Existence of continuous-wave threshold for organic semiconductor lasers
Yifan Zhang, Stephen R. Forrest
Physical Review B
We develop a model that predicts two threshold pump intensities in optically pumped organic semiconductor lasers (OSLs); one for pulsed lasing, I P S , 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
... . 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. Optically pumped organic semiconductor lasers (OSLs) with low thresholds and wide spectral tuning ranges have attracted interest since their demonstration 15 years ago. 1-4 However, a significant obstacle to the application of OSLs has been their limitation to only pulsed operation with a maximum duration of several tens of nanoseconds. 5-7 This limitation is imposed by the buildup of triplet (T ) excitons in the gain region that are generated from intersystem crossing (ISC) of radiative singlets (S). 6, 8, 9 Since relaxation from the triplet to the ground state is quantum-mechanically forbidden, 10 the lifetime of triplet exciton is large (several milliseconds) compared to that of a singlet (several nanoseconds), allowing the triplet population to accumulate over time. The high triplet population, together with overlapping singlet emission and triplet absorption, results in singlet and photon losses that ultimately shut down lasing, thereby preventing continuouswave (CW) operation. While triplet losses in liquid dye lasers can be mitigated by using quencher molecules with triplet energies lower than that of the dye, 11,12 no CW operation has been realized without dye circulation. For OSLs, gain medium circulation is not possible; however, several efforts have been made to mitigate, although not eliminate, triplet losses to the extent that CW operation can be achieved. Bornemann et al. 13 have used a rapidly rotating substrate to demonstrate a CW solid state dye laser, but the output was unstable. Schols et al. 14 have shown that "scavengers" can be used to de-excite triplets, but no lasing improvement was demonstrated. Rabe et al. 15 and Lehnhardt et al. 16 demonstrated that a polymer OSL pumped by very low duty cycle (<0.1%) pulses extended the total duration to 400 μs, although this is not a true CW operation. Here, we introduce a "triplet manager" into the gain region, along with the guest emitter and host molecules. The manager reduces the emitter triplet population, thus extending the lasing duration. The inset of Fig. 1 shows the triplet management concept. The manager has lower triplet energy and higher singlet energy than the emitter. When either the host or the manager molecules are excited, Förster transfer 17 of singlet states to the emitter is highly efficient. Furthermore, Dexter transfer 18 of triplets leads to their trapping on the manager since it has lower triplet energy than both the guest and the host. The manager triplet absorption is shifted from the guest emission; thus, the trapped triplets do not contribute to optical losses 9 or singlet quenching. 6 The 200-nm-thick OSL active region consists of the manager, 9,10-di(naphtha-2-yl)anthracene (ADN), codeposited into the conventional guest-host gain medium consisting of 2 vol% of the red emitting 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2) in tris(8hydroxyquinoline) Al (Alq 3 ). The singlet and triplet energies are determined from fluorescence at room temperature and phosphorescence at 14 K, respectively.    Here, ADN has a lower triplet energy (1.69 eV) and higher singlet energy (2.83 eV) than Alq 3 (T = 1.99 eV and S = 2.38 eV). Furthermore, S = 2.03 eV and T = 1.74 eV for DCM2. This system, therefore, is energetically consistent with Fig. 1 . The manager concentration in (100 − x) vol% Alq 3 is x vol% ADN (x = 0, 10, 30, 50, 70, 100). Blended films were deposited by thermal evaporation in high vacuum (10 −7 torr) on quartz, Si, and 2-μm-thick SiO 2 -on-Si substrates for characterizing absorption, photoluminescence (PL), and triplet absorption, respectively. The same films were deposited on gratings with a period of 430 ± 5 nm and a 50-nm depth on the SiO 2 -on-Si to form distributed feedback (DFB) OSLs. Output from a 0.6-W laser diode at wavelength λ = 405 nm was focused to a 150 × 250-μm spot to optically pump the thin film. Alq 3 and ADN pure film absorption coefficients were measured to be 4.8 × 10 4 and 9.1 × 10 −4 cm −1 , respectively, at λ = 405 nm, and are assumed to contribute to the total blend film absorption proportionate to their volume. All measurements were performed in N 2 ambient to minimize film degradation. Figure 2 shows the PL and lasing transients pumped at 1.6 kW/cm 2 . From Fig. 2(a) , the Alq 3 host undergoes a 55% reduction in PL to its steady state value within 30 μs of the onset of the pump. Previous studies have shown that this intensity roll-off is due to singlet quenching from S-T annihilation.