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Emission properties of radiative chiral nematic liquid crystals



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Mavrogordatos, Themistoklis 


In this work, we calculate the density of photon states (DOS) of the normal modes in dye-doped chiral nematic liquid crystal (LC) cells in the presence of various loss mechanisms. Losses and gain are incorporated into the transmission characteristics through the introduction of a small imaginary part in the dielectric constant perpendicular and along the director, for which we assume no frequency dispersion. Theoretical results are presented on the DOS in the region of the photonic band gap for a range of values of the loss coefficient and different values of the optical anisotropy. The obtained values of the DOS at the photonic band gap edges predict a reversal of the dominant modes in the structure. Our results are found to be in good agreement with the experimentally obtained excitation thresholds in chiral nematic LC lasers. The behaviour of the DOS is also discussed for amplifying LC cells providing an additional insight to the lasing mechanism of these structures. We subsequently investigate the spontaneous emission properties, under the assumption that the electronic transition frequency is close to the photonic edge mode of the structure (resonance). We take into account the transition broadening and the decay of electromagnetic field modes supported by the so-called 'mirror-less' cavity. We employ the Jaynes-Cummings Hamiltonian to describe the electron interaction with the electromagnetic field, focusing on the mode with the diffracting polarization in the chiral nematic layer. As known in these structures, the density of photon states, calculated via the Wigner method, has distinct peaks on either side of the photonic band gap, which manifests itself as a considerable modification of the emission spectrum. We demonstrate that, near resonance, there are notable differences between the behaviour of the density of states and the spontaneous emission profile of these structures. In addition, we examine in some detail the case of the logarithmic peak exhibited in the density of states in 2D photonic structures and obtain analytic relations for the Lamb shift and the broadening of the atomic transition in the emission spectrum. The dynamical behaviour of the atom-field system is described by a system of two first order differential equations, solved using the Green's function method and the Fourier transform. The emission spectra are then calculated and compared with experimental data. Finally, we detail a new technique for the pumping of dye lasers which requires no moving parts or flushing mechanisms and is applicable to both solid state and liquid based devices. A reconfigurable hologram is used to control the position of incidence of a pump beam onto a dye laser and significant increases in device lifetimes are achieved. The technique is also applied to wavelength tune a dye laser. This offers access to higher repetition rates and larger average output powers. With higher repetition rate pump lasers it is feasible that the approach could allow such organic lasers to reach operating frequencies on the order of MHz. The unique nature of the adaptive pumping method also allows precise control of the spatial wave-front and configuration of the pumping wave which allows greater versatility and functionality to be realised. It is possible to envisage that novel pump beam profiles that optimise propagation through the medium could also be demonstrated.





Liquid crystal lasers, Spontaneous emission, Chiral structures, Bandgap


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge