Markedly Improved Performance of Optically Pumped Organic Lasers with Two-Dimensional Distributed-Feedback Gratings

Abstract

Continuous-wave (CW) lasing is still difficult to realize in organic laser dyes, one reason being the thermal degradation caused by intense photoexcitation. A decrease in laser threshold suppresses the thermal degradation and, therefore, leads to long-lasting lasing from organic laser dyes. Here, we show that it is possible to decrease the laser thresholds by combining the organic laser dye 4,4-bis­[(N-carbazole)­styryl]­biphenyl, which has a small spectral overlap between the laser emission and the excited-state triplet absorption, with two-dimensional (2D) distributed-feedback (DFB) gratings. We used second-order 2D cross double and square lattice DFB gratings, which offer light feedback in two orthogonal directions, and second-order and mixed-order circular DFB gratings, which offer light feedback in radial directions. Among these grating structures, the mixed-order circular DFB grating structure led to the lowest lasing threshold of ∼0.015 μJ cm–2 under short-pulse photoexcitation because of the excellent optical feedback. Moreover, a low average-power threshold of 10 W cm–2 (a peak-power threshold of 1 kW cm–2) was obtained when laser devices with this grating structure were operated under 1 s of long-pulse photoexcitation with a repetition rate of 0.01 Hz. Additionally, using the mixed-order circular DFB grating structure improved the laser stability under CW photoexcitation since shorter exciton lifetimes in the optical resonator suppress the chemical decomposition. These results demonstrate the importance of choosing an optical resonator structure for improving organic laser performance.

Toward Nonepitaxial Laser Diodes

“…Systematic progress toward a low loss OSLD resonator began with the Fabry–Perot architecture proposed by Kozlov et al, but it has since focused mainly on DFB gratings since they facilitate straightforward integration of electrical contacts and have enabled the lowest threshold optically pumped OSLs to date. ,− ,, In exploring the impact of electrical contacts on the threshold of second-order polymer DFB lasers, Reufer et al found that the combination of a thick Ag bottom grating and a thin (20 nm) ITO top contact led to only a minor (∼20%) increase in the threshold excitation density relative to a contact-free control (Figure ). Interestingly, the Ag bottom grating on its own (with no ITO top contact) had a ∼30% lower threshold density relative to the control, presumably because the associated increase in DFB coupling coefficient outweighs the added dissipation loss from the metal, as suggested by a more pronounced Bragg dip in the PL spectrum.…”

Materials Design of Organic Lasers Aimed at Low Lasing Threshold

“…The state─ of─ the─ art DFB resonators include mixed─ order DFB structures in which the feedback occurs in the 1 st order DFB, and the light emission occurs from the 2 nd order DFB, leading to the surface emission with a low threshold. 6 Further, the DBRs that are sequential multilayers of high and low refractive index materials provide high re ectivity and low absorption compared to the conventional metallic mirrors, providing a verti-cal─ cavity surface─ emitting laser (VCSEL) (Figure 2e), where the optical feedback is in the direction perpendicular to the laser surface. 2,3,5 In addition, the micro─ ring (Figure 2f) and micro─ sphere (Figure 2g) lasers are ring─ shaped and spherical waveguides including a gain medium that traps light and recirculates it to provide optical feedback based on the constructive interference of light in the resonators, respectively.…”

In Situ Super-Hindrance-Triggered Multilayer Cracks for Random Lasing in π-Functional Nanopolymer Films

“…Introducing interface cracks is an intriguing strategy to manipulate optical scattering and diffraction for the improvement of lasing performances [ 27 ]. Although interface cracks usually decrease mechanical toughness and carrier mobility [ 28 ], they can intrinsically possess large refractive index contrast (∆ n > 0.5 to 0.8) to act as natural cavities [ 29 ].…”