Applications in molecular dynamics and nanofabrication motivate scientists to find ways to measure fields with ever finer temporal resolution. Such measurements rely on our having access to electrical phenomena that respond on ultra-short timescales. So far, researchers have achieved sampling frequencies up to mid-infrared frequencies using nanoscale antennas and electro-optics. In this paper, Kealhofer et al. demonstrate a third strategy. They apply a test voltage to a sharp metallic tip, and illuminate the tip with a pulsed laser. The light excites electrons in the tip, and some of them are ejected. The energies of these electrons can then be measured, and correspond to the test voltage at the instant of time that the laser pulse hit the tip.
Since electron emission of this type happens on very short time scales, and since electrons are only emitted from the very end of the probe, the measurement can be thought of as instantaneous in time compared to a voltage oscillating in the 10 GHz range. The problem is that only a few electrons are ejected each time, so the measurement of their energies tends to be noisy. To compensate, the authors measure the same signal many times. The voltage applied to the tip is oscillatory, and its phase (position with respect to a reference signal at the same repetition frequency) is controlled precisely. The voltage at a single phase is measured for about 200 seconds before advancing the phase to the next voltage point. In this way, they sweep out a measurement of the signal with a temporal resolution of about 22 femtoseconds, or about 4.5*10^13 samples per second - comparable to other recent techniques.
The time scale and accuracy of the measurement depends on the field strength as well as the laser pulse width and repetition rate, so the authors expect the results of this method to improve as it matures.
You must log in to add comments.