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Optical Characterization and Nanophotonics Laboratory

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Apertureless Near-Field Scanning Optical Microscopy

Members: Bennett B. Goldberg

 

Alumni: Wolfgang Bacsa, Georgiy Alekesyevich Kazantsev, Marc Mcguigan, Daniel Shaffer, Harry Theodore Stinson, Andrea Welsh, Seth Benjamin Zuckerman

 

Due to the wave nature of light, most far-field imaging techniques are “diffraction limited” and cannot be used to resolve spatial features that are separated by less than λ/2NA. Waves with a higher spatial frequency have an exponentially decaying amplitude, and cannot be detected in the far-field. By positioning a sharp probe nanometers above a sample, NSOM systems allow these high-spatial frequency waves to be scattered and detected in the far-field.

Near-field scanning optical microscopy (NSOM) provides optical and topographic sample characterization with a spatial resolution on the order of tens of nanometers – providing the benefits of optical microscopy while circumventing the spatial resolution limits of far-field optical microscopy.

Raman spectroscopy is useful for chemical and molecular characterization, but it has a smaller cross section than other spectroscopic techniques. Spatial regions of enhanced Raman scattering (“hot spots”) have been measured on substrates with a rough metallic coating. It is believed that the position of a “hot spot” and the magnitude of the Raman signal enhancement is, in part, due to a large, local, electromagnetic field intensity. In order to take full advantage of this SERS effect, there have been efforts to fabricate substrates with deterministically positioned hot spots. Professor Luca Dal Negro (BU ECE) has recently fabricated such substrates by arranging nanostructures in arrays according to mathematical sequences. A nanoscale investigation of these aperiodic nanostructures is likely to reveal “hot spots” with enhancement factors that are greater than the spatially averaged values that have been reported.

We are building a near-field scanning optical microscope that will characterize the local SERS enhancement on deterministic aperiodic nanostructures. In our set-up, a sample is fixed to a scanning stage that is mounted on an inverted fluorescence microscope. Light will be focused on the probe apex by a high-NA objective below the sample. The probe is glued to one prong of a quartz tuning fork such that it extends 1-2 mm beyond the length of tuning fork. The tuning fork is excited so that it oscillates laterally at its resonant frequency. Piezoceramic tubes are used to lower the tuning fork toward the surface. Nanometers from the surface, the tip will interact with shear forces that will cause a shift in the resonant frequency. This shift can be measured and used to maintain a constant tip-sample separation. We will use apertureless probes; gold wire etched in hydrochloric acid. Apertureless probes can be etched sharper than aperture probes and will allow more localized measurements.

A near-field characterization of these deterministic aperiodic nanostructures is likely to reveal “hot spots” with Raman enhancement factors that are greater than the spatially averaged values that have been reported. The nanoscale study of the light-matter interactions on these samples may lead to the design of substrates that are tuned to yield an even greater Raman signal enhancement.

Figure 1: SEM image of a sharp probe

Figure 2: SEM image of probe sitting on a tuning fork

Collaborators

This project is a collaboration with Nano-optics group at University of Rochester's Institute of Optics.


© 2007 Trustees of Boston University. All rights reserved.  |  Last modified April 16, 2007 at 12:00 AM EDT