Tailored nanoantennas for directional Raman studies of individual carbon nanotubes

Abstract

We exploit the near field enhancement of nanoantennas to investigate the Raman spectra of otherwise not optically detectable carbon nanotubes (CNTs). We demonstrate that a top-down fabrication approach is particularly promising when applied to CNTs, owing to the sharp dependence of the scattered intensity on the angle between incident light polarization and CNT axis. In contrast to tip enhancement techniques, our method enables us to control the light polarization in the sample plane, locally amplifying and rotating the incident field and hence optimizing the Raman signal. Such promising features are confirmed by numerical simulations presented here. The relative ease of fabrication and alignment makes this technique suitable for the realization of integrated devices that combine scanning probe, optical, and transport characterization.

Polarized surface-enhanced Raman spectroscopy of suspended carbon nanotubes by Pt-Re nanoantennas

“…This is often the case: for a given laser frequency ω L , only a small fraction of the CNTs shows a significant signal. This is precisely where the advantage of optical antennas becomes obvious, as the signal amplification compensates for the suppression due to the energy mismatch [22]. As demonstrated in Fig.…”

“…By following the standard assignment procedure we deduce the possible chiral indices for the CNT. We found that the most likely chiral indices are (22,5) or (21,7), two adjacent elements of the CNT family 2n + m = 49 which share very similar properties, as, e.g., diameter, chiral angle, and energy separation between VHSs [29,34].…”

“…Raman and reflectance measurements are performed in a confocal setup [7,22,29]. Light is transmitted through a beam splitter and focused on the sample by a 100× objective lens.…”

“…The light spot size is approximately 2 μm for white light and below 1 μm for the two laser lines, i.e., the spatial resolution is comparable to the array width. The scattered light is transmitted again through the beam splitter and then sent to the detector, as discussed in our previous work [22]. The configuration of light sources, detectors, and polarizers depends on the specific measurement [29].…”

“…These temperatures are sufficient to melt thin films or nanoparticles of the most common plasmonic materials, namely Au and Ag. Therefore, so far the proposed prototypes [19][20][21][22] of optical nanoantennas for CNTs had to be fabricated or deposited after the CNT growth, i.e., they were not compatible with the ultraclean fabrication scheme.…”