Focusing bandwidth of flat lenses in the mid-infrared

Presented is a stand-alone beam focusing flat lens for use in the mid-infrared wavelength range. A rapidly tunable optical parametric oscillator source is used to demonstrate how changing the wavelength of the incident light produces two interfering beams in the far field. For the first time a measured full-width half-maximum bandwidth of ~80 nm is identified over which this focusing is achieved within a field-of-view of 1.8°, which shows good agreement with finite-difference time-domain simulation results.