ABSTRACT
Recently, a terahertz response of single-walled carbon nanotubes has been successfully observed and measured [1,2]. Carbon nanotubes (CNTs) have emerged as one of the most active areas of nanoscale science and technology research [3,4]. They are truly nanoscale materials with typical diameters of 1–2 nm, and can be regarded as being close to ideal 1-D conductors with an unprecedented mean free path of about 1 μm at 300 K. Their cross-sectional dimensions are about an order of magnitude smaller than the limiting dimensions at which complementary metal oxide semiconductor (CMOS) technology is likely to encounter insuperable scaling limits in a few years. Not surprisingly, it has turned out to be quite difficult to realize actual working applications by using materials at such a radically different scale; this is still a largely unmet challenge! For example, CNT field-effect transistors (CNT-FETs) have been predicted to perform up to terahertz frequencies [5,6], but experiments show switching speeds [4] (digital circuits) or cutoff frequencies (analog circuits) that are at least a couple of orders of magnitude below that predicted intrinsic performance. Measurements on CNTs have been extended to about 60 GHz, and much higher frequencies (up to several THz) are required to verify the properties of CNTs as predicted by existing theories. These theories predict plasmon waves propagating along the tubes at slow speeds of roughly 0.01c (c is the speed of light), a unique feature of 1-D conductors [7]. The transmission line model for a metallic single-walled CNT (m-SWCNT) introduced in Ref. [7] includes a unique kinetic inductance element that is about 1000 times greater than the magnetic inductance usually considered for macroscopic conductors. It also incorporates a quantum capacitance. Based on this model, one predicts that plasmon resonances should occur at terahertz frequencies for CNTs as short as 1 μm. It is a current, so far unmet, challenge for experimentalists to measure and take advantage of these resonances. The unusual antenna properties of CNTs have been predicted in Ref. [8]. Surface wave types of fields near the tubes are predicted to be 494enhanced by hundreds of times [9], and this has also not yet been demonstrated experimentally. Such surface waves are promising for many applications, similar to those that are presently under intense development in the optical/NIR range, employing surface plasmon polaritons (SPPs) [10]. The utilization of the plasmon phenomena shows potential for shrinking circuit sizes to a small fraction of a wavelength, at THz as well as in the visible/NIR. Many unique applications of CNTs in the terahertz frequency range thus appear possible when these types of plasmon phenomena are well understood.
