Jun-ichi Shikata, Seigo Ohno, Hiroaki Minamide,
https://www.jstage.jst.go.jp/article/elex/advpub/0/advpub_17.20200133/_article
This paper describes terahertz (THz)-wave generation within forbidden bands in polar crystals, focusing on the A1 phonon modes in lithium niobate. This material exhibits two negative-permittivity frequency ranges at 7.4-12.7 THz and 18.8-25.6 THz for the lowest and highest A1 modes, respectively. Exploiting the finite-difference time-domain simulations, we demonstrate that both the surface phonon modes can be radiative with a structured grating. Fourier analyses of the radiative fields reveal the relevant peaks in the spectrum as well as the dispersion relations. Our results provide a novel method for coherent THz-wave sources at unexplored THz frequencies.
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