Abstract
Cholesteric liquid crystal structures with multiple isotropic defect layers exhibit localized optical modes (defect modes). Coupling effects between these modes were simulated using the finite difference time domain method. Analogous to the well-known result of the tight-binding approximation in solid state physics, splitting of the defect modes takes place, as soon as the structure contains more than one defect layer. The dispersion relation of the mini-bands forming within the photonic band gap of the structure is calculated numerically. The structures might have promising applications for multiwavelength filters and low-threshold lasers.
Highlights
Cholesteric liquid crystals (CLCs) have a periodic helical structure and can be regarded as one-dimensional (1D) photonic crystals but with a photonic bandgap (PBG) only for circularly polarized light of the same handedness as the helix
density of states (DOS) is high at the edge of a PBG, lasing can occur if laser dyes are doped into a CLC [2]
We investigate the coupling of defect modes in a system of multiple defect layers (MDL) in a CLC (MDL-CLC) theoretically
Summary
Cholesteric liquid crystals (CLCs) have a periodic helical structure and can be regarded as one-dimensional (1D) photonic crystals but with a photonic bandgap (PBG) only for circularly polarized light of the same handedness as the helix. DOS is high at the edge of a PBG, lasing can occur if laser dyes are doped into a CLC [2] This is one of the reasons why optical properties of CLCs have been widely investigated in recent years. Yang et al [8] introduced an isotropic layer that disturbs the periodic structure of a CLC Simulations showed that such a defect layer can exhibit a corresponding optical defect mode within the PBG. We investigate the coupling of defect modes in a system of multiple defect layers (MDL) in a CLC (MDL-CLC) theoretically. The eigenmodes of this system have circular polarization. Single mode and multi-mode lasing in the visible light range of the spectrum may be realized by changing the number of defect layers in the MDL-CLC assembly
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