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The field-dependent photocurrent spectrum of individual carbon nanotubes is measured using a displacement photocurrent technique. A series of peaks is observed in the photocurrent corresponding to both excitonic and free carrier transitions. The photocurrent peak corresponding to the ground state exciton increases by a factor of 200 beyond a critical electric field, and shows both red and blue shifts depending on the field regime. This provides evidence for field-induced mixing between excitonic and free carrier states.

The effect of uniaxial strain on the photocurrent spectrum of semiconducting single-wall carbon nanotubes is measured. The energy of the lowest-lying free electron transition is observed to shift with strain as predicted by a simple noninteracting model. The higher-order transitions also shift with strain, but being excitonic, their strain dependence differs from the predictions for the free carrier states. An anomalous photocurrent increase is also observed near the ground-state transition and is attributed to the formation of optically active defect states within the nanotube band gap.