Subwavelength focusing in the infrared range using a planar metallic lens of binary slits with refractive index modulation

In this paper, we demonstrate a plasmonic planar lens structure that can achieve subwavelength focusing of the infrared electromagnetic radiation. The lens is composed of metallic binary slits with different dielectric fillings. The index modulation approach of the filling materials is used to achieve phase modulation of the wavefront of the incident wave. Using this approach, we could achieve a phase range of 0.43π. The structure can focus the incident infrared wave in the subwavelength scale. The focal length attained is 44.69 μm and the achieved Full width at half maximum (FWHM) is 4.28 um for an incident infrared wave of wavelength 8 um. The transmission through the structure is 25.64 % at the design wavelength. The used metal is copper and the dielectric filling materials are silicon and air. Copper has lower losses in the infrared range than the traditional metals used in visible Plasmonics. Silicon has a higher melting point than the common dielectric materials used in refractive index modulation of the visible Plasmonic lenses. This temperature stability is a very important feature when working in the infrared domain. Besides being specifically suitable for the infrared range, copper and silicon are also CMOS compatible. Therefore, the proposed structure is suitable for integration in many potential infrared applications such as thermal imaging, medical diagnosis, thermal photovoltaic cells and heat harvesting. In addition, the fact that many molecules have unique absorption spectra or signature in the infrared range would facilitate the analysis and study of many materials and biological molecules using infrared miniaturized spectrometers.