Optical Properties of Long Wave Infrared Spoof Plasmon using Hexagonal Periodic Silver Hole Arrays

A two-dimensional metal hole array (2DMHA) structure is fabricated by conventional photo-lithography and electron beam evaporation. The transmittance of the 2DMHA is measured at long wave infrared (LWIR) wavelengths (${\lambda}{\sim}10$ to $24{\mu}m$). The 2DMHA sample shows transmittance of 70 and 67% at $15.4{\mu}m$ due to plasmonic resonance with perforated silver and gold thin films, respectively, under surface normal illumination at LWIR wavelengths. The measured infrared spectrum is separated into two peaks when the size of the hole becomes larger than a half-pitch of the hole array. Six degenerated plasmon modes (1,0) at the metal/Si surface split to three modes at an incident beam angle of $45^{\circ}$ with respect to the surface normal direction, and wavelength shifts of the transmitted spectrum are observed in a red shift and blue shift at the same time.