In this work, we present the design of a refractive Axicon lens to be used in refractive index optical sensing. The lens is designed to generate a Bessel – Gauss beam at the wavelength of 3.3 microns. At that wavelength, the absorption of the CH4 is maximum and thus a maximum change in the refractive index due to the CH4 gas is also expected. The intensity profile of the generated beam is quite sensitive to the index of refraction of the surrounding medium. Placing the optical detector at the point of maximum change in the intensity with refractive index allows the measurement of the refractive index change and hence the gas percentage with very high sensitivity. Our design shows sensitivity greater than 970 % per RIU. We also develop an analytical formulation for the intensity variation with the refractive index. The results obtained analytically are confirmed by the finite difference time domain FDTD calculation. From the analysis and the derived expressions, we demonstrate the effect of the Axicon base angle on the sensitivity and hence allow for the lens optimization to achieve maximum sensitivity for a target application.
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.
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