Figure 3: Absorptive 2D spectra and time traces of P3HT/PCBM NP aqueous suspensions.

(a,b) 2D spectra of P3HT/10%PCBM and P3HT/20%PCBM, respectively, at 35, 100 and 300 fs, together with linear absorption to illustrate peak positions. Red and blue squares are guides for the peak positions of time traces in fig. (c,e,f). (c) Representative time trace of diagonal peak with exciting and detecting wavelengths both at 554 nm for P3HT/20%PCBM NPs. The trace is fit with the sum of a constant background, monoexponential function and exponentially damped-cosine function. The amplitude modulations oscillate with a period of ~20 fs. The amplitude modulation is attributed to vibrational coherence of C=C stretching mode. (d) Fourier transformation of oscillatory term, which gives a peak at ~1,506 cm⁻¹. (e,f) Time traces of PIA with exciting wavelength at 556 nm and detecting wavelength at 630 nm for both P3HT/10%PCBM and P3HT/20%PCBM blends. These two time traces are fit with a sum of constant background, two/three exponential functions and exponentially damped-cosine function (shown in Supplementary Tables 2 and 3). Vibrational coherences due to the C=C stretching mode are also observed in the PIA.