  We propose and demonstrate a nanoscale vacuum-tube diode triggered by
few-cycle near-infrared laser pulses. It represents an ultrafast electronic
device based on light fields, exploiting near-field optical enhancement at
surfaces of two metal nanotips. The sharper of the two tips displays a stronger
field-enhancement, resulting in larger photoemission yields at its surface. One
laser pulse with a peak intensity of $4.7\times 10^{11}$ W/cm$^2$ triggers
photoemission of ${\sim }16$ electrons from the sharper cathode tip, while
emission from the blunter anode tip is suppressed by $19$ dB to ${\sim }0.2$
electrons per pulse. Thus, the laser-triggered current between two tips exhibit
a rectifying behavior, in analogy to classical vacuum-tube diodes. According to
the kinetic energy of the emitted electrons and the distance between tips, the
total operation time of this laser-triggered nanoscale diode is estimated to be
below 1 ps.
