Lithium niobate enables optical modulation because of its electro-optical effect. The refractive index can be manipulated by applying an electric field in the area of a propagating light beam in the material. Together with a fabricated domain poling structure, devices such as a polarisation modulator can be realised. This modulator converts the incident polarisation from a vertical to a horizontal polarisation or in the reverse direction by only applying a voltage. We have realised such a polarisation converter in a temperature range of 300K to 0.8K and characterised the change in the modulation performance.
In the video above, the response of a polarisation converter is shown, when applying different voltages around the operation wavelength. The temperature of the polarisation converter is then reduced from 300K to 0.8K over a duration of 16h. Polarised light beam is sent through the sample and changes in the intensity are monitored in the orthogonal and vertical polarisation direction. A change in the applied voltage converts the incident polarisation from on to the other.
In small intervals the wavelength and modulation voltage is swept around the optimal operation voltage. The operation wavelength is shifting from a smaller to a larger wavelength because the refractive index of the material is also shifting with the decrease of temperature. At the (temperature dependent) operating wavelength, the light is converted from the TE to TM mode with an intensity ratio of at least 25dB over the entire temperature range.