Existence of continuous-wave threshold for organic semiconductor lasers

We develop a model that predicts two threshold pump intensities in optically pumped organic semiconductor lasers (OSLs); one for pulsed lasing, $I_{PS}$, and another for continuous-wave (CW) lasing, $I_{CW}$. The theory predicts a decrease in $I_{CW}$ from 32 kW/cm${}^2$, or well above the damage threshold, to 2.2 kW/cm${}^2$, for a laser employing 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran–doped tris(8-hydroxyquinoline) aluminum if the triplets can be effectively removed from the emissive guest. Based on this analysis, we demonstrate that the lasing duration can be extended to nearly 100 μs, ultimately limited by degradation of the lasing medium when a “triplet manager” molecule, 9,10-di(naphtha-2-yl)anthracene, is blended into the gain region of an otherwise conventional distributed feedback OSL. The triplet manager facilitates radiative singlet transfer while suppressing nonradiative triplet transfer to the emitter molecule, thus reducing the triplet-induced losses. Our theory conclusively shows that these lasers have entered the CW lasing regime.

Figure 1

Separate channels for singlet ($S$) and triplet ($T$) formation and transfer in triplet-managed lasers. Singlets are generated (circles) on both Alq${}_3$ and ADN, and Förster transferred (solid arrows) to DCM2. Triplets are generated by ISC and collected by ADN through Dexter transfer (dashed arrows).

Figure 2

(a) PL and (b) lasing transients measured at 1.6 kW/cm${}^2$ pump intensity for different host blends. PL transients are normalized by the peak intensities, and lasing transients are normalized to 1 for $x$ = 0, 10, and 30 ADN blends and to 5 for $x$ = 50 and 70 ADN blends. The fits are obtained by the model described in the text, with the parameters summarized in Table . The inset shows the lasing spectrum of an $x$ = 70 OSL.

Figure 3

Simulated threshold singlet population, $S_{TH}\left(t\right)$ with ($x$ = 70, stars) and without (squares) a triplet manager, and $S\left(t\right)$ (lines) for the $x$ = 70 OSL. Lasing occurs when $S\ge S_{TH}$. The dashed lines correspond to lasers that have exceeded their pulsed threshold singlet population ($S_{PS}$) but not their CW threshold ($S_{CW}$). In the left inset is a streak camera image of laser emission for a triplet-managed OSL ($x$ = 70) measured at a 2.4 kW/cm${}^2$ pump intensity and a 18-μs pulse width. In the right inset is a simulated lasing duration evolution with increasing pump power for a $x$ = 70 triplet-managed OSL.