Consider a solid material such as a conductor. I would like to know if there are ways to determine the voltage at specific spots on the material, without plugging a metallic wire and connect it to a voltmeter. I know voltage is relative and I'm interested in checking voltage differences across different specific spots on a conductive material without making any physical contact that could disturb the potential difference. A high Tc superconductor would do the job, but only for temperatures too low for my interest. I was thinking to use EM waves such as a laser and check the reflectivity for instance, but my knowledge is lacking and I do not know whether the reflectivity or other properties can depend on the local voltage.