Sigurthr wrote ...

LED's ARE diodes. They do the rectification themselves. They are, however, succeptible to reverse breakdown (and destruction) because they are not intended to withstand large reverse voltages.

I've never built a slayer exciter (I went right to full on SSTCing) so I don't know the field strengths and RF voltages present, but you could always put a fast diode with high voltage rating in series with the LED to prevent reverse breakdown of the LEDs.

Remember that output power cannot exceed input power, so given the step up in voltage and losses involved you cannot hope to draw very much current from the RF field. Gasseous discharge lights work so well because the RF field directly excites the gas, very very little current is needed to light to even full brightness. Once you start trying other forms of loads you'll find that there isn't a whole lot of energy to draw upon. I've seen wireless power transfer done effectively with a tuned parallel LC receiver feeding a full wave rectifier with LOADS of filter/energy storage capacitance, but there is still a good amount of loss involved. Remember, the output coil emits RF in a nearly omnidirectional manner, so you would need a spherical receiver that completely encapsulates the Rf field to recover most of the energy. If you use a sufficiently large receiver, you might get 2pi coverage at best - for half the total power.