The authors gratefully acknowledge the financial support of the EPSRC

1. Nonresonant random lasing from a smectic A* liquid crystal scattering device S. M. Morris, A. D. Ford, M. N. Pivnenko and H. J. Coles Centre of Molecular Materials for Photonics and Electronics, Electrical Engineering Division, Cambridge University Engineering Department, 9 JJ Thomson Avenue, CB3 0FA, UK BACKGROUND THRESHOLDS ELECTRONIC CONTROL In highly turbid media, the multiple scattering of light gives rise to long light-paths. If gain is implemented in the scattering medium these long light-paths are then amplified and in some cases this can give rise to a form of laser action generally referred to as random lasing. These random lasers, are incoherent or coherent depending upon whether there is nonresonant or resonant feedback, respectively. An illustration of the processes involved in both cases is shown in Figure 1. Figure 5. Pump energy dependence of the peak emission intensity and the full width at half maximum (FWHM) for the LC sample in the pseudo-planar texture. Figure 4. Micrograph of the optical texture of LC in the absence of an electric field at 44oC. The pesudo-planar texture. Figure 9. A plot of the emission spectra for three different textures for an excitation energy of 30 mJ/pulse. Curve a is the emission spectrum for the planar aligned texture with a zero-field while curve b is the emission spectrum for a homeotropic alignment obtained using a 17 Vmm-1 a.c. electric field. Curve c is the emission spectrum for the field-induced scattering state obtained by applying an electric field of 2 Vmm-1 directly after the homeotropic state. Excitation threshold Ethreshold = 38 mJ/pulse, FWHM = 15 nm Figure 7. Pump energy dependence of the peak emission intensity and the full width at half maximum (FWHM) for the LC in the field induced Scattering texture. Figure 6. Micrograph of the optical texture of LC for an electric field strength of E = 1.9 Vmm-1 when decreasing from the field induced homeotropic state. Figure 1. An illustration of the weak and strong multiple scattering regimes. In (a) an example of a random walk in a nonresonant random laser medium is shown. In this case there is only intensity feedback. In (b) the formation of so-called microcavities in a resonant random laser medium due to strong multiple scattering. (a) Excitation threshold Ethreshold = 15 mJ/pulse, FWHM = 8 nm EMISSION SPECTRUM Increasing the optical path length (b) Figure 10. The plots (a) and (b) show the change in emission intensity with electric field for increasing and decreasing magnitude, resepectively. Figure 8. The pump energy dependence of the emission intensity and the FWHM for the field-induced scattering texture in a 22mm-thick cell.. The increase in cell thickness results in an increase in the average optical path length and therefore a decrease in the excitation threshold. Figure 3. Emission spectrum of the LC in the scattering texture (E = 2 Vmm-1) for an excitation energy of 80 mJ/pulse. The peak emission intensity is centred at approximately 618 nm with a FWHM of 8 nm. S.M.Morris, A.D.Ford, M.N.Pivnenko and H.J.Coles, Appl. Phys. Letts, 86, 141103, 2005 Excitation threshold Ethreshold = 7 mJ/pulse, FWHM = 6 nm The authors gratefully acknowledge the financial support of the EPSRC