Optical Microscope

(continued from Page 3)

the cutoff of propagation by the waveguide modes. The low

of maintaining the integrity of the coating while achieving

light throughput and finite skin depth of the metal limit the

reasonably fast deposition rates. Using electron beam aided

resolution to normally 50 to 100 nm.

deposition they were able to create a dielectric film with

nanometer conformity over complex optical geometries.

Need for Finer Spatial Resolution

Challenges of Tip Design

Many applications require spatial resolutions that are not

attainable with the aperture technique. For example, a spatial

The next step was optimizing the metal tip design. This was

resolution of at least 30 nm is desirable in spectroscopic

accomplished by modeling Chance Prock Silbey (CPS) theory

imaging of photosynthetic membranes in order to resolve

for an emitting Dipole aligned in close proximity to a metal

closely packed individual proteins in a lipid membrane. This

substrate and then applying to a metal tip. In detecting the

has been accomplished by the use of laser‑illuminated metal

fluorescent signal from a molecule the optimum distance from

tips to provide a local excitation source for the spectroscopic

the chromophore to the metal tip is approximately 22 nm for a

response of the sample under investigation. Excitation light of

parallel emitting dipole or 24 nm for a perpendicular emitting

proper polarization induces a strongly enhanced field at the tip.

dipole. If the metal tip is closer than 3 to 5 nm, even with the

The highly localized excitation source has provided resolution

coating, then the fluorescent signal is dramatically quenched,

levels of 15 nm to 25 nm, making it possible, for example, to

making it impossible to detect the signal. This sets up the

resolve tightly packed chromophoric membrane proteins.

primary challenge in tip design, optimizing the near‑field tip

design to deliver the highest possible electromagnetic field

Unfortunately, the presence of the metal tip nanometers

at the appropriate distance. Although the researchers believe

away from the fluorophore leads to fluorescent quenching.

the tip molecule separation would be similar to the bulk

This results in a negative fluorescent image, essentially a dark

metal‑molecule situation, the tip's geometry requires careful

spot at the chromophore position surrounded by a sharp

modeling in order to determine this information. It would be

halo of emission. The team leader Dr. Sánchez along with

very difficult and expensive to accomplish this objective using

colleagues at Harvard University had overcome this problem

conventional build and test methods. The primary problem is

by depositing a homogenous, nanometer‑scale silicon oxide

the cost of fabricating the tips and the difficulty of performing

coating on near‑field probes in order to minimize quenching.

field measurements at the nanometric scale.

An important advancement was achieved when electron

beam assisted deposition (EBAD) demonstrated the ability

For these reasons, the researchers used FDTD to model the

to evenly deposit silicon oxide coatings on the complex

near‑field response of proposed designs. Electromagnetic

three dimensional tips of the apertureless metal probes,

simulation takes only a small fraction of the time and expense

which are typically only a few nanometers wide, in order

involved in building and testing apertureless tips. Simulation

also provides more

information than

physical experiments

a.

b.

c.

by yielding results

at every point in the

solution domain, far

exceeding the results

that can be achieved

with physical

measurements. The

researchers picked

XFDTD software

from Remcom,

Inc., State College,

30 nm

Pennsylvania, because

it is an industry

leader in the ability to

quickly and reliably

Figure 2: FDTD modeling of a sharp TENOM probe tip, (a.) the 3D view of the probe tip with crossection of

turn complicated

E the external electric field magnitude, (b.) the 2D side view of the E field distribution, (c.) and J, the current

geometries

density magnitude. When a horizontally incident (vertical polarization) is applied this probe experiences an

into accurate

increase of local field instensity ( |E | ) of ~1000 times.

electromagnetic

meshes. This ability

to avoid fluorescent quenching. They used a focused ion

has been extended in XFDTD v6.2 with the addition of an

beam (FIB)/scanning electron microscope (SEM) to grow

advanced meshing algorithm that makes meshing of certain

dielectric material on complex three‑dimensional optical

difficult geometry features possible. Adaptive meshing

probe. A DualBeam FIB/SEM manufactured by FEI (Strata

capabilities reduce solution times while maintaining high

DB‑235) uses a liquid metal Gallium ion source accelerated

levels of accuracy by automatically adjusting the mesh to

to 30 kV with specimen currents from 1 pA to 20 nA. The

provide more cells in areas with high transients and reducing

researchers wrestled with and finally overcame the challenge

(Continued on next page)

www.remcom.com

Summer 2006