**4. Picosecond Time-Resolved Spectra of ASE in Toluene**

We began the TRS studies with a PFO-co-PPV-MEHB solution at a concentration of 2.5 mg/mL and a pump energy density of 850 μJ/cm2, as shown in Figure 9. The solution produced only laser-induced fluorescence (LIF) of the CP in the three dimensions when the pump energy density was 3.2 mJ/cm2. The CP started fluorescing at 18 ns and maintained this fluorescence until 69.8 ns. The LIF had two peaks at 507 and 481 nm, and the intensity was unstable, fluctuated unsteadily, and reached a maximum value at 48.19 ns.

**Figure 9.** Three-dimensional profiles of LIF, with the inset showing the temporal profile of LIF for a pump energy density of 850 μJ/cm2.

Figure 10a shows a few frames of the spectral profile from 20 ns after the Q-switch trigger. The peak acquired was very broad, with an FWHM of 55 nm. As the photon flux increased, the band at approximately 509 nm gained intensity due to the large stimulated emission and single-pass optical gain. The FWHM was 21 nm at 22 ns. Threshold spectral narrowing occurred in 2 ns. The full profile of the spectral narrowing spectra over time is shown in Figure 10b. We can compare Figure 6 with Figure 10; the former gives the transition from fluorescence to ASE as a function of pump energy, while the TRS study shows that the transition from fluorescence to spectral narrowing and the onset of ASE takes 2.5 ns at the same pump energy.

**Figure 10.** (**a**) Time evolution of the CP threshold spectrum at different time. (**b**) Three-dimensional profiles of the CP solution threshold spectrum for a pump energy density of 4.25 mJ/cm2.

The ASE of the CP became intense for pump energy densities above the threshold and was recorded using a picomax spectrometer. Figure 11a shows a Z-slice of the ASE of the CP in toluene at a concentration of 2.5 mg/mL under transverse excitation for a pump energy density of 7.75 mJ/cm2. The CP started fluorescing at 24 ns, and the intensity increased with time. The CP produced ASE at 22 ns and reached a maximum intensity at 25 ns. After 25 ns, the ASE intensity decayed up to 34.5 ns and stopped at 35 ns, as displayed in Figure 11a. The ASE peaked at 508 nm with an FWHM of 8 nm, as shown in Figure 11b.

**Figure 11.** (**a**) Temporal profile of CP ASE for a pump energy density of 7.75 mJ/cm2. (**b**) Threedimensional profiles of CP ASE (1.55 mJ/cm2).

Figure 11b shows the temporal dynamics of the CP in toluene at the aforementioned concentration, but the pump energy density was increased to above 7 mJ/cm2. When the pump energy density was 7.75 mJ/cm2, ASE was obtained at 520 nm with an FWHM of 8 nm. The entire ASE event became short due to rapid excitation, population inversion, and stimulated emission. The ASE attained the peak in a rapid phase and lasted for only 4 ns. In the pump pulse tailing phase, the photon flux was lower and produced fluorescence at the vibrational band around 480 nm, producing LIF along with weak ASE 2 ns after the maximum ASE peak intensity.
