'Unknown' by Unknown - Page 5 of 6

Optical Microscope

(continued from Page 4)

cells in areas where there is less variation. In addition, the

contribution. Another way to overcome this problem is to

use of a parallel computational code allows for multiple

non‑radiatively propagate a field to the end of the tip where

computers to be connected in order to perform calculations

the energy of the field could emit radiatively, eliminating the

faster as well as use larger workspaces.

background contribution. Under certain conditions the energy

carried by photons of light is transferred to packets of

electrons, called plasmons, on a metal's surface. The

light's energy is transferred to driving the electrons

resonantly through attenuated total reflection at very

specific conditions.

Enhanced Field,

Guided by electromagnetic simulations, the researchers

Imaging Point

designed a tip that takes advantage of this technique4,

Glass Prism

which was created by A. Otto in the late 1960s.The

angle of the prism, tip shaft length, and gap between

the prism and metal were carefully engineered to

achieve resonance. When the plasmon reaches

the tip end it generates a strong evanescent field

within a region on the order of the tip end diameter.

Evanescent waves are formed when sinusoidal waves

Incident Light

are internally reflected off an interface at an angle

Direction

greater than the critical angle so that total internal

reflection occurs. The intensity of evanescent waves

decays exponentially as they move further from the

interface at which they are formed. This eliminates the

signal generated by far‑field illumination, increasing

Au Probe

the signal to background ratio. PSU researchers are

currently evaluating the performance of these tips and

working to improve their design to deliver even higher

levels of performance.

For more information, contact Remcom: 315

Figure 3: This novel NSOM probe takes advantage of a non-radiatively

South Allen Street, Suite 222, State College, PA 16801;

coupled focused spot of light which creates a resonant Plasmon which

Telephone: 814‑861‑1299, 888‑773‑6266

dramatically improves signal to noise. Light enters the prism and the end of

(888‑7REMCOM) Toll‑Free in U.S. and Canada;

the tip creates a strong localized field.

Fax: 814‑861‑1308, 1‑888‑973‑6266 (888‑9REMCOM)

Toll‑Free in U.S. and Canada; e‑mail: info@remcom.com;

Using Simulation to Iterate to an Optimized Design

Web site: www.remcom.com

The researchers began by simulating an existing tip design in

order to validate the accuracy of the method. They divided the

References

initial design into cubic cells with the appropriate frequency

1. Erik J. Sánchez, Lukas Novotny, and X. Sunney Xie,

dependent behavior. An absorbing boundary condition was

Physical Review Letters 82, 20, pg 4014-4017 May

used. The incident excitation field was set to an electric field

(1999).

strength of 1 V/m at 800 nm wavelength and the space was

discretized with 3 nm cubes. The material of the tip was

2. Achim Hartschuh, Erik J. Sánchez, X. Sunney Xie and

defined to be Au and the spacer was SiO2. The electromagnetic

Lukas Novotny, Physical Review Letters 90, 9, 095503

field in each cell under plane wave illumination was

March (2003).

calculated by the software through time domain integration

3. Derek B. Nowak, John T. Krug II, X. Sunney Xie, and

of Maxwell's equations. The intensity enhancement at the

Erik J. Sanchez, WMSCI Proceedings, 9th World Multi-

end of the tip is 100 without the spacer, approximately 5 nm

conference on Systemics, Cybernetics and Informatics,

in front of the tip. When the 9 nm SiO2 spacer is present, the

Orlando, Florida Conference, July (2005).

enhancement is reduced to 50. This is still high enough for

near‑field imaging at 5 nm in front of the tip. These results

4. E. J. Sánchez, J. T. Jrug, and X. S. Xie, Review of

matched the experimental measurements so the researchers

Scientific Instruments 73, 11, pg 3901-3906,

began using XFDTD in an effort to optimize the tip design.

November (2002).

Normally apertureless near‑field optical probes require

5. John T. Krug II, Erik J. Sánchez and X. Sunney Xie,

direct illumination of the tip apex in order to generate a

Journal of Chemical Physics 116, 24, June (2002).

sub‑diffraction limited light spot. A large background signal

6. John T. Krug II, Erik J. Sánchez and X. Sunney Xie,

originates from the emission of many chromophores in the

Applied Physics Letters 86, 233102, May (2005).

far‑field illuminated volume. Typically, tips with a high field

enhancement are used in order to overcome this background

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