Enhanced-dynamic THz emission in a coupled metal Epsilon Near Zero metasurface

Abstract

Remarkably strong THz field emission from nonlinear metasurfaces excited by femtosecond lasers was recently reported. The field emitted from an ultrathin metasurface was shown to be comparable to that emitted from orders of magnitude thicker Zinc Telluride nonlinear crystal. Here we study this effect thoroughly by comparing the emission from metasurfaces fabricated on top of various substrates. We find that the presence of a thin ITO film, commonly used in the electron beam lithography process leads to two orders of magnitude stronger THz emission compared to the case of metasurface on glass. It also shows a different power law, signifying different dominant emission mechanisms. The enhancement is explained by the large optical nonlinearities of ITO at excitation wavelengths where the permittivity is near zero, in addition to further field confinement. We also show that coupling between the free electrons in the ITO and the surface plasmons on the gold nanoparticles leads to dynamic THz emission phenomena that were not reported to date. Specifically, we show that the generated THz pulse can be shortened in time, and thus broadened in frequency with twice the bandwidth compared to previous studies and to an uncoupled system. This phenomenon is attributed to hot electrons dynamics, which alter the optical response of ITO at sub-picosecond time scale. This modifies the coupling between the plasmonic metasurface and the free electrons in the ITO and drastically affect the nonlinear dynamics of the system. These findings shed new light on the fundamentals of THz emission from metasurfaces and open the door to design efficient and dynamic metasurface THz emitters.