Fabrication Studies On Holographic Polymer Dispersed Liquid Crystals Based Organic Lasers

Holographic polymer dispersed liquid crystal (HPDLC) gratings are formedthrough photo-polymerization induced anisotropic phase separation of liquid crystal(LC) from the polymer matrix. Distributed feedback (DFB) lasers can be made bydoping the laser dye at a suitable concentration into the HPDLC prepolymer syrup.The HPDLC grating film not only provides coherent Bragg scattering at eachcorrugation for positive optical feedback, but also works as the gain medium forlight amplification. Lasing can be obtained once input pump energy exceeds thelaser threshold. These organic lasers have advantages of simple fabrication, lowworking threshold, narrow linewidth and wavelength tunability across the wholevisible range. Thus they are very promising in using as optical sources for highsensitivity spectroscopic analysis.However, the current output lasing from dye-doped HPDLC lasers ismulti-mode with a large divergence. The reported lasing threshold is high and theslope efficiency is ultra-low. In order to address these problems, the correlationsbetween parameters of HPDLC gratings and laser device working performance haveto be considered and the studies into enhancing lasing properties throughoptimization of HPDLC cavities should be put forward. Besides, novel gain mediumshould be introduced to replace the laser dye, as it suffers severely from concentration quenching and provides low absorption efficiency. The mainresearches of this dissertation are listed as follows:1. We successfully fabricated polymer scaffolding morphologic HPDLCtransmission gratings from common acrylate monomers. The phase separated LCsform homogeneous layers instead of spherical domains. Transmittance measurementshows the grating exhibits little scattering which is less than4%. We also show thatdue to the anchoring effect of polymer filaments across the pure LC layer, thesephase separated LC molecules are well aligned along the grating vector. We furtherdesigned a birefringence experiment to measure the LC phase separation degree inHPDLC gratings, so all structural parameters can be obtained. Based on aboveunderstandings of the grating structure, we then analyzed the effect of fabricationconditions, such as polarization state of the writing beams, exposure intensity andgrating period on LC phase separation degree and obtained some fabrication rulesfor efficient HPDLC gratings. At last, the phase separated LC director configurationis altered by recording the grating at a high temperature. In this way, the refractiveindex modulation for light propagating along the grating vector can be increased andthe slope efficiency for output lasing is enhanced from0.3%to0.9%.2. We investigated the mode selection mechanism and found it was based ontwo principles. First, the resonant modes should fulfill the waveguide conditions inthe core layer which is the grating layer in dye-doped HPDLC DFB lasers. So thelaser modes could not leak out through the two glass substrates, and light could beeffectively confined inside the grating layer. Second, the resonant mode shouldfulfill the Bragg condition of the grating, so positive optical feedback can beobtained through coherent Bragg scattering at each corrugation. The mode couldthen be amplified through stimulated emission when it is travelling around. Themode emits lasing when the pump energy reaches the working threshold. Based onthe mode selection mechanism, we modulate or optimize the output lasing modesaccordingly. We show that the lasing wavelength could be continuously tuned over8nm by applying an electrical field. The homogenously aligned LC molecules along the grating vector could be reoriented to the cell normal, thereby increasing theeffective refractive index experiencing by the laser mode. The electrical tunability oflasing wavelength from HPDLC based DFB lasers is an attracting advantage and canbe used in optical spectroscopy. The DFB working structure is optimized from theperspective of refractive index difference between the cladding layer and the corelayer to allow only one lasing mode being resonated.3. Organic semiconductor has been introduced as gain medium to HPDLC basedDFB lasers. Unlike the dye molecules, organic semiconductor is immiscible withHPDLC prepolymer syrup. So we have designed a new DFB working structure inwhich the grating layer is fabricated separately on top of the gain medium layer. Thetwo layers interact with each other by evanescent wave spread into the claddinglayer. We show that the thickness of the organic semiconductor layer has to bearound75nm to balance optical feedback and light amplification in order to obtain ahigh performance lasing output. The output lasing shows single-mode, linearpolarization performance, and can be tuned in the gain spectrum of the gain medium.The working threshold is21μJ/cm2(one tenth that of dye-doped HPDLC lasers) andthe slope efficiency is increased to5.9%(six times that of dye doped HPDLC lasers).We further show the effect of pump polarization on working performance and finds-polarization guarantees best working performance. We then made verticallysurface-emitting organic lasers by employing second-order Bragg scattering. Theperiod of the HPDLC grating is just394nm and emitted lasing is coupled outthrough first-order scattering. Thanks to more efficient light coupling insecond-order Bragg scattering, the laser threshold of this surface-emitting laser isreduced to13μJ/cm2. We also investigate the effect of pump length on laserthreshold, and show the refractive index modulation HPDLC grating is sufficient forefficient feedback as long as the pump length is longer than0.03cm.