Tunable Flat Lenses in the Mid-Infrared

We present a stand-alone beam focusing flat lens for use in the mid-infrared wavelength range. An aperture and second-order grating structure in a thin gold layer 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. A rapidly tunable optical parametric oscillator source is used to demonstrate how changing the wavelength of the incident light from 3.8-4.2µm produces two interfering beams in the far-field caused by the scattering of the propagating SPP interfering with the incident light diffracted by the narrow (sub λ/2) aperture. We identify measured far-field patterns for varying wavelengths using the razor blade edge beam profiling method. The agreement between Finite-Difference Time-Domain (FDTD) modelled and measured results will allow the aperture/grating structures to be integrated directly onto the facets of edge-emitting lasers to dramatically reduce their beam divergence. As edge-emitting lasers have a fixed wavelength, the addition of a thin layer of material such as Silicon or Barium Strontium Titanate to the facets allows tuning by altering the SPP wavelength.