Current reflearch and future development of organic laser materials and devices
Authors: Zhang Qi  , Zeng Wen-Jin  , Xia Rui-Dong  ,   Key Laboratory for Organic Electronics and Information Displays Institute of Advanced Materials(IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts Telecommunications, Nanjing 210023, China
Published: December 2020

Laser has been widely applied in the scientific and industrial areas, including materials, medicine, military and telecommunications, due to its extreflely well-defined frequency, narrow divergence and high intensity. In reflent fifty years, various laser sources have been developed. The laser output power, pulse duration, and attainable wavelengths have been greatly improved. To date, further optimization on laser is mainly focused on the three aspects: an effective gain medium capable of amplifying light, a convenient pump source, and a high efficient resonator (or cavity). Among these aspects, the gain medium plays a very important role in the generation of efficient and high-quality laser.
Lots of laser materials have been explored and developed, among them, organic laser materials, small molecules or polymers based on -conjugated structure, have been attracting more and more attention in the current reflearch of high efficiency laser. Organic laser have advantages such as simple fabrication, low cost, easy integration, and so on. Although the organic lasers with optical pump source have been extensively reflearched, the issues how to achieve electrically pumped organic lasers, or the so-called organic laser diodes, still remain unsolved. Nevertheless, the prospects of organic laser are very promising, such as its application in spectroscopy, chemical sensor (e.g. trinitrotoluene or DNA sequences) and short-haul data communication. In this review, we try to draw a picture of the organic laser reflearch form its first appearence till the end of 2014, with emphasis on the latest progress and variation trends, instead of providing a complete survey of organic laser reflearch. In the first part of this paper, different types of organic materials used for lasers are briefly reviewed. First, basic rules for the selection of suitable materials for organic lasing are summaried as: 1) the appropriate energy level distribution for creating four-level systems; 2) a high-stimulated emission cross-section e, which should affect the gain and threshold; 3) an appropriate radius for host-guest blend if energy transfer system is applied; 4) the low stokes shift to reduce the pump energy converted into heat; 5) a low excited-state absorption to reduce the self-absorbance loss; 6) a low intersystem crossing rate and a low triplet-triplet absorption cross-section to eventually lower the triplet lifetime; 7) a high photoluminescence efficiency in solid-state, i.e. a low - packing; 8) the good stability against oxygen and moisture and photo stability against pump light. Such organic gain media are classified into dyes, semiconductors, and new-concept materials. The active host-guest system is also discussed, which is different from the dispersion chromophore in the inert matrix (e.g. PMMA). This energy transfer strategy has been well proved to be effective to improve the absorption of pump energy and move the absorption band away from the emission band. It is possible, therefore, to reduce the self-absorbance loss to lower the threshold of lasing.
In the second part, different geometries and features of the most commonly used cavity are discussed to investigate the dynamic balance between the gain and loss inside the lasing operating system. We divide the resonator structures into the catalogs of planar waveguides, curved surface cavities, and vertical external cavity solid organic larers (VECSOL). The widely used types of planar waveguides are DFB and DBR. The lasing thresholds of these structures areflextreflely low and their emission wavelength can be tuned by changing the thickness of the organic layer or the period of the modulation.
In the third part, current progress and future reflearch direction of the organic lasers are summarized. The challenge of electrically pumped organic laser (or organic laser diode) remains to be the major driving force for the scientific community to be devoted to the reflearch of organic lasers. Estimation of operating current based on the optical-pumped laser data is only 100 Acm-2. Actually, very high current densities of the order of kA cm-2 (even higher) have been realized both in pulsed OLEDs and light-emitting field-effect transistor (LEFET) devices. But lasing is still not observed. The extra losses brought about by electrical driving can be summarized as follows: 1) the electrodes used for electrical injection; 2) the charge carriers with broad absorption bands overlapping the emission; 3) the triplet excitons with longer lifetime and higher creation probability ratio. LEFET is now the most promising device structure of organic laser diodes. Unfortunately, LEFET is not applicable for dealing with the triplet trouble which is inherent in the organic materials. The proposition of new concept on directly pumped organic lasers seems to be an alternative way to solve this problem.
Finally, we would like to describe the reflent progress in optically pumped organic lasers briefly. Efforts which have been made can be summarized as follows: lowering the lasing threshold, increasing the wavelength coverage (to the deep red or infrared and to the ultraviolet), improving the wavelength sensitivity, enhancing the lifetime of the devices, or improving the conversion efficiency, output power and beam quality. Although these progresses are realized under the condition of optical pumping, all these achievements are meaningful since they constitute the bases of future organic laser diodes.