Conclusion

There have been a number of exciting developments on materials for organic lasers over the last decade. Small molecules that form their own resonator upon crystallization have improved in terms of laser thresholds. Although the crystalline state of the molecules may be expected to be detrimental to lasing due to enhanced self-quenching, this issue can now be managed for certain molecular designs. The constraints of a crystal lattice can then lead to materials with improved stability against photo-induced degradation. However, finding the optimal balance between dense molecular packing and reduced self- quenching through intermolecular separation of π-systems remains a challenge. The unique nanostructure of biologically produced fluorescent proteins, which has evolved in a multi-million year process, may provide important guidance in this context.

Over the years organic gain materials were improved and optimized for their laser performance in various ways. In earlier studies the development of new materials was to some extent serendipitous with

improvements made in a trial and error fashion. By careful analysis of the most potent laser materials available, the community then began to develop methods for morphology control so that the crystalline and amorphous character of the materials can now be tuned. In more recent years scientists have focused on the synthesis of molecularly precise architectures and on materials with specific intermolecular and electronic interactions, leading to further advances in terms of threshold, gain and photo-stability.

Perovskites may also well play a role in future organic/inorganic hybrid laser devices due to their relatively low laser thresholds, high absorption coefficient and high charge carrier mobilities.

The recent development of molecules with carefully designed frontier orbitals led to highly efficient small molecule emitters that offer some of the lowest ASE thresholds reported to date (see Figure 2). Furthermore, the development of triplet converting TADF molecules that can be used as or in conjunction with low threshold emitters may help to overcome the challenge of triplet accumulation and thus improve the chances of achieving CW operation and electrical pumping in organic lasers. (While most TADF materials reported to date are of low molecular weight — typically mixed into a small molecule or polymer host — there are already first reports on polymeric materials supporting TADF.104)

It is intriguing to see that fluorene based materials remain amongst the most powerful laser materials, offering the lowest ASE- and lasing thresholds. It is highly promising to see that further improvements to the performance of fluorene homopolymers can be made by fine tuning the fluorene moiety into to indenofluorene, phenanthrene or by variation of the alkyl periphery.

Figure 2: Thresholds for ASE and lasing of organic gain materials reported since 2007 versus molecular weight of the material. Three “world record” low thresholds from before 2007 are also included, marked with WR69,105,106. Round data points represent ASE thresholds and square data points represent laser thresholds. Red data were obtained from host guest systems, green data from glassy and blue data from single crystalline specimen (except for reference [57], which is liquid crystalline).

This leads to the exciting question of whether generally applicable structure property relations exist that allow identifying molecular structures that facilitate particular low ASE and lasing thresholds. Unfortunately, there is no straightforward answer to this question, as the lasing threshold depends on both the material and the optical feedback structure used (feedback structures will be discussed in the next section). In addition, ASE thresholds depend on film thickness, substrate quality and measurement configuration, which makes them difficult to compare between labs. Due to a wealth of different potential applications and interests, the community has so far unfortunately not been able to adopt a standard configuration to characterize organic materials with regard to their lasing performance.

From the data reported in the literature, one empirically finds that the materials, which have shown the lowest ASE or laser thresholds (< 1 µJ/cm2, Fig. 2), are mostly based on fluorene units such as spirobifluorene,44,45 star-shaped molecules with fluorene arms54 and poly(fluorene)s61,64,106 or on fluorene-vinylene-phenylene-vinylene copolymers105 and poly(phenylene-vinylene)s69. In addition, it appears that for all systems tuning of the intermolecular distance and random molecular orientation or amorphous morphology is crucial for obtaining low laser thresholds. This non-ordered molecular morphology can be achieved by dispersion of the gain medium into dielectric host materials, which is often done with small molecules to prevent crystallization and aggregation or by carefully designing the peripheral side groups of macromolecular gain media. The individual molecules need to be separated far enough to prevent self-quenching but when considering electrically pumped lasing (see section 6) separation can be detrimental to charge carrier mobility.64,69

The thresholds listed in Fig. 2 are given in terms of pump fluence (i.e. in µJ/cm2). When this measure is used, the best laser performances are generally achieved with resonators based on the DFB geometry, with mixed order DFB gratings (for efficient feedback and outcoupling) resulting in the lowest thresholds among these106 (section 3.2.1). When instead looking at the absolute threshold pump pulse energy required to reach the lasing threshold, more compact planar or whispering gallery mode resonators reach the most attractive values. To explore the influence of structure, the following section will therefore review the different optical feedback structures that are used by the community and explain their advantages and disadvantages.