wool, like aerogel, is very light with a huge
surface
area. By treating the glass surface, an enormous
surface area for interacting with biological agents
becomes
available. When connected to a laser light
emitter at one end and a photon counter at the other
extremely
sensitive detection levels can be
established.
By having several fibres, emitters and several
corresponding detectors, a number of different agents could
be
screened, as with a the laser microarray
system.
In confidence some specific thoughts on these
possibilities can be exchanged with
Karl Simpson. Details on all the above ideas
can be
obtained directly from Bénézech -
Simpson.
We are delighted to develop our ideas in partnership with those
having an
established presence in the markets of relevance.