A thermo-optically tunable flat-lens from visible to near-infrared wavelengths

We present a beam focusing flat lens for use in the mid-infrared wavelength range. The layer structure used is a 360 nm thick amorphous silicon layer on 200 nm thick aluminium on a 300 μm thick Borosilicate Glass Substrate. An aperture through the Al and a-Si layers is used to excite and enhance the coupling efficiency of a hybrid-surface plasmon polariton/waveguide mode. A second-order grating structure in the surface of the a-Si layer then scatters the propagating hybrid mode to constructively interfere in the far-field with the diffracted light from the narrow aperture to produce a narrow beam. The high dn/dT of amorphous silicon has the potential to enable thermo-optic focusing and steering.