Optoelectronics with Carbon Nanotubes

DOP = I║ / (I┴ + I║) = 0.77. As we have already seen, EL emission is dominated by the E11
transition, so the maximum at zero degrees agrees well with theory. It is not completely
suppressed at 90 degrees, which is also seen in single-tube PL and/or Raman 137 , PLE 119, 134 , and
PC 27, 135, 136 measurements.
Now we examine the spectra through a polarizer placed in either a parallel or a
perpendicular direction relative to the tube. The spectra in Figure III-15 are from the same
device as in Figure III-10, where blackbody background could be identified. The most
prominent characteristic of the emission is that the E11 peak is significantly polarized in the
tube’s longitudinal direction, as expected from theory. The analysis of E11 peaks (without the
blackbody signal) from this device gives the range of DOP from 0.67 to 0.76, depending on the
power (Figure III-15 (b), inset). The perpendicularly polarized component of the E11 peak is not
zero; a detailed calculation of optical absorption for 29 different types of CNTs showed that the
absorption profile with respect to polarization is highly dependent on the actual tube chirality 138 .
Therefore, depending on the structure of this tube, there may be a relatively large perpendicular
component of the E11 emission. It is also possible that the optical alignment has an error of a few
degrees. Nevertheless, E11 emission in the perpendicular direction was significantly suppressed
for the E11 transition in all the devices on which we conducted polarization-dependent
measurement.
It should be noted that the blackbody background is also polarized, with a large
component in the parallel direction (Figure III-15). This is due to the 1D structure of CNTs, and
is consistent with the observation of incandescent emission from aligned multi-wall nanotubes by
Li et al. 86 . They found that the emission from electrically heated nanotubes emits light that
matches the blackbody radiation spectra very well, and showed via classical electrodynamics that
electrons confined in a 1D structure emits light whose electric field vector is parallel to the
direction of the said structure. This agrees with our observation that the blackbody radiation
originating from the heating of the carriers and the nanotube lattice is also polarized.
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