Artificial retinal glial-like waveguides for biomimetic volume optics

Summary form only given. In the vertebrate eye light must be funnelled through a mangled mass of scattering tissue by Müller cells [1]. In distinction to conventional waveguides, that are essentially tubular, these cells have a double-funnel shape, and can efficiently focus, collect, transfer, and outcouple light without the strong mode selectivity of waveguides. Here we explore the use of biologically inspired funnel index of refraction patterns that mimic retinal Müller cells as versatile volume blueprints for multiple optical functions [2]. Compared to tube-like patterns typical of soliton-based waveguides, funnels are fully three-dimensional structures (illustrated in Fig. 1(LEFT)) that achieve either focusing, guiding, and defocusing: the key ingredient is the changing shape along the propagation direction (say the z axis) that can, depending on circumstances, act as a lens (the cellular ”end-feet”), or as a fiber (the cellular ”body”), thus forming a basic blueprint to multifunctional optics.Funnel patterns are achieved through photorefraction. During the pattern writing phase, light propagates with a polarization orthogonal to the external bias so as to allow for an efficient build-up of the space-charge field Esc without it affecting the beam: the beam diffracts and produces the funnel-like Esc. In the second ”readout phase”, the exposed region in the crystal is used to affect the propagation of light polarized along the x direction, parallel to E0. In this case, the electro-optic response is maximized, and Esc is kept fixed using a low intensity visible beam. The functionality of the pattern is fixed by the duration te of the writing phase (this determines the saturation parameter a in Fig. 1(CENTER) that flags the different response curves).Experiments are carried out in a sample of Cu-doped paraelectric KLTN. During the exposure phase, a 800 nW TEM00 beam of λ = 543 nm polarized along they direction is focused down to w0 6.5 μm at the minimum waist plane at zc. Bias fields range from E0 = -4.6kV/cm to +4.6 kV/cm and the amplitude of the index modulation is Δn ~ 3 χ 10-5, a value insufficient to alter diffraction. In the readout phase, intensity reduced to 10 nW and the optical polarization is rotated in the x direction, and Δn0 ~ 4 10-4 (see Fig. 1 (RIGHT)). We find conditions for tunable lensing, waveguiding, and use the quadratic electro-optic response to rapidly switch functionality, passing, for example, from a focusing to a defocusing regime, and explore the rich variety of multi-funnel patterns that ultimately allow for a remarkably compact optical components, such as switches and splitters. Results can form the basis for more elaborate fully three-dimensional optical circuitry for light control in highly miniaturized environments that are dominated by strong diffraction.