Magneto-Spatial Dispersion Phenomena

Abstract

The effects of spatial dispersion and applied magnetic fields on the propagation of light through media lead to interesting optical phenomena such as the Faraday effect and optical activity. Recently, the magneto-chiral anisotropy, a cross-effect of spatial dispersion and magnetic fields, was discovered.New optical effects are also discovered in photonic crystals. In those materials a spatial modulation of the dielectric constant breaks a continuous translational symmetry into a discrete one. This may result in the opening up of a photonic band gap.In this thesis I studied systems, where some or all of the mentioned symmetries have been broken. The combination of broken symmetries resulted in interesting new magneto-spatial phenomena.The magneto-optics of photonic band gap materials is still lacking groundbreaking experimental and theoretical investigations.In one dimensional photonic crystals, for which I have performed numerical simulations, I observed a Faraday rotation spectrum that has a derivative-type shape with peaks of opposing signs at the edges of the stopband. Strongly resonant Faraday rotation was calculated around specially designed defect modes.The main observation in this part was the measurement of the Faraday rotation of three dimensional photonic crystals. The crystals were impregnated with a Faraday-active liquid. The Faraday rotation outside the stopband follows the spectral behavior of the pure liquid multiplied by its volume fraction. Inside the stopband, the Faraday rotation is enhanced up to a factor of five. Qualitatively, internal reflections are the most likely origin. A fully three dimensional scattering theory needs to be developed in order to understand the observations quantitatively.A small enantiomeric excess can be generated by magneto-chiral anisotropy. The generated excess is usually very small, which drove us to look for a mechanism of amplification. I have shown that crystallization under influence of irradiation (photo-crystallization) provides such an amplification, which was demonstrated here in the growth of nickel sulfate.The produced enantiomeric excess reaches high values within reach of the complete elimination of one handedness. The excess is changing sign when going from left-circularly to right-circularly polarized light and zero for linearly-polarized light. This large excess originates from a rather small natural circular dichroism of my crystals (~8%).I interpret my results in the way that the enantiomer that absorbs more light has, in such conditions, a lower probability to cross the critical nuclei size in the initial nucleation stadium. It is more likely to dissolve due to a local elevation of temperature. A necessary condition is the delicate choice of a proper crystallization point in the solution's phase diagram near the saturation limit.In principle, the same amplification mechanism can be used to enhance the effects of the magneto-chiral anisotropy. The enantioselectivity would then be provided by a magnetic field in combination with unpolarized light. However, a multitude of perturbations present in the used method of crystallization and the very criticality around the initial phase of nucleation make a quick experimental observation of this expectedly much smaller effect unlikely. Future efforts should start with a major investment in crystallization equipment and a change to a steady-state crystallization method. It would also be wise to separate the place of nucleation from the place of growth of already formed crystallites in order to control both processes independently.In this part is was shown that cholesteric liquid crystals are almost ideal systems to exhibit large effects of magneto-spatial dispersion. They somehow combine the two preceding parts in so far as they are both chiral and have the periodic translational symmetry that result in a photonic band gap.The main observation was that magneto-chiral anisotropy takes large values in cholesteric liquid crystals, that can be observed in the transmission of light that is traveling along the optical axis and parallel to an externally applied magnetic field. The mechanism responsible for such a huge amplification was identified as resonant Bragg scattering. A complete theoretical treatment is still missing. A promising project could involve the simulation of the effect in cholesteric liquid crystals with a modified version of the transfer matrix method developed in chapter two of this thesis.Specific chiral structure information should be extractable from resonant magneto-chiral anisotropy measurements, using the Bragg scattering at the crystal lattice planes.