Zero Diffraction: A Novel Approach for High-Contrast Switching of Light

The ultrafast electro-optic Pockels effect is widely used for modulating light in terms of its intensity, phase, and polarization, because controlling these parameters is the basis of modern information and communication technology. However, other applications, including displays, scanners, or solar concentrators, also require the control of geometric parameters of light, such as its pathway in space. Until today, the efficient spatial control of light remains challenging.<br/> <br/>The common approach for achieving spatial control involves modifying optical components. These modifications often rely on the mechanical movement of lenses, mirrors or gratings, which is much slower compared to modulations based on the Pockels effect. Recent reports propose novel approaches based on interference. [1] In these cases, the optical components remain mostly unchanged, but their interaction with light strongly depends on the light’s properties like phase or momentum leading to a limited set of operation parameters.<br/> <br/>Here, we propose a novel approach for controlling the diffraction of light at a leaky waveguide grating using the ultra-fast Pockels effect. The leaky waveguide grating is illuminated by two symmetrical incident plane waves. By tuning the relative phase between the two waves, the diffraction can be entirely suppressed, termed <i>zero diffraction</i>. [2] Therefore, this phenomenon enables to control the diffraction of light with infinite contrast. Remarkably, it not only occurs at singular spectral positions but on continuous curves in the energy–momentum space.<br/> <br/>In analogy to our recent report, we first investigate a standalone grating and a symmetric waveguide grating under symmetric dual-plane wave incidence. Simulations using rigorous coupled wave analysis (RCWA) show that the diffraction efficiency can be controlled with an average contrast below 100. Zero diffraction cannot be found in either of these cases. <br/> <br/>In the next step, we introduce leakiness and find zero diffraction for plane waves. We demonstrate the experimental feasibility of these theoretical findings using real laser beams instead of plane waves. For that purpose, a symmetric waveguide grating is placed between two lithium tantalate (LiTaO₃) wafers of opposite crystal direction. The high refractive index of the LiTaO₃ introduces leakiness, while the opposite crystal direction maximizes the relative phase shift when applying an electric field across the entire waveguide stack. After creating symmetrically propagating beams, we measure the optical power diffracted at the waveguide grating as a function the applied field strength and find a maximum contrast of 1236.<br/> <br/>This way, we also show how zero diffraction can be applied in a nonlinear waveguide in order to control trapping and detrapping of light. The position where a directed laser beam is detrapped from the surface can be selected with an electric field without any mechanics. Laser displays based on zero diffraction would be more efficient than liquid crystal displays as are not based on absorption. Instead, they extract the light only where it is needed. At the same time, such laser displays could be completely transparent and promise immense possibilities in terms of color rendering due to the narrow spectrum of the laser.<br/> <br/>[1] M. Meudt, A. Henkel, M. Buchmüller, and P. Görrn, <i>Optics</i>, <b>2022</b><br/>[2] A. Henkel, S.O. Schumacher, M. Meudt, C. Knoth, M. Buchmüller, and P. Görrn, <i>Adv. Phot. Res.</i>, <b>2023</b>