Electromagnetic Simulation Helps to Optimize Optical Microscope Resolution
A new tip design delivers enhanced sub-20 nanometer performance
Near-field scanning optical microscopy (NSOM) has extended optical measurements past the diffraction limit, making it possible for the first time to view objects and
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Figure 1: Transmission electron micrograph of a metal probe tip coated with 10 nm of electron beam-grown Si02 to prevent fluorescence quenching by the metal
Recent advances in nanotechnology and nanoscience are highly dependent on our newly acquired ability to measure and manipulate individual structures on the nanoscale. A drawback of light microscopy is the fundamental limit of the attainable spatial resolution dictated by the laws of diffraction at about 250 nm. This diffraction limit arises from the fact that it is impossible to focus light to a spot smaller than half its wavelength. The challenge of breaking this limit has led to the development of NSOM. The optical probes originally used in NSOM were created by pulling an optical fiber to a final diameter of 25 to 100 nm, coating it with aluminum, and etching to provide a flat, circular endpoint and aperture. Unfortunately, only a tiny fraction of the light coupled into the fiber is emitted by the aperture because of the cutoff of propagation by the waveguide modes. The low light throughput and finite skin depth of the metal limit the resolution to normally 50 to 100 nm.
Need for finer spatial resolution
Unfortunately, the presence of the metal tip nanometers away from the fluorophore leads to fluorescent quenching. This results is a negative fluorescent image, essentially a
Challenges of tip design
For these reasons, the researchers used FDTD to model the near-field response of proposed designs. Electromagnetic simulation takes only a small fraction of the time and expense involved in building and testing apertureless tips. Simulation also provides more information than physical experiments by yielding results at every point in the solution domain, far exceeding the results that can be achieved with physical measurements. The researchers selected XFDTD software from Remcom, State College, PA, because it can quickly and reliably turn complicated geometries into accurate electromagnetic meshes. This ability is extended with the addition of an advanced meshing algorithm that makes meshing of certain difficult geometry features possible. Adaptive meshing capabilities reduce solution times while maintaining high levels of accuracy by automatically adjusting the mesh to provide more cells in areas with high transients and reducing cells in areas where there is less variation. In addition, the use of a parallel computational code allows for multiple computers to be connected in order to perform calculations faster as well as use larger workspaces.
Using simulation to iterate to an optimized design
Normally, apertureless near-field optical probes require direct illumination of the tip apex in order to generate a sub-diffraction limited light spot. A large background signal originates from the emission of many chromophores in the far-field illuminated volume. Typically, tips with a high field enhancement are used in order to overcome this background contribution. Another way to overcome this problem is to non-radiatively propagate a field to the end of the tip where the energy of the field could emit radiatively, eliminating the 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.
Guided by electromagnetic simulations, the researchers designed a tip that takes advantage of this technique, which was created by A. Otto in the late 1960s.5 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 are internally reflected off an interface at an angle 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 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.
References1. Erik J. Sánchez, Lukas Novotny, and X. Sunney Xie, Physical Review Letters 82, 20, pg 4014-4017 May (1999).
2. Achim Hartschuh, Erik J. Sánchez, X. Sunney Xie and Lukas Novotny, Physical Review Letters 90, 9, 095503 March (2003).
3. Derek B. Nowak, John T. Krug II, X. Sunney Xie, and Erik J. Sanchez, WMSCI Proceedings, 9th World Multi-conference on Systemics, Cybernetics and Informatics, Orlando, Florida Conference, July (2005).
4. IDBR #0500812
5. E. J. Sánchez, J. T. Jrug, and X. S. Xie, Review of Scientific Instruments 73, 11, pg 3901-3906, Nov (2002).
6. John T. Krug II, Erik J. Sánchez and X. Sunney Xie, Journal of Chemical Physics 116, 24, June (2002).
7. John T. Krug II, Erik J. Sánchez and X. Sunney Xie, Applied Physics Letters 86, 233102, May (2005).
Erik J. Sanchez is an assistant professor of physics at Portland State University. He may be reached at editor@ScientificComputing.com.