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  • Final Report Summary - LASERS FORWARD (Implementing Crystalline Materials as the Active Medium in Organic Solid State Lasers: Pushing Forward the Limits of Electrically Driven Lasers)

Final Report Summary - LASERS FORWARD (Implementing Crystalline Materials as the Active Medium in Organic Solid State Lasers: Pushing Forward the Limits of Electrically Driven Lasers)

Lasers Forward aims to achieve a breakthrough in organic semiconductor lasers by using recent advances in organic molecular crystals to make improved lasers and address the major challenge of making an electrically pumped organic semiconductor laser. Molecular crystals are of special interest due to their high degree of molecular order results in high charge carrier mobility, overcoming a serious limitation with existing organic laser materials. The phenomenon of concentration quenching of fluorescence in the condensed state will be overcome using recent advances in crystal engineering to give efficient fluorescence via aggregation induced enhanced emission. The main objectives of the project are. A) Understanding and optimizing the gain medium. b) Understanding and controlling loss mechanisms in the non-/linear regime. c) Optimizing the resonators suitable for lasing in organic crystalline materials. d) Device design and development. In implementing this ambitious project, I will be able to get expertise in the photophysics of laser materials, resonator design and fabrication, and laser implementation and testing in a department internationally recognized for optoelectronics and laser research.
The strategy adopted was to identify suitable materials that shows stimulated emission under optical pumping conditions. The materials with sufficiently low ASE threshold (Pth > 0.5µJ/cm2) will be investigated for the direct/indirect electrical pumping. Three different series of organic materials were investigated for light amplification under optical pumping conditions, a) donor-acceptor molecules, b) solid state emitters based on excited state intramolecular proton transfer (ESIPT) molecules, c) linear conjugated oligomers (Scheme-1 See attached documents). Donor-acceptor system consists of naphthalene monoimide as central acceptor core and piperidinyl group attached to 6th position of the naphthalene ring as donor. Different derivatives with subtle variation in the substitution in the non-conjugated part was investigated to understand the effect of molecular packing in the stimulated emission properties of single crystals. Different types of molecular packing were observed with variation in the substitution due to the steric demand as well as the nearest neighbor electronic interactions. The investigation of gain in these single crystals led us to the conclusion that the molecules with increasing π-π overlap leads to the increase in the ASE/lasing threshold and deter the stimulated emission process. The threshold values ranges from 1mJ/cm2 to 4.5mJ/cm2. The threshold obtained was three orders higher than the preferred conditions suitable for further investigations. However, provides insight into the intermolecular interactions that deter stimulated emission and the for first time showing stimulated emission in co-crystals (compound 5 of scheme1, See attached documents).
Focus was then shifted to ESIPT emitters, where the molecules have interesting photo-physics with different absorbing and emitting species on photoexcitation (Scheme1, b, See attached documents). The objective here is to avoid the accumulation of charged species on the emitting species on electrical excitation. Low fluorescence quantum efficiency is the major shortcoming in ESIPT emitters. Emissive ESIPT materials with photo-luminescence quantum efficiencies (PLQY) up to 30% was investigated for lasing. A series of eight derivatives with subtle variation in substituents were investigated for stimulated emission properties. The variation in substitution pattern induces little changes in the emission characteristics of these derivatives in solution. However, induces drastic changes in the emission properties in single crystals with the emission color tuned along the entire visible region. Under optical excitation only one of the derivative (Scheme1, b) showed stable stimulated emission with a lasing threshold of 1mJ/cm2. Other derivatives also exhibit gain narrowing however cannot be quantified due to the very low damage threshold of these crystals. Charring was observed on photoexcitation that is sufficient enough to show gain in other derivatives. The major reason for higher threshold is expected to be the loses that are of both internal and external in nature. The internal loses might be due to the wastage of energy on downhill migration towards the keto form. The external loses are mainly due to the uneven crystal habit as these crystals were grown from solution. The steps, knicks and the disorders in the crystals habit acts as scattering centers leading in strong out coupling of the emitted light.
The focus was then shifted to crystalline thin films of conventional π-conjugated linear oligomers (Scheme1, c, See attached documents) that are known to form good quality thin films on thermal evaporation. Thin films of these materials were prepared by thermal evaporation. Thermal evaporation on glass substrates at a rate of 2 Å/s leads to films with a microscopic morphology of disordered agglomerates of materials (figure 1a). Uniform thin films with layered morphology of molecular step size was prepared by evaporating the materials at a rate of 2 Å/s on to a heated substrate at 140 C. Thin films were also prepared on freshly cleaved mica substrates to have epitaxial growth that directs the crystallites to grow in specific orientations. Atomic force microscopic images of the thin films of phenylene/thiophene co-oligomer prepared on glass, on heated glass (140 C) and on freshly cleaved mica surface is depicted in figure 1. Thin films were also prepared on distributed feedback structure developed from nano-imprinting technique. However, these techniques reduce the threshold to level of 0.5mJ/cm2. Due to high lasing threshold exhibited by the active materials hinder these materials to be further used in the direct pumping. Indirect electrical pumping was attempted using nitride LED however not shown any gain narrowing. This is mainly due to the very high gain narrowing threshold exhibited by these materials. The strive will continue to find suitable materials that having a lasing threshold below 1µJ/cm2.
In due course of investigation, fellow have set-up femtosecond pump probe, physical vapour transport apparatus and experimental set up to measure gain/loss in single crystals and thin films. The fellow is subjected to a variety of techniques that required for the implementation of the project. For crystal growth from solution, slow evaporation of solvent, diffusion crystal growth, seeding and epitaxial crystal growth on substrates were utilized for single crystal development. Physical vapor transport technique was developed to have crystals with more regular crystal habit than the crystals developed from solution. Fellow is trained on atomic force microscope and different types of optical microscopes for morphology/crystal habit characterization. The fellow got trained on wet deck practices and the practices inside a class 10000 clean room. Training was given on thermal evaporators, magnetron sputterer, spin coaters and the usage of these systems under controlled nitrogen atmosphere in a glove box. Training was also rendered on EVG nanoimprint lithography system for grating preparation. The fellow acquired experience on time correlated single photon counting techniques, transient absorption, streak cameras, fluorescence up conversion, photo-absorption/emission spectrometers for photophysical characterization of the organic molecular materials. Collaborations with several research groups were established such as synchrotron facility at Stanford for grazing angle x-ray scattering (GIWAXS) characterization and analysis, University of Graz for specular and pole XRD measurements, University of Bolonga for naphthalene monoimide based donor-acceptor systems, Seoul national University for ESIPT and AIEE based emitters. Our investigations led to a better understanding of the crystalline gain media and the factors that affect the gain and loses. The results obtained during the period of the fellowships were disseminated within the group as well as to a wider scientific community. The part of the results obtained during the fellowship is presented in MRS Fall meeting at Boston, USA, 2015 and E-MRS 2015 Spring meeting at Lille, France. Three publications from the results obtained during the reporting period is under preparation.

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