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L2.07 - PT Symmetric Metasurfaces and Polarisation Phase Transitions 
December 1, 2014   4:00pm - 4:15pm

PT symmetric quantum mechanics involves the study of open or non-hermitian quantum systems which can still exhibit complete and real eigen spectra, a feature normally reserved for hermitian operators, if they remain invariant under the simultaneous transformations of parity and time reversal. However, unlike hermiticity, PT symmetry doesn’t guarantee real eigenvalues and upon variation of an external parameter the spectrum can suddenly change from real to complex in a process known as spontaneous PT symmetry breaking. While the strict requirement that a physical system be PT symmetric limits experimental investigations in the quantum domain, modern fabrication techniques which allow the spatial profile of both real and imaginary parts of the refractive index to be controlled almost arbitrarily, combined with the similarity between Schrödinger’s and Maxwell’s equations has made PT symmetric optics a hot topic of research. The most notable demonstration of PT symmetry consists of a pair of side coupled waveguides, one with loss and the other with an equal amount of gain. On varying the coupling strength PT symmetry breaking can be observed in the light transmitted through the system, changing from lossless to amplifying or decaying depending on the excitation conditions. Importantly, PT symmetry breaking has also been observed in passive experiments without gain, which can be understood by the fact that a system with a loss contrast maps to a PT structure under a gauge transformation. As well as being of fundamental importance, PT symmetric waveguide systems have also revealed technologically beneficial properties such as anomalous transparency, power oscillations, non-reciprocal Bloch oscillations and unidirectional transparency. Recently, spatiotemporal studies have also highlighted the coexistence of coherent perfect absorption and lasing, realisable via PT symmetric resonators. Here we theoretically and experimentally explore novel polarisation phase transitions induced by PT symmetry breaking in anisotropic terahertz metasurfaces. By using terahertz time domain spectroscopy, giving both amplitude and phase information for all four transmission coefficients, we gain complete knowledge of the polarisation response of our samples. After constructing a metasurface unit cell out of coupled orthogonally orientated split ring resonators with the same resonant frequency but different absorption coefficients, a phase transition is observed for the polarisation eigen states of transmission upon varying the coupling strength. In this case PT symmetry breaking is found to cause a sudden 45° rotation of the eigen polarisation ellipses. Moreover, precisely at the boundary separating these two regimes, known as the exceptional point, the eigen modes coalesce into a single circular polarized state which is highly unusual given the metasurface’s lack of rotational symmetry. Our study points to increased freedom for manipulating polarisation states of light.

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