Distributed Feedback Lasers Based on Thiophene/Phenylene CoOligomer Single Crystals

Organic solid state lasers have attracted considerable attentions because they have the potential to be tunable, flexible, biocompatible, and easily integrated into plastic optoelectronics.[1–8] Optically pumped laser has been reported in a very broad range of conjugated polymers and oligomers. However, for the electrically pumped organic lasers, there still exist many challenging problems to be resolved.[5,9–11] One is that the organic devices have been damaged before the injected current reaches the expected threshold current density (kA/cm2 range) for electrically driven lasing due to the low charge carrier mobilities of the disordered amorphous materials. In the view of carrier transport and charge injection, single crystalline organic semiconductors are recognized as potential building blocks for electrically pumped organic lasers. The long-range order and high chemical purity in crystals make them intrinsically excellent in charge-carrier transport properties, whose mobility could be significantly increased by three orders or more from their amorphous phase to the single-crystal phase without any serious luminescence efficiency decrease.[12–15] Notable achievements on the amplified spontaneous emission (ASE) have been attained in a large number of crystalline materials.[16–22] For a laser device, resonator structure is necessary to apply the positive optical feedback, which may reduce the lasing threshold significantly. Within this framework, several attempts were made to construct a resonator for crystals. Examples include the use of naturally formed cleaved facet as Fabry-Pérot resonator,[23–24] or the microcavities fabricated with electron-beam (EB) lithography and reactive ion etching.[25–27] With the help of these resonators, the lasing threshold could be reduced to half or a fifth of the values observed for the bulk crystals. As counterpart of cavity resonators, diffractive resonators, such as distributed feedback (DFB),[28–32] are extensively investigated. They can be readily incorporated into planar organic semiconductor waveguides and allow surface emission. These kinds of resonators have witnessed the great success in the polymer materials. A variety of fabrication schemes have been demonstrated to construct the DFB cavities, such as UV embossing, nanoimprint lithography, soft lithography, liquid imprinting, micromolding. Unfortunately, most of these methods are limited to organic crystals, because they are generally sensitive to solvent and the fragility makes them difficult to handle. As a dry method, laser ablation has been reported to process the organic materials. Although it has been used to fabricate polymer/amorphous organic DFB lasers,[32–33] organic crystal DFB laser has not been reported, as far as we know. In this work, we have achieved distributed feedback laser based on the organic single-crystalline thin film materials. Diffractive resonators in the high-quality thin film crystals could be easily fabricated with a one-step, noncontact, and dry method – ultraviolet (UV) laser interference ablation (LIA). Different periodic gratings with optical quality could be directly formed in the crystals by changing the incident angle of the interference beam. With the help of the right periodic structures, distributed feedback (DFB) resonators were constructed in the crystals to provide the feedback. In particular, we observed laser emission with threshold of Eth 25 J cm 2 in the distributed feedback resonators. More important, this method avoids both the dissolution of organic crystals and the need of good-quality end facets. This is a crucial aspect for future implementations of high-yield production. Distributed Feedback Lasers Based on Thiophene/ Phenylene Co-Oligomer Single Crystals