Theory of a Carbon-nanotube Polarization Switch

Ken-ichi Sasaki
Materials Science Laboratory

 Polarized light is one of the most-familiar natural phenomena as the origin of variety seen in reflection and transmission of light at the surface of a physical object. On the other hand, as the profound spin degree of freedom (helicity) possessed by a massless gauge boson (photon), two components of polarized light have been used to acquire a new knowledge on so many natural laws [1]. Therefore, it would be beneficial if there is a device by which the polarization direction is switched by an electrical means.
 We predicted a novel phenomenon that the polarization direction of light transmission of carbon nanotube (CNT) changes by charge doping [2]. A CNT absorbs light whose linear polarization is parallel to the tube’s axis, but not when the polarization is perpendicular to it [Fig. 1(left)]. This is a well-known property of an undoped CNT, which enables aligned CNTs to function as an optical polarizer [3]. Charge doping of a CNT inverts the polarization dependence. That is, a doped CNT transmits parallel polarized light and absorbs perpendicular polarized light [Fig. 1(right)].
 To change the polarization direction of light transmitted through a typical Polaroid lens, it is necessary to rotate the lens itself. However, according to the theory of doping dependence, a CNT polarizer can invert the polarization of the transmitted light by 90 degrees without having to spatially rotate the polarizer; in other words, it is expected to function as an electrnoic polarization switch. In regard to cutting-edge optical transmission technology, the two degrees of freedom of polarized light are utilized to double the amount of information being transmitted. Different information like images and sound is transcribed to orthogonal polarization and transmitted. A polarization switch based on CNTs can be used to manipulate information within highly miniaturized structure for optical transmission.

Fig. 1. (left) Undoped CNTs transmit only perpendicular polarized light. (right) Doped CNTs transmit only parallel polarization. Since the polarization of the transmiited light rotates 90 degrees by doping, the aligned CNTs function as a polarization switch.