Nonreciprocal backscattering of millimeter waves by lithium niobate crystals when ultrasonic vibrations are excited in them

Abstract

The results of experimental studies of millimeter waves backscattering with a frequency of 33 GHz by lithium niobate crystals using an installation based on a Doppler radar (microwave sensor) are presented. The measurements were carried out when ultrasonic vibrations with a frequency of 50.3 kHz were excited in the crystals and in their absence. It is found that due to elastic ultrasonic vibrations in the crystals, the phase modulation of the reflected wave from the crystal-dielectric-metal system occurs, as a result of which the effect of non-reciprocity of backscattering occurs. Detection of the phase difference between the voltage of the backscattering wave and the voltage of the incident (radiated) wave gave a useful signal level up to 50 dB higher than the noise level of the receiving and transmitting path of the microwave sensor. We established experimentally that the amplitude and frequency of the maximum of the envelope of the backscattering spectral components depends on the angle between the crystal symmetry axes and the vector polarization of the incident electromagnetic wave and on the orientation of the crystal in space. These measurements provide qualitative information about the piezoelectric properties of the crystal material and the presence of elastic waves. Keywords: Nonreciprocal backscattering, lithium niobate crystals, ultrasonic vibrations, Doppler radar, phase detection.