XUV plasmonic waveguides by near-zero index heterostructures

The lack of transmissive photonic components in the extreme ultraviolet (XUV) constitutes a challenge for micro/nano-metric confinement. Here, we theoretically design a novel approach to attain XUV radiation guidance based on the electromagnetic properties of titanium-aluminum-titanium heterostructures in such a spectral domain. We show that, thanks to the near-zero-index properties of aluminum and titanium, XUV radiation can couple efficiently with plasma oscillations in such heterostructures, enabling the excitation of several distinct plasmon polariton modes. Our predictions, based on the semi-analytical solution of fully vectorial Maxwell's equations, indicate that the dispersion profile of plasmon polariton modes can get efficiently modulated by the aluminum thickness, enabling nanometer confinement and micrometre propagation length. Moreover, we quantify the third-order nonlinearity enhancement factor, finding that it is resonant at the zero-index wavelength. Our results are promising for the development of future devices enabling advanced control and manipulation of XUV radiation.