The Research Of Fabricating A High Conversion Efficiency Liquid Crystal/Polymer Grating Laser

Organic lasers are promising novel lasers because of broad band range lasing and broad band tuning properties,and they can be applied to the field that inorganic semiconductor lasers show disadvantages.Organic emitting materials are synthesized by chemical techniques,and high quality optical films can be obtained by a simple spin-casting method.The lasers using organic materials as active medium possess the advantages such as ease of miniaturization,ease of integration and cost-effectiveness,which shows bright future in the integration optical field such as spectroscopy,sensing and communication.Distributed feedback configurations are extensively used as oscillation cavities for organic lasers because of outstanding spectrum selection and low threshold operation characteristics.Liquid crystal/polymer gratings are alternatively liquid crystal layered and polymer layered structures fabricated by a holographic exposure technique using dual or multi-beams.Liquid crystal/polymer gratings are a suitable candidate used as distributed feedback configurations because of ease of fabrication,cost-effectiveness and ease of design.However,the morphologies of the Liquid crystal/polymer gratings in the literature are rough and unclear,which will lead to more scatterings,elevate the lasing threshold and reduce the conversion efficiency of lasers.The conversion efficiency in the literature for an organic laser is from 0.5 to6%,and the coverage of the lasing range is limited,which restrict the commercial application of organic lasers.The researches were conducted to solve the aforementioned problems as follows.The five functionality monomer DPHPA,which possesses multi-side chains,was used in the pre-polymerization syrup.The monomer DPHPA generated rich monomer radicals during the optically induced chemical reaction period,which helped to construct dense and stable polymer layers.Thus,the phase separation was so complete that the morphology was excellent.As a result,a Liquid crystal/polymer grating suitable used as distributed feedback oscillation cavity,which possessed the diffraction efficiency as high as 91.2%and the scattering losses as low as 1.51%,was fabricated.The non-aromatic strong polarity solvents tetrahydrofuran and chloroform,and the aromatic weak polarity solvents chlorobenzenen,toluene and p-xylene were used to dissolve MEH-PPV so as to investigate the molecular orientation and light emission of the spin-casting MEH-PPV films with distinct dissolution environments.The waveguide ASE experiments indicated that the MEH-PPV films possessed a higher net gain when non-aromatic strong polarity solvents were used to dissolve MEH-PPV,which helped to reduce the lasing threshold and improve the conversion efficicency.The lasing threshold and conversion efficiency were 6.7?J/cm~2 and 9.5%,respectively,when tetrahydrofuran was used to dissolve MEH-PPV.However,when p-xylene was used to dissolve MEH-PPV,the lasing threshold and conversion efficiency were 25.0?J/cm~2 and 4.9%,respectively.The lasing excitation experimental results indicated that the side chains tended to orient non-plane to the main chains when non-aromatic strong polarity solvents were used to dissolved MEH-PPV,which inhibited the interaction of?electron clouds from inter-chains.Thus,the fluorescence quenching process induced by a dual-molecule mechanism was reduced.As a result,the lasing threshold was reduced and the conversion efficiency was improved in comparison to the side chains tending to orient in-plane to the main chains when aromatic weak polarity solvents were used to dissolve MEH-PPV.The DCM-doped liquid crystal/polymer grating was fabricated.The thickness of the MEH-PPV film was controlled at 150nm to inhibit the light emission so as to build the quasi-waveguide geometry.The waveguide ASE experiments indicated that the quasi-waveguide geometry possessed a larger net gain than that of the waveguide geometry.Thus,the light would be amplified intensively when it propagated in the quasi-waveguide geometry,which was beneficial to reduce the lasing threshold and improve the conversion.The 149.1nm lasing ranged from 573.2 to 722.3nm was obtained by varying the grating period in the quasi-waveguide geometry.The dual-wavelength lasing was achieved using a high diffraction order in the quasi-waveguide geometry,and the dual-wavelength lasing covered more than 40nm.At last,a tri-wavelength lasing was achieved by controlling the thickness of the MEH-PPV film at 80nm.The tunable range was as high as 8nm when the temperature was elevated over the clearing point.The works in this dissertation make huge contributions to realize the fabrication of a novel organic tunable distributed feedback laser,which make a further step to the commercial application of distributed feedback organic lasers.