Using liquid crystals as tuneable waveplates in femtosecond laser direct written waveguides

Due to their electro-optical and thermo-optical properties, liquid crystals are used in a wide variety of applications to dynamically change the properties of light. Some widespread examples are the flat-panel display or the spatial light modulator. Liquid crystals are also used in several waveguide types for tuning or sensing applications [1] - [3] . Femtosecond laser direct written waveguides, however, have so far not been combined with liquid crystals. This waveguide type is suitable for rapid prototyping of three-dimensional photonic circuits. To fabricate a waveguide, the focus of a femtosecond laser is moved through a transparent material. Due to nonlinear interactions in the focal volume, local modifications can be achieved, which serve as waveguides. However, after fabrication, the possibilities to actively reconfigure the waveguide properties are limited and often based on thermal effects [4] - [6] . We propose a novel approach of tuning the waveguide properties by embedding a layer of liquid crystals into the femtosecond laser direct written waveguide. We show that it is possible to use liquid crystals as switchable retardation elements in femtosecond laser direct written circuits. The retardation inside the waveguide can be changed using the applied voltage, allowing the rotation of the output polarization state on the Poincaré sphere. The experiments are performed in a commercially available liquid crystal cell (LCC1318-A, Thorlabs) filled with E7. A schematic sketch of the sample is given in figure 1a ). To demonstrate the switchability of the polarization using liquid crystals, a sinusoidal voltage signal with a frequency of 1 kHz is applied to the electrodes of the liquid crystal cell. The Stokes parameters depending on the applied voltage are measured using a polarimeter. A typical measurement taken with a probing laser of wavelength of 808 nm and diagonal polarization is shown in figure 1b ).