Optoelectronics with Carbon Nanotubes

(a)
Figure III-13. (a) Spectra from a device showing E11 and E22 peaks around 0.66 eV
and 1.10 eV, respectively. The solid symbols are experimental data which are fitted
(lines) with a linear combination of two peaks from Equation III.2. (Inset) Intensity
of individual peaks as a function of applied power. (b) Half widths of E11 and E22
peaks as a function of applied power.
iv. Observation of Polarization Effects in Electroluminescence Spectra
Polarization dependence of the absorption spectrum and emission in carbon nanotubes
has been studied both theoretically and experimentally. While it was initially thought under
single-electron theory that the absorption of “cross-polarized” light (i.e., perpendicular to the
long axis of the tube) should be strongly suppressed due to depolarization 131, 132 , a study
considering excitonic effects revealed that there is a prominent absorption peak for perpendicular
polarization, although it is still considerably weaker than in parallel polarization 133 . For parallel
polarization (E11, E22, etc.), selection rules allow only transitions between same sub-bands, while
cross-polarized light couples with non-equal sub-band transitions (i.e., E12, E21, etc.) 32 .
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(b)