Ironically, the premise (laser voltage probes) is physically possible, under rather limited circumstances.
Suppose you have two conductive surfaces, with a known (and equal) work function. Suppose you bring a conductive probe near these surfaces, charged to some voltage V. Finally, you expose one surface to light of a given energy.
The irradiated surface will emit electrons, if the light energy is higher than the work function (photoelectric effect). The electrons can be collected by the conductive probe, but only if its voltage is higher than that of the irradiated surface minus the excess energy due to photoelectric emission.
So if the surface is at, say, +8V, the work function is 3eV, and the light is 4eV (~ low UV), then at 7V on the probe, no current will flow (or at least, no additional current), no matter how intense the light. At 7.1V, very little current will flow (only those electrons with just enough energy to touch it), and at higher and higher voltages, more and more current will flow, until saturation is reached (which might require >100V for a modestly close proximity).
Finally, you repeat the process for the other surface, and now you have a voltage difference.
The whole thing needs to be referenced through ground, somehow or another, because we are talking exchange of electrons. But it need not be a good ground; we're only pulling off tiny currents, so parasitic capacitance can be enough (e.g., the ~150pF of a human body, or similar sized equipment). Obviously, any existing potential (including static charge!) has to be accommodated by the sense probe, so it might have to resolve fractional volts out of a +/-10kV range. Which would be kind of inconvenient.
The three parts that make this ridiculous:
- You need identical surfaces. Atomically clean metals are good. Oxidized surfaces, with adsorbed moisture, and sticky finger prints, are not. The presence of air doesn't really screw with this (except for ionization at high probing voltages). However,
- You need enough energy in the light to achieve the effect. Materials with low work functions include cesium and reactive compounds thereof (which are used in phototubes -- not too common these days, but photomultiplier tubes remain one of the best ways to detect individual photons). Needless to say, these aren't very air stable.
- The energy requirements may also preclude air itself. UV-C and up are required to stimulate emission from most metals (work functions in the 6eV+ range), but oxygen absorbs this.
And, obviously, that probe needing to be close to the detecting surface kind of puts a hindrance on things like isolation and "remote sensing". (You could just as well do the sensing with an electrometer!)
Tim