Towards high performance LC lasers: Monitoring dye depletion, dye diffusion and helix distortion by transient fluorescence measurements J. Schmidtke, C. Gillespie, and H. J. Coles Centre of Molecular Materials for Photonics and Electronics, Engineering Dept., Univ. of Cambridge time-dependent spontaneous emission pump beam is switched on at time t = 0. Observations: - bleaching - (roughly) stable emission after several seconds - shift of resonance wavelength (!)  modified helical order 0 resonant modesband gap  : absorption  : heat conduction  : dye bleaching  : dye diffusion pump switched on at t = 0 effect of the pump beam: 1. heating the sample (counteracted by heat conduction) 2. bleaching the dye (counteracted by dye diffusion) simple model: with  shift of resonant mode seems to be a temperature effect - total number of director turns is conserved (due to surface alignment) - no homogeneous unwinding of the helix - decreasing equilibrium pitch with increasing temperature - decreasing birefringence with increasing temperature - decreasing mean refractive index with increasing temperature Therefore: Why shift to longer wavelengths? pump beam Explanation: inhomogeneous absorption / heating helix contraction helix dilation pump beam detected emission  distorted helix reduced resonator quality modified photonic band structure Variation of pump intensity Introduction Currently, self-organized photonic band edge lasers based on dye doped cholesteric liquid crystals attract considerable interest as candidates for miniaturized, tuneable coherent light sources. However, performance of current systems is limited by low repetition rates (~10 Hz) and low emission energies (~ 10  J), which is not sufficient for most display and telecommunications applications. In order to optimize the performance of LC lasers towards high power emission at high emission rates, a thorough understanding of the structural and photochemical response of the dye doped liquid crystal to high pump intensities is necessary. We studied the response of dye doped CLC films on high power cw pumping, as revealed by time-dependent fluorescence measurements. Selective reflection of circularly polarized light Modified emission of fluorescent guest molecules (very sensitive probe of photonic band structure) Pulsed excitation: Band edge laser emission Effects of the photonic stop band - local nematic order - continuous rotation of the director  twisted birefringent medium  photonic stop band for circularly polarized light  1D self-assembling photonic crystal  A CLC acts as an optical resonator for circularly polarized light.  A CLC doped with a fluorescent dye acts as a mirrorless laser. two resonant optical modes at the band edges Cholesteric liquid crystals Experiment cw laser P  P P intensity adjustment spectrometer sample Shift of band edge resonance: Superposition of fast exponential rise and slow, incomplete exponential relaxation Explanation of the time-dependent helix distortion Variation of dye content, pump spot size - fast initial decay - no equilibrium is reached (simple toy model not sufficient) 50  W 150  W 250  W 349  W 0.5% 1.0% 1.5% 2.0% fast process (heat conduction) slow process (diffusion/depletion) Time dependence of fluorescence intensity Evolution of fluorescence intensity for various pump intensities Assumptions: - “thin” sample (2d problem) - Gaussian beam profile: Model is used to fit measured intensities. Modelling dye depletion and diffusion: Diffusion equation is solved numerically. Result is used to calculate time-dependent emission intensity: “Reduced” diffusion coefficient - expected: ~ I -1 but: levels off at high pump intensities Reasons: - improved dye mobility due to sample heating - heat-induced convection(?) Variation of pump intensityVariation of dye content, pump spot size - roughly constant for given dye concentration - increases with concentration (!)  favourable for device: low dye concentrations - increases for small pump areas (probably due to heating and/or convection) - decreases for high dye concentrations (possibly aggregation of dye molecules)  favourable for device: - low dye concentration - small pump area Dye depletion proportional to pump intensity (as to be expected) - increasing amplitudes with increasing pump intensity - faster response with increasing pump intensity possible reasons: - fast process: heat-induced convection - slow process: enhanced diffusion due to localised heating - increasing amplitudes with increasing dye concentration and decreasing spot size - fast process: faster response with increasing dye concentration - slow process: slower response for large pump spots fast process (heat conduction) slow process (diffusion/depletion) convection? localised heating? p /  W dye concentration: