A processing device may include a display screen and one or more buttons located near the display screen. The method involves providing a zero-mode waveguide which includes a cladding surrounding a core where the cladding is configured to preclude propagation of electromagnetic energy of a frequency less than a cutoff frequency longitudinally through the core of the zero-mode waveguide. The glass substrate is provided with a texture formed to induce magnetic anisotropy in the magnetic layer.