We have used charge-induced absorption to quantify the influence of injected charges on electroabsorption measurements in single-wall carbon nanotube films. The interpretations of experimental measurements of processes in nanotubes are simplified by taking into account the change in electron-electron interactions upon charge injection. Electroabsorption spectra that are properly corrected for charge-induced effects show remarkable agreement with a simple Stark shift of the exciton transitions with no notable second-derivative contributions. Thus, distinguishing electric field effects from carrier density effects allows for a more rigorous calculation of exciton polarizability from electroabsorption measurements, even in heterogeneous films. PACS: 78.67.Ch Nanotubes: optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures. 1. Introduction While much attention has been given to precise optical measurements on individual single-wall carbon nanotubes (SWNTs) or monodispersed populations of SWNTs, devices made from heterogeneous mixtures of nanotubes continue to be of interest for use as field emitters, actuators and transducers, chemical sensors, transparent conductive films, and field-effect transistors [1–6]. Electroabsorption (EA) is a powerful tool for understanding the nature of excited states in materials, and the technique has been applied to various types of nanotube samples over the course of the last decade. The absence of Franz-Keldysh oscillations in EA spectra of nanotubes was used as evidence of the excitonic nature of the photoexcitations in SWNTs [7–9]. Furthermore, EA spectra have provided a quantitative estimate of exciton polarizability and size in individual species, evidence for charge trapping in SWNT bundles, and evidence for the existence of？ ？low-energy “dark” excitons in order to explain the low photoluminescence quantum efficiency in SWNTs [10–12]. Unfortunately, EA spectra of heterogeneous mixtures of SWNTs suffer from a variety of complex interactions. Perturbation of orbitals due to bundling, intertube charge transfer or trapping, and other competing effects can obfuscate individual tube responses to external fields. Because of these interactions, the analysis of SWNT EA spectra is less straightforward than for many other materials. Charge-induced absorption (CA) is a complementary technique that elucidates the influence of excess charge carriers independent of photoexcitations. This technique has been used in conducting polymers to measure carrier transport and recombination dynamics and to
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