Performance Studies On Holographic Polymer Dispersed Liquid Crystals Based Organic Lasers

Holographic polymer dispersed liquid crystal(HPDLC) grating is composed of alternating layers of the polymer and the phase separated LC, and forms a refractive index periodic structure. We can easily obtain dye-doped HPDLC DFB lasers by doping laser dyes into the pre-polymer mixture and exposing in the interference fields. The organic DFB lasers based on HPDLC grating have advantages of narrow linewidth, low threshold and wavelength tenability. Thus thay are very promising in the research fields of integrated optics and spectroscopic analysis.However, there are still some problems with these lasers. The lowest threshold reported previously was 5 ?J/pulse, and the slope efficiency was lower than 0.5%, which severely hinders its practical process. To solve the above problems, this dissertation develops studies on the improvement the performance of the organic DFB laser based on HPDLC grating.Firstly, we investigated the performance of dye-doped HPDLC DFB lasers. Due to the low curing intensity, the fabricated grating shows a scaffolding-like morphology, in which the phase-separated LC exists in homogeneous layers instead of spherical domains. The scattering loss of the grating is less than 6%, thus the grating can be a good cavity for laser. By measuring the properties of Amplified Spontaneous Emission(ASE) of different dyes, we can find high energy conversion efficiency dye as gain medium of laser. However, the self-absorption of a single dye is usually very high, which will influence the fluorescence quantum efficiency. The Forster energy transfer from host emitter to guest emitter could cause the redshift of the ?uorescence spectrum relative to the absorption band of the blend, which could lower down the losses caused by self-absorption. Based on this theory, we ?rst doped two dyes into HPDLC transmission gratings, and achieved better performance of output laser with an optimised doping concentration of the guest dye. The slope efficiency for output lasing is enhanced from 0.3% to 0.7%, and the laser threshold is lowered from 3.21μJ/pulse down to 1.35 μJ/pulse. We also demonstrate that the lasing wavelength could be tuned over 11.3 nm continuously by applying an external electrical field.Nevertheless, dyes need substantial dilution to avoid quenching, which limits optical performance of lasing output from dye–doped HPDLC grating. To avoid quenching, the concentration of dyes is usually lower than 1wt.%, and the pump light utilization rate is less than 20% correspondingly. Gain medium MEH-PPV with higher absorption to pump light is adopted in DFB HPDLC laser, thus the pump light utilization rate can be 70%. Due to the insolubility of MEH-PPV and HPDLC grating system, a new laser structure with HPDLC grating over MEH-PPV laser is proposed, and it is so called DFB MEH-PPV HPDLC laser. The output lasing shows single-mode, s linear polarization performance, and the direction is concerned with Bragg order. The slope efficiency of three-order laser is 2.3%, and the working threshold is 0.71μJ/pulse.In order to improve light feedback for light propagating along the grating vector, we carried out the research on increasing refractive index modulation Δn of the grating. Adopting low functional monomers and rubbed PI alignment layer, the orientation of phase separated LCs along the direction of the holographic planes will be altered. So the refractive index difference Δn can be improved, and the slope efficiency is increased to 4.6%, the working threshold is 0.25μJ/pulse. Furthermore, decreasing grating period is proposed to change the orientation of phase separated LCs due to the geometrical effect of small-period grating. By using s-polarization pump light, the slope efficiency is increased to 7.1%, and the working threshold is 0.14 μJ/pulse. The output lasing is vertically surface-emitting by employing second-order Bragg scattering.In this dissertation, the fabrication and performance of both dye-doped HPDLC DFB laser and DFB MEH-PPV HPDLC laser are studied, and we further improve the performance of laser by increasing refractive index difference ?n. The research conclusions presented in this dissertation will lay a solid foundation for the practical process of organic DFB HPDLC laser.