Instigating chiral-selective nonlinear optical phenomena in metamaterials

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Rodrigues, Sean P.
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As naturally occurring chiral materials demonstrate limited circularly dichroic contrast, enhancement of these polarization dependent signals has been the focus of chiral metamaterial research. By manipulating the geometric chirality of resonant plasmonic nanostructures and utilizing the state of the art in metamaterial design, we are capable of creating sophisticated chiral structures with enormous optical activity and chiral-selective light confining ability. Here, a set of chiral nanoarcs demonstrates a transmission contrast of 0.5 between left and right circular polarizations and a 20× contrast between second harmonic responses from the two incident polarizations; nonlinear and linear imaging is provided. In a later set of experiments, these chiral nanoarcs are opened up to place emitters in the most light-confining and chirally sensitive regions of the meta-structure. Tested with an ultra-fast laser, the resulting two-photon emission profiles under circularly polarized excitation display mirrored symmetry for the two enantiomeric, nanoarcs. The profile of the nonlinear signal correlates to the chiral resonance in the linear regime. The nonlinear emission signal is enhanced by 40× that of the emitters not embedded in the metamaterial and displays a 3× contrast for the opposite circular polarization. These results expose new methods for the enhancement of chiral signal contrast and extraction. Chiral metamaterials, in particular, offer immediate distinction between left and right circular polarizations, prompting an ideal starting place for active switching efforts – a central thrust of active metamaterial research. By exposing a chiral metamaterial to a change in 15 mW of power beyond the structure’s linear optical regime, the two absorptive resonances belonging to the chiral meta-structure are spectrally shifted by 10 nm. These results demonstrate the strongest nonlinear optical rotation to date, a value 106 stronger than the most recent studies. The research presented herein, leverages the higher order components of the electric susceptibility tensor to yield large optically active responses and nonlinear phenomena. The last project of this thesis designs a chiral metamaterial mirror, which has the unique property of maintaining the spin state of a circularly polarized wave upon reflection from its surface. Chiral meta-mirrors, exhibiting large chiroptical responses and spin-selective near field enhancement, promise applications in polarization sensitive electro-optical information processing and biosensing.
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