Measurement of field emission current variations caused by an amplitude modulated laser

Abstract

A laser diode ( λ=658 nm, 30 mW) was focused on a tungsten field emitter tip in vacuum. The time dependent emitted current was measured while the laser diode was TTL-amplitude modulated at frequencies from 10 Hz to 50 kHz. It was found that below 100 Hz, the rise and fall times of the field emitted current ranged from 2 to 3 ms, but beginning at 300 Hz, the rise and fall times become shorter as the frequency is increased. The incremental change in the total emitted current is greater at 5 kHz than at any of the other frequencies. Since the only parameter changed was the modulation frequency, we conclude that these effects are not consistent with the assumptions from photofield studies that (1) the increase in the total current is mainly thermal and therefore slow, (2) the current increase is small, and (3) the effect is negligible at the red end of the spectrum. From the carried out experiments, we conclude that the current increase is not caused by tip heating, but rather, they show the significance of the circuit parameters of the field emitter tube itself.