Figure 2.

(a) Time dependence of ESA (λ_det = 550 nm) of β-carotene in octane (squares) observed after TPE (λ_exc = 1,310 nm). Monoexponential fit: 9 ± 0.2 ps (solid line). (b) Power dependence of ESA. Power fit: Exponent = 2.2 ± 0.3 (solid line). (c) Relative intensity of ESA excited with linear and circular polarized light (Ω = 0.84 ± 0.07). The “coherence spike” is very large in the two-photon experiments (off scale) and has a quadratic dependence on the excitation power and a linear dependence on the Car concentration. No spike was seen with a pure buffer sample. In experiments with chlorophyll a, which has no two-photon allowed states, the coherence spike was found to be substantially smaller in magnitude (data not shown).