Abstract: We present results examining the scattering of THz radiation from multi-walled carbon nanotubes oriented perpendicularly to the substrate plane. Despite the wavelength of the terahertz radiation being significantly larger that the diameters of the nanotubes, significant scattering of the incident terahertz radiation occurs. In fact, when compared to measurements performed on bulk metals, the scattering from the oriented nanotubes is significantly higher.
A commercial THz time-domain spectroscopy system with fiber-coupled photoconductive antennas is used to perform the scattering characterization. THz pulses are normally incident on the sample and the scattered radiation is detected in reflection at several off-axis angles varying from 20 to 80 degrees. Wire-grid polarizers are placed in front of the transmitter and receiver antennas to insure which polarization component is incident on the samples and detected. The multi-walled carbon nanotubes were grown via chemical vapor deposition to form carpet-like films consisting of nanotubes aligned approximately parallel to one another and normal to the plane of the glass substrate. The approximate density of the carbon nanotubes in the carpet film is 5x106 mm-2.
These results are counter-intuitive as the wavelength of the THz radiation is large enough that scattering from the nanotube carpets should not differ from the bulk metal with such significance. The conductivity of the nanotubes is not expected to be larger than for the metal. In fact, measurements of the back-reflected THz pulses (A pellicle beam-splitter was added to the setup) from both the metal and nanotube carpet shows that the reflected pulse from the metal is greater. It is only at off-axis points that the scattered radiation is greater from the nanotube carpet. Further studies will explore the effects of nanotube density, polarization dependence, and the variation of the incidence angle on this interesting scattering phenomenon.