Space-variant polarization conversion with artificial birefringent metallic elements

We present an artificial birefringent space-variant polarization converter for the near infrared, λ = 1550 nm. Each hollow waveguide has a rectangular shape with lateral dimensions of 1550 nm in the x-direction and 1034 nm as the largest length in the y-direction. The whole device consists of approximately 2000 × 2500 hollow waveguides realized in a 2-µm-thick gold structure. They are separated by sidewalls with a width of less than 500 nm. By proper choice of the lateral widths of the individual holes, a pixel-wise polarization conversion of an incoming wave field is possible. By suitable choice of the fabrication parameters, a birefringent phase shift up to 2π can be achieved. Hence, the structure is able to fully convert the state of polarization, e.g., from linear to circular. For fabrication of the device, femtosecond 3D direct laser writing was combined with electroplating. Here, we describe the operation of our device as a space-variant polarization converter by measuring the angle-dependent transmitted power and by calculating the ellipticity and the phase delay dependent on position as well as the azimuth angle from the experimentally determined powers.