*3.3. ASE (Mirrorless Lasing) from CO PFO-co-PPV-MEHB in Toluene*

Amplified spontaneous emission occurs due to the stimulated emission and amplification of spontaneously emitted photons under a high population inversion with the single-pass gain of the laser media. ASE without feedback can be considered mirrorless lasing, since most of the features are laser features [40]. Many CPs and COs are capable of producing optically pumped ASE [41,42].

PFO-co-PPV-MEHB in toluene at a concentration of 2.5 mg/mL was transversely excited with a 355 nm Nd: YAG laser. At a minimal pump energy density (3.2 mJ/cm2), the laser-induced fluorescence (LIF) was verified and had two peaks at 507 and 481 nm, as shown in Figure 6. After the pump energy density was increased to 4.5 mJ/cm2, the spectrum became narrower and peaked at 508 nm with an FWHM of 20 nm. When the pump energy density was 7.75 mJ/cm2, ASE was obtained with an FWHM of 8.75 nm and a peak at approximately 508 nm, as presented in Figure 6.

**Figure 6.** LIF, threshold, and ASE spectra of PFO-co-PPV-MEHB in toluene.

Figure 7 shows the spectral narrowing and increasing of emission intensity of the CP in toluene as a function of the input pump energy density. At 4.5 mJ/cm2, the FWHM of the spectrum reduced from 60 to 8.4 nm and the surge of intensity was linear. A additional rise in the input energy density boosted the output strength, however the spectral bandwidth (nm) of ASE was sustained. The reduction in bandwidth (nm) occurred due the net gain maximization near the vibronic transition peaks of the fluorescence spectrum; so, the spectrum displayed intensity surge as the pump energy density raised.

**Figure 7.** Spectral narrowing and rapid output intensity increase in the CP in toluene as a function of the input pump energy density.

Commonly, photostability is estimated by recording the total ASE intensity emitted under a constant pump energy as a function number of pulses or time. The photodegradation is predictable when a decline in the total ASE output intensity is detected [43]. The ASE stabilities of PFO-co-PPV-MEHB (CP) and the laser dye Coumarin 510 (C510) in its best solvent (methanol) were examined and related, as displayed in Figure 8. Both the CP and C510 were kept at a concentration of 2.5 mg/mL, and the pump energy density was 12.75 mJ/cm2. The C510 shows signs of degradation, even though the output was high. On the other hand, both the stability and high output of CP did not change much; this shows that CP is a very stable laser material. The CP output was not significantly reduced even after twenty thousand (20 × 103) shots. The CP output was almost maintained, as presented in Figure 8.

**Figure 8.** Photochemical stability of the PFO-co-PPV-MEHB ASE in toluene and C510 in methanol at a pump energy density of 12.75 mJ/cm2.
