Optimize Optical Microscope Resolution
M ODELING & SIMULATION
18
June 2006
Scientific CO M P UTI N G.com
Electromagnetic Simulation
Helps to Optimize Optical
Microscope Resolution
A new tip design delivers enhanced sub-20 nanometer performance
N
ear-field scanning optical
ance without the need to build a physi-
■ Erik J. Sanchez, Ph.D.
microscopy (NSOM) has
cal model.
extended optical measure-
Recent advances in nanotechnology
ments past the diffraction
and nanoscience are highly dependent
limit, making it possible for the first
on our newly acquired ability to meas-
time to view objects and features in the
ure and manipulate individual struc-
50 to 100 nanometer (nm) range.
tures on the nanoscale. A drawback of
Recent research has demonstrated that
light microscopy is the fundamental
the use of apertureless probes can fur-
limit of the attainable spatial resolution
ther improve spatial resolution to below
dictated by the laws of diffraction at
25 nm.1-3 The
about 250 nm. This diffraction limit
next challenge is
arises from the fact that it is impossible
optimizing the
to focus light to a spot smaller than half
Si02
tip design in
its wavelength. The challenge of break-
order to strong-
ing this limit has led to the development
ly illuminate the
of NSOM. The optical probes originally
sample at dis-
used in NSOM were created by pulling
tances from the
an optical fiber to a final diameter of 25
aperture that
to 100 nm, coating it with aluminum,
are hundreds of
and etching to provide a flat, circular
20 nm
times closer
endpoint and aperture. Unfortunately,
than the dimen-
only a tiny fraction of the light coupled
sion of the
into the fiber is emitted by the aperture
wavelength of
because of the cutoff of propagation by
the light that is
the waveguide modes. The low light
employed. Trial
throughput and finite skin depth of the
and error meth-
metal limit the resolution to normally
F I G U R E 1 : Transmission electron micrograph of
ods are highly
50 to 100 nm.
a metal probe tip coated with 10 nm of electron
beam-grown Si02 to prevent fluorescence quenching
undesirable
by the metal
because of the
Need for finer spatial resolution
great challenges involved in building
Many applications require spatial res-
and testing optics at nanometer scales.
olutions that are not attainable with the
Researchers at Portland State University
aperture technique. For example, a spa-
(PSU), led by Erik Sánchez and funded
tial resolution of at least 30 nm is desir-
by the NSF4 are overcoming this prob-
able in spectroscopic imaging of photo-
lem by using a commercial finite differ-
synthetic membranes in order to resolve
ent time domain electromagnetic simu-
closely packed individual proteins in a
lator (XFDTD) to analyze tip perform-
lipid membrane. This has been accom-