Professor Mitsuru Takenaka and his research group have succeeded in observing for the first time in the world the nonvolatile modulation of the refractive index induced by an external electric field in the ferroelectric material hafnium zirconium dioxide (ZrHfO2). Detailed analysis using a transmission electron microscope revealed that this non-volatile refractive index modulation is caused by a change in spontaneous polarization in ZrHfO2. In addition, an optical phase shifter with ZrHfO2 deposited on a silicon nitride optical waveguide was fabricated, and non-volatile modulation of the optical phase was successfully achieved by applying a voltage. The resulting paper was published in the online edition of the British scientific journal Nature Communications on May 9.