Energy Efficiency Studies On Holographic Polymer Dispersed Liquid Crystals Based Distributed Feedback Lasers

Holographic polymer dispersed liquid crystals(HPDLCs)based distributed feedback(DFB)laser is a new type of organic lasers.In the interference light field,the miscible liquids composed of liquid crystals and photoactive polymer monomers are subjected to a photochemical reaction to form a periodic refractive index distributed grating structure in which the liquid crystal layer and the polymer layer are alternating arranged as a “resonant cavity” of lasers.Organic laser dyes with wide fluorescence wavelengths are uniformly incorporated into this "resonant cavity" structure and serves as the gain medium for organic lasers.Finally,the dye produces the inversion population,under the pumping of the pulsed laser,and laser is emitted from the grating.This type of lasers has the advantages of small size,light weight,narrow line width,low threshold and wide wavelength.At the same time,due to the introduction of liquid crystal layers,the laser wavelength and energy can be tuned by electric field or thermal effect.Therefore,the HPDLCs based DFB laser has broad application prospects in the fields of integrated optics,spectral analysis,optical communication and others.However,the low energy conversion efficiency of DFB lasers based on HPDLCs severely limits its application.At present,in the reports of such laser in other groups,the energy conversion efficiency is only 0.8%.To solve this problem,we carried out the studies on the energy efficiency of HPDLCs based DFB lasers.Firstly,we investigated the “resonant cavity” feedback efficiency and fabricated the scaffolding-like morphology grating,under small exposure intensity(2.5 mW/cm2),with low scattering loss of less than 6%.Besides,the orientation of the liquid crystal layers in the small periodic grating,with a period of 405.5 nm,are controlled by friction orientation,and the refractive index modulation is increased from 0.004 to 0.041.The energy conversion efficiency of dye laser is enhenced from 0.7% to 1.3%,and the laser threshold is 3 uJ/pulse.However,considering that the concentration of the laser dye is too low(no more than 1%),the absorption of pump laser is only 16%,slightly higher concentration will produce a quenching effect to reduce the intensity of emitted lasers.To solve the dye concentration quenching problem and improve the utilization ratio of the pump energy,we used a novel semiconductor luminescent polymer material MEH-PPV as the gain medium.This luminescent material does not have concentration quenching problem,and the pump absorptivity reached 70% with 85 nm MEH-PPV film,which is 4.4 times for that of laser dye.The HPDLCs grating structure is formed upon MEH-PPV film to form a new type of organic semiconductor laser with gain layer and feedback layer separated because of the immiscibility between MEH-PPV and liquid crystals.The laser energy conversion efficiency is further increased to 5.8% which is 4.5 times for that of laser dye,this tells one fact that the feedback efficiency is not decreased,and the laser threshold is reduced to 0.33 uJ/pulse.On this basis,we carried out vacuum thermal annealing treatment on MEH-PPV films,which improved the degree of film order and enhanced the luminous efficiency.After annealing at 120 °C for an hour,the absolute value of the photoluminescence quantum yield(PLQY)of MEH-PPV films were increased from 23% to 32%.The laser energy conversion efficiency increased to 6.7% and the threshold reduced to 0.2 uJ/pulse with 45 ° polarized pulsed laser pumped.Furthermore,we fabricated the other DFB laser based on semiconducting luminous film MDMO-PPV which was pretreated with the same technique.The laser energy conversion efficiency was 7.2% and laser threshold was 0.15uJ/pulse,which proved MDMO-PPV was better than MEH-PPV.Finally,we developed two types of DFB lasers with blended gain media to investigate the energy transfer between them and enhance the energy efficiency of dominant wavelength.The first type laser was made by combining MEH-PPV with MDMO-PPV in proportion as 2:1,share similar emitted energy levels,to form the mixed gain film,and the dual-wavelength laser with wavelengths of 629.9 nm and 640 nm was achieved by fabricated a 405.5 nm HPDLCs grating,which illustrated there was no energy transfer between them.The energy conversion efficiency of the former laser,emitted from MEH-PPV,was 4.8%,and that of MDMO-PPV was 2.8%.The second type laser was formed by doping DCM into the HPDLCs grating formed upon the MDMO-PPV film.The energy conversion efficiency of MDMO-PPV and DCM were increased from 7.8% to 8.0% and decreased from 1.4% to 1.0%,respectively.When the MDMO-PPV film was formed with thickness gradient,more interesting phenomenon apperaed,the laser emitted from DCM located at 614 nm and blue shifted 7 nm under electric field,and laser from MDMO-PPV,changed with film thickness,ranged from 625 nm to 640 nm.In this dissertation,the HPDLCs grating structure characteristics,the laser dyes and semiconductor polymer luminescence properties are investigated in detail.We further improve the laser performance by increasing the refractive index modulation,enhancing the absorptivity and optimizing the luminescence performance of gain film.The conclusions show in the dissertation will lay a solid foundation for the application process of organic DFB lasers based on HPDLCs grating.