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Posted: Sep 17, 2013

A new method of measuring optical near-fields within 1 nanometer of a metal surface

(Nanowerk News) Researchers at the Max-Planck-Institut für Quantenoptik have demonstrated a new method of probing optical near-fields within 1 nm distance from the surface of a nanoscale metal tip. The method is based on rescattering of electrons driven by short laser pulses. The length scale on which the near-field is measured reaches down to dimensions that are of utmost interest in the emerging field of quantum plasmonics.
The team reports their findings in the September 13, 2013, online edition of Nano Letters ("Probing of Optical Near-Fields by Electron Rescattering on the 1 nm Scale").
Illustration of electron rescattering
Illustration of electron rescattering. Electrons are emitted in the optical near-field of a metal nanotip. A fraction of the emitted electrons is driven back to the tip surface, where they can scatter elastically. The kinetic energy gained during the rescattering process depends sensitively on the electric field near the tip surface. Thus the strength of the optical near-field is mapped to the kinetic energy of the emitted electrons. (© ACS)
In this work, they demonstrate a nanometric field sensor based on electron rescattering, a phenomenon well-known from attosecond science. It allows measurement of optical near-fields, integrating over only 1 nm right at the structure surface, close to the length scale where quantum mechanical effects become relevant.
The excitation of enhanced optical near-fields at nanostructures allows the localization of electromagnetic energy on the nanoscale. At nanotips, this effect has enabled a variety of applications, most prominent among them are scanning near-field optical microscopy (SNOM), which has reached a resolving power of 8 nm, and tip-enhanced Raman spectroscopy (TERS). Because of the intrinsic nanometric length scale, measuring and simulating the tips’ near-field has proven hard and led to considerably diverging results.
Experimental results for the field enhancement factors of tungsten and gold tips agree well with Maxwell simulations. On the basis of these results, we give a field enhancement map for a wide range of materials. Furthermore, the simulations reveal that geometric effects are the predominant mechanism of optical field enhancement at nanotips in most cases. Exceptions exist close to plasmon resonances. In the future, a tomographic reconstruction of the near-field, likely in three dimensions, will be possible by measuring the cutoff energy of the rescattered electrons while varying the laser power or wavelength.
Source: American Chemical Society
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