I am a materials science student and my project is to design a nanomaterial. Now I place my material under ultrasonic stimulation and measure its voltage response with an oscilloscope. It is expected that the magnitude of this voltage response should be around 5 volts. Unfortunately, I have noticed that the oscilloscope is experiencing an industrial frequency interference of 50Hz and I have tried a number of methods but I am unable to get rid of it, which is affecting my measurements.

I think this is caused by the high internal resistance of my material. In fact, my other previous electrical measurements show that my material has a resistance of about two million ohms. What can I do about the circuit (e.g., grounding) to eliminate the interference and continue my measurements?

The first image shows the image where the interference occurred. The second image is the connection of the line when the interference occurs, where the square transparent object is my large resistance material. The third image shows this interference disappearing from the oscilloscope when I connected the measurement probe directly to the ground probe. Also, when I disconnected the test probe from the material under test, the oscilloscope still showed this interference.