by Profesor Aaron Lewis
The confinement of light in nanometric domains was only a dream when I first
considered that such confinement might be possible. The idea occurred to me
in 1979, while I was at Cornell University in the department of Applied Physics,
attending a seminar given by Professor Lance Taylor then of Harvard University.
Professor Taylor was trying to determine the distance between two protein receptors
confined to a cell membrane. The protein receptors were situated at a distance
that was smaller than the resolution limit of the microscope. The resolution
limit - which had been defined by Lord Rayleigh at the end of the 1800s - was
approximately 0.25 microns or 250 nanometers in x and y, and about five times
worse in the z direction. Essentially, Lance Taylor had reached the limit of
optical resolution and consequently was stymied in his research efforts to resolve
the two proteins on the cell membrane.
After this seminar I immediately drew a simple diagram that described an approach
to optics that would allow one to break this limit in Physics. This diagram
has been replicated many times throughout the world in the last thirty years.
Since the time of Galileo the confinement and the placement of light was defined
by three optical elements: Lenses, prisms and mirrors. The idea that is illustrated
in the figure below uses a very small hole to pass light onto the surface of
If the sample is brought very close to this hole within the red/pink rectangle,
it can capture the confined light of the hole before the light spreads out.
This confined area of light is called the Near-field. With this innovation,
two points that were closer than the Rayleigh limit and could not be resolved
with a lens could now be interrogated point-by-point with the very fine, relative-motion
of the sample or the hole. We named the resulting technique Near-Field Scanning
Optical Microscopy, or NSOM.
In parallel with these developments, Dieter Pohl at IBM Zurich developed a
similar idea, originally called Optical Stethoscopy and later called Scanning
Near-Field Optical Microscopy, or SNOM. Truth be told, approximately fourteen
years before our paper and Dieter Pohl's 1984 paper, Eric Ash at Imperial
College in London pioneered using a small hole and microwave radiation that
relied on the same near-field principal. However, it was far from obvious that
this research could be extended to optics, since the metal film in which a small
hole is fabricated has infinite conductivity in the microwave regime, rendering
the metal film opaque to microwave radiation. In the visible regime however,
metal films have finite conductivity and therefore are partially transparent.
Thus, it was not evident that a hole of nanometric dimension would have enough
contrast to be able to see light coming through such a hole.
It took us nearly three years to make the first small hole in a metal film,
for the simple reason that the tools for fabricating nanometric dimension holes
in metal films were only just being invented. I recall the first night that
I saw red light from a Helium Neon Laser coming through the smallest holes that
we were able to fabricate, which were 15 nanometers. Nonetheless, we were still
far from the dream of the object of this experiment, which was to develop a
light microscope based on light coming through this small hole. That took another
twelve years of research and development which led to the establishment of a
company that focused on providing the world with these instruments. The company
is owned by the three academic institutions of which I had been a part during
this period of invention, research and product development: Cornell University
- where I had been a professor before I moved to Israel - Hebrew
University and the Hadassah University Hospital.
Without boring the reader with the difficulties of the research and development
process that led to the realization of this form of microscopy, suffice it to
say that the most successful near-field optical probe was built on the technology
that had been prevalent in electrophysiology for decades. In electrophysiology,
glass elements could be tapered to ultra-small dimensions using heat, a pulling
force and controlled tension. Realizing the difficulty of applying nanofabricatioin
tools, that were still in development, for the creation of nanometric holes
- and the difficulty of using these holes to track real, rough surfaces
- we invented a technique in 1986 using glass-pulling technology together
with specific metal-coating protocols to form sub-wavelength apertures at the
tip of glass structures.
The year 1986 also heralded one of the most important developments of the twentieth
century: Atomic Force Microscopy, developed by Gerd Bennig and Calvin Quate.
Using this invention, we rapidly advanced our method of creating apertures,
and pioneered a methodology of cantilevering glass structures to produce a force-sensing
optical waveguide with a sub-wavelength aperture at its tip. This enabled bringing
a sub-wavelength aperture to sub-nanometer distances from a surface that could
be rough, without destroying the nanometric light probes. As a penultimate step,
these instruments were finally fully integrated into standard far-field optical
It has taken a considerable period of time for people to fully appreciate
the importance of these developments. First, the difficulty in development has
only eased in the last several years; and second, the mindset in optics was
one that did not appreciate the importance of nanophotonics in the early years
near-field optics. In 1984, there were only two papers on nanophotonic applications.
Last year, nearly eight hundred papers were published in the area of nanophotonics.
Today, one field after another is appreciating the critical role that NSOM
can play in fundamental and applied developments. For example, new development
in plasmonics allow light propogation to extend from nanometric to micrometric
dimensions. This can impact everything from optical computing to solar energy
concentration to biological sensing. Where would the development of nanophotonic
devices based on plasmonics be without the invention of near-field optics? Much
of the experimental device fabrication in plasmonics requires near-field optics
for understanding the distribution of light in nanometric dimensionalities.
Aside from the sensing applications that we have mentioned above, only recently
has it been possible to extend the power of near-field optics to biological
imaging of soft samples. This is a result of a breakthrough that we have achieved
that allows tuning forks with near-field optical probes to be completely immersed
in biological liquid. Now fundamental investigations of the properties of cellular
membranes - where 85% of the body's reactions take place -
are open to near-field optical investigations, such as the original problem
of Lance Taylor.
In addition to the above, there are applications as well in the area of Fluorescence
Correlation Spectroscopy, whose signal to noise depends upon the size of illumination
and near-field optics allows zeptoliter or 10-21 liters illumination volumes.
This is three orders of magnitude smaller than the smallest available illumination
volumes that can be achieved with standard optics. There is even the possibility
that such small illumination volumes of the optical near-field can be used to
delineate one of the most difficult problems in science - the nanometric
structure of solutions.
Finally, there are important developments in chemistry, which can profitably
employ such small illumination volumes. For example, the illumination of a region
of a photocatalytic surface, while monitoring its electrochemical properties
can be achieved with an NSOM probe. Such research is extremely important for
the future of solar energy conversion and photocatalysis in general.
These are only a few of the wide spectrum of areas that the invention of near-field
optics - now in its rising phase - has the ability to advance forward.
Copyright AZoNano.com, Professor Aaron Lewis (Nanonics Imaging)
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