Tunable flat lenses in the infrared

We present a beam focusing flat lens for use in the mid-infrared wavelength range. An aperture and second-order grating structure in thin gold and silicon layers is used to excite and then scatter a propagating surface plasmon polariton (SPP) to constructively interfere in the far-field to produce a narrow beam. The silicon layer acts as a mechanism for enhancing the surface plasmon polariton coupling efficiency and as a material whose refractive index can be increased by heating. We have shown through heating the silicon layer to 900 ºC, we can tune the beaming angle of the lens through −7º to 7º.