Electrochromism

LMO-based integrated non-volatile phase shifters

Non-volatile integrated phase shifters based on the charge-dependent refractive index of lithium manganese oxide  (LiMn₂O₄)

In integrated photonics, thermo-optic phase shifters are typically used to compensate for manufacturing tolerances or to adjust long-term effects. However, these require a constant supply of electrical power for heating and are sensitive to fluctuations in ambient temperature.

To counteract this continuous energy consumption, this project investigates how well the charge-dependent refractive index change of lithium manganese oxide (LMO) is suited for the realization of an integrated phase shifter. LMO is a common cathode material in lithium-ion batteries and exhibits a significant change in its refractive index depending on the state of charge.

For initial measurements, an existing chip was used, whose top oxide was removed and then coated with LMO using sputtering in a masking process. Figure 1 shows the coated structures. These are cutback structures, which are used to determine the losses of the waveguide by means of different lengths.

Figure 2 shows a scanning electron microscope image of a waveguide cross-section without top oxide, which was coated with LMO. There are some irregularities in the coating, which need to be examined more closely in further steps.

Simulations on a silicon nitride platform at a wavelength of 900 nm show that a significant refractive index shift can be achieved.

A specially developed silicon-integrated chip designed for comprehensive measurements of this material system is expected in the near future. In addition to structures for determining attenuation, modulator structures have also been designed. After characterizing the basic functionality, the properties at the interfaces and the implementation of a suitable battery cell enclosure will be investigated.