Assuming we have a rectangular shaped potential that modulates in reductive direction between 0 V and -1 V having a period of 2π as displayed for the red curve in Figure . The lock-in amplifier is fed with the same waveform as reference signal and sets the reference phase of 0° to the falling potential at position 0π. Let us further assume that an applied voltage of 0 V is located within the bandgap of the investigated material and -1 V is sufficient negative to reduce the conduction band edge. Therefore, the conduction band is half of the time occupied and half of the time unoccupied. As a result, the absorption behavior of the material under investigation also changes over time. Two cases can be distinguished: