Quasi‐Continuous‐Wave Organic Thin‐Film Distributed Feedback Laser

Abstract

wileyonlinelibrary.com COMMUNICATIONoverlap between emission and triplet absorption in BN-PFO. [ 10 ] Therefore, the development of organic laser dyes with less spectral overlap between the excited-state absorption and emission is crucial for the realization of cw and quasi-cw lasing with low threshold.In our group, we have continuously investigated the optical and amplifi ed spontaneous emission (ASE) characteristics of many organic materials with the aim of realizing electrically pumped organic laser diodes. [21][22][23][24][25][26][27] Among them, 4,4′-bis[( Ncarbazole)styryl]biphenyl (BSBCz) is one of the most promising candidates because a vacuum-deposited fi lm of the host material 4,4′-bis( N -carbazolyl)-1,1′-biphenyl (CBP) blended with 6 wt% BSBCz, the chemical structures of which are shown in Figure 1 a, possesses outstanding optical and ASE characteristics, such as a high photoluminescence quantum yield ( Φ PL ) of nearly 100% and short PL lifetime ( τ PL ) of about 1.0 ns, leading to a large radiative decay constant ( k r ) of about 10 9 s −1 and low ASE threshold energy of about 0.3 µJ cm −2 . [ 23,26 ] In this paper, we report quasi-cw surface-emitting lasing in a distributed feedback (DFB) device based on this BSBCz:CBP blend fi lm. In this laser device, we obtained the highest repetition rate (up to 8 MHz) and the lowest threshold (on the order of 0.25 µJ cm −2 ) ever reported for a quasi-cw laser based on organic thin-fi lm systems. The incorporation of triplet quenchers is not necessary in our blend fi lm because of its high Φ PL and no signifi cant spectral overlap between the emission and triplet absorption of BSBCz. [ 24 ] In a DFB structure, a laser oscillation takes place when the following Bragg condition is satisfi ed: mλ Bragg = 2 n eff Λ , where m is the order of diffraction, λ Bragg is the Bragg wavelength, n eff is the effective refractive index of the gain medium, and Λ is the period of the grating. [ 28,29 ] When considering a second-order mode ( m = 2), the grating period is calculated to be Λ = 280 nm using the reported n eff and λ Bragg for BSBCZ. [ 21,22 ] A grating with Λ = 280 nm provides surface-emitting lasing in a direction normal to the substrate plane as shown in Figure 1 b. Although a second-order grating typically leads to a higher lasing threshold compared to a fi rst-order grating, surface-emitting lasing using a second-order grating is suitable for the fabrication of electrically pumped organic laser diodes having an organic light-emitting diode structure showing the same surface emission. [ 30,31 ] Using electron beam lithography and reactive ion etching, such gratings were directly engraved onto silicon dioxide surfaces over a 5×5 mm 2 area. Figures 1 c,d shows scanning electron microscopy (SEM) images of a representative grating fabricated in this study. We obtained Λ = 280 ± 2 nm and a grating depth of d = 70 ± 5 nm from the SEM images, which are in perfect Since the discovery of organic solid-state lasers, [1][2][3][4][5][6] great efforts have been devoted to th...

Molecular‐Barrier‐Enhanced Aromatic Fluorophores in Cocrystals with Unity Quantum Efficiency

“…100 fs, which relies on chromophore interactions . Efforts to avoid a slow ISC process and control the formation of triplets have been suggested by either utilizing the intermittent excitation methods in engineering (replenishing dye fluids, pulse photoexcitation, or spinning micro‐disk) or using a blend of fluorophores and scavengers that could partially quench the triplets . However, the residual triplet states retain significant losses and are hitherto the major problem in evolving organic optics, for instance, in developing compact organic optical devices.…”

“…100 fs,which relies on chromophore interactions. [9] Efforts to avoid aslow ISC process and control the formation of triplets have been suggested by either utilizing the intermittent excitation methods in engineering (replenishing dye fluids, pulse photoexcitation, or spinning micro-disk) [10,11] or using ab lend of fluorophores and scavengers that could partially quench the triplets. [12,13] However,t he residual triplet states retain significant losses and are hitherto the major problem in evolving organic optics,f or instance,i nd eveloping compact organic optical devices.H erein, we report an innovative synthetic approach where high-energy-gap molecular barriers are intercalated in ap eriodic p-conjugated molecular array ( Figure 1A), which effectively cut off the interaction of the low-energy-gap fluorophores,r esulting in the absence of triplet states.…”

Light Amplification in Molecules Exhibiting Thermally Activated Delayed Fluorescence

“…[16][17][18][19][20] Thus, since DABNA-2 has a favorable window for q-CW lasing around the 0-1 transition band, i.e., no presence of σ T , q-CW lasing with an enhanced contribution from triplet excitons is expected to be possible. In the ASE measurement, a nitrogen gas laser (KEN2020, Usho Optical Systems Co., Ltd.) at an excitation wavelength of λ = 337 nm with a 0.8 ns pulse (an operating frequency of 20 Hz) was used as optical excitation source.…”

Degradation Mechanism and Stability Improvement Strategy for an Organic Laser Gain Material 4,4′‐Bis[(N‐carbazole)styryl]biphenyl (BSBCz)

“…A low laser threshold of 1.0 µJ cm −2 was demonstrated from 4‐dicyanomethylene‐2‐methyl‐6‐sp‐dimethylaminostyryl‐4H‐pyran (DCM) doped into a tris(8‐hydroxyquinoline)aluminum (Alq 3 ) host film . Since then, organic laser performance has significantly improved from the development of new gain materials, device architectures, and triplet managing materials and clarification of the underlying physics of laser oscillation …”