Boston College Researchers Extend Range of Near-Field Scanning Optical Microscopy with Nanoscale Magnifiers

Nanoscale optical probes using carbon nanotubes that facilitate sub-wavelength, sub-diffraction limit, and spatial resolution for near-field scanning optical microscopes are disclosed in U.S. Patent 7,623,746.  Boston College researchers developed a nanoscale optical probe for use with a near-field scanning optical microscope that includes an inner conductor with a top end, a bottom end, and a body; a dielectric material which surrounds the inner conductor; and an outer conductor which surrounds the dielectric material.  The inner conductor is longer at a tip surface of the probe than the dielectric material and the outer conductor. The nanoscale optical probe can achieve resolutions of less than about 10 nanometers (nm) in a transverse direction. In the art for nanoscale optical probes, the Boston College probe extends the range of measurements possible compared with conventional near-field scanning optical microscopy techniques.

Professor Michael J. Naughton, Krzysztof J. Kempa and Zhifeng Ren created a magnifying element for use with a near-field scanning optical microscope.  The magnifying probe is made of a film with a top surface, a bottom surface and cylindrical channels and an array of nanoscale optical probes penetrating the film through cylindrical channels.   Each nanoscale optical probe has an inner nanowire with a top end, a bottom end, and a body; a dielectric material which surrounds the inner nanowire; and an outer metal material which surrounds the dielectric material. Because the width of the inner conductor may be significantly smaller than the wavelength of visible light, which is in the range of about 300 nm to about 700 nm, the nanoscale optical probe may be used to image objects with spatial resolution well under this range.

The inventors developed a method of fabricating a nanoscale optical probe which involves growing a carbon nanotube (CNT) on the optical fiber; depositing a dielectric material over the carbon nanotube; and depositing an outer metal material over the dielectric material. They also fabricated a nanoscale optical probe by electrodepositing a catalytic transition metal on an AFM-type tip; growing a carbon nanotube (CNT) on the optical fiber; depositing a dielectric material over the carbon nanotube; and depositing an outer metal material over the dielectric material.

Plasma enhanced chemical vapor deposition (PECVD) is used grow the carbon nanotube. A dielectric photovoltaic material having both electrical conductivity and transparency (for example silicon- and non-silicon-based materials) is deposited over the carbon nanotube via (for example, via PECVD, sputtering, or evaporation). Typically, the dielectric material is coated to yield a thickness of about 10 nm to about 200 nm. An outer metal (for example, aluminum) is then deposited (via CVD, sputtering or evaporation) over the dielectric material, forming a nanoscale optical probe having a coplanar waveguide configuration. If desired, the outer metal may be removed from the bottom surface of the probe (via focused ion beam or wet etch), thus exposing the photovoltaic material and the carbon nanotube, yielding a nano-optical antenna at the substrate surface of the probe. 

Near-field scanning optical microscopy (NSOM) is a type of microscopy where a sub-wavelength light source, usually a fiber tip with an aperture smaller than 100 nm, is used as a scanning probe over a sample. Near-field scanning optical microscopy is one in a family of scanned probe techniques that includes scanning tunneling microscopy and atomic force microscopy (AFM) where an image is obtained by raster scanning a probe across a surface collecting data at an array of points during the scan. In order to achieve an optical resolution better than the diffraction limit, the scanning probe has to be brought within the near-field region (that part of the radiated field nearest to the antenna, where the radiation pattern depends on the distance from the antenna). NSOM is based upon the detection of non-propagating evanescent waves in the near-field region. The probe is scanned over a surface of the sample at a height above the surface of a few nanometers and allows optical imaging with spatial resolution beyond the diffraction limit.

The scanning probe can either detect in the near-field directly, by means of the sub-wavelength size aperture (collection mode), or by using the probe as a waveguide with a sub-wavelength scattering source and detecting the evanescent waves as they propagate into the far-field (transmission mode). The achievable optical resolution of NSOM is mainly determined by the aperture size of the scanning probe and the probe-surface gap. NSOM may, in theory, be combined with any spectroscopic technique to gather spectra from small regions of a sample. Infrared (IR), Raman, visible, and V, as well as NSOM fluorescence, photoluminescence, photoconductance, and magnetooptical (MOKE) spectroscopies have been investigated.