Visual Observation of Zener Tunneling

We experimentally investigate photonic Zener tunneling between the bands of a waveguide array by directly monitoring the propagating light inside this structure. For strong transverse index gradients we observe Zener breakdown as regular outbursts of radiation escaping from the Bloch oscillations. Tunneling to higher order photonic bands and Bloch oscillations in different bands have been detected.

Figure 1

(a) Schematic drawing of the polymer waveguide array used in the experiment (b) Corresponding band structure of the waveguide array where the second band already intersects with the spectral continuum of the cladding modes. The gap between the first and second band is minimal at the edge of the Brillouin zone.

Figure 2

Bloch oscillations and Zener tunneling in a waveguide array when several waveguides are excited by a $25\mu \mathrm{m}$ wide elliptical beam. (a) Experiment, (b) corresponding simulation (beam propagation method) for a temperature difference of $\Delta T=24\mathrm{K}$. (c),(d) Positions of outbursts of radiation caused by Zener tunneling in dependence of the applied temperature difference $\Delta T$. In correspondence to the Bloch oscillation period, a $1/\Delta T$ dependence is observed.

Figure 3

Bloch oscillations and Zener tunneling in a waveguide array when the entire Brillouin zone spectrum is excited by shining light in a single waveguide. (a) Experiment, (b) corresponding numerical simulation (temperature difference $\Delta T=35\mathrm{K}$).