Standard LEDs will work into the GHz. You also need a return path for the current (diode rectifies in one direction).. preferably a resistor that puts the decay

Standard LEDs will work into the GHz. You also need a return path for the current (diode rectifies in one direction).. preferably a resistor that puts the decay time constant in a visible period (e.g. few ms). However, you're correct old crystal radios worked like this. Condenser Mics might also work with some rectification.