Abstract
Organic lasers have undergone decades of development. A myriad of materials with excellent optical gain properties, including small molecules, dendrimers, and polymers, have been demonstrated. Various resonator geometries have also been applied. While sharing the advantages of the solution processability and mechanical flexibility features of organic materials, organic optical gain media also offer interesting optical properties, such as emission tunability through chemical functionalization and inherent large optical gain coefficients. They offer prospects for different applications in the fields of bioimaging, medicine, chemo‐ and biosensing, anticounterfeit applications, or displays. However, the realization of electrically pumped organic lasers still remains a challenge due to the inherent drawbacks of organic semiconductors, e.g., modest carrier mobility, long‐lived excited‐state absorption, and extra losses which originate in the device (e.g., absorption from metal electrodes). Herein, the past developments of organic lasers are discussed, highlighting the importance of materials and cavities with regard to the goal of electrically pumped organic lasers. The latest progress and the possible ways to address the challenge are discussed.
Highlights
In an organic sandwich-type structure, the estimation of minimum current density to reach the lasing threshold is %100 A cmÀ2.[12b,37] Typical organic light-emitting diodes (OLEDs) devices are usually driven under much smaller current densities in the range of 0.01 AcmÀ2,[38] yet current densities of the order kA cmÀ2 or 100 kA cmÀ2 can be reached in OLEDs[39a] and in light-emitting field-effect transistors (LEFETs), respectively.[39b,c] Considering the extra losses mentioned above, the current density needed to achieve net gain can exceed this value
Quasi-CW lasing was demonstrated with a threshold of 0.25 μJ cmÀ2 on the same optical gain medium in second-order distributed feedback (DFB) upon photoexcitation at 365 nm with 10 ps and 8 MHz repetition rates.[48e]. A further improvement was later reported in mixed-order DFBs and improved encapsulation, achieving lasing at the threshold of 0.25 μJ cmÀ2 upon 80 MHz pump pulse repetition rate
The very first trial with thermally activated delayed fluorescence (TADF) materials in light amplification involved the use of 3-(9,9-dimethylacridin-10(9H)-yl)-9H-xanthen-9-one (ACRXTN) as a triplet harvester codoped together with 2,3,6,7tetrahydro-1,1,7,7,tetramethyl-1H,5H,11H-10-(2-benzothiazolyl) quinolizino-[9,9a,1gh] coumarin (C545T) in a 3,3-di(9H-carbazol9-yl)biphenyl host.[34a]. The resultant ternary blend exhibited superior optical gain and electroluminescence properties with a lower threshold and higher efficiency compared with C4C5T:mCBP without ACRXTN dopant
Summary
When enumerating the key optical properties of organic semiconductors, strong absorption and efficient luminescence are often mentioned. The typical absorption cross-section (σabs) value for π-conjugated molecules and polymers is in the order of 10À16 cm2 This value guarantees 90% light absorption for film thickness in the 100 nm range, which brings two major benefits. It constitutes a strong asset for optical-pumping operation as it allows the achievement of population inversion at low pumping fluences in films.[6b] Second, the SE cross section (σSE) is considered to be correlated with the absorption cross section (σabs), enabling high optical gain.[13] Broad fluorescence spectra allow, on the other hand, for broadband amplification.[14,15] In addition, the photoluminescence (PL) spectra can be tailored by backbone modification, e.g., by controlling the conjugation length[16] or by the introduction of electron-donor or electron-acceptor groups,[17] in contrast to inorganic semiconductors where the emitting wavelengths are restricted to lattice-matching requirements. Geminate polarons located on different chains show a more persistent detrimental effect over optical gain.[19c]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
